DESIGN AND CHARACTERISTICS OF A LARGE BOUNDARY- LAYER WIND TUNNEL WITH TWO TEST SECTIONS

Transcription

1 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan DESIGN AND CHARACTERISTICS OF A LARGE BOUNDARY- LAYER WIND TUNNEL WITH TWO TEST SECTIONS Kai Chen 1, Xin-Yang Jin 2 and Ji-Da Zhao 3 1 Associate Researcher, China Academy of Building Research Beijing, China, chenkai@cabrtech.com 2 Researcher, China Academy of Building Research Beijing, China, jinxinyang@cabrtech.com 3 Researcher, China Academy of Building Research Beijing, China, zhaojida@cabrtech.com ABSTRACT A large boundary-layer wind tunnel for wind engineering research has been designed and constructed by China Academy of Building Research (CABR). This wind tunnel has two features: the open-circuit blow-down layout and two tandem test sections. The open-circuit blow-down layout can avoid the contamination in the wind tunnel in some special tests, such as the wind-rain-induced vibration test, diffusion simulation of the pollution and flow visualization experiment with tracer particles. Furthermore, this wind tunnel is suitable for the model test with different sizes, as the dimensions (W H L) of the two test sections are 4.0m 3.0m 22.0m and 6.0m 3.5m 21.0m, respectively. A series of measurements were performed in the empty wind tunnel. The results showed that a flow field with uniform velocity distribution and low turbulence intensity was achieved and the flow characteristics fulfilled the design requirements. Besides, a new method to simulate the atmospheric boundary layer with some modified spires is developed in this paper. This new method is flexible and simplifies the simulation work of ABL with different exponents. KEYWORDS: BOUNDARY-LAYER WIND TUNNEL, WIND ENGINEERING, WIND-TUNNEL DESIGN Introduction A boundary-layer wind tunnel is an essential equipment for wind engineering research [Cermak (2003)]. Compared with the aeronautical low-speed wind tunnel, the boundary-layer wind tunnel has longer test section for simulating the atmospheric boundary layer. With the rapid development in wind engineering, many boundary-layer wind tunnels have been constructed in the world [Witter et al. (2000) and Balendra et al. (2002)]. Besides some special boundary-layer wind tunnels were designed, such as the multiple-fans wind tunnel [Nishi and Miyagi (1995)] and the non-conventional wind tunnel for both aeronautical and wind engineering field [Diana et al. (1998)]. According to the test purpose and building size, different test section sizes and maximum wind speeds are required in practice. Therefore two-test-section configuration is commonly used in many boundary-layer wind tunnels. In most close-circuit wind tunnels with two parallel test sections, the flow characteristics requirements can hardly be satisfied simultaneously in both sections due to the difficulties in aerodynamics design. If these two test sections are tandem, only one section s length can meet the requirement because of the space limit of close-circuit design. Moreover, both the close-circuit and the suction opencircuit wind tunnels are not appropriate for the experiment with solid or liquid test medium. Under these considerations, a new design concept was consequently applied on the construction of the wind tunnel of CABR.

2 Compared with other familiar boundary-layer wind tunnels, the distinctive design characteristics of this wind tunnel are the open-circuit blow-down layout and two tandem test sections configuration. The open-circuit blow-down layout can avoid the contamination in the wind tunnel in some special tests, such as the wind-rain-induced vibration test, diffusion simulation of the pollution and flow visualization experiment with tracer particles. The two tandem test sections ensure the consistency of the flow characteristics and provide a flexibility to carry out the different size model tests according to the test purpose. The photograph of the wind tunnel was shown in Fig. 1. Figure 1: The photograph of the wind tunnel Main features of the wind tunnel Wind tunnel configuration and layout The conventional low-speed wind tunnel has open-circuit and close-circuit two systems. Both systems have their own advantages [Mehta and Bradshaw (1979)]. Considering the requirements for the different wind engineering experiments, the open-circuit system was adopted in this wind-tunnel and the fan was installed at the upstream to avoid the possible contamination on the drive system. Both the intake and the exhaust ends were arranged in the experiment hall so that the quality of the return flow was not affected significantly by the circumstantial air. When the special tests with liquid or solid test medium were conducted, the doors near the intake and exhaust could be open to permit the contaminant to be vented to the outside. The layout of the wind tunnel was shown in Fig.2 and the main aerodynamic design parameters were listed in Tab.1. Intake Fan Wide-angle Diffuser Settling Chamber Contraction Low-speed Test Section Contraction High-speed Test Section Exit Diffuser Figure 2: Schematic Diagram of Wind Tunnel

3 Table 1. Main Designed Parameters Issue Content Parameter Outline Layout Open-circuit and Blow-down Total length (m) 96.5 Contraction ratio 3.7 Maximum Power (kw) 370 Fan Number of Vane 12 Maximum Rotating Speed (r/min) 400 Maximum Pressure Rise (Pa) 800 Wide-angle Diffuser Diffusion Angle ( o ) 21 Settling Chamber L/D Ration of Honeycomb 12 Screen Size(mesh/inch) 20 High-speed Test Section Dimension(W D L, m) Low-speed Test Section Dimension(W D L, m) Aerodynamic design of the wide-angle diffuser and test sections of the wind tunnel The diameters of fan section and the settling chamber are 4.5m and 7.5m, respectively, which lead to a diffusion area ratio of 2.8. If the traditional small-angle diffuser design was adopted, the length of the diffuser would be 24.5m with a diffusion angle 7. It is not acceptable. For reducing the length between the fan section and the settling chamber, wideangle diffuser is selected, which was widely used in both low-speed and high-speed wind tunnels. Many researches show that with the technique of flow separation prevention, the flow field quality could be satisfying. According to the compromise of the length of each section, the wide-angle diffuser with length 8.0m and diffusion angle 21 was used in this wind tunnel. An inner cylinder tube was installed at the center of the diffuser as a splitting plate. The effect of the splitting plate on the flow was investigated by CFD simulation and illustrated in Fig. 3. Without the splitting plate, the flow separation occurred and the recirculation zone could be found at the downstream part of the diffuser. However the inner tube reduced the diffusion angle to 7 and the separation was avoided effectively; accordingly the uniformity of the flow field at the outlet of the diffuser was improved significantly. Figure 3: Comparison of Flow in Wide-angle Diffuser With (right) and Without (left) Splitting Plates Similar design was used in exit diffuser. Two vertical splitting plates and one horizontal splitting plate in this section were used to reduce the length of exit diffuser. The dimensions (W H L) of the two test sections were 4.0m 3.0m 22.0m and 6.0m 3.5m 21.0m respectively. They were large enough to simulate the natural winds using Counihan method [Counihan (1979)] and suitable for the model test with different sizes. Usually, the large section could be used for the topographical model test and evaluation of community wind environment while the smaller one could be used for the wind loading test. Little-angle diffusion was designed in both test sections to eliminate the axial static pressure gradient. The exit width of these two test section were extended to 6.25m and 4.20m corresponding to diffusion angle 0.34 and 0.26 respectively.

4 Other features of the wind tunnel The wind tunnel was constructed with steel and the structural weight is 230 ton approximately. The total length of the wind tunnel is 96.5m and the maximum diameter of the tunnel is 10.5m. The central axis of the wind tunnel is 5.5m in the height from ground level. The finite element analysis was performed to validate the structural design of the wind tunnel. Different load case combinations were considered, including the deadweight of the structure, the negative pressure acted on the wall under the maximum wind speed, and the weight of several persons standing on the floor in wind tunnel. It was shown that the maximum vertical deflection of the large test section is only 4.06mm under these load cases which meet the steel structure design specification requirement. Flow field quality of the empty wind tunnel Maximum wind speed and background turbulence intensities Two standard wind pipes were fixed at the central axis of the wind tunnel for the measurement of the wind speed. It was verified that the maximum wind speed at the test sections could reach 35.5m/s and 20.5m/s respectively which was high enough to conduct the different wind engineering tests. The turbulence intensity was measured by the hot-wire anemometer. The sampling rate was 1 khz and sampling time was 10s. Under five different wind speed of oncoming flow, the maximum and minimum turbulence intensity were 0.98% and 0.63% at the large test section and 0.35% and 0.33% at the small test section. It was a satisfying result for the boundary-layer wind tunnel. Axial static pressure gradient Five measurement points were arranged along the test section and the static pressure coefficient was calculated accordingly. It was shown that the axial static pressure gradient was rather low. The results in detail were listed in Tab. 2. Wind speed stability The stability of the wind speed was characterized by the wind variation in a specific duration. The stability coefficient of the wind speed η was introduced here. It was defined as Vmax Vmin η = Vmax + Vmin where V max and V min were the maximum and minimum wind speed respectively among the total 120 wind speed samples in one minute. The test results were presented in Tab. 2. The low stability coefficient implied the high stability of the wind speed. Table 2. Flow Characteristics of Empty Wind Tunnel Low-speed Test Section High-speed Test section Wind Speed (m/s) Stability Coefficient η 2.4% - 1.3% - 1.9% 0.74% 0.51% Axial Static Pressure Gradient dcp/dx (1/m) Flow Field Coefficient Pitching Angle α ( o ) Yaw Angle β ( o )

5 Uniformity of the flow field Twelve seven-hole probes were used to investigate the wind speed field at the specific cross-section of the wind tunnel. The fields at the inlet and the model test region were chosen to be measured and the coefficient of wind speed field was calculated. It was defined as vi v vi μi = = 1 v v where v i represented the wind speed at the measurement point i and v represented the averaged wind speed of all these points. Figure 4 showed the distribution of the departure from the mean wind speed at the turnable table of the high-speed test section. The wind speed of oncoming flow was 20m/s. Each horizontal line in the figure represented a base wind speed 20m/s and each vertical grid represented 0.5m/s. The averaged absolute coefficients at four locations were listed in Table 2. H(mm) W(mm) Figure 4: The distribution of the departure from mean wind speed The orientation of the flow vector was also investigated based on the same data obtained by the seven-hole probes. The average yaw angle and pitching angle were listed in Tab 2. Both results showed a uniformity velocity distribution. Noise Sound pressure level (SPL) of the wind tunnel noise was tested in test section, air intake, air exhaust, operation room and outside the workshop building with different wind speeds in empty wind tunnel by using portable noise level meter. The results showed that the noise of this wind tunnel was low in usual speed range. Under the wind speed of 20m/s at high-speed test section, the SPL in operation room is around 65dBA, while workshop hall near test section 75dBA below and outside workshop 65dBA below. The simulation of the atmospheric boundary layer The simulation of the atmospheric boundary layer is a key point in wind tunnel test. The existing methods are divided into two types: passive method and active method. At present, a passive simulation method with spires and rough elements is most widely used due to its convenience and cost saving. This method could practically ensure the similarities of simulated atmospheric boundary layer in wind tunnel, such as mean wind speed profile, turbulence profile, integral scale, wind spectrum, etc. Spire has great influence on mean wind speed profile. Generally speaking, different spires were used to simulate the different wind profiles. However, It was inconvenient to change the spire when a different profile was needed. A new method to simulate the

6 atmospheric boundary layer with some modified spires was developed here. This spire is made of two rotatable right triangle plates, which is shown in Fig 5. Different mean wind profiles are produced by different front face areas adjusted by changing the plate angle, which is much more simply for the simulation of atmospheric boundary layer. The detailed information of this method will be presented elsewhere. Fig. 6 shows the A, B, C profile type by Chinese structure loading code simulated in our wind tunnel α=0.12 α=0.16 α= Z(mm) 600 Z(mm) Fig 5: The modified spire U/U g σ/u(%) Fig 6: Three typical wind profiles Conclusions The CABR wind tunnel aims at the wind engineering tests in the practical buildings and structures. The open-circuit and blow-down aerodynamic layout was adopted to avoid the contamination in the wind tunnel in some special tests. The two tandem test sections were suitable for the model tests with different sizes according to the test purpose. The total length of the wind tunnel was limited effectively by employing the wide-angle diffuser. In consequence, the flow characteristics, the total investment and the experimental capability reached a compromise. It was verified by the measurements in the empty wind tunnel that the flow field was uniform and the turbulence intensity was low. Most of the design requirements were fulfilled. Besides, the new passive method to simulate the atmospheric boundary layer is successfully developed to obtain different wind profile. References Balendra T., Shah D.A. and Tey K.L., et al. (2002), Evaluation of flow characteristics in the NUS-HDB Wind Tunnel, Journal of Wind Engineering and Industrial Aerodynamics, 90(6), Cermak J. E. (2003), Wind-tunnel development and trends in applications to civil engineering, Journal of Wind Engineering and Industrial Aerodynamics, 91(3), Counihan J. (1979), A method of simulating a neutral atmospheric boundary layer in a wind tunnel, Conf. on the Aerodynamics of Atmospheric Shear Flows, 48, Pap. 14 Diana G., Ponte S. De and Falco M., et al. (1998), A new large wind tunnel for civil-environmental and aeronautical applications, Journal of Wind Engineering and Industrial Aerodynamics, 74-76, Mehta R. D. and Bradshaw P. (1979), Design rules for small low speed wind tunnels, Aeronautical Journal, 83(827), Nishi A. and Miyagi H. (1995), Computer-controlled wind tunnel for wind-engineering applications, Journal of Wind Engineering and Industrial Aerodynamics, 54/55, Wittwer A. R. and Möller S. V. (2000), Characteristics of the low-speed wind tunnel of the UNNE, Journal of Wind Engineering and Industrial Aerodynamics, 84(03),