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Wind tunnel simulation of an opencut tunnel airflow field along the Linhe-Ceke Railway, China
Aeolian Research 39 (2019) 66–76
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Wind tunnel simulation of an opencut tunnel airflow field along the LinheCeke Railway, China
T
Min Yana, Haibing Wanga, Hejun Zuoa, , Gangtie Lia ⁎
a
Inner Mongolia Key Laboratory of Aeolian Physics and Desertification Control Engineering, College of Desert Control Science and Engineering, Inner Mongolia Agricultural University, 29 Erdos East Street, Hohhot, Inner Mongolia Autonomous Region 010011, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Airflow field Opencut tunnel for sand prevention Wind tunnel simulation Linhe-Ceke railway
Opencut tunnels are linear arch constructions that are being used as new structures to prevent sand encroachment on railway lines. The large size and shape of the contained structure affect the wind field around the structure in a manner different to how traditional constructions prevent sand accumulation. The present study identified the characteristics of wind velocity and flow fields in opencut tunnels in a wind-tunnel simulation. Results show that airflow having different wind angles (15°–90°) had different effects on the sand-accumulationprevention characteristics of the opencut tunnel. The inflection point of a rapid change in wind speed was 6H (where H is the vertical height of the model) from the windward side of the opencut tunnel, and the position of an area of strong wind on the upper part of the opencut tunnel gradually moved to the windward side with an increase in the wind angle. This trend was not affected by a change in the indicated wind speed. The reattachment distance on the leeward side decreased with an increase in the wind angle, being 1H at 75°–90°, 2H at 30°–60° and 6H at 15° or less. The protection effect was best for a large angle of the main wind direction (exceeding 75°). Observed characteristics of the wind field show that the opencut tunnel is effective at all wind angles in preventing and controlling railway sand flow hazards and can thus ensure smooth railway operation.
1. Introduction Statistics presented in the fifth Chinese bulletin on desertification show that 1.83 million square km of land has undergone wind-eroded desertification, accounting for 69.93% of the total area of desertification in China. The characteristics of sparse vegetation, strong winds, long wind periods, more sand matter carried in the wind, less rainfall and uneven rainfall distribution in time and space has led to frequent dust storms, which seriously affect production activities such as traffic and construction in the affected area (Xie et al., 2017). Railways are not only a popular means of transportation, but also play an important role in economic and social development, acting as the arteries of national development. Many railways in arid regions of China are seriously affected by sandstorms. As of December 31, 2017, the total length of railways nationwide was 128,000 km, and about 18,131 km of railway lines had sandstorm hazards or potential sandstorm hazards, accounting for 16.5% of the total length of railway operations in China. The main railways affected are the Qinghai-Tibet railway, Lan-Xin railway, Bao-Lan railway, Lin-Ce railway and some other trunk lines (Xu et al., 2006; Cheng et al., 2010; Xiao et al., 2015; Luca et al., 2018). Because railways usually require the roadbed to be
⁎
higher or lower than the ground, in a sandstorm area the railway often becomes an artificial sand barrier which intercepts the blown sand material in the vicinity of the railway. This makes railways more vulnerable to sand damage than highway facilities and, therefore, railways in a sandstorm area are often seriously damaged by sandstorms (Zhang et al., 2012a). The damage caused to railways by sandstorms are mainly reflected in derailments and slower transport because of the accumulation of sand in the ballast bed. After the wind-blown sand material has been deposited on the ballast bed, the accumulated sand will slowly infiltrate the cracks in the ballast due to the vibration of the ballast bed, impeding drainage and increasing how often the ballast bed needs to be cleaned. Irregularities in the ballast bed also cause a triangular pit and low joint phenomenon in the track. To prevent the pollution of the ballast bed and maintain the quality of the line, it is necessary to screen the ballast frequently and remove the accumulated sand, which requires both labor and financial resources. With the development of western China and the implementation of the “One Belt, One Road” strategy, the railway mileage in sandstorm areas will increase. Effectively solving the safe operation of the railways in sandstorm areas has become an urgent problem. The Lin-Ce railway, which is an important line connecting
Corresponding author. E-mail address: [email protected] (H. Zuo).
https://doi.org/10.1016/j.aeolia.2019.04.007 Received 4 February 2019; Received in revised form 27 April 2019; Accepted 29 April 2019 Available online 09 May 2019 1875-9637/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic route of the Linhe-Ceke Railway and wind directions from 1981 to 2014 in the Guaizi Lake region.
Fig. 2. Schematic of the wind speed measurement system and experimental setup.
northwest and north China, passes through the Ulanbuhe, Yamalek and Badain Jaran deserts and the vast Gobi desert (rock desert and gravel desert) on the Alxa plateau, with a total length of 768.415 km (Fig. 1). When selecting the route for the Lin-Ce railway line, the line design unit chose to detour along the south side of the nature reserve to the Badain Jaran desert to protect the Ejin Populous euphractica forest national nature reserve and avoid the destruction of the P. euphractica desert riparian forest. Although the detour protected the P. euphractica forest
to the greatest extent, it meant that the railway passed through a 10 km mobile dune area on the northwest margin of the Badain Jaran desert. To prevent sandstorm hazards after the completion of this section, the design unit compared several types of construction, and finally completed an opencut tunnel railway of 8.08 km in the densest area of mobile dunes (Fig. 1). The near-surface airflow field is a dominant factor affecting aeolian sand deposition, and the sand-control function of the majority of 67
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Fig. 3. Wind tunnel equipment and opencut tunnel model. (a), (c) and (e) show the model of the opencut tunnel, (b) is the wind tunnel body, (d) is the model size and the observation position, and (f) is the field picture of opencut tunnel.
measures is realized in the turbulent near-surface boundary layer in railway sand control (Zhang et al., 2007, 2016). Therefore, to prevent sandstorm disasters, many measures have been taken along railways (Wang et al., 2007; Zhang et al., 2010, 2012b; Cheng and Xue, 2014), including mechanical sand barriers, roadbed windbreaks, sand barriers, bridge windbreak shelters and windbreak tunnels. To further strengthen the measures of wind control engineering, a series of sand control engineering systems has been set up on the windward side of railways in areas experiencing frequent sandstorms (Cheng et al., 2016; Wang et al., 2017). So far, many wind-proofing measures and sediment characteristic have been studied and flow field characteristics and sediment transport modes have been determined (Bergstrom et al., 1992; Lee et al., 2002; Jiang et al., 2018). The opencut tunnel, as a new type of linear sand control measure compared with traditional engineering sand control measures, has the basic characteristics of a sand control wall (Fig. 3). The impermeability of the sand control and the large construction size, however, make it difficult to apply traditional engineering sand control measures and wind and sand resistance mechanisms. So far, no papers or reports on the mechanisms, effective periods and effects of opencut tunnels for sand prevention have been published. As a new type of linear engineering sand control measure, the characteristics of the airflow variation on the windward side and leeward side become an important basis for determining whether sand is buried. Previous studies have shown that the motion characteristics of airflow are different at different locations on the surfaces of transverse dunes. Topographic rotation flows, for example, form on the windward slopes of dunes while separation flows, reattachment deviation flows, and reverse flows form on leeward slopes (Lynch et al., 2010; Bauer et al., 2012). The reattachment distance of leeward airflow is closely related to the morphological characteristics of dunes, such as the grade of the windward slope, dune height and dune spacing. However, there is no clear quantitative relationship between the airflow reattachment distance and a transverse dune (or other horizontal sand retaining measures) (Schatz and Herrmann, 2006; Fenton et al., 2014). Therefore, to better understand the characteristics of wind along the opencut railway tunnel, we simulated the flow field characteristics in a wind tunnel. This paper studied a sand protection opencut tunnel combined with the different models of wind direction angles, variations in the airflow velocity, different gradients of flow profile variations, windward air flow speed and leeward flow separation and weight attached to the process. The wind tunnel simulation results provided some baselines for
designing railways and highways. 2. Material and methods 2.1. Background of the research area The Lin-Ce (Linhe to Ceke) railway (Fig. 1) is located in the arid region of northwest China, east of Linhe station in Bayannur city, and west of Ejin banner in the Alashan league of the Inner Mongolia Autonomous Region. The Lin-Ce railway passes through the Ulanbuhe, Yamalek and Badain Jaran deserts and the Gobi Desert from the Alxa plateau, with a total length of 768.415 km (Fig. 1). About 456 km is within the wind-sand disaster area. Sandstorms happen annually, and obvious sand accumulation phenomena occur on both sides of the line. The sand accumulation in the section with the most serious sand pollution exceeds 1.5 m on the rail surface. The natural environment along the railway is harsh, with strong winds and violent sandstorms. The average wind speed over the years 1981–2014 was 4.68 m/s, and the average maximum wind speed over this period was 9.39 m/s. The wind direction at speeds greater than 5 m/s is shown in Fig. 1, and the main wind directions are W, WNW, E and ENE, which account for 64.11% of all wind direction frequencies. 2.2. Wind tests A wind tunnel experiment was carried out in the Shapotou soil wind tunnel laboratory in the Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences. The dc held blowing low speed wind tunnel was 37.08 m, the length was 21 m, the cross-section was 1.2 × 1.2 m, and lifting the whole section of the bottom can make a gradient of 1°. The wind energy ratio was 0.55, with 0.6% average turbulence intensity, a turbulence scale epsilon ε < 1‰, as the axial static pressure gradient delta p/ x = 0 . The free-stream wind velocity in the wind tunnel ranged from 1.2 to 30.8 m/s. The working principle is shown in Fig. 2. According to the similarity characteristics of airflow motion, generally, if the Reynolds number is controlled around Re = 106, the wind tunnel can reach the aerodynamic self-modeling state, and the Reynolds number is calculated as (Wu, 1982)
Re = 68
vL , µ
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2.3. Data analysis
Table 1 Characteristics of sand particle size distribution. Grain size/μm
< 100
100–200
200–300
300–400
(1) The wind tunnel speed is calculated as (Bagnold, 1941)
Percentage/%
4.31
76.20
19.14
0.35
v = 4.05
where ρ is the flow density, v is the wind speed in the tunnel, L is the model dimension and μ is the viscosity coefficient. When designing a model of a sand prevention tunnel, both the accuracy of the model size and the operability of the experiment should be considered to determine whether the model meets the requirements of wind-tunnel simulation and a similarity ratio of 1:30 should be adopted. We therefore selected the model of the opencut tunnel as shown in Fig. 3. The model was 100 cm long, 26.7 cm wide and 30 cm high and the top vent of the model was 10 × 10 cm. The model was made of sheet iron and welded by molding. It was a double-layer hollow structure. The design of the wind tunnel experiment did not involve the response characteristics of different particle sizes in the sand material to the sand prevention tunnel. The source of the sand material sample did not affect the progress of the experiment plan, nor did it affect the analysis, summary and extraction of the test accuracy and general rules of the experiment. Therefore, eolian sand samples were obtained from the edge of the Tengger desert where the laboratory was located. The particle size distribution characteristics are shown in Table 1. The wind tunnel experiment was conducted to determine the airflow velocity field characteristics under the condition of a clean wind with different angles between the model and the wind direction. Ten angle designs (15°, 30°, 35°, 40°, 45°, 55°, 60°, 65°, 75°, 90°) and four wind speed levels (6 m/s, 8 m/s, 10 m/s and 12 m/s) were selected and the net wind blew steadily at a certain pace and angle for 5 min. The wind speed was converted by a pitot tube differential pressure measurement system. The measured wind heights were 0.3 cm, 0.6 cm, 1.5 cm, 3 cm, 12 cm, 20 cm, 35 cm and 60 cm. The location of measurement points, in relation to the model height of H = 30 cm, were 0.5H, 1H, 2H, 3H, 4H, 6H, 8H, 10H before the model, and 0.5H, 1H, 2H, 4H, 6H, 8H after the model (Fig. 3d). We likewise used multiples of h for the vertical position.
P T × × h p 293
where v is wind speed, P is standard atmospheric pressure (mmHg), p is atmospheric pressure (mmHg), T is temperature (K), and △h is pitot tube differential pressure (mmH2O); (2) The reattachment distance (l) is used to describe the horizontal distance between the model and the airflow reattachment point when the horizontal velocity is zero. (3) The separation bubble height and its area are used to describe the separation of airflow on the leeward side of the model. The separation bubble height (h) is the upper edge of the closed streamline group while the separation bubble area (S) is the outer boundary area surrounding the separation bubble divided by H2 for dimensionless treatment (Dong et al., 2011). (4) The shape ratio (A) is the ratio between the height of the separation bubble and the reattachment distance, indicating the flatness of the separation bubble. The ratio is calculated as A = h/l. 3. Results 3.1. Influence of wind speed and direction on the airflow velocity field of an opencut tunnel The characteristic curves of the airflow velocity field under different wind velocity gradients when the model of the opencut tunnel and inlet angle was 90° can be seen in Fig. 4. The upper airflow on the windward side rose gently, and the inflection point of airflow changes was at −6H. There was an inflection point of the airflow changes in the lower layers at −2H, which formed a zone of weak wind. The range of the airflow uplift zone in the section before the model expanded, and there was uplift at roughly −4H. A strong wind area formed above the model, and the strong wind area moved to the left of the center of the model with reduced intensity. The number of eddies on the leeward side decreased, and the upper airflow had diffusion changes. With an increase in the indicated wind speed, the change trend of the airflow on
Fig. 4. Characteristics of the opencut tunnel airflow velocity with different wind speed gradients. The example shows airflow at 90° to the tunnel. 69
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Fig. 5. Characteristics of the opencut tunnel airflow velocity at different wind angles. The wind speed is 12 m/s in this example.
the windward side was the same. The change in the upper airflow on the leeward side was gentle, and the vortex formed by the lower airflow gradually became larger. The trend of connecting multiple weak wind zones into a single weak wind zone or a quiet wind zone was strengthened. When the wind speed was 12 m/s, the vortex on the leeward side of the 0–4H range became connected into one piece, and the height increased. In conclusion, the position of airflow field did not change with the change in the indicated wind speed. The velocity results of the wind flow field showed that the angles 15°, 30°, 45°, 60°, 75° and 90° well reflect different wind angles across the 0°–90° interval Fig. 5 shows the airflow velocity field characteristics of the opencut tunnel model and the air inlet at different angles under a 12 m/s indicated wind speed. It is seen that when the airflow acted on the opencut tunnel, the windward side airflow could be divided into a lower deceleration airflow, a middle uplift accelerated airflow and an upper turbulence, which formed a weak wind zone. When the airflow crossed the opencut tunnel, the boundary layer separated and part of the airflow continued to be uplifted and accelerated. In addition, a strong wind zone formed at the top of the opencut tunnel, while another part of the airflow decelerated on the leeward side and formed a reverse airflow, producing a weak or quiet wind zone. When the airflow angle was between 15° and 90° relative to the opencut tunnel, airflow around the opencut tunnel model operation was prevented, there was a differentiation regularity on the windward side, and the angle of the lower airflow disturbance was greater than of the upper airflow disturbance. From the wind speed change on the windward side of opencut tunnel and weak wind zone formed on the leeward, it can be seen that the place 6H away from the opencut tunnel was always the inflection point where the wind speed on the windward side changed sharply. From the position and range of the strong wind area on the upper part of the
opencut tunnel, as the wind direction angle increased, the position of the strong wind area gradually moved to the windward side, and the position where the wind speed lifted changed from −2H to −4H. On the leeward side of the opencut tunnel, the wind speed change and the formation of the weak area in the wind under different wind angle conditions showed similar leeward air change rules, all exert that weak with the increase of wind direction angle between the wind zone showed increasing trend (more than 60°). 3.2. Influence of wind direction on the variation of the wind velocity profile of an opencut tunnel Under the action of the indicated wind speed of 6 m/s, 8 m/s, 10 m/ s and 12 m/s, the wind velocity profile changes at different wind angles and positions on the windward side and leeward side are shown in Figs. 6 and 7. The airflow velocity on the windward side increased with an increase in the indicated wind speed and the gradient at the same horizontal position (Fig. 6). Under the same wind direction angle, the smaller the indicated wind speed, the stronger the effect of increasing wind speed. The variation trend on the leeward side was similar to that on the windward side. Under the same wind direction angle conditions, the greater the airflow velocity, the more obvious the change was. According to the airflow velocity profile with different wind direction angles, the trend of the airflow velocity profile with the four groups of indicated wind speed was similar. This indicated that the wind direction angle had a direct influence on the wind velocity profile, while the change in the indicated wind speed only reflected on the scale of the airflow change. Under the action of different indicated wind speeds on the windward side, the wind velocity profile presented a logarithmic function increasing trend with the increase of gradient, while the 70
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Fig. 6. Wind speed profile on the windward side (−10H, −6H, −2H, −1H, −0.5H) with different wind directions.
change on the leeward side was more complex and variable, and there was no obvious functional relationship. The wind velocity profiles at different horizontal positions on the windward side and different wind directions were accelerated at different degrees, among which airflow rose faster within the range of 0.01–0.10 h from the near stratum. Within the range of 0.10–0.70 h, the airflow rose slowly and then accelerated. The airflow above 0.70 h had
lower acceleration. The effect of this process gradually strengthened as the wind speed approached the opencut tunnel model while the airflow uplift strengthened with an increase in the wind angle. At horizontal positions of −10H and −6H, the airflow velocity increased logarithmically with the vertical gradient but the increasing degree was closer to the direction of the model. At horizontal positions of −2H and −0.5H, the airflow velocity fluctuated with an increase in the vertical 71
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Fig. 7. Wind speed profile on the leeward side (1H, 2H, 4H, 6H, 8H) with different wind directions.
gradient. The wind velocity profile with different horizontal positions and different wind directions on the leeward side varied greatly with the vertical gradient. A wind velocity below 0.70 h was significantly affected by the model of the opencut tunnel, and the wind velocity near the formation at 0.01 h generally increased under the influence of the leeward eddy. From 0.02 h to 0.70 h, it was squeezed by the eddy flow of the lower airflow, the weak wind zone appeared to varying degrees,
and the wind speed even reached zero. With the increase of the wind direction angle, this effect gradually increased. Above 0.70 h, the influence of the opencut tunnel model decreased, and the airflow rose slowly in constant fluctuation. The overall change in the wind velocity profile on the leeward side was significantly affected by the opencut tunnel model, and as the wind direction angle increased, the fluctuation of airflow in the vertical gradient gradually weakened. This indicated that with an increase in 72
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the included angle of wind direction, the impact force between the windward side airflow and the opencut tunnel model gradually increased. After passing the model, the wind velocity was affected by the air pressure difference. The larger the included angle of the wind direction, the faster the airflow velocity on the leeward side decayed, and the greater the distance from the model, the smaller the action process. As the wind direction angle increased, the airflow changed to a typical position on the leeward side, and the airflow velocity even appeared to be zero, but this was not directly related to the change of indicated wind speed. Locations at 2H and 4H showed typical air velocity that first decreased with increasing vertical gradient, but the change trend could be divided into 15°, 30°, 45°, 60°, 75° and 90° with four types of curve, although the changing process at 4H at 90° did not follow this relationship. This was because the 90° direction angle of flow and sediment control in the opencut tunnel model had the biggest impact force, which caused the air flow around the operation process to become more fluid. At the typical positions of 6H and 8H, the fluctuation of airflow velocity gradually decreased with the increase in vertical gradient.
3.5. Sand buried validation of the opencut tunnel We consider a real opencut tunnel that had been in operation for nearly 10 years. Google Earth satellite images (Fig. 9) taken in 2007, 2012 and 2016 show that there are no appreciable differences in the aeolian geomorphology, the surrounding area or in the lines of the opencut tunnel. The figure reveals a crescent-shaped dune chain distributed in the northeast and southwest and inclined toward the opencut tunnel. Although there are quicksand deposits in the transition zone on the northern side of the opencut tunnel, the accumulation amount is not large and the zone is a certain distance from the opencut tunnel. At the same time, it is seen that there is less quicksand on the southern side of the opencut tunnel and this quicksand is farther from the opencut tunnel. It is thus judged from the theoretical determination of the wind tunnel experiment and the field investigation that it is unlikely that the opencut tunnel will be buried. 4. Discussion Construction of the opencut tunnel makes the primary airflow flowing over the surface become a special secondary flow. As a result, the airflow near the opencut tunnel changes greatly in both flow velocity and flow direction. In particular, the momentum and energy transfer mode of the airflow becomes more complex, and this change affects the deposition and migration of the wind-sand flow around the opencut tunnel (Mulhearn and Bradly, 1997; McEwan and Willetts, 1993; Liu and Dong, 2004). The opencut tunnel is an artificial obstruction above the natural surface, and the dynamic process of its sand prevention system is controlled by complex interactions between the opencut tunnel body, topography, wind direction, airflow and sandaccumulation process (Dong et al., 2004). The results of this paper show that when the airflow goes over the obstacle, the airflow on the windward side is likely to separate due to the blocking effect of the obstacle, forming a partial uplift and accelerated airflow. After passing the obstacle, the air pressure difference on the leeward side further affects the air flow. The main factors controlling the air flow reattachment include the speed of the air flow, the angle between the air flow and the obstacle, and the form of the obstacle. When the airflow acts on the opencut tunnel, the windward side airflow is divided into a lower deceleration airflow, a middle uplift accelerated airflow and upper turbulence, which forms a weak wind zone. The boundary layer separates when the airflow crosses the opencut tunnel, and part of the airflow continues to be uplifted and accelerated, forming a strong wind area over the top of the opencut tunnel. The other part of the airflow on the leeward side decelerates or produces reverse airflow, forming a weak wind zone or quiet wind zone. Results of the wind-tunnel simulation experiment showed the direction of the vertical wind gradient under the condition of different gradients of transverse dunes where the airflow reattachment average distance was commonly 4.8H–10.8H. With an increase in the wind angle degree of the windward slope, the reattachment distance of the airflow increased rapidly and reached a maximum at 15°. This paper further studied the conditions of different wind angles on the opencut tunnel with the airflow reattachment average distances at 1.0H–6.0H. An increase in the wind direction angle of airflow reduced the reattachment distance: the reattachment distance was 6H at 15° and below, but when the angle was greater than 75°, reattachment distance was small, only 1H. The air reattachment distance is affected by many factors. According to Sweet and Kocurek (1990), the air reattachment distance largely depends on wind speed and direction, dune morphology, airflow incidence angle and atmospheric stability. The differences in the reported airflow reattachment distances are due to different experimental and observational conditions. Although the reattachment distance of fluid is affected by different factors under the action of wind and running water, the research results of this experiment are essentially consistent with the previous research results in air
3.3. Influence of wind direction on the reattachment distance of airflow of the opencut tunnel The roof of the opencut tunnel was arched, and the airflow above the vertical height of 0.7 h was more likely to form accelerated airflow and then form reattachment airflow on the leeward side. Fig. 8 shows the air reattachment characteristics at the height of 0.7 h at different wind speed gradients with different angles. It can be seen that the air reattachment phenomenon occurred at different positions on the leeward side of the airflow with different wind direction angles after deceleration and acceleration on the windward side. When the indicated wind speed was 6 m/s, 8 m/s, 10 m/s or 12 m/s with the opencut tunnel, the increased distance for air reattachment between the six groups with different wind direction angles did not significantly change with wind speed, and the air reattachment distance was relatively constant. However, a change in the angle of wind direction created a significant difference in the position of the attachment point. With the gradual decrease of the angle between airflow and the tunnel, the attachment point for air currents gradually increased from the leeward side. For example, when the angle was 90°, the reattachment distance of airflow was 1H, at 60° it was 2H, and at 15° it was 6H. Therefore, the attachment distance of airflow changes in relation to the wind direction angle could be approximately divided into three areas: when ε = 75°–90°, the attachment distance of airflow was 1H, when ε = 60°–30°, the attachment distance of airflow was 2H, and when ε = 15° or below, attachment distance of airflow was 6H. 3.4. Determining the effective protection and airflow area of the opencut tunnel Table 2 shows the relationship curve between the wind direction angle and effective protection range. We used the separation bubble height and area to describe the separation of airflow on the leeward side of the model. It is seen that the separation bubble height, area and shape ratio of the separation cell all increased with the wind angle. These values were almost consistent within the wind angle range of 30°–75°. When the angle increased to 90°, the effective protection range of the opencut tunnel was the largest, reaching up to 1.5H, 2.3H2, 1.5 for the height, area and shape ratio of the separation cell, respectively. However, the reattachment distance decreased with an increase in the wind direction angle. The maximum distance was at 15° with 6H, but at 75°, the distance was only 1H. Regardless of the reattachment distance or the height of the separation cell, the area of the separation cell and the shape ratio did not significantly change with different wind speeds. 73
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Fig. 8. Reattachment distance for airflow under different wind directions.
and water (Engel, 1981; McLean and Smith, 1986; Nelson and Smith, 1989; Frank and Kocurek, 1996a). For example, through the observation of flag tracer and sand particle trajectories, Frank and Kocurek (1996a) showed that the reattachment distance of airflow on the leeward side of dunes was generally from 1.6H to 5.4H of the dune height, with an average of 4H, which was consistent with the observation results of McLean and Smith (1986) and Nelson and Smith (1989) in water. Walker and Nickling (2002) found through field observations and wind tunnel simulation experiments that the airflow reattachment distances corresponding to the free wind speeds of 8 m/s, 13 m/s and
18 m/s were 6.5H, 8.0H and 7.5H, respectively. Parsons et al. (2004) studied airflow structure passing an ideal transverse dune by means of numerical simulation. The change of its reattachment distance was between 3.25H and 14.63H, and it increased with the increase of the upwind slope. This also confirmed that the reattachment distance of airflow under vertical wind speeds in this paper was consistent with the results of previous studies. We further obtained the results under different wind direction conditions, which was conducive to the promotion and application of linear sand damage engineering sections under the background of multi-wind climates. 74
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Table 2 Effective protection range for different wind direction angles. Wind angle
Indicate wind speed (m/s) 6
8
10
12
Reattachment distance (l/H) δ = 90° δ = 75° δ = 60° δ = 45° δ = 30° δ = 15°
1.0H 1.0H 2.0H 2.0H 2.0H 6.0H
1.0H 1.0H 2.0H 2.0H 2.0H 6.0H
1.0H 1.0H 2.0H 2.0H 2.0H 6.0H
6
8
10
12
6
1.5H 1.3H 1.3H 1.3H 1.3H 0.9H
1.2H 1.0H 1.0H 1.0H 1.0H 0.6H
1.0H 0.9H 0.9H 0.9H 0.9H 0.4H
10
12 2
Height of separate cell (h/H) 1.0H 1.0H 2.0H 2.0H 2.0H 6.0H
8
Area of separate cell (S/H ) 0.9H 0.8H 0.8H 0.8H 0.7H 0.3H
2.3H2 1.7H2 1.7H2 1.7H2 1.7H2 0.8H2
1.4H2 1.0H2 1.0H2 1.0H2 1.0H2 0.4H2
1.0H2 0.8H2 0.8H2 0.8H2 0.8H2 0.2H2
6
8
10
12
Shape ratio of separation cell 0.8H2 0.6H2 0.6H2 0.6H2 0.5H2 0.1H2
1.5 1.3 0.7 0.7 0.7 0.2
1.2 1.0 0.5 0.5 0.5 0.1
1.0 0.9 0.5 0.5 0.5 0.1
0.9 0.8 0.4 0.4 0.4 0.1
Fig. 9. Google Earth satellite images of opencut tunnel in 2007 (a), 2012 (b) and 2016 (c).
The wind profile gives the change in wind velocity with height and has been studied in detail (Bauer et al., 2004; Zhang et al., 2016). The relationship between the dimensionless height of the separation cell and wind direction angle is shown in Table 2. The angle of the height of the separation cell in the wind changed more obviously with the wind speed. In our experimental conditions, the dimensionless separation cell height was 0.3H–1.5H, which was consistent with the range of 0.61H–1.48H of the separation cell with different lateral dune angles in the wind simulation (Dong et al., 2011). It has been proved that the separation cell area can be dimensionless by H2. The experimental results showed that the dimensionless separation cell area increased with the wind angle, and the maximum value appeared when the model was vertical. Previous studies have shown that under field conditions, the wind direction can change at any time, and the flow dynamics over the dune surface are controlled by topographic forcing and steering effects and are sensitive to changes in the incident flow angle (Zhang et al., 2017). Based on this, our study also used the value of the separation cell flat shape ratio to analyze its relationship with the wind angle. The results showed the same change rule, which was that the wind direction angles of 15° and 75° gave the minimum and maximum effective protective range based on the change of inflection point. Results of the wind-tunnel simulation experiment show that, in the vicinity of −2H to −1H on the windward side of the opencut tunnel, the longitudinal airflow begins to uplift and accelerate. Because most of the sediment-carrying materials have been unloaded in the deceleration process before, the airflow has less sediment-carrying capacity and is in an unsaturated state, and there is no quicksand accumulation when air flows through the vents at the top of the opencut tunnel. Changes in the vertical airflow velocity profile show that at the horizontal position of 0.5 h on either side of the opencut tunnel, the airflow in the height range of 0.1 h–1 h in the opencut tunnel has begun to accelerate. The area above the height of 1 h is a zone of strong wind, and the wind speed is higher than the indicated wind speed. This law of motion is in line with the movement characteristics of transverse dunes and a tall measure airflow field (Frank and Kocurek, 1996b; Walker and Nickling, 2003; Zhang et al., 2010). Therefore, the airflow can only be transported by wind erosion without accumulation. The present study also used three Google Earth images to verify the problem of sand burial, finding that the opencut tunnel has operated for 10 years without being buried by sand.
5. Conclusions The present study conducted a wind-tunnel simulation experiment and found an inflection point at a distance 6H to windward, where there were rapid changes in wind speed, when air flowed at an angle to the opencut tunnel ranging between 15° and 90°, and not change with instructions wind speed. The area of strong wind on the upper part of the opencut tunnel moved to the windward side with an increase in the wind angle, while there was no effect of changing the indicated wind speed. Under the conditions of different wind angles, the trends of changes in leeward airflow were the same while the area of weak or quiet wind increased with an increase in the indicated wind speed. The airflow below a vertical height of 1 h on the windward side was decelerating, with the deceleration being greater closer to the opencut tunnel. The airflow above a vertical height of 1 h had an acceleration trend near the opencut tunnel. The airflow reattachment distance on the leeward side was 1H when ε = 75°–90°, 2H when ε = 60°–30° and 6H when ε = 15° or less. The characteristics of the wind field and the scope of effective protection show that the construction of an opencut tunnel for the prevention of sand accumulation can isolate a railway from an environment characterized by multiple wind directions and sandstorms and avoid the phenomenon of sand burying a tunnel. The problem of sand filling and burying a channel at the vents of the opencut tunnel is impossible to appear. The best scope of protection was achieved for a wind angle exceeding 75°, but the effective prevention and control of railway sand flow hazards can be realized for different angles, ensuring smooth railway operation. Acknowledgements This work was supported by the National Key Research and Development Program of China (No. 2016YFC0501009). We thank Leonie Seabrook, PhD, and Glenn Pennycook, MSc from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript. Declarations of interest None. 75
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Appendix A. Supplementary data
35, 344–353. McEwan, I.K., Willetts, B.B., 1993. Adaptation of the near-surface wind to the development of sand transport. J. Fluid Mech. 252, 99–101. McLean, S.R., Smith, J.D., 1986. A model for flow over two-dimensional bed forms. J. Hydraul. Eng. 112, 300–317. Mulhearn, P., Bradly, E.F., 1997. Secondary flow in the lee of porousshelt erbelts. Boundary Layer Meteorol. 12, 75–92. Nelson, J., Smith, J.D., 1989. Mechanics of flow over ripples and dunes. J. Geophys. Res. 94, 8146–8162. Parsons, D.R., Walker, I.J., Wiggs, G.F.S., 2004. Numerical modelling of flow structures over idealized transverse Aeolian dunes of varying geometry. Geomorphology 59, 149–164. Schatz, V., Herrmann, H.J., 2006. Flow separation in the lee side of transverse dunes: a numerical investigation. Geomorphology 81, 207–216. Sweet, M.L., Kocurek, G., 1990. An empirical model of Aeolian dune lee-face airflow. Sedimentology 30, 567–578. Walker, I.J., Nickling, W.G., 2002. Dynamics of secondary airflow and sediment transport over and in the lee of transverse dunes. Prog. Phys. Geogr. 26, 47–75. Walker, I.J., Nickling, W.G., 2003. Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel. Earth Surf. Proc. Land. 28, 1111–1124. Wang, X.L., Zhang, D.X., Jiang, Y.H., Jiang, F.Q., Li, Y., 2007. Drifting sand disasters and engineering control along south Xinjiang railway line. Chin. J. Geol. Hazard Control 18, 59–63. Wang, T., Qu, J.J., Ling, Y.Q., Xie, S.B., Xiao, J.H., 2017. Wind tunnel test on the effect of metal net fences on sand flux in a Gobi desert, China. J. Arid Land 9, 888–899. Wu, W.Y., 1982. Hydromechanics. Peking University Press, Beijing. Xiao, J.H., Yao, Z.Y., Qu, J.J., 2015. Influence of Golmud-Lhasa section of Qinghai-Tibet railway on blown sand transport. Chin. Geogr. Sci. 25, 39–50. Xie, S.B., Qu, J.J., Pang, Y.J., 2017. Dynamic wind differences in the formation of sand hazards at high and low altitude railway sections. J. Wind Eng. Ind. Aerodyn. 169, 39–46. Xu, X.L., Zhang, K.L., Kong, Y.P., Chen, J.D., Yu, B.F., 2006. Effectiveness of erosion control measures along the Qinghai-Tibet Highway, Tibet plateau. China. Transp. Res. Part D 11, 302–309. Zhang, K.C., Qu, J.J., Liao, K.T., Niu, Q.H., Han, Q.J., 2010. Damage by wind-blown sand and its control along Qinghai-Tibet railway in China. Aeolian Res. 1, 143–146. Zhang, K., Qu, J., Han, Q.J., Xie, S., Kai, K., Niu, Q., An, Z.S., 2012a. Wind tunnel simulation of windblown sand along China‘s Qinghai-Tibet railway. Land Degrad. Dev. 25, 244–250. Zhang, K.C., Qu, J.J., Han, Q.J., An, Z.S., 2012b. Wind energy environments and aeolian sand characteristics along the Qinghai-Tibet railway, China. Sed. Geol. 273, 91–96. Zhang, C.L., Zou, X.Y., Pan, X.H., Yang, S., Wang, H.T., 2007. Near-surface airflow field and aerodynamic characteristics of the railway-protection system in the Shapotou region and their significance. J. Arid Environ. 71, 169–187. Zhang, Z.C., Dong, Z.B., Zhao, A.G., Qian, G.Q., 2016. Field observations of the vertical distribution of sand transport characteristics over fine, medium and coarse sand surfaces. Earth Surf. Process Landforms. https://doi.org/10.1002/esp.4045. Zhang, Z.C., Dong, Z.B., Wu, G.X., 2017. Field observations of sand transport over the crest of a transverse dune in northwestern China Tengger Desert. Soil Tillage Res. 166, 67–75.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.aeolia.2019.04.007. References Bauer, B.O., Houser, C.A., Nickling, W.G., 2004. Analysis of velocity profile measurements from wind-tunnel experiments with saltation. Geomorphology 59, 81–98. Bauer, B.O., Davidson-Arnott, R.G.D., Walker, I.J., 2012. Wind direction and complex sediment transport response across a beachdune system. Earth Surf. Proc. Land. 37, 1661–1677. Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen, New York. Bergstrom, D.J., Boucher, K.M., Derksen, D., 1992. Wind flow over an elevated roadway. J. Wind Eng. Ind. Aerodyn. 41–44, 2697–2698. Cheng, J.J., Jiang, F.Q., Yang, Y.H., Xue, C.X., 2010. Study on the hazard characteristics of the drifting sand along the railway in Gobi aera and the efficacy of the control engineering measures. China Railway Sci. 31, 15–20. Cheng, J.J., Lei, J.Q., Li, S.Y., Wang, H.F., 2016. Disturbance of the inclined insertingtype sand fence to wind-sand flow fields and its sand control characteristics. Aeolian Res. 21, 139–150. Cheng, J.J., Xue, C.X., 2014. The sand- damage-prevention engineering system for the railway in the desert region of the Qinghai-Tibet plateau. J. Wind Eng. Ind. Aerodyn. 125, 30–37. Dong, Z.B., Wang, H.T., Zhang, X.H., Michael, A., 2004. Height profile of particle concentration in an aeolian saltating cloud: a wind tunnel investigation by PIV MSD. Geophys. Res. Lett. 30, 1–4. Dong, Z.B., Su, Z.Z., Qian, G.Q., Luo, W.Y., Zhang, Z.C., Wu, J.F., 2011. Aeolian Geomorphology of the Kumtagh Desert. Science press, Beijing. Engel, P., 1981. Length of flow separation over dunes. ASCE J. Hydraul. Div. 107, 1133–1143. Fenton, L.K., Michaels, T.I., Beyer, R.A., 2014. Inverse maximum gross bedform-normal transport: how to determine a dune-constructing wind regime using only imagery. Icarus 230, 5–14. Frank, A.J., Kocurek, G., 1996a. Airflow up the stoss slope of sand dunes: limitations of current understanding. Geomorphology 17, 47–54. Frank, A.J., Kocurek, G., 1996b. Toward a model for airflow on the lee side of Aeolian dunes. Sedimentology 43, 451–458. Jiang, Y.S., Gao, Y.H., Dong, Z.B., Liu, B.L., Zhan, L., 2018. Simulations of wind erosion along the Qinghai-Tibet Railway in northcentral Tibet. Aeolian Res. 32, 192–201. Lee, S.J., Park, K.C., Park, C.W., 2002. Wind tunnel observations about the shelter effect of porous fences on the sand particle movements. Atmos. Environ. 36, 1453–1463. Liu, X.P., Dong, Z.B., 2004. Experimental investigation of concentration profile of a blowing sand cloud. Geomorphology 60, 371–381. Luca, B., Marko, H., Lorenzo, R., 2018. Windblown sand along railway infrastructures: a review of challenges and mitigation measures. J. Wind Eng. Ind. Aerodyn. 177, 340–365. Lynch, K., Jackson, D.W.T., Cooper, J.A.G., 2010. Coastal foredune topography as a control on secondary airflow regimes under offshore winds. Earth Surf. Proc. Land.
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Wind tunnel simulation of an opencut tunnel airflow field along the LinheCeke Railway, China
T
Min Yana, Haibing Wanga, Hejun Zuoa, , Gangtie Lia ⁎
a
Inner Mongolia Key Laboratory of Aeolian Physics and Desertification Control Engineering, College of Desert Control Science and Engineering, Inner Mongolia Agricultural University, 29 Erdos East Street, Hohhot, Inner Mongolia Autonomous Region 010011, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Airflow field Opencut tunnel for sand prevention Wind tunnel simulation Linhe-Ceke railway
Opencut tunnels are linear arch constructions that are being used as new structures to prevent sand encroachment on railway lines. The large size and shape of the contained structure affect the wind field around the structure in a manner different to how traditional constructions prevent sand accumulation. The present study identified the characteristics of wind velocity and flow fields in opencut tunnels in a wind-tunnel simulation. Results show that airflow having different wind angles (15°–90°) had different effects on the sand-accumulationprevention characteristics of the opencut tunnel. The inflection point of a rapid change in wind speed was 6H (where H is the vertical height of the model) from the windward side of the opencut tunnel, and the position of an area of strong wind on the upper part of the opencut tunnel gradually moved to the windward side with an increase in the wind angle. This trend was not affected by a change in the indicated wind speed. The reattachment distance on the leeward side decreased with an increase in the wind angle, being 1H at 75°–90°, 2H at 30°–60° and 6H at 15° or less. The protection effect was best for a large angle of the main wind direction (exceeding 75°). Observed characteristics of the wind field show that the opencut tunnel is effective at all wind angles in preventing and controlling railway sand flow hazards and can thus ensure smooth railway operation.
1. Introduction Statistics presented in the fifth Chinese bulletin on desertification show that 1.83 million square km of land has undergone wind-eroded desertification, accounting for 69.93% of the total area of desertification in China. The characteristics of sparse vegetation, strong winds, long wind periods, more sand matter carried in the wind, less rainfall and uneven rainfall distribution in time and space has led to frequent dust storms, which seriously affect production activities such as traffic and construction in the affected area (Xie et al., 2017). Railways are not only a popular means of transportation, but also play an important role in economic and social development, acting as the arteries of national development. Many railways in arid regions of China are seriously affected by sandstorms. As of December 31, 2017, the total length of railways nationwide was 128,000 km, and about 18,131 km of railway lines had sandstorm hazards or potential sandstorm hazards, accounting for 16.5% of the total length of railway operations in China. The main railways affected are the Qinghai-Tibet railway, Lan-Xin railway, Bao-Lan railway, Lin-Ce railway and some other trunk lines (Xu et al., 2006; Cheng et al., 2010; Xiao et al., 2015; Luca et al., 2018). Because railways usually require the roadbed to be
⁎
higher or lower than the ground, in a sandstorm area the railway often becomes an artificial sand barrier which intercepts the blown sand material in the vicinity of the railway. This makes railways more vulnerable to sand damage than highway facilities and, therefore, railways in a sandstorm area are often seriously damaged by sandstorms (Zhang et al., 2012a). The damage caused to railways by sandstorms are mainly reflected in derailments and slower transport because of the accumulation of sand in the ballast bed. After the wind-blown sand material has been deposited on the ballast bed, the accumulated sand will slowly infiltrate the cracks in the ballast due to the vibration of the ballast bed, impeding drainage and increasing how often the ballast bed needs to be cleaned. Irregularities in the ballast bed also cause a triangular pit and low joint phenomenon in the track. To prevent the pollution of the ballast bed and maintain the quality of the line, it is necessary to screen the ballast frequently and remove the accumulated sand, which requires both labor and financial resources. With the development of western China and the implementation of the “One Belt, One Road” strategy, the railway mileage in sandstorm areas will increase. Effectively solving the safe operation of the railways in sandstorm areas has become an urgent problem. The Lin-Ce railway, which is an important line connecting
Corresponding author. E-mail address: [email protected] (H. Zuo).
https://doi.org/10.1016/j.aeolia.2019.04.007 Received 4 February 2019; Received in revised form 27 April 2019; Accepted 29 April 2019 Available online 09 May 2019 1875-9637/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic route of the Linhe-Ceke Railway and wind directions from 1981 to 2014 in the Guaizi Lake region.
Fig. 2. Schematic of the wind speed measurement system and experimental setup.
northwest and north China, passes through the Ulanbuhe, Yamalek and Badain Jaran deserts and the vast Gobi desert (rock desert and gravel desert) on the Alxa plateau, with a total length of 768.415 km (Fig. 1). When selecting the route for the Lin-Ce railway line, the line design unit chose to detour along the south side of the nature reserve to the Badain Jaran desert to protect the Ejin Populous euphractica forest national nature reserve and avoid the destruction of the P. euphractica desert riparian forest. Although the detour protected the P. euphractica forest
to the greatest extent, it meant that the railway passed through a 10 km mobile dune area on the northwest margin of the Badain Jaran desert. To prevent sandstorm hazards after the completion of this section, the design unit compared several types of construction, and finally completed an opencut tunnel railway of 8.08 km in the densest area of mobile dunes (Fig. 1). The near-surface airflow field is a dominant factor affecting aeolian sand deposition, and the sand-control function of the majority of 67
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Fig. 3. Wind tunnel equipment and opencut tunnel model. (a), (c) and (e) show the model of the opencut tunnel, (b) is the wind tunnel body, (d) is the model size and the observation position, and (f) is the field picture of opencut tunnel.
measures is realized in the turbulent near-surface boundary layer in railway sand control (Zhang et al., 2007, 2016). Therefore, to prevent sandstorm disasters, many measures have been taken along railways (Wang et al., 2007; Zhang et al., 2010, 2012b; Cheng and Xue, 2014), including mechanical sand barriers, roadbed windbreaks, sand barriers, bridge windbreak shelters and windbreak tunnels. To further strengthen the measures of wind control engineering, a series of sand control engineering systems has been set up on the windward side of railways in areas experiencing frequent sandstorms (Cheng et al., 2016; Wang et al., 2017). So far, many wind-proofing measures and sediment characteristic have been studied and flow field characteristics and sediment transport modes have been determined (Bergstrom et al., 1992; Lee et al., 2002; Jiang et al., 2018). The opencut tunnel, as a new type of linear sand control measure compared with traditional engineering sand control measures, has the basic characteristics of a sand control wall (Fig. 3). The impermeability of the sand control and the large construction size, however, make it difficult to apply traditional engineering sand control measures and wind and sand resistance mechanisms. So far, no papers or reports on the mechanisms, effective periods and effects of opencut tunnels for sand prevention have been published. As a new type of linear engineering sand control measure, the characteristics of the airflow variation on the windward side and leeward side become an important basis for determining whether sand is buried. Previous studies have shown that the motion characteristics of airflow are different at different locations on the surfaces of transverse dunes. Topographic rotation flows, for example, form on the windward slopes of dunes while separation flows, reattachment deviation flows, and reverse flows form on leeward slopes (Lynch et al., 2010; Bauer et al., 2012). The reattachment distance of leeward airflow is closely related to the morphological characteristics of dunes, such as the grade of the windward slope, dune height and dune spacing. However, there is no clear quantitative relationship between the airflow reattachment distance and a transverse dune (or other horizontal sand retaining measures) (Schatz and Herrmann, 2006; Fenton et al., 2014). Therefore, to better understand the characteristics of wind along the opencut railway tunnel, we simulated the flow field characteristics in a wind tunnel. This paper studied a sand protection opencut tunnel combined with the different models of wind direction angles, variations in the airflow velocity, different gradients of flow profile variations, windward air flow speed and leeward flow separation and weight attached to the process. The wind tunnel simulation results provided some baselines for
designing railways and highways. 2. Material and methods 2.1. Background of the research area The Lin-Ce (Linhe to Ceke) railway (Fig. 1) is located in the arid region of northwest China, east of Linhe station in Bayannur city, and west of Ejin banner in the Alashan league of the Inner Mongolia Autonomous Region. The Lin-Ce railway passes through the Ulanbuhe, Yamalek and Badain Jaran deserts and the Gobi Desert from the Alxa plateau, with a total length of 768.415 km (Fig. 1). About 456 km is within the wind-sand disaster area. Sandstorms happen annually, and obvious sand accumulation phenomena occur on both sides of the line. The sand accumulation in the section with the most serious sand pollution exceeds 1.5 m on the rail surface. The natural environment along the railway is harsh, with strong winds and violent sandstorms. The average wind speed over the years 1981–2014 was 4.68 m/s, and the average maximum wind speed over this period was 9.39 m/s. The wind direction at speeds greater than 5 m/s is shown in Fig. 1, and the main wind directions are W, WNW, E and ENE, which account for 64.11% of all wind direction frequencies. 2.2. Wind tests A wind tunnel experiment was carried out in the Shapotou soil wind tunnel laboratory in the Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences. The dc held blowing low speed wind tunnel was 37.08 m, the length was 21 m, the cross-section was 1.2 × 1.2 m, and lifting the whole section of the bottom can make a gradient of 1°. The wind energy ratio was 0.55, with 0.6% average turbulence intensity, a turbulence scale epsilon ε < 1‰, as the axial static pressure gradient delta p/ x = 0 . The free-stream wind velocity in the wind tunnel ranged from 1.2 to 30.8 m/s. The working principle is shown in Fig. 2. According to the similarity characteristics of airflow motion, generally, if the Reynolds number is controlled around Re = 106, the wind tunnel can reach the aerodynamic self-modeling state, and the Reynolds number is calculated as (Wu, 1982)
Re = 68
vL , µ
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2.3. Data analysis
Table 1 Characteristics of sand particle size distribution. Grain size/μm
< 100
100–200
200–300
300–400
(1) The wind tunnel speed is calculated as (Bagnold, 1941)
Percentage/%
4.31
76.20
19.14
0.35
v = 4.05
where ρ is the flow density, v is the wind speed in the tunnel, L is the model dimension and μ is the viscosity coefficient. When designing a model of a sand prevention tunnel, both the accuracy of the model size and the operability of the experiment should be considered to determine whether the model meets the requirements of wind-tunnel simulation and a similarity ratio of 1:30 should be adopted. We therefore selected the model of the opencut tunnel as shown in Fig. 3. The model was 100 cm long, 26.7 cm wide and 30 cm high and the top vent of the model was 10 × 10 cm. The model was made of sheet iron and welded by molding. It was a double-layer hollow structure. The design of the wind tunnel experiment did not involve the response characteristics of different particle sizes in the sand material to the sand prevention tunnel. The source of the sand material sample did not affect the progress of the experiment plan, nor did it affect the analysis, summary and extraction of the test accuracy and general rules of the experiment. Therefore, eolian sand samples were obtained from the edge of the Tengger desert where the laboratory was located. The particle size distribution characteristics are shown in Table 1. The wind tunnel experiment was conducted to determine the airflow velocity field characteristics under the condition of a clean wind with different angles between the model and the wind direction. Ten angle designs (15°, 30°, 35°, 40°, 45°, 55°, 60°, 65°, 75°, 90°) and four wind speed levels (6 m/s, 8 m/s, 10 m/s and 12 m/s) were selected and the net wind blew steadily at a certain pace and angle for 5 min. The wind speed was converted by a pitot tube differential pressure measurement system. The measured wind heights were 0.3 cm, 0.6 cm, 1.5 cm, 3 cm, 12 cm, 20 cm, 35 cm and 60 cm. The location of measurement points, in relation to the model height of H = 30 cm, were 0.5H, 1H, 2H, 3H, 4H, 6H, 8H, 10H before the model, and 0.5H, 1H, 2H, 4H, 6H, 8H after the model (Fig. 3d). We likewise used multiples of h for the vertical position.
P T × × h p 293
where v is wind speed, P is standard atmospheric pressure (mmHg), p is atmospheric pressure (mmHg), T is temperature (K), and △h is pitot tube differential pressure (mmH2O); (2) The reattachment distance (l) is used to describe the horizontal distance between the model and the airflow reattachment point when the horizontal velocity is zero. (3) The separation bubble height and its area are used to describe the separation of airflow on the leeward side of the model. The separation bubble height (h) is the upper edge of the closed streamline group while the separation bubble area (S) is the outer boundary area surrounding the separation bubble divided by H2 for dimensionless treatment (Dong et al., 2011). (4) The shape ratio (A) is the ratio between the height of the separation bubble and the reattachment distance, indicating the flatness of the separation bubble. The ratio is calculated as A = h/l. 3. Results 3.1. Influence of wind speed and direction on the airflow velocity field of an opencut tunnel The characteristic curves of the airflow velocity field under different wind velocity gradients when the model of the opencut tunnel and inlet angle was 90° can be seen in Fig. 4. The upper airflow on the windward side rose gently, and the inflection point of airflow changes was at −6H. There was an inflection point of the airflow changes in the lower layers at −2H, which formed a zone of weak wind. The range of the airflow uplift zone in the section before the model expanded, and there was uplift at roughly −4H. A strong wind area formed above the model, and the strong wind area moved to the left of the center of the model with reduced intensity. The number of eddies on the leeward side decreased, and the upper airflow had diffusion changes. With an increase in the indicated wind speed, the change trend of the airflow on
Fig. 4. Characteristics of the opencut tunnel airflow velocity with different wind speed gradients. The example shows airflow at 90° to the tunnel. 69
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Fig. 5. Characteristics of the opencut tunnel airflow velocity at different wind angles. The wind speed is 12 m/s in this example.
the windward side was the same. The change in the upper airflow on the leeward side was gentle, and the vortex formed by the lower airflow gradually became larger. The trend of connecting multiple weak wind zones into a single weak wind zone or a quiet wind zone was strengthened. When the wind speed was 12 m/s, the vortex on the leeward side of the 0–4H range became connected into one piece, and the height increased. In conclusion, the position of airflow field did not change with the change in the indicated wind speed. The velocity results of the wind flow field showed that the angles 15°, 30°, 45°, 60°, 75° and 90° well reflect different wind angles across the 0°–90° interval Fig. 5 shows the airflow velocity field characteristics of the opencut tunnel model and the air inlet at different angles under a 12 m/s indicated wind speed. It is seen that when the airflow acted on the opencut tunnel, the windward side airflow could be divided into a lower deceleration airflow, a middle uplift accelerated airflow and an upper turbulence, which formed a weak wind zone. When the airflow crossed the opencut tunnel, the boundary layer separated and part of the airflow continued to be uplifted and accelerated. In addition, a strong wind zone formed at the top of the opencut tunnel, while another part of the airflow decelerated on the leeward side and formed a reverse airflow, producing a weak or quiet wind zone. When the airflow angle was between 15° and 90° relative to the opencut tunnel, airflow around the opencut tunnel model operation was prevented, there was a differentiation regularity on the windward side, and the angle of the lower airflow disturbance was greater than of the upper airflow disturbance. From the wind speed change on the windward side of opencut tunnel and weak wind zone formed on the leeward, it can be seen that the place 6H away from the opencut tunnel was always the inflection point where the wind speed on the windward side changed sharply. From the position and range of the strong wind area on the upper part of the
opencut tunnel, as the wind direction angle increased, the position of the strong wind area gradually moved to the windward side, and the position where the wind speed lifted changed from −2H to −4H. On the leeward side of the opencut tunnel, the wind speed change and the formation of the weak area in the wind under different wind angle conditions showed similar leeward air change rules, all exert that weak with the increase of wind direction angle between the wind zone showed increasing trend (more than 60°). 3.2. Influence of wind direction on the variation of the wind velocity profile of an opencut tunnel Under the action of the indicated wind speed of 6 m/s, 8 m/s, 10 m/ s and 12 m/s, the wind velocity profile changes at different wind angles and positions on the windward side and leeward side are shown in Figs. 6 and 7. The airflow velocity on the windward side increased with an increase in the indicated wind speed and the gradient at the same horizontal position (Fig. 6). Under the same wind direction angle, the smaller the indicated wind speed, the stronger the effect of increasing wind speed. The variation trend on the leeward side was similar to that on the windward side. Under the same wind direction angle conditions, the greater the airflow velocity, the more obvious the change was. According to the airflow velocity profile with different wind direction angles, the trend of the airflow velocity profile with the four groups of indicated wind speed was similar. This indicated that the wind direction angle had a direct influence on the wind velocity profile, while the change in the indicated wind speed only reflected on the scale of the airflow change. Under the action of different indicated wind speeds on the windward side, the wind velocity profile presented a logarithmic function increasing trend with the increase of gradient, while the 70
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Fig. 6. Wind speed profile on the windward side (−10H, −6H, −2H, −1H, −0.5H) with different wind directions.
change on the leeward side was more complex and variable, and there was no obvious functional relationship. The wind velocity profiles at different horizontal positions on the windward side and different wind directions were accelerated at different degrees, among which airflow rose faster within the range of 0.01–0.10 h from the near stratum. Within the range of 0.10–0.70 h, the airflow rose slowly and then accelerated. The airflow above 0.70 h had
lower acceleration. The effect of this process gradually strengthened as the wind speed approached the opencut tunnel model while the airflow uplift strengthened with an increase in the wind angle. At horizontal positions of −10H and −6H, the airflow velocity increased logarithmically with the vertical gradient but the increasing degree was closer to the direction of the model. At horizontal positions of −2H and −0.5H, the airflow velocity fluctuated with an increase in the vertical 71
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Fig. 7. Wind speed profile on the leeward side (1H, 2H, 4H, 6H, 8H) with different wind directions.
gradient. The wind velocity profile with different horizontal positions and different wind directions on the leeward side varied greatly with the vertical gradient. A wind velocity below 0.70 h was significantly affected by the model of the opencut tunnel, and the wind velocity near the formation at 0.01 h generally increased under the influence of the leeward eddy. From 0.02 h to 0.70 h, it was squeezed by the eddy flow of the lower airflow, the weak wind zone appeared to varying degrees,
and the wind speed even reached zero. With the increase of the wind direction angle, this effect gradually increased. Above 0.70 h, the influence of the opencut tunnel model decreased, and the airflow rose slowly in constant fluctuation. The overall change in the wind velocity profile on the leeward side was significantly affected by the opencut tunnel model, and as the wind direction angle increased, the fluctuation of airflow in the vertical gradient gradually weakened. This indicated that with an increase in 72
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the included angle of wind direction, the impact force between the windward side airflow and the opencut tunnel model gradually increased. After passing the model, the wind velocity was affected by the air pressure difference. The larger the included angle of the wind direction, the faster the airflow velocity on the leeward side decayed, and the greater the distance from the model, the smaller the action process. As the wind direction angle increased, the airflow changed to a typical position on the leeward side, and the airflow velocity even appeared to be zero, but this was not directly related to the change of indicated wind speed. Locations at 2H and 4H showed typical air velocity that first decreased with increasing vertical gradient, but the change trend could be divided into 15°, 30°, 45°, 60°, 75° and 90° with four types of curve, although the changing process at 4H at 90° did not follow this relationship. This was because the 90° direction angle of flow and sediment control in the opencut tunnel model had the biggest impact force, which caused the air flow around the operation process to become more fluid. At the typical positions of 6H and 8H, the fluctuation of airflow velocity gradually decreased with the increase in vertical gradient.
3.5. Sand buried validation of the opencut tunnel We consider a real opencut tunnel that had been in operation for nearly 10 years. Google Earth satellite images (Fig. 9) taken in 2007, 2012 and 2016 show that there are no appreciable differences in the aeolian geomorphology, the surrounding area or in the lines of the opencut tunnel. The figure reveals a crescent-shaped dune chain distributed in the northeast and southwest and inclined toward the opencut tunnel. Although there are quicksand deposits in the transition zone on the northern side of the opencut tunnel, the accumulation amount is not large and the zone is a certain distance from the opencut tunnel. At the same time, it is seen that there is less quicksand on the southern side of the opencut tunnel and this quicksand is farther from the opencut tunnel. It is thus judged from the theoretical determination of the wind tunnel experiment and the field investigation that it is unlikely that the opencut tunnel will be buried. 4. Discussion Construction of the opencut tunnel makes the primary airflow flowing over the surface become a special secondary flow. As a result, the airflow near the opencut tunnel changes greatly in both flow velocity and flow direction. In particular, the momentum and energy transfer mode of the airflow becomes more complex, and this change affects the deposition and migration of the wind-sand flow around the opencut tunnel (Mulhearn and Bradly, 1997; McEwan and Willetts, 1993; Liu and Dong, 2004). The opencut tunnel is an artificial obstruction above the natural surface, and the dynamic process of its sand prevention system is controlled by complex interactions between the opencut tunnel body, topography, wind direction, airflow and sandaccumulation process (Dong et al., 2004). The results of this paper show that when the airflow goes over the obstacle, the airflow on the windward side is likely to separate due to the blocking effect of the obstacle, forming a partial uplift and accelerated airflow. After passing the obstacle, the air pressure difference on the leeward side further affects the air flow. The main factors controlling the air flow reattachment include the speed of the air flow, the angle between the air flow and the obstacle, and the form of the obstacle. When the airflow acts on the opencut tunnel, the windward side airflow is divided into a lower deceleration airflow, a middle uplift accelerated airflow and upper turbulence, which forms a weak wind zone. The boundary layer separates when the airflow crosses the opencut tunnel, and part of the airflow continues to be uplifted and accelerated, forming a strong wind area over the top of the opencut tunnel. The other part of the airflow on the leeward side decelerates or produces reverse airflow, forming a weak wind zone or quiet wind zone. Results of the wind-tunnel simulation experiment showed the direction of the vertical wind gradient under the condition of different gradients of transverse dunes where the airflow reattachment average distance was commonly 4.8H–10.8H. With an increase in the wind angle degree of the windward slope, the reattachment distance of the airflow increased rapidly and reached a maximum at 15°. This paper further studied the conditions of different wind angles on the opencut tunnel with the airflow reattachment average distances at 1.0H–6.0H. An increase in the wind direction angle of airflow reduced the reattachment distance: the reattachment distance was 6H at 15° and below, but when the angle was greater than 75°, reattachment distance was small, only 1H. The air reattachment distance is affected by many factors. According to Sweet and Kocurek (1990), the air reattachment distance largely depends on wind speed and direction, dune morphology, airflow incidence angle and atmospheric stability. The differences in the reported airflow reattachment distances are due to different experimental and observational conditions. Although the reattachment distance of fluid is affected by different factors under the action of wind and running water, the research results of this experiment are essentially consistent with the previous research results in air
3.3. Influence of wind direction on the reattachment distance of airflow of the opencut tunnel The roof of the opencut tunnel was arched, and the airflow above the vertical height of 0.7 h was more likely to form accelerated airflow and then form reattachment airflow on the leeward side. Fig. 8 shows the air reattachment characteristics at the height of 0.7 h at different wind speed gradients with different angles. It can be seen that the air reattachment phenomenon occurred at different positions on the leeward side of the airflow with different wind direction angles after deceleration and acceleration on the windward side. When the indicated wind speed was 6 m/s, 8 m/s, 10 m/s or 12 m/s with the opencut tunnel, the increased distance for air reattachment between the six groups with different wind direction angles did not significantly change with wind speed, and the air reattachment distance was relatively constant. However, a change in the angle of wind direction created a significant difference in the position of the attachment point. With the gradual decrease of the angle between airflow and the tunnel, the attachment point for air currents gradually increased from the leeward side. For example, when the angle was 90°, the reattachment distance of airflow was 1H, at 60° it was 2H, and at 15° it was 6H. Therefore, the attachment distance of airflow changes in relation to the wind direction angle could be approximately divided into three areas: when ε = 75°–90°, the attachment distance of airflow was 1H, when ε = 60°–30°, the attachment distance of airflow was 2H, and when ε = 15° or below, attachment distance of airflow was 6H. 3.4. Determining the effective protection and airflow area of the opencut tunnel Table 2 shows the relationship curve between the wind direction angle and effective protection range. We used the separation bubble height and area to describe the separation of airflow on the leeward side of the model. It is seen that the separation bubble height, area and shape ratio of the separation cell all increased with the wind angle. These values were almost consistent within the wind angle range of 30°–75°. When the angle increased to 90°, the effective protection range of the opencut tunnel was the largest, reaching up to 1.5H, 2.3H2, 1.5 for the height, area and shape ratio of the separation cell, respectively. However, the reattachment distance decreased with an increase in the wind direction angle. The maximum distance was at 15° with 6H, but at 75°, the distance was only 1H. Regardless of the reattachment distance or the height of the separation cell, the area of the separation cell and the shape ratio did not significantly change with different wind speeds. 73
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Fig. 8. Reattachment distance for airflow under different wind directions.
and water (Engel, 1981; McLean and Smith, 1986; Nelson and Smith, 1989; Frank and Kocurek, 1996a). For example, through the observation of flag tracer and sand particle trajectories, Frank and Kocurek (1996a) showed that the reattachment distance of airflow on the leeward side of dunes was generally from 1.6H to 5.4H of the dune height, with an average of 4H, which was consistent with the observation results of McLean and Smith (1986) and Nelson and Smith (1989) in water. Walker and Nickling (2002) found through field observations and wind tunnel simulation experiments that the airflow reattachment distances corresponding to the free wind speeds of 8 m/s, 13 m/s and
18 m/s were 6.5H, 8.0H and 7.5H, respectively. Parsons et al. (2004) studied airflow structure passing an ideal transverse dune by means of numerical simulation. The change of its reattachment distance was between 3.25H and 14.63H, and it increased with the increase of the upwind slope. This also confirmed that the reattachment distance of airflow under vertical wind speeds in this paper was consistent with the results of previous studies. We further obtained the results under different wind direction conditions, which was conducive to the promotion and application of linear sand damage engineering sections under the background of multi-wind climates. 74
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Table 2 Effective protection range for different wind direction angles. Wind angle
Indicate wind speed (m/s) 6
8
10
12
Reattachment distance (l/H) δ = 90° δ = 75° δ = 60° δ = 45° δ = 30° δ = 15°
1.0H 1.0H 2.0H 2.0H 2.0H 6.0H
1.0H 1.0H 2.0H 2.0H 2.0H 6.0H
1.0H 1.0H 2.0H 2.0H 2.0H 6.0H
6
8
10
12
6
1.5H 1.3H 1.3H 1.3H 1.3H 0.9H
1.2H 1.0H 1.0H 1.0H 1.0H 0.6H
1.0H 0.9H 0.9H 0.9H 0.9H 0.4H
10
12 2
Height of separate cell (h/H) 1.0H 1.0H 2.0H 2.0H 2.0H 6.0H
8
Area of separate cell (S/H ) 0.9H 0.8H 0.8H 0.8H 0.7H 0.3H
2.3H2 1.7H2 1.7H2 1.7H2 1.7H2 0.8H2
1.4H2 1.0H2 1.0H2 1.0H2 1.0H2 0.4H2
1.0H2 0.8H2 0.8H2 0.8H2 0.8H2 0.2H2
6
8
10
12
Shape ratio of separation cell 0.8H2 0.6H2 0.6H2 0.6H2 0.5H2 0.1H2
1.5 1.3 0.7 0.7 0.7 0.2
1.2 1.0 0.5 0.5 0.5 0.1
1.0 0.9 0.5 0.5 0.5 0.1
0.9 0.8 0.4 0.4 0.4 0.1
Fig. 9. Google Earth satellite images of opencut tunnel in 2007 (a), 2012 (b) and 2016 (c).
The wind profile gives the change in wind velocity with height and has been studied in detail (Bauer et al., 2004; Zhang et al., 2016). The relationship between the dimensionless height of the separation cell and wind direction angle is shown in Table 2. The angle of the height of the separation cell in the wind changed more obviously with the wind speed. In our experimental conditions, the dimensionless separation cell height was 0.3H–1.5H, which was consistent with the range of 0.61H–1.48H of the separation cell with different lateral dune angles in the wind simulation (Dong et al., 2011). It has been proved that the separation cell area can be dimensionless by H2. The experimental results showed that the dimensionless separation cell area increased with the wind angle, and the maximum value appeared when the model was vertical. Previous studies have shown that under field conditions, the wind direction can change at any time, and the flow dynamics over the dune surface are controlled by topographic forcing and steering effects and are sensitive to changes in the incident flow angle (Zhang et al., 2017). Based on this, our study also used the value of the separation cell flat shape ratio to analyze its relationship with the wind angle. The results showed the same change rule, which was that the wind direction angles of 15° and 75° gave the minimum and maximum effective protective range based on the change of inflection point. Results of the wind-tunnel simulation experiment show that, in the vicinity of −2H to −1H on the windward side of the opencut tunnel, the longitudinal airflow begins to uplift and accelerate. Because most of the sediment-carrying materials have been unloaded in the deceleration process before, the airflow has less sediment-carrying capacity and is in an unsaturated state, and there is no quicksand accumulation when air flows through the vents at the top of the opencut tunnel. Changes in the vertical airflow velocity profile show that at the horizontal position of 0.5 h on either side of the opencut tunnel, the airflow in the height range of 0.1 h–1 h in the opencut tunnel has begun to accelerate. The area above the height of 1 h is a zone of strong wind, and the wind speed is higher than the indicated wind speed. This law of motion is in line with the movement characteristics of transverse dunes and a tall measure airflow field (Frank and Kocurek, 1996b; Walker and Nickling, 2003; Zhang et al., 2010). Therefore, the airflow can only be transported by wind erosion without accumulation. The present study also used three Google Earth images to verify the problem of sand burial, finding that the opencut tunnel has operated for 10 years without being buried by sand.
5. Conclusions The present study conducted a wind-tunnel simulation experiment and found an inflection point at a distance 6H to windward, where there were rapid changes in wind speed, when air flowed at an angle to the opencut tunnel ranging between 15° and 90°, and not change with instructions wind speed. The area of strong wind on the upper part of the opencut tunnel moved to the windward side with an increase in the wind angle, while there was no effect of changing the indicated wind speed. Under the conditions of different wind angles, the trends of changes in leeward airflow were the same while the area of weak or quiet wind increased with an increase in the indicated wind speed. The airflow below a vertical height of 1 h on the windward side was decelerating, with the deceleration being greater closer to the opencut tunnel. The airflow above a vertical height of 1 h had an acceleration trend near the opencut tunnel. The airflow reattachment distance on the leeward side was 1H when ε = 75°–90°, 2H when ε = 60°–30° and 6H when ε = 15° or less. The characteristics of the wind field and the scope of effective protection show that the construction of an opencut tunnel for the prevention of sand accumulation can isolate a railway from an environment characterized by multiple wind directions and sandstorms and avoid the phenomenon of sand burying a tunnel. The problem of sand filling and burying a channel at the vents of the opencut tunnel is impossible to appear. The best scope of protection was achieved for a wind angle exceeding 75°, but the effective prevention and control of railway sand flow hazards can be realized for different angles, ensuring smooth railway operation. Acknowledgements This work was supported by the National Key Research and Development Program of China (No. 2016YFC0501009). We thank Leonie Seabrook, PhD, and Glenn Pennycook, MSc from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript. Declarations of interest None. 75
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Appendix A. Supplementary data
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