Wind tunnel tests on the characteristics of wind fields over a simplified gorge

Abstract

Characteristics of wind fields over the gorge or valley terrains are becoming more and more important to the structural wind engineering. However, the studies on this topic are very limited. To obtain the fundamental characteristics information about the wind fields over a typical gorge terrain, a V-shaped simplified gorge, which was abstracted from some real deep-cutting gorges where long-span bridges usually straddle, was introduced in the present wind tunnel studies. Then, the wind characteristics including the mean wind speed, turbulence intensity, integral length scale, and the wind power spectrum over the simplified gorge were studied in a simulated atmospheric boundary layer. Furthermore, the effects of the oncoming wind field type and oncoming wind direction on these wind characteristics were also investigated. The results show that compared with the oncoming wind, the wind speeds at the gorge center become larger, but the turbulence intensities and the longitudinal integral length scales become smaller. Generally, the wind fields over the gorge terrain can be approximately divided into two layers, that is, the gorge inner layer and the gorge outer layer. The different oncoming wind field types have remarkable effects on the mean wind speed ratios near the ground. When the angle between the oncoming wind and the axis of the gorge is in a certain small range, such as smaller than 10°, the wind fields are very close to those associated with the wind direction of 0°. However, when the angle is in a larger range, such as larger than 20°, the wind fields in the gorge will significantly change. The research conclusions can provide some references for civil engineering practices regarding the characteristics of wind fields over the real gorge terrains.

Keywords

oncoming wind direction oncoming wind field type simplified gorge wind fields wind tunnel test

Introduction

In the literature, the characteristics of wind fields over complex terrains have been extensively carried out by methods of theoretical studies (Jackson and Hunt, 1975), field measurements (Huang et al., 2015, 2016; Hui et al., 2009; Zhu et al., 2011), numerical simulations (Cao et al., 2012; Iizuka and Kondo, 2006; Tamura et al., 2007), and wind tunnel tests (Cao and Tamura, 2006, 2007; Shiau and Hsu, 2003). Not only can these studies help understand and analyze the mechanism of wind fields over these complex terrains, but they can also guide the design principles for engineering projects located in these complex terrains. However, it should be pointed out that most of these studies are aimed at the hilly or mountainous terrains, and they are rarely aimed at the gorge or valley terrains (Hu et al., 2015). Actually, the wind fields over gorge or valley terrains are very important to the civil structures, such as studying the wind parameters of a long-span bridge straddling a gorge (Chen et al., 2008; Li et al., 2011). With the increase in civil structures building in these gorge terrains (Hu et al., 2015), studying the characteristics of wind fields over gorge or valley terrains has become very important to the structural wind engineering.

For civil structures such as the long-span bridges straddling gorge terrains, the wind characteristics will be greatly influenced by the gorge terrain, and the wind characteristics around the bridge main beam and towers are very complicated (Li et al., 2010). To study the wind characteristics at these bridge sites, Chen et al. (2008) took a super long suspension bridge with a main span of 1176 m located in a deep-cutting gorge that is V shaped as the research background. They investigated the effects of the gorge terrain on the wind speed distributions along the bridge main beam in a wind tunnel with the scale ratio of 1:500 and the terrain diameter of 4.0 m. The wind tunnel test results show that the mean wind speed profiles in the gorge, obviously changing along the bridge main beam, are not uniform. The attack angles around the bridge main beam are higher than the range of −3° to +3° prescribed by the Chinese specification (CCCC Highway Consultants Company Ltd., 2004), and when considering the speed-up effect of wind in the gorge, the effects of the measurement positions and the shape of the gorge terrain should be taken into account. Li et al. (2011) investigated the wind field distributions at the bridge site in another deep-cutting gorge that is V shaped, and the terrain model around the bridge site was modeled with the range of 8.0 km × 8.0 km using computational fluid dynamics (CFD) commercial software. According to the numerical simulation with different cases, the variations of wind velocities along the vertical direction and along the bridge main beam were obtained, and the wind speeds and wind attack angles along the bridge main beam changing with the different oncoming wind directions were also analyzed. The results showed that the wind fields at the bridge site in the deep-cutting gorge are different from those near the sea or over a wide river, and they are very complex with obviously spatial ambient flows.

Although the deep-cutting gorges are generally shaped as a V, they cannot be identical to each other in shapes, which will result in different wind characteristics at the bridge sites. In other words, case-by-case studies on the wind characteristics at these bridge sites should be conducted. To avoid this, an effective method is to obtain the fundamental information about the wind characteristics over a typical or simplified gorge terrain, which will then provide the long-span bridge located in a real deep-cutting terrain with some reference information. The fundamental information from the typical or simplified gorge terrain can help explain the complex wind fields over the real deep-cutting gorge terrains. Therefore, it is important to study the characteristics of wind fields over a typical or simplified gorge terrain. Li et al. (2010) investigated the wind fields over four simplified valley models in a simulated atmospheric boundary layer (ABL) wind tunnel to gain understanding of the characteristics of wind fields over the mountainous valley terrain and further applications to determine the design wind speed for the long-span bridge straddling the mountainous valley terrain. The results showed that it is not suitable to describe the mean wind speed profiles by the power law using the same roughness exponent along the span-wise direction in the valley terrain. Meanwhile, the mean wind speed ratios in the middle of the valley decrease faster with the increase in height than those at the positions near the hillside. Furthermore, the longitudinal turbulence intensity near the ground level in the valley is reduced due to the speed-up effect. Although the fundamental results from the above simplified valley models can be useful for understanding the characteristics of wind fields over a real valley or gorge terrain, the shapes of these simplified valley models were all conical, while the real valley or gorge terrains usually extend over a long distance along the axis. Furthermore, the effects of different oncoming wind field types or different oncoming wind directions on the wind fields over these simplified terrains were not investigated. Generally speaking, the oncoming wind field type in the valley terrains probably changes with the atmospheric environment, and the oncoming wind direction is not always along the axis of the valley or gorge terrain.

To address the above problems in the previous studies, a V-shaped simplified gorge was introduced in this study to obtain some wind fundamental characteristics information that will give the possibility of evaluating the general effects introduced by the gorge terrain geometry and providing some basis for the wind-resistant design of civil structures such as the long-span bridges located in a gorge terrain. Based on the wind tunnel tests, the wind characteristics such as the mean wind speed, turbulence intensity, integral length scale, and the wind power spectrum over the V-shaped simplified gorge were studied in a simulated ABL wind field. Furthermore, the effects of the oncoming wind field type and oncoming wind direction on the characteristics of wind fields over the V-shaped simplified gorge were also investigated. Finally, some main conclusions were presented.

Setup of wind tunnel test

As discussed earlier, a simplified terrain is more appropriate to obtain the wind fundamental characteristics information. Therefore, a V-shaped simplified gorge which was abstracted from some real deep-cutting gorges (Chen et al., 2008; Huang et al., 2016; Li et al., 2010; Zhu et al., 2011) was introduced here. In this article, the wind tunnel test method was employed to study the characteristics of wind fields over the simplified gorge, and the wind tunnel tests were conducted in the XNJD-1 wind tunnel at Southwest Jiaotong University, China, of which the test section is 3.6 m wide and 3.0 m high, and the wind speed ranges from 0.0 to 22.0 m/s. To make full use of the test section size and to conveniently investigate the effects of oncoming wind directions on the wind fields over a simplified gorge terrain, the simplified gorge terrain was made in a circular shape with a diameter of 3.066 m and a height of 0.25 m. Furthermore, to make the oncoming wind flow over the edge of the terrain model smoothly and reasonably, a transition section from the wind tunnel floor to the edge top of the terrain model must be established (Hu et al., 2015). Therefore, a curved transition section with an equivalent slope of 30° suggested by Hu et al. (2015) was adopted around the circular terrain. Then, a V-shaped groove with an angle of 120° was cut along the center of the circular terrain to simulate a simplified gorge terrain. The finally V-shaped simplified gorge terrain model is shown in Figure 1, and it results in about a 5.4% blockage ratio of the wind tunnel test section.

Figure 1.

The simplified gorge terrain model.

Some researchers (Cao and Tamura, 2006; Derickson and Peterka, 2004; Miller and Davenport, 1998) have emphasized that the surface roughness has remarkable effects on the characteristics of wind fields over the terrain. Considering that there exist a lot of vegetation and trees on the surface of the real gorge terrain which can create a certain surface roughness, in this study, approximately 3500 small roughness elements (10-mm cubes) were placed in a staggered pattern on the terrain surface to simulate the real surface roughness appropriately, as shown in Figures 1 and 2. This arrangement gives a roughness density λ = 8.0%, where λ is defined as the total roughness front area per unit ground area (Cao and Tamura, 2006).

Figure 2.

Arrangement of roughness elements on the terrain surface (mm).

The Cobra probe is a multi-hole pressure probe able to resolve the wind speed in real time, and it is very suited to the measurements of turbulent flow and mean flow (Turbulent Flow Instrumentation Pty Ltd., 2012). Therefore, Cobra probes were used to measure the wind speeds in the simplified gorge terrain. In the tests, the sampling frequency was 1250 Hz, and the sampling time was about 60 s for each measurement position. Considering that a real gorge terrain usually exists in a very complex terrain, the surface roughness of the terrain and the oncoming wind field should belong to the type IV (refer to very rough terrain) according to the related Chinese specification (CCCC Highway Consultants Company Ltd., 2004). Accordingly, the oncoming neutrally ABL was simulated by a passive method in a scale ratio of 1:600, namely, using a combination of spires and floor roughness arrays in the wind tunnel. The simulated wind speed profile which is well fitted with the power law exponent (denoted by a) of 0.296 agreed well with the target one represented by the power law exponent of 0.30 (CCCC Highway Consultants Company Ltd., 2004), as shown in Figure 3, where the simulated values were fitted by the software of Origin (OriginLab Corporation, 2010). The simulated longitudinal turbulence intensity profile also agreed with the target values, and the longitudinal wind power spectra at the height of 0.132 and 0.262 m above the ground generally agreed well with the corresponding values obtained by Kaimal model (Kaimal et al., 1972) and Karman model (Von Kármán, 1948), as shown in Figures 4 and 5, respectively. Therefore, the simulated oncoming wind fields were generally in accordance with the specification.

Figure 3.

Mean wind speed profile and its fitted results by power law model.

Figure 4.

Turbulence intensity profile.

Figure 5.

Simulated wind power spectra compared with the target values: (a) 0.132 m from the ground and (b) 0.262 m from the ground.

Characteristics of wind fields over a simplified gorge

For a long and straight gorge terrain, the wind usually flows along the gorge (Bullard et al., 2000). From the viewpoint of wind-resistant design for civil structures such as the long-span bridges, the wind characteristics, including the mean wind speed, turbulence intensity, integral length scale, and wind power spectrum, in the gorge were studied in detail first, with the simulated oncoming wind flowing along the gorge (as shown in Figure 1).

Mean wind speed and turbulence intensity

To investigate the wind fields changing along the gorge terrain, the profiles of the mean wind speed and the turbulence intensity at five positions were studied as shown in Figures 6 and 7, that is, 0.64 m upstream the center, 0.32 m upstream the center, 0.32 m downstream the center, and 0.64 m downstream the center, from the first position to the fifth position, respectively. It can be seen that the mean wind speed profiles at different positions are very close. Although there are some differences between the profiles of the turbulence intensity, the differences are relatively small, and the profiles change almost evenly with the positions. Furthermore, the influences of the two ends of the gorge terrain on the mean wind speed and turbulence intensity profiles at the center position are smaller than those at the other four positions. Therefore, the wind characteristics at the center position should be the most representative parameters for the present gorge terrain, and the wind characteristics at the center position are carefully and mainly analyzed in the further studies.

Figure 6.

Mean wind speed profiles along the axis of the gorge.

Figure 7.

Turbulence intensity profiles along the axis of the gorge.

Figures 8 and 9 show the profiles of the mean wind speed and the turbulence intensity at the gorge center together with those of the simulated oncoming wind. Compared with the oncoming wind, the wind speeds at the gorge center become larger, but the turbulence intensities at the gorge center become smaller. To further investigate the changes of wind fields at the gorge center in comparison to the oncoming wind, the ratios of the mean wind speeds and the turbulence intensities at the gorge center to those of the oncoming wind are also shown in Figures 8 and 9, respectively. It can be seen that with the measurement positions moving away from the ground, the mean wind speed ratio first increases and then decreases but gradually becomes stable at a value of about 1.05. In comparison, the turbulence intensity ratio first decreases and then increases but gradually also becomes stable at a value of about 0.9. It is noted that the wind fields over a flat plate can be generally divided into two layers, that is, the inner shear layer produced by the viscous effects and the outer potential flow layer where the viscous effects can be ignored (Currie, 2003). Similarly, the wind fields over the gorge terrain can also be approximately divided into two layers, that is, the gorge inner layer and the gorge outer layer. In the gorge inner layer, the characteristics of wind fields are very complex due to the influences mainly caused by the two side slopes of the gorge and also caused by the oncoming wind. For this study, the gorge inner layer is about just below the range of 0.205–0.231 m or about 0.85 times the height of the terrain, where the mean wind speeds accelerate, but the acceleration is not stable. Also, the variations of the turbulence intensity fluctuate remarkably. For the gorge outer layer which is about above the range of 0.205–0.231 m, the variations of the mean wind speed and turbulence intensity are stable and simple. The reason is that the influences of the two side slopes of the gorge on the wind fields in the gorge outer layer are small, and the wind fields in the gorge outer layer mainly depend on the oncoming wind field. Therefore, the mean wind speeds and turbulence intensities in the gorge outer layer are very consistent with those of the oncoming wind.

Figure 8.

Mean wind speed profiles at the oncoming wind and the gorge center.

Figure 9.

Turbulence intensity profiles at the oncoming wind and the gorge center.

From the above analysis, if a long-span bridge is located in the gorge inner layer, the wind parameters around the bridge main beam will be mostly influenced by the two side slopes of the gorge, and they are probably very complex and variable. Inversely, if a long-span bridge is located in the gorge outer layer, the influences of the gorge terrain on the wind parameters around the bridge main beam become small, and the wind parameters will be relatively simple and explicit. In this case, the wind parameters could be investigated by the CFD numerical simulation method (Hu et al., 2016; Li et al., 2011).

Integral length scale and wind power spectrum

The integral length scale measures the average eddy size of the turbulent flow, and it is very important to the wind loads on the structures. When the oncoming wind flows into the gorge, the turbulent eddy and the integral length scale will change. According to Taylor’s (1938) hypothesis, the longitudinal integral length scale can be computed as

L_{u}^{x} = \frac{U}{σ_{u}^{2}} \int_{0}^{\infty} R_{u} (τ) d τ

(1)

where R_u(τ) is the autocorrelation function of the longitudinal fluctuating wind u and σ_u is the root mean square of u. It is generally believed that when the value of R_u(τ) is very small, then the errors will increase. To improve the precision, the upper limit of integral in equation (1) is suggested to be $0.05 σ_{u}^{2}$ (Flay and Stevenson, 1948).

It should be noted that equation (1), which is valid for the homogeneous and stationary turbulence (Mizuno and Panofsky, 1975), is not held in the complex terrain. Therefore, the integral length scale in the gorge terrain computed by equation (1) may not be accurate. However, from the viewpoint of engineering applications, the integral length scale results computed by equation (1) can still represent the average sizes of the turbulent eddies of the flow in the gorge terrain. As such, at least they can be taken as the basis for comparing the different average sizes of the turbulent eddies in the gorge terrain. Therefore, the longitudinal integral length scales at the gorge center and the oncoming wind are calculated based on equation (1) and shown in Figure 10. It can be seen that the longitudinal integral length scales generally become large with the measurement positions moving away from the ground. More importantly, the integral length scales at the gorge center are remarkably smaller than those of the oncoming wind. The main reason is that when the oncoming wind flows into the gorge, the turbulent eddies break into the smaller eddies due to the disturbances mainly caused by the two side slopes of the gorge. Therefore, the integral length scales at the gorge are smaller compared with those of the oncoming wind.

Figure 10.

Integral length scales at the oncoming wind and the gorge center.

Wind power spectrum is also a very important parameter to the civil structures. To investigate the changes in the wind power spectrum at the gorge center in comparison to the oncoming wind, the power spectra values with two typical heights of 0.132 and 0.262 m from the ground are shown in Figure 11. It can be seen that the power spectra values in the low-frequency part associated with the gorge center are smaller than those associated with the oncoming wind. The reason is that the large eddies which can cause the low-frequency fluctuations break into small eddies when oncoming wind flows into the gorge as discussed earlier. As a result, on the one hand, the low-frequency energy of the turbulent flow decreases at the gorge center, as shown in Figure 11. On the other hand, the high-frequency energy of the turbulent flow is caused by the small eddies which are mainly determined by the viscosity effect (Simiu and Scanlan, 1996). Because the measurement positions are all in the middle plane of the gorge center, they have certain distances away from the two side slopes of the gorge. Therefore, the influences of the viscosity effect on the small eddies at the measurement positions are very small. Then, the power spectra values in the high-frequency part associated with the gorge center show little change compared with those associated with the oncoming wind. Note that the heights of 0.132 and 0.262 m from the ground are in the gorge inner layer and gorge outer layer, respectively. Obviously, the influences of the two side slopes of the gorge on the power spectra values associated with the measurement positions in the range of the gorge inner layer are larger than those in the range of the gorge outer layer, as shown in Figure 11.

Figure 11.

Comparison of wind power spectra values at the gorge center and the oncoming wind with several typical measurement positions moving away from the ground: (a) 0.132 m from the ground and (b) 0.262 m from the ground.

Effects of some factors on the characteristics of wind fields over a simplified gorge

To further investigate the characteristics of wind fields over a simplified gorge, the effects of some factors such as the oncoming wind field type and the oncoming wind direction on the mean wind speed, turbulence intensity, and wind power spectrum over a simplified gorge were analyzed. Here, the different oncoming wind field types mainly refer to the oncoming wind with different mean wind speeds, turbulence intensities, wind power spectra values, and so on. While the different oncoming wind directions refer to the mean wind speeds, turbulence intensities, wind power spectra values, and other parameters of the oncoming wind do not change, but the angles between the oncoming wind and the axis of the gorge change.

Effects of the oncoming wind field type

When investigating the characteristics of wind fields over the complex terrains, the oncoming wind field types are always difficult to determine. Usually, they are roughly determined based on the experience (Derickson and Peterka, 2004; Li et al., 2011; Maurizi et al., 1998). In this article, to investigate the effects of different oncoming wind field types on the characteristics of wind fields over the simplified gorge, the tests with the uniform oncoming wind were also carried out in this study. The wind speed of the uniform flow was 10.6 m/s, and the turbulence intensity was about 1%. To enhance the comparability of the mean wind speeds in the gorge center between the different oncoming wind field types, that is, the above simulated type IV ABL (IV ABL) and the uniform flow field (UFL), the ratios of the mean wind speeds at the gorge center to those of the oncoming wind with these two oncoming wind field types are calculated and shown in Figure 12. It can be seen that in the range of the gorge inner layer, the mean wind speed ratios associated with the IV ABL are much larger than those associated with the UFL. However, the mean wind speed ratios are almost the same for these two oncoming wind field types in the range of the gorge outer layer.

Figure 12.

Mean wind speed ratios at the gorge center with three different oncoming wind field types.

Generally, all the wind characteristics of different oncoming wind field types can be divided into two parts, that is, the mean wind characteristics and the turbulence wind characteristics. To investigate the effects of the mean wind characteristic of the UFL such as the mean wind speed, the tests of the UFL with the mean wind speed of 6.8 m/s were also conducted, and the corresponding mean wind speed ratios are also shown in Figure 12. It can be seen that the mean wind speed ratios with different mean wind speeds are very close to each other, which indicates that the effects of the mean wind speed of the oncoming wind on the mean wind speed ratios at the gorge center are very small, and then the turbulence wind characteristics of the oncoming wind probably have remarkable effects on them.

The turbulence intensity is the simplest parameter to describe the turbulence wind characteristics, and it is also the most important parameter to distinguish the oncoming wind field types of IV ABL and UFL. Figure 13 shows the turbulence intensities at the gorge center with these two types of oncoming wind field, where the turbulence intensities at the gorge center associated with the IV ABL are much larger than those associated with the UFL. However, the turbulence intensities at the gorge center associated with the UFL with different mean wind speeds are almost the same. From Figures 12 and 13, the variation of mean wind speed ratios associated with the IV ABL and UFL can be explained below. In the range of the gorge inner layer, especially near the ground, the mean wind speeds are very small due to the presence of the small roughness elements on the two side slopes of gorge. However, when the oncoming wind of IV ABL with a high turbulence intensity flows into the gorge, the wind near the ground will get more kinetic energy supplies due to the effects of the turbulent mixing. As a result, the speeds of the wind near the ground increase. While the oncoming wind of the UFL with low turbulence intensity flows into the gorge, the effects of the turbulent mixing become very small and then the speeds of the wind near the ground cannot increase. Therefore, the mean wind speed ratios caused by the IV ABL with a high turbulence intensity are much larger than those caused by the UFL with a low turbulence intensity. However, in the range of the gorge outer layer, as the measurement positions are far away from the ground and the two side slopes of the gorge, the effects of the turbulent mixing become small for different oncoming wind field types. Therefore, the mean wind speed ratios for the IV ABL are almost the same as those for the UFL in such circumstances. Actually, the mean wind speeds in the gorge outer layer mainly depend on the mean wind speeds of the oncoming wind as discussed earlier.

Figure 13.

Turbulence intensity profiles at the gorge center with three different oncoming wind field types.

To investigate the effects of different oncoming wind field types, that is, the oncoming wind of IV ABL and the UFL with the mean wind speed of 10.6 m/s, on the wind power spectra at the gorge center, the wind power spectra values with several typical measurement positions away from the ground are shown in Figure 14. It can be seen that when the measurement positions are very near to the ground, the power spectra values in the low-frequency part associated with the UFL are smaller than those associated with the IV ABL. However, the power spectra values in the high-frequency part associated with the UFL are larger than those associated with the IV ABL, such as the height of 0.075 m from the ground, as shown in Figure 14(a). Furthermore, with the heights moving away from the ground, the whole power spectra values associated with the UFL become small, and they are gradually smaller than those associated with the IV ABL, as shown in Figure 14(b) to (d).

Figure 14.

Comparison of wind power spectra values at the gorge center with different oncoming wind field types: (a) 0.075 m from the ground, (b) 0.132 m from the ground, (c) 0.262 m from the ground, and (d) 0.323 m from the ground.

The differences of the power spectra values associated with the IV ABL and UFL can be explained below. Since the average size of the turbulent eddies of the UFL is very small, it naturally results in relatively smaller power spectra values in the low-frequency part and larger power spectra values in the high-frequency part at the gorge center. With the heights away from the ground increasing, the effects of the disturbances caused by the gorge terrain on the turbulent flow decrease and so does the production of the eddies. As a result, the small eddies from the oncoming wind of the UFL begin to decay and dissipate rapidly due to the viscosity effect of the flow. Therefore, the whole power spectra values associated with the UFL become small, especially for the heights of 0.262 and 0.323 m from the ground, and they are smaller than those associated with the IV ABL.

From the above analysis, it can be concluded that the different oncoming wind field types have great effects on the characteristics of wind fields over the gorge terrain. Therefore, the oncoming wind field type should be determined and simulated as accurately as possible when investigating the wind characteristics at the sites of some civil structures located in the gorge terrains, or there will be large errors in the research results.

Effects of the oncoming wind direction

The different oncoming wind directions have remarkable effects on the wind fields at the bridge site located in the gorge terrain (Li et al., 2011). Therefore, the tests with the angles between the oncoming wind and the axis of the gorge equaling to 10° and 20°, respectively, were carried out. In the tests, the above oncoming wind of IV ABL was adopted, the angle changes were completed by rotating the circular terrain, and the Cobra probes were also rotated with the circular terrain. Figure 15 shows the mean wind speed profiles at the gorge center with different oncoming wind directions. The mean wind speeds associated with the wind direction of 10° are very close to those associated with the wind direction of 0° (refer to the oncoming wind flowing along the axis of the gorge). Especially for the measurement positions near the ground, such as in the range of the gorge inner layer, the mean wind speeds associated with these two oncoming wind directions are almost the same. However, the mean wind speeds associated with the wind direction of 20° are obviously smaller than those associated with the wind directions of 0° and 10°. By comparing the mean wind speeds of the oncoming wind with those induced by the three oncoming wind directions, it is found that the wind speeds associated with the wind directions of 0° and 10° are more significantly larger than those associated with the wind direction of 20°, and the mean wind speeds at the gorge center associated with the wind direction of 20° even decrease at the higher measurement positions.

Figure 15.

Mean wind speed profiles at the gorge center with different oncoming wind directions.

Figure 16 shows the turbulence intensities at the gorge center with different oncoming wind directions. It can be seen that the turbulence intensities associated with the wind directions of 0° and 10° are close to each other, and they are both smaller than those associated with the oncoming wind. However, the turbulence intensities associated with the wind direction of 20° are very large, and they are generally larger than those associated with the oncoming wind. The main reason is probably that the flow separation occurs in the gorge when the oncoming wind flows into the gorge with large wind directions, such as the oncoming wind direction of 20°, and it results in very large turbulence intensities under such circumstances.

Figure 16.

Turbulence intensity profiles at the gorge center with different oncoming wind directions.

To investigate the effects of different oncoming wind directions on the wind power spectrum at the gorge center, the wind power spectra values with several typical measurement positions moving away from the ground are shown in Figure 17. For the measurement positions near the ground, such as in the range of the gorge inner layer, the whole wind power spectra values associated with the wind direction of 20° are significantly larger than those associated with the wind directions of 0° and 10°, as shown in Figure 17(a). However, with the heights moving away from the ground, the differences of power spectra values associated with the different oncoming wind directions gradually become small. For instance, when the height away from the ground reaches 0.323 m, the power spectra values associated with the different oncoming wind directions are very close, as shown in Figure 17(d). The main reason is that when the oncoming wind with the wind direction of 20° flows into the gorge, lots of eddies with different sizes are produced due to the flow separation in the gorge, which results in the increase in the wind fluctuation, and then the power spectra values also increase. With the heights away from the ground increasing, on the one hand, the effects of the flow separation on the power spectra values will gradually decrease, and on the other hand, the oncoming wind direction of 20° is not large enough to significantly influence the power spectra values of the current measurement direction. As a result, the power spectra values in the higher positions are close to those associated with the wind directions of 0° and 10°. Furthermore, the power spectra values associated with the wind direction of 0° are always close to those associated with the wind direction of 10°. The main reason is probably that a significant flow separation has not occurred in the gorge for the oncoming wind direction of 10°, and the wind fields change slightly compared with those associated with the oncoming wind direction of 0°.

Figure 17.

Comparison of wind power spectra values at the gorge center with different oncoming wind directions: (a) 0.075 m from the ground, (b) 0.132 m from the ground, (c) 0.262 m from the ground, and (d) 0.323 m from the ground.

From the above observations, it can be concluded that when the angle between the oncoming wind and the axis of the gorge is in certain range, such as smaller than 10°, the wind fields including the mean wind speed, turbulence intensity, and wind power spectra in this case are very close to those associated with the wind direction of 0°. However, when the angle between the oncoming wind and the axis of the gorge is in a certain higher range, such as larger than 20°, the wind fields in the gorge will significantly change. To be specific, with the increase in the angle, the mean wind speed will decrease, the turbulence intensity will increase, and the power spectra values also increase. Furthermore, the smaller the measurement positions move away from the ground, the greater the wind fields change.

Conclusion

To study the fundamental characteristics of wind fields over the deep-cutting V-shaped gorge, a simplified gorge terrain was made in a wind tunnel, and the wind characteristics such as mean wind speed, turbulence intensity, integral length scale, and the wind power spectra over the simplified gorge were investigated. The main conclusions are summarized as follows:

Compared with the oncoming wind, the wind speeds at the gorge center become larger, but the turbulence intensities at the gorge center become smaller. When the measurement positions move away from the ground, the mean wind speed ratios first increase and then decrease but gradually become stable, while the turbulence intensity ratios first decrease and then increase but also gradually become stable.

The wind fields over the gorge terrain can be approximately divided into two layers, that is, the gorge inner layer and the gorge outer layer. In the gorge inner layer, the wind fields are very complex due to the influences mainly caused by the two side slopes of the gorge and also caused by the oncoming wind. While in the gorge outer layer, the wind fields are relatively stable and simple because the influences caused by the two side slopes of the gorge are small. Therefore, when a civil structure is located in the gorge outer layer, its wind parameters could be easier to obtain.

The longitudinal integral length scales at the gorge center are remarkably smaller than those of the oncoming wind. The power spectra values in the low-frequency part associated with the gorge center are smaller than those associated with the oncoming wind, but the power spectra values in the high-frequency part show little change.

The mean wind speed ratios in the range of the gorge inner layer associated with the oncoming wind of IV ABL are much larger than those associated with the oncoming wind of UFL. With the heights moving away from the ground, the power spectra values associated with the UFL become small, and they gradually become smaller than those associated with the IV ABL. Therefore, the oncoming wind field type should be determined and simulated as accurately as possible when investigating the wind characteristics at the sites of some civil structures located in the gorge terrains.

When the angle between the oncoming wind and the axis of the gorge is in a certain range, such as smaller than 10°, the wind fields are very close to those associated with the wind direction of 0°. However, when the angle between the oncoming wind and the axis of the gorge is in other higher ranges, such as larger than 20°, the wind fields in the gorge will significantly change. Furthermore, with the increase in the angle, the smaller the measurement positions move away from the ground, the greater the wind fields change.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the National Natural Science Foundation of China under grant Nos 51408496 and 90915006 and also by the National Basic Research Program of China under grant Nos 2015CB057706 and 2015CB057701.

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