Experimental study on enhancement of supersonic twin-jet mixing by vortex generators

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Experimental study on enhancement of supersonic twin-jet mixing by vortex generators Aqib Khan, Saif Akram, Rakesh Kumar ∗ Indian Institute of Technology Kanpur, Uttar Pradesh, 208016, India

a r t i c l e

i n f o

Article history: Received 25 September 2019 Received in revised form 28 September 2019 Accepted 27 October 2019 Available online xxxx

a b s t r a c t Experimental results on the mean flow evolution and the control of single and twin compressible jets at Mach 1.6 are presented. The jets issue from conical CD nozzles closely placed side-by-side resembling the twin nozzle configuration of supersonic aircrafts. The results are relevant to scenarios where turbulent jet mixing, supersonic core length, thermal radiation and acoustic loading are of concern. Experiments show that closely spaced twin jets grow, merge and interact near the inter-nozzle region that influences the characteristic decay and jet spread. Moreover, the deviation in centerline characteristic decay is more significant at off-design conditions. Vortex generators in the form of small metallic rectangular tabs mounted at the nozzle exit plane in different azimuthal orientations are used to control the mixing characteristics and the spread of these jets. Abrupt reduction in the core length and suppression of shock cell structure is achieved in over to under-expanded conditions. Furthermore, the orientation of vortex generators is found to significantly influence the development of jet flow field. The jet bifurcation and formation of daughter streams with distorted quasi-periodic shock cells structure are visualized using the schlieren technique. The underlying mechanisms for the observed effects and the behavior of daughter streams are discussed. © 2019 Elsevier Masson SAS. All rights reserved.

1. Introduction Subsonic and supersonic jets are ubiquitous to the aerospace industry. They have their role to play in turbojet engines for commercial and military aviation, ram/scram-jet engines for hypersonic cruise vehicles, rocket engines for air-to-space transitions, etc. The performance of these aircraft engines depend to a great extent on the ambient pressure and the interaction of exhaust gases with the structural components in the immediate surroundings. Thrust, drag, acoustic waves and side loads are the typical byproducts of these interactions. Free shear layers in general and circular jets in particular, for the aforesaid reasons, have been studied in great detail since the advent of the aviation industry. Jets are a typical case of free shear flows, which develop in the absence of a solid surface. A very simplistic and first step towards understanding such flows is the two-dimensional mixing layer formed when two parallel streams with different velocities merge downstream of a splitter plate [1,2]. Just downstream of the trailing edge, the mixing layer develops an inflection in the velocity profile, as shown in Fig. 1(a). Such flows are unstable at

*

Corresponding author. E-mail address: [email protected] (R. Kumar).

https://doi.org/10.1016/j.ast.2019.105521 1270-9638/© 2019 Elsevier Masson SAS. All rights reserved.

high Reynolds numbers and even a slight perturbation may trigger Kelvin-Helmholtz type instability. The instability waves are amplified and the shear layer rolls up into discrete identifiable structures (vortices), convecting, growing and retaining their identity before getting fragmented [1]. This onsets the turbulence downstream. Shear layer growth and primary instabilities follow similar mechanism in axisymmetric jets with differences attributed mainly to the axisymmetric nature of the flowfield. The large-scale vortical structures formed are found to be coherent for a wide range of Reynolds numbers [3]. As the convective velocity increases, these structures become less stable and less coherent. For supersonic jets, the growth rate of the shear layer and the vortical structures depends on the Mach number, temperature and the compressibility effects [4]. During the process of vortex formation, there is a significant increase in the entrainment of freestream fluid that results in faster decay of the jet. Linear and nonlinear instability analysis of jets using realistic velocity profiles with perturbations superimposed provide important details regarding the effects of flow parameters on the growth of perturbations [5–8]. For a good review on the topic, readers may refer to Ref. [9]. The growth of the shear layer and the dynamics of large-scale structures play an important role in the behavior of jets. The presence of these structures is not only closely related to the acoustic radiations but often dominates the other sources of acoustic waves

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Fig. 1. Representation of flow structures, (a) Growth of instabilities in free shear layer flows and b) Experimental image of a jet showing the evolution of vortices. (b) is taken from Ref. [50].

in subsonic and correctly expanded supersonic jets. Early experiments by Tam et al. [10–12] and Plascho [13] showed strong correlation between turbulent structures/instability waves and the acoustic waves. Later, to support the argument that large-scale instability waves are indeed the dominant sources of high-speed jet noise, Tam et al. [14,15] and Mclaughlin et al. [16] showed that the frequency at the peak of the noise spectrum is nearly equal to the frequency of the most amplified instability wave of the jet and that the peak noise radiations and the Mach wave radiations of the most amplified instability are aligned [17]. Their calculations were in close agreement with the experimental measurements of Tanna et al. [18]. At conditions that are far from the correct expansion, sonic and supersonic jets have a quasi-periodic shock cell structure in the core region. These shock waves oscillate and are the sources of broadband shock associated noise and the screech tones. The interaction between broadband turbulent structures and shock waves generates acoustic waves over a large range of frequency commonly termed as the broadband shock associated noise, which is the most dominant of the noise components in the upstream direction of a supersonic jet [18–20]. Screech tones are intense, discrete frequency harmonics that appear as a sudden jump in the noise spectrum. The intensities of screech tones are affected by the Mach number, the jet temperature and the level of expansion. A good review on supersonic jet noise is given by Tam [21]. Further experimental studies demonstrated that the peak noise in the acoustic far field is primarily generated by the large-scale coherent structures [22–24]. The studies so far discussed in the present section highlighted the role of turbulent eddies and shockwaves in the noise generation. It is important to design control techniques that can be realized for practical applications where the presence of multiple jet streams can hamper the performance of engineering systems (military aircrafts with supersonic twin jet, for example). Shock associated noise and screech tones that propagate in upstream direction have been experimentally shown to produce intense pressure fluctuations in twin jet proximity [25–27]. Phase coupling of large-scale structures of each jet is responsible for intense internozzle pressures. Such loadings are attributed for the structural failure of aircraft like F-15 and B-1B that comprises of very closely spaced engine configuration [28]. A significant number of studies have been performed to understand the dynamics of closely placed twin jet dynamics. The jets exiting the twin nozzles with initially thin shear layer near the nozzle lip drag the surrounding fluid due to viscous forces. The shear layer thickens and the jet width increases due to mass and momentum exchange at the free shear boundaries and within the jet core due to mixing. The two individual jets then interact with each other, merge and undergo a strong interaction around the midplane, transporting momentum from individual jet core to the internozzle region. Further downstream, the

jets become fully developed and resemble a fully developed single jet flow [29–35]. Wleizen [36], Shaw [37] and Moustafa [38,39] reported that the acoustic waves produced by the mutual interaction of two supersonic jets are a strong function of nozzle spacing, pressure ratio, and jet Mach number, with a significant reduction of screech tone amplitude when the nozzles are 3 to 5 diameter apart. Detrimental effects of off-design aerodynamic and aero-acoustic characteristics of jets have compelled researchers to devise active and passive control techniques like microjets, pulsed jets, alterations in the nozzle exit shape like notches and chevrons, vortex generators/tabs etc. For brevity, only tabs will be discussed here. Interested readers can refer to the scientific literature for detailed discussions on various other control techniques [40–49]. Tab-like vortex generator (VG) is a simple flat metallic structure that can increase the mixing in free shear flows by shedding spanwise vortices which later align streamwise due to convection, and thus serves as an effective passive control device. The role of streamwise structures in the dynamic and static properties of the shear layer is studied by Liepmann and Gharib [50]. For mixing enhancement in jets, early transition to turbulence and sustenance of large scales streamwise vortical structures is important. The introduction of perturbations such as by vortex generators (VGs) introduces vortical structures and cause transition of shear layer to the turbulent state at the very exit of the nozzle. The generation of vortices at the nozzle exit results in a faster spreading of flow by enhanced mass entrainment and crossstream mixing of fluids across the shear layer [51–54]. Based on the extensive flow visualizations, Zaman [55] attributed the jet distortion and enhanced mixing to the upstream pressure hill and a series of vortex filaments along the edges of the tab. Reeder et al. [56] and Gretta et al. [57] probed deeper into the flow physics and revealed that a tab generates a pair of counter-rotating transverse vortices that become streamwise after some residual time due to a differential pressure gradient. These vortices are accompanied by secondary vortices, which together result in a rapid crossstream mixing and cause shear layer thickening [56,57]. Motivated by their results, Samimy et al. [58] and Lou et al. [59] examined the use of tabs for impinging jet flows. They noticed that micro tabs were able to reduce the flow unsteadiness, particularly for underexpanded flows. The size and strength of the vortices strongly depend on the shape, size and orientation of the tab. Although much of the studies pertaining to the use of tabs have focused on single jets (for example [60], [61] etc.), attempts have been made to use them for twin jets as well. Seiner et al. [28] successfully used a small tab-like device mounted arbitrarily at the nozzle exit with an objective of decoupling the twin jet instability modes that are responsible for large dynamic pressures in the internozzle region. Another study by Shaw [37] focused on the

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suppression of screech tones. He observed that the screech tone is dependent on the shock-cell spacing, which in turn is dependent on jet Mach number. His experimental results demonstrated that coupling of twin jets caused amplified screech tone, and the placement of tabs at the nozzle exit was very effective in decoupling the jets and thus suppressing the screech tones. From the preceding discussions, it can be summarized that the aerodynamic and aeroacoustic characteristics of free supersonic jets largely depend on the dynamics of the coherent structures and the shock waves in the supersonic jet core. Thus, in order to enhance the mixing characteristics of a supersonic jet, it is quintessential to alter the strength and shape of the waves prevailing in the nearfield by introducing some small-scale vortices by the application of tabs at the exit of twin nozzles. The objective of the present work is to investigate the viability of tabs as vortex generators (VGs) for improved performance of twin supersonic jets with respect to mixing characteristics, which has important implications in the aerospace industry. Moreover, unlike a single circular nozzle, the twin jet is asymmetric. Therefore, the orientation of VGs also becomes important. Two different orientations namely horizontal (VGs in series) and vertical (VGs in parallel) are studied here. Before studying the influence of VGs and the effect of their relative azimuthal orientation, uncontrolled single and twin jet characteristics are discussed in brief. 2. Experimental details 2.1. Jet flow facility and nozzle Experiments are conducted in the open jet facility at the high-speed laboratory, Indian Institute of Technology, Kanpur. The measurements are conducted using two identical axisymmetric, convergent-divergent(CD) nozzles with a design Mach number of 1.6 each. The divergent part of the nozzle is a straight walled conical section with 8.39 degrees semi-divergence angle from throat to the nozzle exit. The nozzles are machined in a circular brass block of 50 mm height and 90 mm diameter. A flange at the base of the block assists in mounting the block on the jet facility. Both

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the nozzles are identical and are separated by a distance of 1.5de , where de is the nozzle exit diameter. The nozzles were manufactured by CNC electric discharge machining for precision. The throat and exit diameter of nozzles are measured to be 10 ± 0.02 and 10.84 ± 0.02 mm, respectively. Nozzles are calibrated over a range of nozzle pressure ratios. The calibration results show that the Mach number at the exit plane of the nozzles is almost uniform. The Reynolds number of the Mach 1.6 jet issuing out of the nozzle at designed NPR of 4.25 based on the exit diameter is approximately 106 . Any losses due to viscous effects are neglected while calculating the Reynolds number. In the present study, two rectangular VGs are deployed at the nozzle exit at two different relative azimuthal orientations, as shown in Fig. 2(b). VGs are made from 1 mm thick brass sheet. The percentage of the momentum thrust loss suffered due to the VGs is approximately equal to the total projected area of the VGs [62]. Therefore, the size of the VGs should in general be minimum that can achieve the desired mixing enhancement and shock-cell suppression. Two plain rectangular VGs of length 3 mm and width 1.5 mm offering a blockage of about 5% to the exiting flow are used in the present investigation. 2.2. Schlieren setup To gain an insight into the physics of the wave structure for controlled and uncontrolled jets, optical flow visualization is done using schlieren technique. Schlieren is a non-intrusive flow visualization technique and gives a correct representation of the compression and expansion waves prevailing in the flow field. Schlieren setup detects the changes in refractive index of the medium brought about by the first derivative in density. A knife edge is used to cut off a fraction of light at the focus. The beam displacement normal to the knife edge will translate into an enhanced intensity variation on the screen. Variations in density gradients in the respective directions can be retrieved by suitably orienting the knife edge. The technique is very suitable for supersonic flows where strong gradients across the shock waves and mild gradients in expansion waves and the shear layer region all need to be resolved in the same image. The setup used is a Z-type with two concave mirrors of 1.5 m focal lengths and a helium spark arc light source. In the present experiments initially both vertical and horizontal orientations were tried, but the images with vertical knife edge were more revealing than the one with horizontal knife edge. Hence, the vertical knife edge was finally adopted for all the experiments. Effect of knife edge orientation on the schlieren images for one case (NPR = 6) is shown in Fig. 3. 2.3. Instrumentation and Data accuracy

Fig. 2. Schematic of nozzle and VGs arrangement.

The pressure in the jets is measured along the jet centerline and across the jet cross section at several axial locations using a pitot tube of 0.4 mm inner diameter and 0.6 mm outer diameter. The schematic of the coordinate system and the location of measurement are shown in Fig. 4. The probe is mounted on a

Fig. 3. Schlieren images with different knife edge orientation at NPR = 6.

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Fig. 4. Illustration of coordinate axes and data acquisition location. Dotted lines show the location of transverse data measurement corresponding to Figs. 15, 16 and 17.

three-dimensional traverse with a resolution of 0.1 mm in linear translation. It is ensured that the sensing probe hole is well aligned with the flow in every test run. The ratio of nozzle exit to the probe outer diameter is (10.844/0.6)2 = 327 and is very well above the probe blockage limit of 64 [63]. The pitot pressures are measured with an accuracy of ±3%. The repeatability of pressure measurements is ±2%. One of the advantages of using pitot tube for measurements in supersonic flow is that the error caused due to slight misalignment relative to the flow direction is negligible. A 16 channel Pressure Systems, Inc., 9010 transducer with a range of 0 - 300 psi was used for pressure measurement. The transducer provides the options to choose the number of samples to be averaged by means of dip-switch settings. The specified accuracy for the transducer is ±0.15% of the full scale [64]. The desired NPR was achieved by controlling the settling chamber pressure P 0 while the backpressure P b (atmospheric pressure) remained constant. The pressure in the settling chamber was maintained within ±2% of the desired NPR. The software used for data acquisition has a separate window that plots the real-time values of the stagnation chamber. The uncertainty in the measured pressure values was around ±3%. The uncertainty associated with the Mach number based on the pitot pressure measurements at the nozzle exit is estimated to be ±2%. For details of the methodologies of calculating the uncertainties, readers can refer to Ref. [63]. 3. Results Pitot probe is used to obtain the pressure values in the flow field. As and when the probe is exposed to the supersonic jet, a strong bow shock is formed ahead. The shock wave that is essentially curved can be considered normal to the flow in the small region immediately upstream of the probe tip. Hence, the conditions across the shock, i.e., between points 1 and 2 in Fig. 5 are related by the normal shock relations. Furthermore, since the flow behind the normal shock wave is always subsonic, the deceleration from point 2 to point 3 in Fig. 5 may be assumed to be an isentropic process. The pressure sensed by the probe is neither static nor the stagnation pressure of the freestream, rather, it is the total pressure of the subsonic flow behind the shock wave standing ahead of the probe. If the actual pressure of the freestream is required, one has to correct the pitot pressures for the pressure loss across the shock. Since the supersonic jet core is highly wave-dominated, the Mach number in the core varies from point to point and also the shock waves in different cells are of varying strength. Nevertheless, the data are accurate enough to capture the overall features of the jet, such as the core length, characteristic decay, fully developed or self similar zone, the number of shockcells and the spacing between them [65–68]. The Mach number at the nozzle exit can be calculated using the normal shock relation given below.

Fig. 5. Schematic of supersonic flow around a pitot probe.

Pt P0

 = 1+

2γ

γ +1

( M e2

 −1  γ −1 − 1)

(γ + 1) M e2 (γ − 1) M e2 + 2

 γ γ−1 (1)

Using this model of the supersonic flow, the pressure at the stagnation point can be calculated for the specified upstream conditions. The complex interaction of waves and the shear layer results in a complex flow field with mixed supersonic and subsonic flow fields in the jet. For such flows, the usual practice to use the measured pitot pressure is by non-dimensionalizing it with the settling chamber pressure. Same is also adopted in the present analysis. The non-dimensionalized pitot pressure data can be used for quantifying the mean flow characteristics of free jets, such as the core length, characteristic decay, shock-cells and the far-field, and is particularly suitable for comparative studies [66,67]. The variation of dimensionless pitot pressure P t / P 0 along the jet axis with dimensionless streamwise distance, x/d is termed as centerline pressure decay (CPD). Noteworthy is the fact that CPD is a good measure of jet mixing, indicating how fast the ambient fluid mass is entrained into the high-speed jet fluid. Higher entrainment results in higher jet spread, stronger decay and weakening of shock waves in the supersonic jet core. Therefore, to investigate the mixing characteristics of uncontrolled and controlled twin jets, with two rectangular VGs placed at the nozzle exit, the pitot pressure P t is measured along the jet centerline upto an axial distance of 16d. NPR is varied from 3 to 7, ranging from over-expanded to highly under-expanded cases. 3.1. Twin jet vs single jet 3.1.1. Single jet flow field The centerline pitot pressure decay results for the uncontrolled single jets for NPRs 3 to 7 are shown in Fig. 6. It is clear from the plots that both the shock-cell size and the spacing increase as the nozzle pressure ratio increases. Fig. 7 presents the time-averaged schlieren images for NPR = 4.0 and 7.0. A train of shock cells is observed downstream of the nozzle exit, and the same can be inferred from the oscillatory nature of the CPD plots given in Fig. 6

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for different pressure ratios. From the one-dimensional isentropic flow analysis, the NPR required for correct expansion of M 1.6 nozzle is 4.25. Therefore, at the NPR of 4.0, the jet is slightly overexpanded. Oblique over-expanded shock wave envelope is formed close to the nozzle exit, which converges into a point termed as shock cross-over point, as shown in Fig. 7(a). Although the flow at the nozzle exit is over-expanded, still some expansion near the centerline close to the nozzle exit occurs until the shock cross-over point is reached. This expansion that is due to the relaxing effect of the internal fluid exiting from the nozzle causes an increase in the Mach number, which in turn increases the strength of the normal shock wave ahead of the pitot probe. Stronger shock wave results in higher stagnation pressure losses that reflect in low pitot pressures. It is for this reason that the pitot pressure for NPR = 4.0 (in fact, the explanation holds for all the NPRs) decreases till the flow reaches the shock cross-over point at x = 1.2d for NPR = 4.0 (Fig. 6). At NPR = 7, since the nozzle is under-expanded, strong expansion waves bounded by the barrel shock wave are formed at the nozzle exit, as shown in Fig. 7(b). The barrel shock wave terminates into a Mach disk that appears as a normal shock in the 2-dimensional schlieren images. The radius of the Mach disk increases as the NPR increases. The triple point (essentially a triple line for circular jet-like flows) formed by the intersection of the incident shock wave, Mach disk and the reflected shock wave can be clearly seen. Mach disk and the reflected shock wave causes strong deceleration of the flow to subsonic Mach numbers. Schlieren image for NPR = 6 with horizontal knife (Fig. 3(b)) reveals the slip line originating from the triple point. The formation of Mach disk is also evident from the minimum pitot pressure value at an axial location of around 2d in the CPD plot given in Fig. 6.

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3.1.2. Twin jet flow field Fig. 9 shows the schlieren images of a single jet at NPR = 5 and a corresponding image of twin jet. The flow-field from the interaction of closely placed twin jet plumes is very complicated, particularly at higher NPRs. Where a single jet is free to expand in all the directions, a twin jet has an azimuthal asymmetry and is influenced by the presence of the neighboring jet. The region of the flow lying between the two jets is referred to as the internozzle region. The shear layer entrains fluid from the surroundings that causes thickening in the shear layer and increase in the jet spread. The internozzle region because of the slight suction effect caused by mass entrainment results in the unbalanced lateral force on the jet structure causing the jet axes to lean towards each other, albeit very slightly. The flow field in the internozzle region of a twin jet is different from the flow field of the single jet at the same corresponding radial location due to the interaction of the inner shear layer. These flow features, to some extent are reflected in Fig. 8, where centerline pressure decay of single and twin jet are plotted together for comparison. The plots show the differences in the jet flow structure due to their mutual interaction. A single jet undergoes faster characteristic decay than their twin counterpart for all the NPRs. This is because of the fact that a single jet is free to entrain fluid mass from all the directions, unlike a twin jet. Moreover, the twin jet in over-expanded case, as shown in Fig. 8 (a) does not undergo much interaction resulting in centerline decay rate not significantly different from the corresponding single jet. The two plots overlap each other in the vicinity of the nozzle exit, and start deviating as the jets start spreading more and more. The difference is significant beyond x/d = 5.0 for NPR = 3. The difference in the characteristics of single and twin jet is more contrasting for under-expanded case as it is evident form Fig. 8(b). Since the spacing between the two nozzles is kept fixed in present experiments, the limited internozzle space is more influenced at higher NPRs. At higher NPRs, jets expand more downstream of the nozzle exit resulting in more interaction in the internozzle region from the very beginning. Further, since the single jet is free to entrain fluid from all directions, the single jet decay faster than the twin-jet in the characteristic decay zone for both (over-expanded and under-expanded cases). 3.2. CPD of uncontrolled and controlled twin jets

Fig. 6. Centerline pressure decay of single uncontrolled M 1.6 jet.

To enhance the mixing characteristics of the twin jet, passive control techniques in the form of vortex generators have been studied since long by different researchers for single jets. Although there are a few pieces of literature on controlled twin jets, they are of preliminary nature and considering the complexity of dynamics involved in twin jet problem, there is a vast scope for exploring their characteristics. In the present work, the passive control of the jet is achieved by deploying small VGs offering 5% blockage to the outgoing stream at the nozzle exit as shown in Fig. 2(b). The

Fig. 7. Schlieren images of uncontrolled single jet.

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Fig. 8. Centerline pressure decay comparison of single and twin jet.

Fig. 9. Schlieren images at NPR 5.

VGs are placed diametrically opposite at the exit of each nozzle in two orientations, one in series (horizontal orientation) and the other in parallel (vertical orientation). These orientations are selected for investigation as the extreme cases of all possible relative azimuthal orientations. Figs. 10(a) and 10(b) show the CPD plots for NPR = 3 and 4 respectively. The nozzle is designed for correct expansion at NPR = 4.25. Thus at NPR = 3 and 4, the nozzle is over-expanded. Because of the over-expansion, oblique shock wave envelope is formed near the nozzle exit. Even though the nozzle is overexpanded, expansion fans are present at the corners of the nozzle owing to the relaxation effect when the fluid experiences a sudden change in area while leaving the nozzle walls. However, the pressure at the nozzle exit is lower than the ambient pressure that results in the formation of an oblique shock wave. This shock wave decelerates and compresses the flow to equilibrate with the atmosphere. The combination of oblique shocks and expansion waves turn the flow parallel to the jet axis. The location where oblique shock envelope converges into a point at the jet axis is termed as shock cross-over point, represented by the first minima in the CPD plots of uncontrolled jets. Even though the individual oblique shock wave is weak, their amplified effect at the cross-over point proves to be very strong, causing the flow to become subsonic just downstream. The subsonic flow again accelerates due to momentum exchange from the surrounding fluid and attains the transonic values. The first peak after the shock cross-over point in the CPD plots corresponds to the sonic state achieved by the jet downstream of the cross-over point. Fig. 11(a) shows the position of the first crossover point. At higher pressure ratios, the shock crossover point is stretched and becomes a Mach disk, as can be seen in Fig. 11(d) for uncontrolled twin jet at NPR = 7. Downstream of the Mach disk, the flow is essentially subsonic in nature. The sonic flow further accelerates to higher Mach number due to the coupled

effect of two mechanisms. The first mechanism is the momentum exchange from high speed surrounding fluid and the second is the conversion of jet thermal energy into kinetic energy brought about by the expansion waves formed in response to the interaction of shock waves and the constant pressure jet boundary. These mechanisms result in continuous increase in the jet velocity near the centerline, until it reaches the second shock cross-over point. This cycle repeats as long as the flow in the jet core remains supersonic. The length measured from nozzle exit to the axial location where the local flow Mach number drops to 1.0 and the flow becomes permanently subsonic all the way downstream is defined as the core length for supersonic jet. The oscillatory nature of the CPD plots ceases and characteristic decay begins where supersonic core ends. Supersonic core length is primarily dependent on the nozzle pressure ratio (NPR) and the nozzle dimensions and can be estimated using the pitot probe measurements. The detailed procedure for core length estimation can be seen elsewhere [67,68]. Table 1 shows the core lengths of the uncontrolled as well as controlled twin jets at different NPRs and the corresponding reduction in core lengths by the application of vertical and horizontal VGs. The length of the wave-dominated supersonic core is an indirect measure of the mixing rate of a jet in the near-field region. Shorter the core length, the higher is the mixing rate. Therefore, it can be inferred from Fig. 10 and Table 1 that by placing small VGs at the exit of a converging-diverging nozzle, the mixing characteristic of a twin jet is drastically affected. The data shows that a core length reduction of around 70 to 85% is achieved by the use of vortex generators in the form of VGs. Thus the controlled twin jets have much higher mixing rate in the nearfield, as compared to uncontrolled jets. This higher mixing can be attributed to the fact that as soon as VGs are placed at the nozzle exit, bow shock appears ahead of each tab that leads to higher pressure on the upstream

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Fig. 10. centerline pressure decay comparison of uncontrolled and controlled twin jet.

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Fig. 11. Schlieren images of uncontrolled twin jet. Table 1 Core lengths of uncontrolled and controlled twin jets at different NPRs. Nozzle pressure ratio NPR

Core length of uncontrolled jet ( L / D )unc

Core length of twin jet with vertical VGs ( L / D )cv

Core length of twin jet with horizontal VGs ( L / D )ch

Change in core length by vertical VGs ( L / D )cv (%)

Change in core length by horizontal VGs ( L / D )ch (%)

3 4 4.25 5 7

8.57 8.79 8.00 11.18 16.85

1.26 2.28 2.28 3.24 4.19

1.78 1.93 3.32 3.34 4.81

85.29 74.06 71.50 71.21 75.27

79.23 78.04 58.5 71.5 70.13

side of the tab [56]. This high pressure forces the fluid stream to curve around the tab sides resulting in the formation of transverse vortices. These transverse vortices travel downstream and after a due amount of time transform into axial vortices due to the pressure differential prevalent in the flow field. Since these vortices are of comparatively larger size, they create suction effect leading to mass entrainment from the surrounding atmosphere. Due to this continuous mass entrainment, they assist in higher spreading of the jet in radial direction and eventually break down into smaller vortices as they become more unstable. These small-scale vortices are efficient mass transporters, and thus result in a higher mixing rate of the jet [51,52]. Fig. 14 shows the schematic representation of the vortices at the nozzle exit in the presence of vortex generators. The VGs shed small-scale vortices from their edges, which significantly contributes to an enhancement in the mixing ability of the twin jet. Furthermore, it is apparent from the schlieren images that the VGs mitigate the shock and expansion waves at the nozzle exit leading to considerably weaker waves. 3.3. Flow visualization The conventional Z-type schlieren setup with a vertical knife edge is employed in this work. Fig. 11 shows the schlieren images of the uncontrolled twin jet for over, correct and under-expanded modes. Flow direction is from left to right. Because of the vertical orientation of the knife edge, the density variations in the horizontal direction are enhanced, and the compression and expansion regions appear with dark and bright contrast respectively. Shock

cells formed by repetitive reflections of compression and expansion waves, as long as the reaccelerated flow reaches the supersonic speed, are visible in the images. The dark and bright regions in the schlieren images appear as triangular patches because of the orientation of the oblique shock waves. Shock cross-over point where the conical three-dimensional shock front converges into a point is shown in the Fig. 11(a) and the shock cell length can be taken as the distance between two successive shock cross-over points. Weak shock cells prevail for low NPRs, as can be seen in Fig. 11(a), whereas, for higher NPRs, the strength increases with increase in the NPR. For the designed condition, the uncontrolled nozzle was not found to undergo shock free expansion. This is because the nozzle wall was not contoured for aerodynamically clean flow. The nozzle at NPR = 7 is highly under-expanded. The flow at such high NPRs exits with strong expansion waves as can be seen from Fig. 11(d). The flow downstream of the nozzle exit, being at high pressure, expands to high supersonic Mach numbers. The shock and expansion waves due to lateral interaction of the twin streams do not exhibit a typical diamond structure. Flow velocity along the nozzle centerline increases until the flow reaches the Mach disk. This acceleration phenomenon results in the low pitot pressure values in the CPD plots, as shown in Fig. 8 for uncontrolled twin jet case. Katanoda et al. [65] studied the structure of M = 1.5 and 2.0 jet using pitot probe. Their results demonstrated that insertion of pitot probe does not disturb the position of Mach disk in supersonic free jets.

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Fig. 12. Schlieren images of twin jet controlled by vortex generators in vertical orientation.

Fig. 13. Schlieren images of twin jet controlled by vortex generators in horizontal orientation.

The overall performance of the VGs depends on their relative orientation with respect to the jets axis. Also, the influence of the relative azimuthal orientation of the VGs at the nozzle exit is dependent on the nozzle pressure ratio. Based on the present observations and the background understanding from the previous literature on the jet mixing, we explain the underlying mechanism that governs the mixing enhancement caused by the VGs used in the present study. For a single nozzle case, such studies are done in details and some of them are referred in section 1. When the rectangular tab type vortex generators are placed at the nozzle exit, as shown in Figs. 2(b), they alter the nozzle exit geometry, mak-

ing it asymmetric that influences the vortex size and distribution, as discussed earlier in section 3.2. Also, the VGs break the original clean shock-cell structure into more staggered one, as can be seen in Figs. 12 and 13. The flow field becomes complicated because of the presence of two jets in proximity. The fluid from the stagnation chamber is pushed into the nozzle by virtue of pressure difference imposed across it. The fluid accelerates to sonic conditions at the throat and then expands further to the supersonic speeds based on the NPR. Except for very low NPRs, the flow at the exit of the nozzle is in supersonic or near sonic state. The high speed fluid exiting the nozzle is shocked by the presence of the vortex generators. The

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Fig. 14. Schematic of flow at the nozzle exit demonstrating the distribution of vortices at the nozzle wall and along the VGs. The center arrow shows the flow direction. The figure is drawn with tab size exaggerated for representation purpose.

Fig. 15. Transverse pressure profiles of the uncontrolled and controlled twin jets at different x/d locations for NPR = 3.

fluid is abruptly turned sidewise around the tab surface through a strong shock envelope. The entire fluid stream that was earlier circular in cross-section is bifurcated into two non-circular daughter streams on either side of the VGs (see Fig. 12. These daughter streams have their own supersonic core with quasi-periodic shock cell structure that decays faster than the parent stream in uncon-

trolled case. In the present case of twin jets, four daughter streams are formed. These newly formed streams bound a low momentum region near the nozzle centerline and immediately downstream of the tab surface. The orientation of VGs actually governs the orientation of these daughter streams, which ultimately dictates the evolution of the entire flowfield downstream. For vertically ori-

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Fig. 16. Transverse pressure profiles of the uncontrolled and controlled twin jets at different x/d locations for NPR = 4.25.

ented VGs (Fig. 2), the streams are bifurcated laterally in the plane of the jet axes such that the two streams on the outer side are directed further away from the internozzle region, whereas the other two daughter streams are pushed towards each other in the internozzle region. The outer streams have more opportunity to entrain low momentum fluid from the surroundings as compared to the inner streams. The inner streams interact and almost merge into a single stream in the internozzle region. For low NPRs, such as NPR = 3 as shown in Fig. 12(a), the jet diameter is small and the inner streams initially move side by side gradually mixing and merging into one at some downstream location. On the other hand, for NPR = 7, the jet diameter is larger because of strong expansion at the exit. For such high NPRs, the inner streams merge earlier near the nozzle exit, as shown in Fig. 12(d). Shock wave reflection and the coupling of the two daughter streams resulting in higher momentum in the internozzle region can be seen from the schlieren images. High momentum near the centerline region of the controlled twin jets is also reflected in the transverse pressure profiles shown in Figs. 15 to 17 and discussed later in Sec. 3.4 in detail. For uncontrolled jets, on the other hand, the internozzle region near the nozzle exit is the dead region with slight suction until the inner shear layers of the two jet streams merge. Figs. 13 do not show jet bifurcation since VGs are aligned normal to the camera view line. The jets, in this case, are bifurcated vertically upside-down. As compared to the vertical orientation case, there is less interaction between the two jet streams near the internozzle region. The jet streams seem to evolve independently and undergo faster decay due to the ample amount of low momentum fluid in the surrounding region. For highly underexpanded case, i.e., for NPR = 7, as shown in Fig. 13(d), the two jet streams

seem to interact near the nozzle exit plane due to larger jet diameter caused by strong expansion waves. For high NPRs, the jet shear layer tends to bulge outwards resulting in larger jet diameter fostering early merging of the two streams near the centerline region. However, this interaction near the internozzle region is less severe. There is a drastic reduction of the supersonic core length and the shock cells are suppressed at all the pressure ratios. Comparing Fig. 13(b) and (c), the jet spread is larger for vertically oriented VGs. 3.4. Transverse pressure profiles The CPD plots shown in Fig. 10 give a good qualitative idea of the decay rate and the core lengths. However, since the measurements pertaining to these plots are done along the centerline of any one of the two jets, these do not give the full picture of the flow field evolution of the controlled cases. For instance, the placement of vortex generators at the very exit of the nozzle results in counter rotating vortices and the jet bifurcation with two high momentum streams bounding the low momentum region near the jet center as demonstrated by the schlieren images in Sec. 3.3. Heeb et al. [69] in a recent investigation using PIV data demonstrated the formation of counter rotating vortices form vortex generators of different sizes. Their results showed that the streamwise vortices influenced the jet cross section and caused an appreciable reduction in shock cell spacing. Of particular interest in the present case is to study the jet evolution in the cross-stream direction giving a broader picture of the shear layer thickening and the jet spread. With this objective, the transverse pressure variation in y-direction was measured for selected NPRs. A representative set of profile

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Fig. 17. Transverse pressure profiles of the uncontrolled and controlled twin jets at different x/d locations for NPR = 5.

plots at NPR = 3 (overexpanded), NPR = 4.25 (correctly expanded) and NPR = 5 (underexpanded) are presented in Figs. 15 to 17 respectively. All the measurements were done at six different axial locations (x/d = 0 to x/d = 10.0) covering the nearfield and the characteristic decay regimes. There is a marked difference between the profile plots of uncontrolled and controlled twin jets. The peak pressure value of the twin jets controlled by both vertical and horizontal VGs rapidly falls down as we move away from the nozzle exit towards farfield. This is unlike uncontrolled jet for which the dimensionless peak pressure ( pt / p 0 ) at x/d = 10.0 lies close to 0.7, whereas it reduces to around 0.4 for controlled jet at all the NPRs studied. This rapid decay in pressure is a sign of improved mixing characteristics by the application of VGs. Figs. 15 to 17 show that very close to the nozzle exit, the pressure profiles are similar with almost same peak values. But as we move away, the shape of the profiles of jet controlled with vertical VGs suddenly changes. At x/d = 0.0, we have two broad peaks with a valley at the center. At x/d = 2.0, instead of two peaks, we have four peaks for vertical VGs. This is due to the jet bifurcation. As was discussed in Sec. 3.3, the twin jet bifurcates into four jets by placing two sets of vertical VGs. As we move further downstream (x/d = 5.0 and 10.0), three peaks with two intermediate valleys are visible. This is due to merging of the two inner jets, as can be seen in schlieren images (Fig. 12). Although the twin jet is bifurcated into four jets by the application of horizontal VGs also, only two peaks are visible since we are moving our pitot probe in the y-direction (which is perpendicular to the direction of jet bifurcation). Moreover, since the pitot probe moves in the low-pressure central region for horizontally tabbed jets, the peak values are slightly lower than compared to that of vertically tabbed jets. The intense fluctuations in the cen-

tral region of the vertical tabbed jet at x/d = 1.0 indicates that the two jets begin to merge at around 1d. As we move to around 5d, the two merged jets stabilize, resulting in a smooth curve (Fig. 15(d)). The jet spread in y-direction very close to the nozzle exit is almost same for the uncontrolled jet and jet controlled with vertical VGs. As we move slightly downstream, the jet spread of the vertically tabbed twin jets is the highest, as is visible from Figs. 15 to 17. For example, at NPR = 3, the jet spread for the controlled jet is around 5d, whereas it is only 3d for uncontrolled jet at an axial distance of 5d. Therefore, it can be inferred from the profile plots that the vertical VGs lead to a considerably higher rate of mass entrainment from the ambient. The addition of VGs makes the pressure profiles asymmetric in nature, unlike the uncontrolled jet which are almost symmetric. The asymmetricity is the result of small and large-scale vortices being introduced into the jet flow field from the sides of the VGs and due to the formation of daughter streams. Since the flow inside the nozzle is supersonic, bow shocks are formed upstream of the VGs leading to a sudden increase in pressure which in turn results in vortex shedding. These vortices continuously interact with the comparatively larger azimuthal vortices shed from the nozzle lip and the shear layer. Furthermore, these vortices, as they convect downstream, also interact with the shocks and expansion waves prevalent in the flow field resulting in enhanced mixing of the jet from the surroundings. This enhanced mixing is ultimately responsible for reduction in supersonic core length and higher characteristic decay of the jet.

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4. Conclusions The investigations in the present experimental work has brought about certain key aspects of the effects of using small tab like vortex generators (VGs) as passive means for controlling twin jet flowfield, which are interesting and of much practical relevance. It was observed that the VGs increase the spread and significantly enhance the decay rate of the twin jet at all the NPRs. While the orientation of the VGs affect the characteristic decay along the centerline of the jet, the influence on the overall jet spread is even more pronounced. The enhanced mixing and the faster characteristic decay was attributed to the vortices shed by VG, which are convected and reoriented streamwise by the surrounding high momentum fluid in the jet. While convecting, these vortices entrain and drag the surrounding low-speed fluid causing cross-stream momentum exchange leading to faster jet decay. The supersonic twin jet upon encounter with the VGs bifurcates into four daughter supersonic streams separated by the subsonic fluid in the wake of the VGs. The bifurcation of the jet results in the formation of daughter streams that are deflected away from the jet centerline. The orientation of these daughter streams depends on the azimuthal orientation of the VGs, and influences the mutual interaction of the twin jets. To further study the evolution of daughter streams, transverse data are plotted for controlled and uncontrolled cases. It was observed that the daughter streams behave as separate supersonic jets that undergo characteristic decay hand-in-hand. For the vertically oriented VGs, the inner streams merge into one (unlike the horizontal VGs) resulting into three streams in total. The results showed that the orientation of VGs when used for twin jets will give directivity to the flow field that is very well represented in the mean flow characteristics such as jet spread and decay rate. This directivity is expected to have implications on the acoustic spectrum and the directivity of the sound waves that has been the topic of much concern in the recent decades. Further research on the topic will be carried out in the future using more sophisticated flow diagnostic techniques with a particular focus on the unsteady nature of the flowfield and the acoustic radiations. Declaration of competing interest On behalf of all authors, the corresponding author states that there is no competing interest of any kind. Acknowledgements The research performed in this work was financially supported by the Department of Aerospace Engineering at the Indian Institute of Technology (IIT) Kanpur, India, which is gratefully acknowledged. We also acknowledge the help and support provided by Mr. Suresh Mishra for carrying out the experiments. References [1] R.W. Miksad, Experiments on the nonlinear stages of free-shear-layer transition, J. Fluid Mech. 56 (4) (1972) 695–719. [2] H. Li, J.-g. Tan, J.-w. Hou, D.-d. Zhang, Investigations of self-excited vibration in splitter plate with a cavity in the supersonic mixing layer, Aerosp. Sci. Technol. 74 (2018) 120–131. [3] P.E. Dimotakis, G.L. Brown, The mixing layer at high Reynolds number: largestructure dynamics and entrainment, J. Fluid Mech. 78 (3) (1976) 535–560. [4] S. Barre, C. Quine, J. Dussauge, Compressibility effects on the structure of supersonic mixing layers: experimental results, J. Fluid Mech. 259 (1994) 47–78. [5] A. Michalke, Survey on jet instability theory, Prog. Aerosp. Sci. 21 (1984) 159–199. [6] D. Crighton, M. Gaster, Stability of slowly diverging jet flow, J. Fluid Mech. 77 (2) (1976) 397–413. [7] R. Petersen, M. Samet, On the preferred mode of jet instability, J. Fluid Mech. 194 (1988) 153–173.

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[8] P.K. Ray, L.C. Cheung, S.K. Lele, On the growth and propagation of linear instability waves in compressible turbulent jets, Phys. Fluids 21 (5) (2009) 054106. [9] M. Samimy, N. Webb, M. Crawley, Excitation of free shear-layer instabilities for high-speed flow control, AIAA J. (2018) 1–22. [10] C.K. Tam, On the noise of a nearly ideally expanded supersonic jet, J. Fluid Mech. 51 (1) (1972) 69–95. [11] C.K. Tam, P. Chen, Turbulent mixing noise from supersonic jets, AIAA J. 32 (9) (1994) 1774–1780. [12] C. Tam, Supersonic jet noise generated by large scale disturbances, J. Sound Vib. 38 (1) (1975) 51–79. [13] P. Plaschko, Stochastic model theory for coherent turbulent structures in circular jets, Phys. Fluids 24 (2) (1981) 187–193. [14] C.K. Tam, P. Chen, J. Seiner, Relationship between the instability waves and noise of high-speed jets, AIAA J. 30 (7) (1992) 1747–1752. [15] H. Tanna, P.D. Dean, R.H. Burrin, The Generation and Radiation of Supersonic Jet Noise, vol. 3: Turbulent Mixing Noise Data, Tech. rep., Lockheed-Georgia Co Marieta, 1976. [16] D.K. McLaughlin, W.D. Barron, A.R. Vaddempudi, Acoustic properties of supersonic helium/air jets at low Reynolds numbers, in: 14th DGLR/AIAA Aeroacoustics Conference, vol. 1, 1992, pp. 308–316. [17] J.M. Seiner, M.K. Ponton, B.J. Jansen, N.T. Lagen, The effects of temperature on supersonic jet noise emission, in: 14th DGLR/AIAA Aeroacoustics Conference, vol. 1, 1992, pp. 295–307. [18] H. Tanna, An experimental study of jet noise part II: shock associated noise, J. Sound Vib. 50 (3) (1977) 429–444. [19] C.K. Tam, H. Tanna, Shock associated noise of supersonic jets from convergentdivergent nozzles, J. Sound Vib. 81 (3) (1982) 337–358. [20] T. Norum, J. Seiner, Broadband shock noise from supersonic jets, AIAA J. 20 (1) (1982) 68–73. [21] C.K. Tam, Supersonic jet noise, Annu. Rev. Fluid Mech. 27 (1) (1995) 17–43. [22] M. Kearney-Fischer, A. Sinha, M. Samimy, Intermittent nature of subsonic jet noise, AIAA J. 51 (5) (2013) 1142–1155. [23] J.I. Hileman, B.S. Thurow, E.J. Caraballo, M. Samimy, Large-scale structure evolution and sound emission in high-speed jets: real-time visualization with simultaneous acoustic measurements, J. Fluid Mech. 544 (2005) 277–307. [24] A. Sinha, H. Alkandry, M. Kearney-Fischer, M. Samimy, T. Colonius, The impulse response of a high-speed jet forced with localized arc filament plasma actuators, Phys. Fluids 24 (12) (2012) 125104. [25] D. Berndt, Dynamic Pressure Fluctuations in the Internozzle Region of a TwinJet Nacelle, Tech. rep., SAE Technical Paper, 1984. [26] C.-W. Kuo, J. Cluts, M. Samimy, Exploring physics and control of twin supersonic circular jets, AIAA J. (2016). [27] B.T. Warwick, I.Y. Kim, C.K. Mechefske, Substructuring verification of a rear fuselage mounted twin-engine aircraft, Aerosp. Sci. Technol. 93 (2019) 105305. [28] J.M. Seiner, J.C. Manning, M.K. Ponton, Dynamic pressure loads associated with twin supersonic plume resonance, AIAA J. 26 (8) (1988) 954–960. [29] J. Gao, X. Xu, X. Li, Numerical simulation of supersonic twin-jet noise with high-order finite difference scheme, AIAA J. (2017) 290–300. [30] K. Goparaju, D.V. Gaitonde, Dynamics of closely spaced supersonic jets, J. Propuls. Power (2017) 1–13. [31] Y. Lin, M. Sheu, Interaction of parallel turbulent plane jets, AIAA J. 29 (9) (1991) 1372–1373. [32] V.B. Sabareesh, K. Srinivasan, T. Sundararajan, Acoustic characteristics of equal and unequal twin circular slot jets, J. Sound Vib. 342 (2015) 90–112. [33] G. Bell, J. Soria, D. Honnery, D. Edgington-Mitchell, An experimental investigation of coupled underexpanded supersonic twin-jets, Exp. Fluids 59 (9) (2018) 139. [34] M. Faheem, A. Khan, R. Kumar, S.A. Khan, Experimental study of supersonic multiple jet flow field, in: Proc. of the 32nd International Symposium on Shock Waves, ISSW32, 2019. [35] M.B. Alkislar, A. Krothapalli, I. Choutapalli, L.M. Lourenco, Structure of supersonic twin jets, AIAA J. 43 (11) (2005) 2309–2318. [36] R. Wlezien, Nozzle geometry effects on supersonic jet interaction, AIAA J. 27 (10) (1989) 1361–1367. [37] L. Shaw, Twin-jet screech suppression, J. Aircr. 27 (8) (1990) 708–715. [38] G.H. Moustafa, Experimental investigation of high-speed twin jets, AIAA J. 32 (11) (1994) 2320–2322. [39] G.H. Moustafa, Interaction of axisymmetric supersonic twin jets, AIAA J. 33 (5) (1995) 871–875. [40] D. Parekh, V. Kibens, A. Glezer, J. Wiltse, D. Smith, Innovative jet flow controlmixing enhancement experiments, in: 34th Aerospace Sciences Meeting and Exhibit, 1996, p. 308. [41] K. Zaman, Spreading characteristics and thrust of jets from asymmetric nozzles, in: 34th Aerospace Sciences Meeting and Exhibit, 1995, p. 200. [42] M. Samimy, J.-H. Kim, P. Clancy, S. Martens, Passive control of supersonic rectangular jets via nozzle trailing-edge modifications, AIAA J. 36 (7) (1998) 1230–1239. [43] R.W. Wlezien, V. Kibens, Influence of nozzle asymmetry on supersonic jets, AIAA J. 26 (1) (1988) 27–33.

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14

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[44] F.S. Alvi, H. Lou, C. Shih, R. Kumar, Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets, J. Fluid Mech. 613 (2008) 55–83. [45] E. Gutmark, F. Grinstein, Flow control with noncircular jets, Annu. Rev. Fluid Mech. 31 (1) (1999) 239–272. [46] A. Valdivia, K. Yuceil, J. Wagner, N. Clemens, D. Dolling, Active control of supersonic inlet unstart using vortex generator jets, AIAA Pap. 4022 (2009) 2009. [47] M. Samimy, J.-H. Kim, J. Kastner, I. Adamovich, Y. Utkin, Active control of highspeed and high-Reynolds-number jets using plasma actuators, J. Fluid Mech. 578 (2007) 305–330. [48] T. Lapushkina, A. Erofeev, Supersonic flow control via plasma, electric and magnetic impacts, Aerosp. Sci. Technol. 69 (2017) 313–320. [49] H. Pourhashem, S. Kumar, I.M. Kalkhoran, Flow field characteristics of a supersonic jet influenced by downstream microjet fluidic injection, arXiv:1901. 00559, 2019. [50] D. Liepmann, M. Gharib, The role of streamwise vorticity in the near-field entrainment of round jets, J. Fluid Mech. 245 (1992) 643–668. [51] L. Bradbury, A. Khadem, The distortion of a jet by tabs, J. Fluid Mech. 70 (4) (1975) 801–813. [52] K. Zaman, M. Reeder, M. Samimy, Control of an axisymmetric jet using vortex generators, Phys. Fluids 6 (2) (1994) 778–793. [53] A. Khan, R. Kumar, Experimental study and passive control of overexpanded plug nozzle jet, J. Spacecr. Rockets (2017) 1–7. [54] P. Arun Kumar, M. Aileni, E. Rathakrishnan, Impact of tab location relative to the nozzle exit on the shock structure of a supersonic jet, Phys. Fluids 31 (7) (2019) 076104. [55] K. Zaman, Streamwise vorticity generation and mixing enhancement in free jets by ‘delta-tabs’, 3rd Shear Flow Conference (1993) 3253. [56] M. Reeder, M. Samimy, The evolution of a jet with vortex-generating tabs: realtime visualization and quantitative measurements, J. Fluid Mech. 311 (1996) 73–118.

[57] W. Gretta, C. Smith, The flow structure and statistics of a passive mixing tab, J. Fluids Eng. 115 (2) (1993) 255–263. [58] M. Samimy, K. Zaman, M. Reeder, Effect of tabs on the flow and noise field of an axisymmetric jet, AIAA J. 31 (4) (1993) 609–619. [59] H. Lou, F.S. Alvi, C. Shih, Active and adaptive control of supersonic impinging jets, AIAA J. 44 (1) (2006) 58–66. [60] S. Akram, E. Rathakrishnan, Control of supersonic elliptic jet with ventilated tabs, Int. J. Turbo Jet-Engines (2017). [61] S. Akram, E. Rathakrishnan, Corrugated tabs for enhanced mixing of supersonic elliptic jet, J. Aerosp. Eng. 32 (1) (2018). [62] P. Lovaraju, E. Rathakrishnan, Subsonic and transonic jet control with crosswire, AIAA J. 44 (11) (2006) 2700–2705. [63] E. Rathakrishnan, Instrumentation, Measurements, and Experiments in Fluids, CRC Press, 2016. [64] A. Khan, R. Panthi, R. Kumar, S.M. Ibrahim, Experimental investigation of the effect of extended cowl on the flow field of planar plug nozzles, Aerosp. Sci. Technol. 88 (2019) 208–221. [65] H. Katanoda, Y. Miyazato, M. Masuda, K. Matsuo, Pitot pressures of correctlyexpanded and underexpanded free jets from axisymmetric supersonic nozzles, Shock Waves 10 (2) (2000) 95–101. [66] A. Khan, R. Kumar, S.B. Verma, C. Manisankar, Effect of cross wire tab orientation on twin jet mixing characteristics, Exp. Therm. Fluid Sci. 99 (2018) 344–356. [67] K. Phalnikar, R. Kumar, F. Alvi, Experiments on free and impinging supersonic microjets, Exp. Fluids 44 (5) (2008) 819–830. [68] A. Kumar, E. Rathakrishnan, Triangular tabs for supersonic jet mixing enhancement, Aeronaut. J. 118 (1209) (2014) 1245–1278. [69] N. Heeb, E. Gutmark, K. Kailasanath, An experimental investigation of the flow dynamics of streamwise vortices of various strengths interacting with a supersonic jet, Phys. Fluids 26 (8) (2014) 086102.