Unstart phenomena induced by flow choking in scramjet inlet-isolators

Progress in Aerospace Sciences xxx (2017) 1–21

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Unstart phenomena induced by flow choking in scramjet inlet-isolators Seong-kyun Im a, *, Hyungrok Do b, ** a b

Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN, USA Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul, South Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Hypersonic propulsion Scramjet inlet-isolator Unstart Flow choking Shockwave-boundary layer interaction Flow control

A review of recent research outcomes in downstream flow choking-driven unstart is presented. Unstart is a flow phenomenon at the inlet that severely reduces the air mass flow rate through the engine, causing a loss of thrust and considerable transient mechanical loading. Therefore, unstart in a scramjet engine crucially affects the design and the operation range of hypersonic vehicles. Downstream flow choking is known to be one of the major mechanisms inducing inlet unstart, as confirmed by recent scramjet-powered flight tests. The current paper examines recent research progress in identifying flow choking mechanisms that trigger unstart. Three different flow choking mechanisms are discussed: flow blockage, mass addition, and heat release from combustion reactions. Current research outcomes on the characteristic of unstarting flows, such as transient and quasi-steady motions, are reviewed for each flow choking mechanism. The characteristics of unstarted flows are described including Buzzing phenomena and oscillatory motions of unstarted shockwaves. Then, the state-of-the-art methods to predict, detect, and control unstart are presented. The review suggests that further investigations with highenthalpy ground facilities will aid understanding of heat release-driven unstart.

1. Introduction 1.1. Background Supersonic combustion ramjets (scramjets) have been developed to power the next generation of hypersonic air-breathing aircraft [1–57]. Like other air-breathing combustion engines, the scramjet demands for a stable oxygen supply from the atmosphere to the combustor being captured at the inlet. Unstart in a scramjet is a flow phenomenon at the inlet that limits the oxygen delivery of the intake air flow to the supersonic combustor [6,8,14,35,41,49,58–93]. Unstart accompanies an unwanted and abrupt oxygen deficit in the combustor and unsteady flow spillage at the inlet, which leads to a loss of both thrust and vehicle control [58,68,73,94]. Consequently, unstart can severely impair the operation of the scramjet. The recent X-51 scramjet-powered hypersonic vehicle flight test, for example, suffered from the inlet unstart [95]. The scramjet flow passage mainly consists of four parts: inlet, isolator, combustor, and exhaust nozzle [96]. The converging inlet compresses and decelerates the incoming supersonic or hypersonic air flow. The isolator, connecting the inlet to the supersonic combustor further decelerates the flow and prevents disturbance propagations from the combustor to the inlet. The combustor supplies thermal energy to the

flow via combustion reactions. Finally, the diverging exhaust nozzle accelerates the supersonic flow to generate thrust enabling hypersonic flights [10]. Once the internal flow is choked in the combustor or isolator, the static pressure and temperature quickly rise at the choked location (throat) due to the rapid deceleration of the supersonic internal flow [97–101]. The sudden rises in the pressure and temperature affect the flow in both the upstream and downstream regions. The rapid pressure rise at the choked throat increases the pressure in the subsonic portion (e.g., subsonic regions of boundary layers, subsonic corner flow areas, separated boundary layer regions) of the upstream region via the reverse propagation of sonic pressure waves [75,102,103]. The increased pressure extends the subsonic portion, and under a certain range of conditions, the virtual area of the choked throat decreases. The inlet eventually unstarts when the choked throat reaches the critical condition causing unsteady flow spillage [73,75,78–81,102,104,105]. Conversely, the flow is un-choked at a downstream location. Thus, the sonic point reappears due to the large pressure difference between the choked flow and the atmosphere at a typical altitude for hypersonic flights (see Fig. 1). The conditions causing unstart can be formulated with i) incoming air flow conditions such as freestream Mach number (Ma), pressure, temperature, and turbulence properties, ii) geometric (design) parameters of the inlet and flow passages such as inlet contraction ratio (CR), leading

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.-k. Im), [email protected] (H. Do). https://doi.org/10.1016/j.paerosci.2017.12.001 Received 31 July 2017; Received in revised form 27 November 2017; Accepted 22 December 2017 Available online xxxx 0376-0421/© 2017 Published by Elsevier Ltd.

Please cite this article in press as: S.-k. Im, H. Do, Unstart phenomena induced by flow choking in scramjet inlet-isolators, Progress in Aerospace Sciences (2017), https://doi.org/10.1016/j.paerosci.2017.12.001

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Ma, and the ratio of specific heats, respectively. When the inlet CR of an axisymmetric scramjet vehicle exceeds the critical CR defined in Eq. (1), excessive compression in the inlet can potentially induce the inlet unstart. Nevertheless, determining a universal critical CR at a given freestream Ma as an unstart boundary, e.g., simply a function of only freestream Ma as in the Kantrowitz limit, is not practically feasible. For example, in cases with non-axisymmetric 3D inlets, the inlet CR cannot solely define a starting/unstarting criterion [57,115,116]. This is because the criterion depends not only on the inlet CR but also on the geometry of the inlet and the isolator as a whole (Fig. 2). Fig. 2 represents the contraction ratio limits for inlet starting/unstarting, Kantrowitz, isentropic, and empirical limits. As seen in the figure, the Kantrowitz limit alone cannot represent a starting/unstarting criterion. The 3D inlet geometry along the flow path controls the compression process that affects the resulting stagnation pressure loss while the flow contracts through the inlet. The inlet geometry also determines whether the strong incident shockwaves from the leading edges (lips) enter the isolator and the strength of these shocks. Once the incident shockwave enters the isolator, sequential reflections of the shockwave along the flow passage will further compress the flow [43,57,107,117–119]. The flow will be choked when the compression through the inlet and the isolator is excessive (Fig. 2). Furthermore, the shape of the cross-section in the inlet and the isolator can also affect the unstart boundary [45,51,107,120–128]. The shape will determine the structure, interaction, and strength of the incident and the reflected shockwaves, as well as the development of the boundary layers on the internal surfaces, corner flows, and their interactions with shockwaves. These important internal flow features significantly influence the formation and distribution of subsonic flow area in the internal flow passages that will potentially deliver pressure waves from the downstream high-pressure region toward the inlet when the unstart process is triggered [75,102,103]. The internal flow choking and unstart are also caused by the downstream pressure build-up due to flow deceleration [129], fuel mass addition [104], and combustion heat release [109]. The pressure rises from fuel mass injection and accompanying combustion heat release are typically much higher than that from the flow deceleration depending largely on the geometry of flow passages [109]. Therefore, fuel mass injection rate and combustion heat release have been regarded as the crucial scramjet operation conditions that can actively trigger or prevent the internal flow choking and unstart [10,98–100,109]. Fuel mass injection and the following combustion heat release are closely correlated, i.e., the combustion heat release will monotonically increase with the fuel injection rate presuming operations in overall fuel-lean conditions. The correlation would, however, vary with the combustor geometry, fuel injection strategy, injection location, and injection direction [128, 130–138]. Ultimately, the scramjet combustor would be designed to maximize the combustion efficiency (ratio of burnt fuel to the injected fuel) and to minimize the simultaneous and conflicting stagnation pressure loss that occurs in practice [56,57,136,137,139–141]. Hence, there is no generic or representative correlation between the fuel mass injection rate and the combustion heat release. Therefore, it is practically unavoidable to separate the two, mass and heat additions, in formulating the unstart conditions.

Nomenclatures Ma or M CR A γ TR f D FA Ff V Vi

αi

as Φ fn a M L

Mach number contraction ratio flow area (m2) specific heat ratio throttling ratio friction coefficient hydraulic diameter (m) area change coefficient friction influence coefficient local freestream speed (m/s) jet injection speed (m/s) jet injection angle speed of sound (m/s) equivalence ratio resonance acoustic frequency (Hz) mean speed of the sound (m/s) mean Ma in the channel characteristic length of the channel

Fig. 1. Air-breathing hypersonic vehicle flight trajectory (altitude and speed) and operational limits [96].

edge angle, and isolator length and iii) downstream pressure build-up from combustion heat release, fuel mass injection, and shockwave/ boundary layer-induced deceleration/separation of the internal flow [66]. The i) freestream flow conditions and the ii) geometric parameters passively govern the starting/unstarting criterion; i) and ii) are invariable in steady cruise flights and predetermined by the designated flight trajectory and the corresponding vehicle design. There have been numerous studies delineating the freestream flow conditions and the geometric parameters to define an unstart boundary [61,104,106–109]. In particular, the freestream Ma and the inlet CR have been most frequently used to describe the unstart boundary [36,53,107,110–113]. For example, the Kantrowitz limit [114] determines the critical inlet CR causing inlet unstart as a function of freestream (incoming air flow) Ma (Eq. (1)):
CRKantrowitz ¼

Ainlet Athroat

1.2. Scope of the review The current paper aims to provide an overview of flow chokingdriven unstart in scramjets and the resulting flow phenomena. As addressed above, the causes of unstart are typically categorized by three factors, freestream conditions, an inlet-isolator geometry, and downstream flow choking. Chang et al. [107] recently outlined the research progress on these three unstart mechanisms. Freestream condition- and geometry-driven unstart phenomena primarily concern the flow at the lip of the scramjet inlet. Consequently, the phenomena are relatively simpler than those induced by downstream flow choking because unstart driven by flow choking involves the flow phenomena of the entire internal flow

 Kantrowitz

" # γþ1 1 γ  γ1 1 2 2ðγ1Þ 1 þ γ  2M 1 ðγ þ 1ÞM 2 γ1 γþ1 ¼ 2 M ðγ  1ÞM 2 þ 2 2γM 2  ðγ  1Þ γ þ 12 

(1) where Ainlet, Athroat, M, and γ are the inlet area, throat area, freestream 2

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Fig. 2. Inlet starting regimes for various Ma and contraction ratios. Kantrowitz, isentropic, and empirical maximum contraction ratio limits for inlet starting are shown [114]. Note: Diffuser area ratio in the plot is the reciprocal of the contraction ratio.

inlet near the leading edge (lip) appears in certain flight situations, e.g., the vehicle is in the unsteady pitching motion. Vehicle pitching can move the shock impinging locations in the inlet, and prompt unexpected shockwave/boundary layer interactions (SBLI) inducing a large flow separation zone close to the inlet lip [53,146], which can cause unsteady and local flow spillage at the inlet. In this instance, the core flow entering the internal flow channel, however, remains supersonic, and the scramjet internal flow is not choked. Even the local flow spillage due to the flow separation at the inlet can deem the vehicle uncontrollable and further increase the chance of inlet unstart. Fortunately, the situation causing the flow separation-driven local flow spillage is highly improbable, particularly, under high-Ma flight conditions, when the maneuverability of the vehicle (e.g., changing directions) is considerably limited. Therefore, the discussion in the current paper focuses only on the flow spillage caused by internal flow choking. Strategies to detect and control the inlet unstart have been investigated, to mitigate the adverse effects of inlet unstart on scramjet flight operations [35,62,63,77,85,86,91,147,148]. Inlet unstart causes severe fluctuations in the flow at the inlet and the isolator [147–149]. The fluctuations will then induce instabilities in combustion processes [49, 109]. Although, these fluctuations negatively affect thrust generation and vehicle controllability, they can serve as the useful indications of the inlet unstart. Thus, most of the previous unstart detection studies have used the deviation as a measure of the fluctuation in combustion and pressure to capture unstart. Due to the limited knowledge regarding unstart, unstart control strategies have not yet been extensively studied and tested. However, in recent years, some novel studies have successfully demonstrated the implementations of various flow control methods employing vortex generators and plasma actuators to suspend or prevent unstart effectively. Examples of research efforts in this regard are introduced later in this review. The topics discussed in the paper are as follow. Unstart induced by downstream flow choking and the characteristics of the unstarting flows are discussed in Section 2. The flow behaviors of the unstarting flows are described for three different flow choking mechanisms (flow blockage, mass injection, and combustion heat release). The characteristics of the unstarted flows are described in Section 3. Then, the detection and control of unstart are presented in Section 4. Finally, Section 5 presents the conclusion and future study suggestions.

path of the engine. Therefore, giving fresh perspective of flow choking-driven unstart phenomena would aid to understand the flow physics of scramjet unstart and to develop better scramjet inlet-isolators. The first part of the current review focuses on the dominant flow choking mechanisms such as flow blockage [79], fuel mass injection [75, 104,109], and combustion heat release for a fixed vehicle geometry [99, 109], a steady cruise flight condition, and zero yaw and pitch angles. Here the choking of the supersonic internal flow implies that the Ma is equal to or below the unity in the entire cross-sectional area of the scramjet flow channel, at least at a certain location. During a normal scramjet operation, the internal flow remains in the supersonic regime throughout the flow passage, but not necessarily in the entire cross-section. It is well-known that subsonic regions exist within boundary layers in supersonic internal flows, and the intensive combustion and shockwave impingements can induce a local subsonic area [142,143]. Herein, the transient unstarting flow phenomena will be discussed in detail, e.g., the onset of unstart due to internal flow choking and propagations of unstart shockwaves toward the inlet. The unstarting flow features including the unstart shockwave behaviors and interactions between the unstart shockwave and pre-existing shockwaves will be compared. These characteristics strongly depend on the choking mechanism, i.e., via flow blockage, mass injection, and heat release. It should be noted that the current paper will primarily discuss unstart phenomena in scram-mode operations. Thus, pre-existing shockwave structures mainly consist of the incident inlet shockwave and its subsequent reflections. After the completion of the transient unstart phenomenon, new quasi-steady shockwave structures are formed near the inlet, and the characteristics of the flow structures also depend on the choking mechanism. Again, the unstarted flow features induced by the three internal flow choking mechanisms will be compared. Another important aspect of the unstarted flow is the flow spillage at the inlet [109,144]. As introduced above, inlet unstart is a flow phenomenon at the scramjet inlet that limits the oxygen delivery of the intake air flow to the supersonic combustor. Namely, catastrophic ‘inlet unstart’ occurs when the approaching flow toward the inlet cannot be fully captured or swallowed through the internal flow passages of the vehicle, leading to the flow spillage. Therefore, the flow spillage is one indicator of the inlet unstart. The flow spillage phenomenon, however, could also result from local adverse pressure gradient at the inlet, rather than the flow choking [145]. The local adverse pressure gradient at the 3

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2. Flow choking and unstarting flow

using the total backpressure rise [84,91,118,150]. For example, the throttling ratio (TR) is a commonly used to describe flow choking when mechanical flow blockages are used to trigger unstart [62,68,79,87,93]. Jet momentum ratios [104], jet mass flow rates [75,151], and equivalence ratios [49,56,99,140,143] are other common parameters that have been used to quantify flow choking and unstart thresholds. The flow behaviors at the time of flow choking could vary considerably, depending on the dominant flow choking mechanism. Laurence et al. [143] compared the required flow area change and heat release that can trigger flow choking using theoretical one-dimensional (1D) analysis. The analysis shows that the necessary flow area change drastically increases with an upstream Ma away from the unity. Thus, severe flow separation is required to induce unstart when flow blockages are used to increase the backpressure as seen in Wagner et al. [68,79]. Conversely, thermal choking driven unstart does not accompany severe flow separation because heat release serves as a dominant pressure rise source, which is confirmed by Laurence et al. [142,143,152]. Im et al. [109] further the discussion, using experimental results and the following simplified 1D equation (Eq. (2)), for the factors affecting backpressure rise and for the portion of each factor contributing to the total backpressure rise.

This section discusses downstream flow choking and the unstarting flow features. Internal flow choking can be induced by any physical phenomena that can change downstream pressure such as flow separation, friction, mass addition, and heat release. Under flight conditions, all physical phenomena could simultaneously contribute to triggering flow choking, although the degrees of contribution differ for each flight condition. Therefore, it would be ideal to collectively investigate the physical phenomena affecting internal flow choking, for any given flight condition. However, the realization of the flight flow conditions (high-Ma and high-enthalpy) on the ground is extremely challenging. Furthermore, it is still more challenging to investigate the effects of each physical phenomenon on internal flow choking independently. Accordingly, various studies have simplified unstart using various flow choking mechanisms. One of the popular ways to mimic internal flow choking has been downstream pressure rise using flow blockages at low-enthalpy conditions, given flow blockages can be easily achieved by using a mechanical component and low-enthalpy conditions can provide simple but valuable flow diagnostics. Although using flow blockages as a flow choking mechanism at low-enthalpy conditions does not allow investigating the flow choking dynamics, the simplified studies could provide overall knowledge on the unstart process. In recent years, there have been efforts to replicate unstart close to real flight conditions. For example, a series of studies replaced mechanical flow blockages by mass addition through jet injection. Although the studies were still performed at low-enthalpy conditions, the investigation of the unstarting and unstarted flows close to the actual flight flow configuration was possible. The studies of unstart using combustion heat release as a flow choking mechanism are relatively rare compared to other flow choking mechanisms, mainly due to the limited available ground facilities that can reproduce high-enthalpy flows of scramjet flights, yet, incorporating combustion would give flow conditions closest to the actual scramjet flights. Nevertheless, recent progress in high-enthalpy flow facilities and flow diagnostics allow us to investigate unstart that could be closest to actual unstart in practice. In this section, flow choking and unstarting flow features induced by three different choking mechanisms including mechanical flow blockage, jet injection, and combustion, are discussed.

  M 2 dT0 2γM 2 ½1 þ ðγ  1ÞM 2  dx dp γM 2 dA γM 2 1 þ γ1 2 ¼   f p 1  M2 D 1  M2 1  M2 A T0 ( )   2 2 2 2γM 2 γ1 M γM ½1 þ ðγ  1ÞM  V cos α d m _ i i 2  þ 1  M2 m_ V 1  M2 ¼ FA

dA dT0 dx dm_ þ FT þ Ff f þ Fm   A D m_ T0

(2)

where f , D, FA , Ff , V, Vi , and αi are friction coefficient, hydraulic diameter, area change coefficient, friction influence coefficient, local freestream speed, jet injection speed, and jet injection angle. Three different flow choking conditions, low-enthalpy freestream and mass addition, high-enthalpy freestream and mass addition, and high-enthalpy freestream and heat release, were investigated to identify the threshold and the dominant source of flow choking in a model scramjet. It was argued that both mass addition and heat release significantly affect backpressure rise leading to flow choking [109]. However, both the experiments and theoretical analysis showed that mass addition results in a higher pressure rise than heat release for the same ratio changes in mass addition or heat release. The effect of mass addition becomes much greater than that of heat release as upstream Ma increases from the unity. However, this observation is partially caused by the flow configuration, so a rich-mixture should be used to induce unstart. In practice, excessive combustion heat release is known as the key cause of scramjet inlet unstart [10,98–100,109,134,141,143,153–155]. It is evident that the injected fuels should be fully combusted in the combustor to maximize the achievable thrust [56,57,136,137,139–141]. Consequently, there have been numerous attempts to secure the reliable thrust maximized in flight and, thereby, to maintain stable combustion reactions [20,50,134,135,138,156–158]. Assuming that the combustion reaction is successfully stabilized in the combustor, the total heat release rate depends simply on the fuel injection rate if the combustor is not operated under a fuel-rich condition which is less probable in practice. At a fixed incoming air intake rate into the combustor, as the fuel injection rate increases, the heat release rate and the flow temperature will increase accordingly. The speed of sound (as) is ideally proportional to the square root of the static temperature of the medium. Thus, the as will also increase with the elevated temperature [159]. When as exceeds the combustor flow speed on an entire cross-section of the combustor at a location, the internal flow is thermally choked. Once the flow is choked, downstream pressure waves can freely propagate upstream until they reach the choked location that is virtually a ‘choked throat.’ The pressure waves will then find alternative propagation channels such as the subsonic boundary layer cores, corner flow regions, and separated flow

2.1. Flow choking mechanisms The onset of flow choking-driven unstart is caused by backpressure rise. Although there are many sources (flow choking mechanisms) that could increase backpressure, the major reasons are effective flow area change, mass addition, and heat release. It is evident that the flight and freestream flow conditions also strongly affect unstart. For example, a higher freestream Ma number operation is relatively immune from unstart, due to the higher backpressure threshold for flow choking. However, this review paper focuses on flow choking-driven unstart. Thus, only the sources of backpressure rise are considered in the discussion. Also, flow choking alone does not induce unstart. Flow choking could result in ramjet-like flow structures, downstream flow choking and strong upstream termination shockwave. Therefore, here, we only consider flow choking with a sufficiently high backpressure rise that triggers complete inlet-isolator unstart. Quantifying the portion of backpressure rise by each mechanism is extremely difficult. It requires a fully resolved velocity field, pressure distribution, and temperature distribution along the entire scramjet flow path. Such information, however, does not exist to date either from the most advanced experimental or computational investigations. Thus, most of the unstart studies qualitatively discuss the flow choking mechanisms [6,58,60,69,97–101]. Due to the challenge of quantifying pressure rise by each mechanism, most of the unstart studies have used the total backpressure rise to discuss flow choking phenomena without quantifying the source of pressure rise. Instead, many studies report flow choking using quantities from the methods used for backpressure rise or 4

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mechanisms are addressed.

regions that are just in front of the choked throat being connected with the downstream subsonic area behind the choked location. The pressure waves that penetrate into the alternative subsonic channels will enlarge the subsonic portion and increase static temperature to extend the subsonic region toward upstream and against the supersonic internal flow. Under a range of the flow condition defined by the unstart threshold, the subsonic region (along with separation shocks in cases with significant flow separations at the forefront of the extended subsonic region [54]) would move to the inlet to cause flow spillage, which is referred to as the inlet unstart [109,144]. With the analogy in the discussion above, the fuel injection rate or the fuel concentration in the supersonic combustor could be the one dominant parameter defining the unstart threshold, which is true at least with a fixed vehicle geometry, fuel injection strategy, flight Ma/altitude, and yaw/pitch angles. As an example, Dalle and Driscoll [160] have shown in a numerical study as seen in Fig. 3 that the ram mode (thermally choked) to scram mode (not choked) transition can be triggered by slightly reducing the overall fuel concentration (overall equivalence ratio) in the combustor from 0.145 to 0.144. Such an abrupt change in combustion dynamics, i.e., the transition from subsonic combustion to supersonic combustion, could, however, be induced by a slight reduction of fuel injection rate. Zhang et al. [54] also experimentally observed that the combustion mode can be abruptly changed from supersonic to subsonic due to the flow choking when the fuel concentration in the supersonic combustor is increased. In the experimental investigation, three different combustion modes were identified with the increasing fuel concentration, including scramjet mode (supersonic combustion), weak ramjet mode (partial subsonic combustion), and strong ramjet mode (subsonic combustion in choked flows), in concurrence with previous numerical investigations. Emory et al. [161] numerically analyzed a full-system scramjet vehicle (HyShot II model) and predicted that the subsonic portion in the scramjet combustor increases with fuel concentration as depicted in Fig. 4. A similar phenomenon was also numerically investigated earlier with a strut injector (McDaniel and Edwards [98]), which was described as the growth of the separation region or the expansion of reacting shear layers, with increasing fuel concentration.

2.2.1. Mechanical flow blockage The use of mechanical flow blockages mimics the downstream pressure build-up by the effective flow area change, due to boundary layer growth and flow separation. Although unstart induced by the flow blockage would omit flow behaviors induced by jet injection and heat release in practice, understanding of the overall unstart process could be investigated. Once the flow choking occurs due to downstream pressure build-up, unstart shockwaves are spawned to match the pressure rise. Then, the unstart shockwaves propagate upstream toward the inlet as downstream pressure build-up continues. The onset of unstart and the transient behaviors of unstart shockwave propagation in a model inletisolator were observed in detail by Wagner et al. [68,79,102] using pressure measurements and qualitative schlieren flow visualization (see Fig. 5). When a mechanical flap chokes the flow, a sudden rise in pressure at a downstream sensor location is observed. The sudden rise in pressure indicates that there exist unstart shockwaves near the location of the flow choking (the location of the mechanical flap). The onset of unstart accompanies flow separation due to the high adverse pressure gradient. The separation increases viscous effects that reduce the effective flow area, promoting local flow choking. Therefore, the flow separation accelerates the initiation and progression of unstart. After the formation of the unstart shockwave structures, the unstart shockwaves move upstream (as depicted by the schlieren images in Fig. 5) because the strong adverse pressure gradient cannot be matched with stable shockwaves. The propagating unstart shockwaves interact with pre-existing shockwaves and boundary layers. When the unstart shockwaves reach the location of the upstream shockwaves, the unstart shockwaves coalesce with the pre-existing shockwaves. Then, the merged shockwaves become stronger shockwaves, resulting in steeper shockwave angles. Consequently, the merged shockwave increases the pressure gradient across the impinging location of the shockwaves and induces larger flow separation. Enlarged flow separation pushes the unstart shockwaves further upstream, and the propagated unstart shockwaves remerge with upstream shockwaves. The shockwave merging and flow separation repeat during the progression of unstart. Similar transient motions were observed by other studies for various inlet-isolator configurations and flow conditions using mechanical flow choking mechanisms such as flaps and plugs. Tan and Guo [78] reported shockwave propagation using high-resonance frequency pressure sensors. Li et al. [144] and Tan et al. [69] visualized the transient shockwave motions for various throttling cases using high-speed schlieren imaging. Su and Zhang [163] simulated backpressure effects on the unstart process using flap as the backpressure rise mechanism. Each group reported different unstart shockwave structures and propagation speed. An

2.2. Unstarting flows Once the flow is choked with a sufficiently high backpressure, the onset of unstart occurs, and the unstart process begins [84,162]. Often, the state between the onset of unstart and the completion of unstart is called the “unstarting” state. The unstarting flows exhibit many interesting flow phenomena including, flow separation, oscillatory flows, and transient and quasi-steady shockwave propagation. In this section, the flow behaviors of the unstarting flows induced by various flow choking

Fig. 3. Subsonic combustion in ram mode at overall equivalence ratio (Φ) 0.145 and supersonic combustion in scram mode at Φ ¼ 0.144 when the flight Mach number is fixed at 4.87 [160].

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Fig. 4. Ratio of subsonic cross-sectional area to total cross-sectional area. Nominal condition Φ ¼ 0.3, thick continuous line; Φ ¼ 0.35, dashed line; Φ ¼ 0.4, dash-dot line; Φ ¼ 0.45, dotted line; and Φ ¼ 0.5, dash-dot-dot line [161].

Fig. 5. High-speed schlieren imaging (left) and PIV measurements (right) for an unstarting flow. Unstart shockwave propagation, shockwave merging, and boundary layer separation are shown in schlieren and PIV measurements [79,102].

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the upstream shockwaves (leading edge shockwave and subsequent reflected shockwaves). The static pressure at the location of upstream shockwave impingement is relatively higher than other regions. Thus the unstart shockwaves experience lower local pressure ratios across the unstart shockwave when they approach the upstream shockwaves. The reduced local pressure ratio decreases the local driving force for shockwave propagation. Then, the unstart shockwave anchors at the impinging location of the upstream shockwave until the backpressure builds up enough to push the unstart shockwave again. The quasi-steady flow motions were further studied by Do et al. [58,75] and Im et al. [109] using mass injection as a flow choking mechanism, which is addressed in the following section.

oblique shockwave [58,127,164–166], a normal shockwave [117,164], a pseudo-shockwave [75], or multiple oblique shockwaves [102,165] appear as propagating unstart shockwaves, and the speed of propagation ranges from a few m/s to tens of m/s. It is apparent that the differences in the unstart shockwave structures and the propagation speed originate from the dependency of unstart processes on flow geometries and flow conditions (e.g., Ma and boundary layer conditions). Although there are differences in unstart processes due to geometries and flow conditions, there exist similarities across all the studies of unstart triggered by the mechanical flow blockages. The flow choking occurs at the location of the flow blockage, the flow choking spawns the unstart shockwaves, and the unstart shockwaves propagate upstream interacting with the boundary layers and the pre-existing shockwaves. Studies about the onset of unstart and unstarting flows have primarily focused on the transient motions of the flow. However, several research groups [58,68,75,79,102,103,109] reported another important flow phenomenon, namely, the quasi-steady state of the unstarting flow motions such as anchoring of the unstart shockwaves at a certain location. The deceleration of the unstart shockwave propagation was observed by Wagner et al. [79,102] using high-speed schlieren and PIV measurements (Fig. 5) when the unstart shockwave reaches the impinging location of

2.2.2. Mass injection Few studies [58,69,75,109] describe using mass injection as a flow choking mechanism for unstart, although mass injection is a more suitable way to reproduce backpressure rise in practice than the mechanical flow blockages. Ideally, incorporating all backpressure rise sources including heat release from supersonic combustion for studying unstart is desirable, but reproducing supersonic combustion in ground facilities is extremely challenging. Therefore, using jet injection as a flow choking

Fig. 6. Time sequential CO2 Rayleigh scattering images (top) and wall surface pressure measurements at various locations (bottom) for an unstarting flow, S1: farthest upstream pressure sensor, S8: farthest downstream sensor [58]. 7

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the location of mechanical devices. However, for jet injection driven flow choking, flow choking is induced downstream of the jet. Consequently, the unstart shockwave is spawned downstream of the jet and interacts with the jet, while the unstart shockwave propagates upstream. The interaction between the jet and the unstart shockwaves was investigated by Im et al. [109] using pressure measurements and high-speed schlieren imaging (Fig. 7). Similar to the outcomes from other unstart studies, the authors reported the transient and quasi-steady motions of unstarting shockwaves. Flow choking occurs downstream of the jet and induces propagating shockwaves. The unstart shockwaves decelerate when they approach the location of the jet, due to the pressure gradient imposed by the jet injection. Once there is sufficiently high backpressure to push the unstart shockwaves upstream again, the unstart shockwaves pass the jet with a high acceleration and change the characteristics of the jet injection (Fig. 7). After the unstart shockwaves propagate upstream of the jet, severe fluctuation of the jet occurs. Then, the jet reaches relatively steady motions indicating the completion of the unstart process.

mechanism would be a choice to provide more realistic flow conditions for investigating unstart. Do et al. [58,75] investigated unstart dynamics using jet injection as a flow choking mechanism. The onset of unstart and unstart shockwave propagation were visualized by using CO2 Rayleigh scattering imaging as depicted in Fig. 6 [58]. The shockwave propagation process is similar to those observed in the studies using mechanical flow blockages. Continuous propagation of the unstart shockwave was seen during the unstart process induced by jet injection, and the propagation of the unstart shockwave monotonically increased static pressure downstream of the unstart shockwave as shown in Fig. 6. The authors also showed quasi-steady motions of unstarting shockwaves when the unstart shockwaves approach impinging locations of the upstream shockwaves. Do et al. [75] further investigated the transient and quasi-steady motions of unstarting flows at various boundary layer conditions. They showed that the boundary layer conditions dictate the types of the unstart shockwaves and their propagation speeds. A strong oblique shockwave was observed as an unstarting shockwave in the presence of thicker turbulent boundary layers while a pseudo-shockwave appears as an unstart shockwave in the presence of thinner laminar boundary layers. The unstarting pseudo-shockwave has much longer quasi-steady motions during the unstart process. Thus, the time to the completion of unstart by the unstarting pseudo-shockwave takes longer than unstart by an unstarting oblique shockwave. Fike et al. [104] simulated the unstart process induced by mass injection using unsteady Reynolds-averaged Navier-Stokes (RANS). The authors observed downstream flow choking, unstart shockwave spawning, and unstart shockwave propagation while backpressure increases due to flow blockage and mass injection by the jet. Although overall flow features of unstarting shockwaves are similar between the unstarting flow driven by the mass injection and the mechanical flow blockage, the location of the flow choking differs. When the mechanical blockage is used to induced unstart, flow choking occurs at

2.2.3. Combustion With various scramjet vehicle geometries corresponding to their design purposes, there is no generic unstart threshold applicable (e.g., generic fuel concentration threshold), and the behaviors of the flame and flow during or just before the beginning of the unstart process are nonmonotonic and multi-dimensional [49,92,160]. The flame and flow in the supersonic combustor are turbulent and fully 3D [46,51,75,99,100, 117,120,125,164,167,168]. Therefore, to fully understand the unstart mechanism, several critical factors need to be considered together. In order to investigate the 3D flame and flow dynamics in the scramjet combustor while the vehicle undergoes the inlet unstart, it is essential to use a high-enthalpy and high-Ma ground test facility with optical accesses for resolving the 3D and transient flame and flow structures [169].

Fig. 7. Time sequential high-speed schlieren images for an unstarting flow induced by mass addition near the jet location, 0 ms: the time of jet injection, 9 ms: fully developed jet, 27 ms: unstart shockwave passing across the jet, and 48 ms: after the completion of unstart [109].

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Moreover, the test time of the facility should be longer than the typical time scale of the transient inlet unstart phenomenon that is on the order of tens of milliseconds [49,75]. For this reason, considerably limited number of ground test facilities allow for the experimental investigation on the inlet unstart [170]. Expansion tubes and shock tunnels typically having less than a few milliseconds test time are ideal for investigating the 3D flame and flow structures with fast responding optical diagnostics tools [45,59,89,118, 171–175]. An example of high-enthalpy shock tunnels is shown in Fig. 8 [176]. These relatively low-cost test facilities are optimized for providing high quality, high speed, and high-enthalpy flows with full optical accesses. Like other ground test facilities, stagnant air needs to be heated and compressed before being expanded and accelerated to reach the designated freestream Ma in the test section. This is primarily because directly heating the fast moving supersonic or hypersonic flows without perturbing the freestreams is extremely difficult. For example, in order to reproduce realistic scramjet flight conditions, the high-pressure stagnant air (>6 bar) should be heated up to 1400 K (stagnation temperature). This condition corresponds to the flow enthalpy at flight Ma ¼ 5 (1900 K at Ma ¼ 6) and altitude ¼ 30 km where the static temperature ¼ 227 K and static pressure ¼ 1.12 kPa (US standard atmosphere). The expansion tubes and shock tunnels employ shockwaves for rapidly heating and compressing the stagnant test air and pushing the heated high-pressure gas into the test section being expanded and accelerated. Scramjet flights can be simulated using the facility when a model scramjet is installed in the test section during the test time while the high-Ma freestream is sweeping the test model. Presuming that the shockwaves in the inlet and the isolator of the model scramjet convert the flow enthalpy into the thermal energy effectively, thus, sufficiently elevating the static temperature and pressure of the combustor flow, excessive fuel injection into the combustor can cause the thermal choking [10,98–100,109,153,155]. Unsteady flow behaviors and the movement of the intensive combustion zone toward the inlet or the isolator of the model scramjet were observed in several of studies [49,89,109,142,143, 152,177] as depicted in Fig. 9. In these studies, the fast-evolving compressible flow structures and the flame dynamics were clearly resolved when the flow was thermally choked due to the combustion heat release. As predicted in the previous experimental studies, which partly reproduced the unstart situations under low-enthalpy and high-Ma number conditions [58,68,75,79,102], as soon as the flow was choked, the downstream high-pressure region near the choked throat starts to perturb the upstream flow to alter the upstream shock structure in the isolator. Later the intensive combustion reaction zone along with the preceding unstart flow structure (e.g., high-pressure subsonic area) moved upstream until the end of the test time. With the shock-heating ground test facilities, it is impossible to investigate the entire unstart procedure until the unstart flow structure reaches the inlet to cause flow spillage, simply because of the limited test

Fig. 9. Transient movement of the combustion zone to upstream observed in a shock tunnel. Time-resolved schlieren (top) and OH* chemiluminescence (bottom) images for each time frame are shown [143].

time. It was shown that the completion of scramjet unstart procedure takes longer than several or tens of milliseconds [49,73,74,102]. Unlike the shock-heating ground test facilities, high-enthalpy wind tunnel facilities using arc-heating can provide high-enthalpy and high-Ma flows during the test time longer than the characteristic time scale of the inlet unstart [170]. Liu et al. [49] and Im et al. [109] used an arc heated hypersonic wind tunnel facility (ACT-1, see Fig. 10) for investigating the 3D flame and flow dynamics. When the model scramjet was thermally choked, and an unsteady shockwave in front of the inlet appeared, due to the inlet flow spillage. It was shown that the inlet unstart strongly depends on the fuel injection rate that is proportional to the overall fuel concentration in the supersonic combustor. This finding is consistent

Fig. 8. Photographs of high enthalpy shock tunnel G€ ottingen (HEG) of the German Aerospace Center (DLR) [176]. 9

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Fig. 10. The ACT-1 facility installed at Notre Dame (left) and a detailed view of the arc-heater (right) [170].

speed of propagation depend on the geometry of inlet-isolator models and the degree of the backpressure rise, overall unstarting processes are similar. Once unstart completes, however, distinct differences are observed between unstarted flows induced by flow blockages, jet injection, and combustion. In this section, the flow phenomena of unstarted flows including buzzing within inlet-isolators will be discussed. Then, the characteristics of unstarted shockwaves upstream of the cowl (or inlet lips) will be addressed. For the unstarted shockwaves, the characteristics of unstarted shockwaves induced by flow blockages will be addressed

with other numerous experimental and numerical investigations that correlate the combustion heat release with the thermal choking [98–100, 143,154,155]. It was also reported that the location of an intensive combustion reaction zone moves upstream as the fuel injection rate increases, thus the combustion heat release increases. Eventually, the combustion zone reaches the inlet to cause the inlet flow spillage when the overall fuel concentration exceeds an unstart threshold defined with the experimental setup. Interestingly, the strong combustion zone moves into both the isolator and the inlet that are placed upstream of the fuel injection location as seen in Fig. 11. It implies that a reverse flow due to flow separation appeared in the model scramjet that could deliver the fuel toward the upstream region. As the overall equivalence ratio further increases above the unity (fuel-rich operation), the combustion zone moves back (downstream) into the combustor, due to the reduction of the combustion heat release. 2.3. Generic flow choking and unstarting mechanism The dominant source of flow choking depends strongly on the specific scramjet operation conditions [92,96,160]. Freestream Ma, type of fuel, and flow path geometry will determine the dominant pressure rise mechanism. The dominant pressure rise mechanism will dictate flow features at the time of flow choking. Consequently, defining generic unstart mechanisms for flow choking and unstart shockwave propagation is extremely challenging, and each research group reported a range of flow choking mechanisms, shockwave structures, and unstart propagation speeds [163]. While quantifying flow choking mechanisms and the generalization of the unstarting process are very strenuous tasks, the overall flow features of the unstarting process can be investigated using any flow choking mechanism. For example, unstart shockwave spawning, transient behaviors, and quasi-steady motions are commonly seen in unstart driven by any flow choking mechanism. Therefore, it is reasonable to use non-heat release flow choking mechanisms to investigate unstarting flows, although reproducing combustion driven unstart is ideal to study real flow conditions. It is also recommended to include some discussion on the flow choking mechanisms at least in a qualitative manner for any future unstart studies. 3. Characteristics of unstarted flow Unstarted scramjet inlet flows have been actively studied because the characteristics of unstarted flow dictate combustion processes in the combustor and aerodynamic loads to the vehicle. As addressed in the previous section, the unstart shockwave propagations were similar each other, when unstart is triggered by different backpressure rise mechanisms, flow blockage, mass injection, and combustion. Although the location of the flow choking, the unstart shockwave structure, and the

Fig. 11. Upstream propagation of the intensive subsonic combustion zone when the scramjet flow is thermally choked. The static pressure sequentially jumps up from downstream (P5) to upstream (P1) locations [49].

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location of flow choking is acoustically closed. Also, the source of the resonance is single because the location of flow choking is quasi-steady and the flow inside the scramjet engine becomes subsonic after the flow undergoes unstart. Therefore, the frequencies of the resonance can be calculated by the length of the unstarted internal flow, average Ma within the internal flow, and the speed of sound. The resonance frequency estimation, however, becomes inaccurate when it is applied to the hypersonic flow regime [69,78,80]. For a hypersonic condition (Ma > 5), the unstarted flows typically accompany a severe flow separation upstream of the inlet cowl due to the high adverse pressure gradient. The separation acts as another resonance source because it induces additional flow instabilities [54]. Furthermore, for the hypersonic conditions, the unstarted flows inside the scramjet inlet-isolator can be partially supersonic. Tan et al. [78,80] reported the existence of intermittent supersonic flows traveling downstream of the inlet due to flow instabilities. The experiments showed that there are multiple resonance frequency sources in the unstarted flows and the authors proposed a new estimation method for the dominant oscillatory frequencies incorporating these resonance sources [80]. The estimation of dominant oscillatory acoustic-convection motions would be even more challenging in practice since there are other factors affecting flow instabilities: mass injection and heat release from chemical reactions. Reverse propagation of the mass injected by the jet interacts with incoming flows generating instabilities in the shear layer [58,75, 109]. The pressure disturbances induced by downstream heat release propagate upstream and affect the upstream portion of the unstarted flows. Then, the disturbed upstream flows induce instabilities such as a wide range of eddy motions propagating downstream. Ultimately, the flow instabilities alter the turbulent combustion that again affects upstream flows. Thus, chemical reactions are coupled with the unstarted flow as a closed feedback loop [107]. Since the flow instabilities can have a diverse range of sizes and frequencies, the origin of acoustic resonance would fluctuate. Therefore, studies for unstart using mass injection and combustion as flow choking mechanisms obtained various fluctuation frequencies other than buzzing frequencies [148]. Recently, there have been studies using high-speed flow visualization to investigate the unstarted flow features as seen in Figs. 5–8. The disgorgement of the designed inlet-isolator flows is observed once unstart completes. Compression waves and pre-existing shockwave systems in the inlet and isolator disappear after the unstart shockwave propagates upstream of the flow. The flow visualization shows that the unstarted flow becomes primarily subsonic and very turbulent, due to the interactions between the unstarted shockwave and upstream boundary layer and the coupling between upstream and downstream flows. Consequently, there exist a wide range of flow instabilities that cause the turbulent flow. In a certain occasion, a local supersonic flow was observed by Zhang et al. [101] when the side compression is used at the

first. Then, the unstarted flows triggered by mass injection and combustion will be compared to those induced by flow blockages. 3.1. Unstarted flows within an inlet-isolator As noted above, most of the studies for the unstarted flows relied on the pressure measurements to identify the oscillatory motions of the unstarted flows in the early years of the unstart investigation [11,13,14, 60]. Once unstart completes, the mean pressure distribution within scramjet inlet-isolators significantly increases due to the deceleration of the incoming flow. Given the unstarted flow spills incoming oxidizer from the scramjet combustor, the thrust generated by the propulsion system decreases. Therefore, the rise of pressure in the scramjet inlet-isolator and the thrust reduction served as the indications of unstart in both experimental and computational studies [11,13,14,18,60,100]. Considering the threshold of unstart highly depends on the incoming flow conditions and scramjet inlet-isolator geometry, it is highly challenging to generalize the threshold and the margin of unstart. However, the measured values such as pressures and thrusts can inform if the flow is unstarted. An absolute backpressure or an upstream and downstream pressure ratio has been used to determine the threshold of unstart [91, 162,163]. The common unstarted flow features that can be easily captured by using pressure sensors (Fig. 12) are oscillatory motions within the internal portion of the scramjet flow [11,14,69,109,147,178,179] beyond the inlet cowl as depicted in Fig. 12 [69]. Given that most of the oscillatory motions are caused by the acoustic-convection resonance of unstarted flows, the oscillations of the flows are often referred to as buzz phenomena [47,83,148,178,180,181]. It is apparent that there would be multiple fluctuation motions, depending on the flow conditions and inlet-isolator geometry. For example, the dominant frequency of the oscillation is dictated by the pressure ratio between the upstream and downstream flows [173], as depicted in Fig. 13. It can be seen that a higher pressure ratio induces higher frequencies. The inlet buzz was found by Oswatitsch [182] in the 1940s, and has been widely studied [47,80,83,148,178,180,181]. In the 1980s, Newsome [183,184] proposed Eq. (3) that estimates the resonance acoustic frequencies of the unstarted flows. fn ¼ ð2n þ 1Þ

a  2 1M 4L

n ¼ 0; 1; 2; …

(3)

Where a, M, and L are the mean speed of the sound, the mean Ma in the channel, and the characteristic length of the channel, respectively. The estimated results from Eq. (3) agree well with experiments when the freestream Ma is on the lower side of supersonic flows (Ma < 5), i.e., nonhypersonic conditions. Since the source of resonance radiates from the location of the downstream flow choking, it can be assumed that the

Fig. 12. Pressure oscillation in unstarted inlet-isolator flows. C and R represent sensors downstream and upstream of the inlet throat, respectively, higher sensor number for farther downstream locations [69]. 11

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Fig. 13. Frequency dependency of the unstarted flows on backpressure. Pressure traces from two sensor locations (CH4: most upstream and CH10: most downstream) are shown for various throttling ratios (TR) [173].

flow and spills the mass out of the inlet to match the downstream pressure and the mass flow rate of the choked flow. The angle and the location of the unstarted shockwave are governed by the incoming flow conditions and backpressure. Flow separation is occasionally observed near the location of the unstarted shockwave due to the strong adverse pressure gradient [58,75,185]. The unstarted flows induced by the mass injection accompanies additional flow structures, as opposed to the unstarted flows by flow blockage. A strong primary unstarted shockwave still occurs to match the pressure and the mass flow rate. In addition to the primary unstarted shockwave, there exist multiple secondary unstarted shockwaves as seen in Fig. 14 [109]. For a mechanical flow blockage driven unstart, there is no mass added to the flow within inlet-isolator flows. Consequently, the mass flow rate of the choked flow can be matched by spilling incoming flows. However, for a mass injection driven unstart, there exist reverse flow motions within the inlet-isolator flows spilling mass from the internal flow to match the mass flow rate of the choked flow because mass is added to the internal flows. Thus, the reverse flow motions induce

inlet. During a buzz cycle, the authors reported that the intermittent restart of the inlet and a supersonic reverse flow region transiently exist [127]. Despite some exceptions, it is widely accepted that the majority of the unstarted flows is subsonic. Since the characteristics of the flow are drastically changed due to unstart, the fuel jet also exhibits sudden changes in its motions as seen in Im et al. [109]. 3.2. Unstarted shockwave structures Studies using flow visualization confirmed that the unstart shockwave propagates upstream the lip (cowl) of the inlet and anchors at a certain location [109]. This state is known as unstarted, and the anchored shockwave upstream of the inlet is called an unstarted shockwave. The anchored shockwave exhibits a relatively stationary state when a flow blockage is used as the flow choking mechanism [79]. The shockwave structures of the unstarted flows were investigated by many research groups [72,78,80,94,109,185]. Typically, a single strong shockwave acts as an unstarted shockwave. The shockwave decelerates the incoming

Fig. 14. Time-resolved schlieren images of unstarted shockwave structures induced by mass addition (top) and the schematic of the unstarted flows (bottom). Relatively stationary primary and unsteady multiple secondary unstart shockwaves are seen [109]. 12

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initiates the unstart process strongly depends on the flight conditions such as the flight Ma and altitude [160].

multiple secondary shockwaves. The secondary unstarted shockwaves severely fluctuate while the primary unstarted shockwave remains relatively steady. Stronger secondary unstarted shockwaves and a higher fluctuation are observed when a higher mass flow rate is injected, which imposes a higher backpressure. Investigations of the unstarted shockwaves induced by combustiondriven flow choking are limited. There have been studies on unstarting flows using optical diagnostics and pressure measurements [58,73,75,79, 102,143], as discussed in the previous section. However, the unstarted shockwaves upstream of the inlet have rarely been investigated because computation cost is extremely high, and very limited experimental facilities exist, with an optical access and a sufficiently long test time to reproduce combustion-driven unstarted flows. Im et al. [109] (see Fig. 15) visualized the unstarted shockwaves at the inlet of a scramjet model induced by the backpressure rise due to combustion reactions using high-speed schlieren imaging in an arc-heated hypersonic facility. The unstarted shockwaves induced by heat release have similar structures to those initiated by mass injection. The primary and secondary shockwaves appear to match the backpressure and the mass flow rate of the choked flow. One noticeable difference between unstarted flows by combustion and those by other flow choking mechanisms is coupling between upstream unstarted shockwaves and downstream chemical reactions. Im et al. [109] argued that the coupling changes the captured oxidizer, affecting the amount of heat release from the chemical reactions in time. Thus, there are coherent fluctuation motions between the unstarted shockwaves and the jet. Note, the thermal choking is not a necessary condition for the completion of the unstart process or for causing disastrous inlet unstart. The engine with the choked flow within the isolator, but without flow spillage at the inlet, is still operational in a ram mode. The flow spillage that is asymmetric and unsteady prompts the reduction of oxygen supply to the combustor and asymmetric drag on the vehicle, thus will result in sudden loss of thrust and uncontrollable vehicle motion. In other words, the inlet unstart becomes disastrous mainly due to the flow spillage at the inlet. Conversely, the impact of the thermal choking that potentially

4. Prediction, detection, and control The prediction, detection, and control of unstart have been extensively studied, to delineate operable ranges of scramjet inlet-isolators and to extend the limit of operation. Theoretical, experimental, and computational studies have been performed to assess the thresholds and the margin of unstart, using flow phenomena, engine operation conditions, and even flight dynamics [61,104,106–109,186]. Typically, a higher mass flow rate and higher temperature at the exit of a scramjet combustor produce a higher thrust. Hence, the maximum thrust of the scramjet propulsion system can be achieved at the boundary of unstart. Therefore, quantitatively predicting the threshold of unstart and determining the margin of a scramjet operation play significant roles in designing the geometry and flow conditions of the scramjet propulsion system. The operation of the scramjet propulsion system should have a good margin to avoid unstart. However, operating under the boundary of unstart does not guarantee the safe operation due to uncertainties in prediction and margin quantifications [106,161,187,188]. Consequently, there is always the risk of unstart during the operation of the scramjet propulsion system. In this way, the detection and control of unstart become crucial for the safe operation of the scramjet. The detection of unstart must be sufficiently fast that the flow control can be activated to mitigate unstart before unstart completes [62,63,77,80,85–87,107,147]. In this section, a brief overview is addressed on recent research activities on the unstart prediction, detection, and control, then a more detailed discussion is provided for the detection and control of unstart. Although the general overview is provided, the emphasis is given to unstart driven by flow choking. 4.1. Unstart prediction and margin The prediction of unstart has been primarily investigated using

Fig. 15. Time-resolved schlieren images of unstarted shockwaves and fuel jet coupling in unstart induced by heat release. Coherent motions between unstart shockwave and fuel jet in time were observed [109]. 13

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et al. [108] explored the stability margin of an isolator-combustor with isolator-combustion interactions. The authors used backpressure and the pressure distribution area integration to characterize the unstart margin. It was found that the unstart margin has a relatively linear relationship with the equivalence ratio for their isolator-combustor model.

theoretical and computational methods. Among various factors that could affect unstart, backpressure and changes in the vehicle motions such as the angle of attack have mostly been studied for the prediction of unstart. The boundary of start/unstart conditions is determined in terms of flight motions and flow conditions. After the determination of unstart boundaries, the margin of unstart, which represents the closeness of current conditions to the unstart boundary, is established. For example, Chang et al. [88] used surface pressure distribution to determine the unstart boundaries (Fig. 16), followed by characterization of the margin of unstart. It is seen in Fig. 16 that a higher pressure was observed at upstream locations for unstarted flows. However, the pressure distribution alone does not provide a distinct inlet operation classification, started or unstarted. Thus, Chang et al. [88] used two-categories classification to determine the unstart threshold of the upstream and downstream pressure ratio and argued that unstart can be predicted by surveying pressure ratios. The studies on the unstart prediction became very active in the 1990s. Mayer and Paynter [67] developed an 1D linear analysis procedure to analyze inlet unstart behaviors. Unstart caused by freestream disturbances was investigated and the authors concluded that any changes in flow conditions, except a pressure increase can lead to inlet unstart. In 1995, Cox et al. [66] investigated the prediction of unstart using artificial neural networks and other nonlinear signal processing techniques. The start/unstart boundary was approximated and validated with wind tunnel data. It was argued that the prediction of backpressure-driven unstart is the easiest to predict, considering pressure information is easy to measure and the lead time of unstart is the longest. As computation technologies advanced in the 2000s and the 2010s, predicting and quantifying margin of unstart have extensively been studied to date using RANS and large-eddy simulation (LES) for both 2D [35,61,84,88,92] and 3D flows [104,154,162,189–191]. Although the reported unstart boundaries among the literature studies differ due to the dependency of unstart on flow conditions and inlet-isolator geometries, all studies show that the flow choking induces the incipient of unstart. Therefore, it is crucial to obtain an accurate prediction of the flow choking conditions that limit the operation of the scramjet propulsion system. Considering that the accuracy of the prediction is highly dictated by numerical methods, uncertainty quantification in computed results has also been a popular research subject [106,161,188]. Experimental studies on the prediction of unstart are relatively rare compared to the computational studies due to the limited ground highenthalpy facilities. Nonetheless, a few research groups have experimentally investigated the margin characterization of unstart. Fike et al. [104] characterized the unstart boundary using jet injection pressures and quantified the margin of unstart using the leading edge of a pseudo-shockwave position and jet injection pressure. The linear relationship between the shock position and jet pressure was observed. Qin

4.2. Unstart detection It is apparent that a larger margin of unstart could help to prevent unstart of the scramjet propulsion system. However, having a large margin of unstart results in the reduction of the thrust if the margin of unstart is obtained by reducing overall equivalence ratio (i.e., lower heat release). Thus, in practice, the operation of the scramjet is more likely very close to the boundary of unstart, and any additional downstream pressure disturbances could trigger flow choking, leading to unstart. It should be noted that the propulsion system could operate under certain conditions in the ramjet mode as the isolator could hold a shock train matching downstream pressure and preventing upstream shockwave propagation. However, further pressure disturbances could still trigger unstart. Accordingly, the detection and the control of unstart become crucial in practice. The propulsion system needs to equip rapid sensing systems that can detect the incipient of unstart. The flow control or vehicle motion control has to be applied to avoid the onset of unstart or to mitigate the problem caused by the unstarting and the unstarted flows. In this section, we will discuss the detection mechanisms of unstart, followed by the control of unstart in the subsequent section. One highly attractive and simple way to detect unstart is to use a pressure signal [35,41,77,80,91,107,147]. Fig. 17 shows typical pressure traces at the inlet (T2 sensor) and at the isolator (T5 sensor) during the unstart process [62]. Downstream flow choking accompanies a sharp pressure rise due to unstart shockwave formation. The shock-induced instabilities increase the amplitude of the pressure fluctuations. The spatial pressure increase is sequentially detected (T5 to T2) while the unstart shockwave propagates upstream. The sequential pressure increase along upstream could be caused by the propagation of either a pre-existing shock train (acting as unstart shockwave) or unstart shockwaves disgorging pre-existing shockwave structures. Nonetheless, a sudden pressure rise and severe pressure oscillations are captured at the upstream location (T2 in Fig. 17) of the scramjet inlet-isolator. Consequently, the pressure ratios between downstream and upstream locations [192,193], the derivative of a static pressure [192,193], the standard deviation of a static pressure [40,192,193], and power spectra of frequencies [39,77,192,193] have served as the criteria for the unstart

Fig. 17. Pressure traces during unstart process, T2: sensor at the inlet and T5: sensor at the isolator [62].

Fig. 16. Unstart threshold prediction using surface pressure computed by CFD [88]. 14

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costly to implement in practice [181]. Although the pressure signal has most commonly been used for the unstart detection studies, there are investigations using combustion characteristics to detect unstart. Rieker et al. [44] used the diode laser absorption technique to measure temperature. The Fourier analysis of the time-resolved temperature in the combustor provides a power spectrum of the temperature fluctuations. The authors used the temporal power spectrum of low-frequency contents to capture changes in temperature fluctuation as visualized in Fig. 20 [44]. Similarly, Carter et al. [195] and Donbar et al. [196] used tunable diode laser absorption spectroscopy (TDLAS) to detect potential unstart per-cursors. A greater lead time for shockwave detection could be achieved by using the non-resonant portion of the TDLAS signals than either the resonant TDLAS signal or the high-frequency pressure measurements. Both studies suggest that the use of laser-based techniques for the unstart detection could lead to a better performance than using pressure measurements.

detection. Using a pressure ratio between certain upstream and downstream locations is the simplest way to detect unstart. However, a pressure ratio-based detection highly depends on freestream conditions, thus setting a universal criterion for unstart is difficult [40]. Chang et al. [88, 147] proposed using the derivative of a filtered pressure signal for the unstart detection. As seen in Fig. 17, there is a sharp rise in pressure at the onset of unstart, which generates high derivative values. The average of the characteristic value is used in the ratio-based detection methods at a steady start mode as a criterion, and the method compares the criterion with the measured pressure signal. The standard deviation-based detection method has been explored by a few research groups [40,62,192, 193]. Srikant et al. [79] computed the standard deviation of the pressure data over a moving window, as seen in Fig. 18. The authors reported that the unstart and restart phenomena could be detected by setting the proper thresholds for the standard deviation values. The use of downstream and upstream sensor signals was also investigated, and it is found that using a downstream sensor is not feasible for detecting unstart due to occasional spikes. The use of an upstream sensor signal with a relatively large moving window is suggested for the accurate detection of unstart. Later in 2014, Chang et al. [91] confirmed that the most upstream pressure sensor signal results in a better accuracy. However, using the most upstream sensor for detecting unstart could be problematic although the standard deviation-based detection provides a very reasonable accuracy for detecting unstart. Given unstart will be detected near the completion of unstart, the lead time for control mechanisms will be reduced. Thus, there is a higher chance of engine failure. The power spectrum-based detection technique has been studied because it provides a similar accuracy to the standard deviation-based detection and is relatively free from the detection locations [39,77,192,193]. Analogous to the standard deviation-based detection, the power spectrum-based detection uses the peak [39] and distribution changes [77] (see Fig. 19) of a power spectrum from a steady operation. Trapier et al. [181] investigated the use of more sophisticated methods such as the cumulative sum (CUSUM) and the generalized likelihood ratio (GLR) algorithms for the unstart detection. Later Chang et al. [91] confirmed the efficacy of detection using the CUSUM algorithm. The CUSUM is a commonly used algorithm that detects a change in the probability density of the analyzed signal. Although the CUSUM can easily detect unstart in advance of spawning the unstart shockwaves, the probability density function of the pressure date should be known. The GLR algorithm does not require knowledge of the probability density function. Thus, this method can provide a simpler way to detect unstart [194]. However, the GLR algorithm requires a high memory and could be

4.3. Flow control The flow control is mandated for the safe operation of a scramjet propulsion system while ensuring a high-performance. When it comes to unstart, the flow control becomes crucial because it could prevent catastrophic engine failures. The flow control can mitigate the unstart issue in two ways, namely, extending operable flow boundaries and suppressing the unstart process. Firstly, the flow control can prevent unstart under the conditions that could trigger unstart without the flow control. It means that the boundary and the margin of unstart can be extended by applying the flow control either passively or actively. The second advantage of using the flow control is suppression of the unstart process. Once unstart is detected, the next natural step is applying the flow control to unstarting or unstarted flows so the scramjet propulsion system can restart the inletisolator flow. It should be noted that the flow control discussed in the current review paper is for the flows with the geometry design optimization. The design of inlet and isolator geometry (length and shape) can significantly affect the threshold and propagation of unstart [10]. Thus, the suppression and prevention of unstart should be accounted for the design choices of the inlet and isolator. In the current review, as the paper focuses on flow phenomena, the paper discusses the applicable flow control methods beyond the geometry design or optimization. The flow control methods are typically categorized by how they are applied, passive or active. A passive control acts as an open-loop system. Flow control strategies based on fixed geometry methods such as microramps and vortex generators can fall into the passive control category.

Fig. 18. Standard deviation of time averaged pressure during unstart process for an inlet sensor location (T2) and an isolator sensor location near the mechanical flap (T7) that triggers unstart [62]. 15

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Fig. 19. Power spectrum for oscillation frequency in pre-ignition (left) and passed shockwave (right) conditions. Increased energy in the 0–500 Hz range is observed in unstarted flow [77].

Therefore, boundary layer bleeding increases the threshold of unstart and suppresses the unstart process. Kodera et al. [197] investigated the use of boundary layer bleeding for the suppression of flow separation. Later, Mitani et al. [200] experimentally demonstrated the suppression of unstart using boundary layer bleeding through a porous plate (see Fig. 21) at freestream Ma ¼ 4 and 6 flows. For the Ma ¼ 4 condition, the extension of the unstart boundary was observed by bleeding 3% in the captured air. Bleeding approximately 0.6% in the captured air for the Ma ¼ 6 condition increased the threshold of unstart from Φ ¼ 0.45 to Φ ¼ 1. The extension of the unstart boundary and the suppression of unstart propagation were confirmed by Kouchi et al. [198] using numerical simulation. Fan et al. [83] and Chang et al. [82] investigated the application of the boundary layer bleeding to the unstarted flows. The boundary layer bleeding weakens the shockwave-boundary layer interactions, which reduces the adverse effect of buzzing on hypersonic inlets [83]. The oscillatory motions of the unstarted flow disappear when 10% of the captured flow is removed by boundary layer bleeding. Valdivia et al. [87,93] investigated the use of mass injection to control unstart. The jet injection served as a vortex generator, which increased the threshold of unstart in certain conditions by increasing effective flow areas. However, the jet injection alone triggered unstart when larger flow blockage was applied due to the jet-in-cross flow bow shockwave. As discussed above, boundary layer bleeding is an effective way to control the unstart threshold, to suppress unstarting process, and to reduce the aerodynamic load oscillation of unstarted flows. However, the thrust generation is sacrificed due to the reduced internal mass flow rate. Thrust reduction due to the loss of captured mass could be compensated by operating at higher equivalence ratio at the combustor. However, higher equivalence ratio would result in faster fuel consumptions, reducing the range of the propulsion system. The jet injection vortex generator is also capable of controlling unstart, but jet injection adversely affects unstart under certain conditions. Therefore, non-mass exchanging flow control methods such as micro-ramps, vortex generators, and plasma-based actuators, have been investigated to achieve a successful flow control without reducing thrust or the range of the propulsion system. Valdivia et al. [87] employed ramp-type vortex generators in a Wheeler doublet configuration and its combination with jet injection vortex generators as depicted in Fig. 22. Actuators are placed at the inlet of a model scramjet inlet-isolator. The reduction of pressure fluctuation and increased unstart triggering backpressure were demonstrated. The use of dielectric barrier discharge (DBD) actuators was proposed to suppress the propagation of the unstart shockwave [151]. This flow control strategy is based on the boundary layer dependency of unstart and the boundary layer thinning effects of the DBD actuators. Do et al.

Fig. 20. Power spectrum for low-frequency content (1–50 Hz) of temperature measured by TDLAS for stable (Φ ¼ 0.7) and unstable (Φ ¼ 0.85) conditions. The increased fraction of frequency content in the unstable condition indicates higher temperature fluctuation due to unstart [44].

When a closed-loop system is used, the flow control method is called active. The active flow control methods use downstream sensing and control actuators. Although categorizing flow control methods into passive and active flow control is the most common way to discuss the flow control, an alternative approach is taken in this paper. Given the review paper concerns flow choking-driven unstart, two flow control strategies are discussed: mass exchanging and non-mass exchanging methods. For mass exchanging flow control methods, boundary layer bleeding has been the most popular way to control the flow [57,82,83,197–199]. Boundary layer bleeding removes fluids from the slower regions of the flow out of the flow path, which thins the boundary layer and, hereby, increases the effective flow area. By removing only a few percent of the mass flow rate (even less than 1%), the flow characteristics could be drastically changed [200]. One disadvantage of using boundary layer bleeding is the reduction of the mass flow rate of the propulsion system, which may decrease the thrust. However, removing mass can positively affect unstart, and thrust reduction due to mass removal can be compensated by operating at higher equivalence ratio that could not be achieved without boundary layer bleed. Given that boundary layer bleeding removes the mass out of the flow path and increases the effective flow area, the downstream pressure could decrease (see Eq. (2)). 16

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Fig. 21. A schematic of a scramjet model engine with boundary-layer bleed and two-staged injection of H2 (left) and a schematic of boundary-layer bleed (right) [200].

mimic the flow choking (mechanical blockage or mass injection) that induces unstart shock structures and their movement to the inlet, ii) highMa, high-enthalpy, and short-test time shock tunnels or expansion tubes simulated thermal choking situation from excessive combustion heat release, and iii) high-Ma, high-enthalpy, and long-test time arc-heated hypersonic wind tunnels are used for investigating the entire unstart process. Also, numerical approaches could interpret the complex unsteady flow behavior that cannot be observed in experiments, e.g., the Ma and temperature contours in the high-speed scramjet flows. The inlet unstart is a transient fluid mechanical phenomenon that is initiated by thermal choking from excessive combustion heat release in a combustor and completed by flow spillage at the inlet. The primary cause of the flow choking is combustion heat release, for this reason, the unstart threshold and margin have often been defined as the fuel concentration or fuel injection rate with a given flight Ma, altitude, vehicle geometry and injection strategy. Once the flow is choked, the scramjet internal flow becomes subsonic in an entire cross-section at least at a location. Consequently, the downstream pressure waves originating from the highpressure combustor region can propagate upstream, push subsonic boundary layers and subsonic corner flow areas to extend toward the inlet along with the unsteady unstart shock structure, and eventually induce a strong adverse pressure gradient at the inlet to cause flow spillage. The flow spillage will result in an oxygen deficit in combustors and unsteady asymmetric drag on the vehicle, resulting in engine malfunction and aircraft crash. The unstarted flows have different features depending on the cause, e.g., mechanical blockage, mass injection, and combustion heat release, which have also been investigated to simulate the situations encountered after the unstart process is completed.

[75] observed a much slower unstarting shockwave propagation with a thinner boundary layer. Im et al. [201] demonstrated the use of the DBD actuators to reduce the thickness of the turbulent boundary layer and to re-laminarize the turbulent boundary layer. The DBD actuators do not directly control unstart but delay the propagation of unstarting shockwaves through thinning the boundary layers. Im et al. [151] investigated three flow configurations, base, boundary layer tripping, and boundary layer tripping with the DBD actuator, as displayed in Fig. 23. The authors demonstrated the successful delay of unstart using the DBD actuator. The unstarting oblique shockwave in the tripping case is replaced by the unstarting pseudo-shockwave when DBD actuation is activated. The controlled unstarting shockwaves exhibited relatively long quasi-steady states, thus the time to the completion of unstart increases. 5. Conclusion It is evident that the scramjet inlet unstart should be effectively avoided in operation, detected earlier, and actively prevented in using the next generation of air-breathing hypersonic propulsion system. There have been numerous studies to reveal the unstart mechanism that can help in predicting, detecting, and controlling this catastrophic event that can potentially cause the aircraft to crash. Due to the inherently 3D turbulent flame and flow structures in the supersonic combustor, the experimental investigations heavily rely on the sophisticated nonintrusive optical measurements. Thus, the methods are rather qualitative. However, in conjunction with high-speed pressure measurements, detailed inlet unstart mechanisms have been revealed: i) high-Ma number, low-enthalpy, and long-test time wind tunnels have been used to

Fig. 22. The vortex generator used to form Wheeler doublets (left) and arrangement of the vortex generators (right) [87]. 17

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Fig. 23. DBD flow control of unstarting duct flows. Exemplary CO2 Rayleigh scattering images for three different cases, no boundary layer tripping-no actuation, boundary layer trippingno actuation, and boundary layer tripping-actuation [151].

Once the choked flow initiates the unstart process, the process will be completed within tens of milliseconds with flow spillage at the inlet, therefore, detecting the precursor or indicator of the unexpected inlet unstart as early as possible is critical for preventing or stopping the unstart process. Pressure signals at critical locations directly indicate the unsteady flow behavior, and are, therefore, most frequently used to detect the unstart process. For example, abnormal pressure rises and movements of the high-pressure region are evident unstart indicators or precursors. As one of the earliest precursors, abnormal combustion behaviors also precede the unstart process that can be optically detected. Some mass exchanging or non-mass exchanging flow control methods were tested to prevent the inlet unstart in unstarting or nearing the unstart situations. The use of boundary layer bleeding, micro-ramps, vortex generators, and plasma actuators was reported. As reviewed in this article, the inlet unstart mechanism has been actively investigated, and some detail unstart procedures and their causes-consequences were revealed. Based on the information provided with the previous numerical and experimental investigations, guidelines can be achieved for determining the flight trajectory, altitude, Ma, and fuel injection rate adequate for a previously tested given vehicle geometry. However, it is still necessary to find improved unstart precursors and systematic flow control strategies to ensure safe and stable flights. This can be enabled by ground test facilities providing high-enthalpy and high-Ma flows for a sufficient period and accurate optical measurement methods. The supersonic combustion phenomenon along with the turbulent compressible flows are the most complex and fast-evolving flow phenomenon observed to date. In future studies, further efforts for detecting and controlling the unsteady flow behaviors will need to be attempted to facilitate the practical use of the new scramjet propulsion system.

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