实验流体力学

Comprehensive investigation into wind-induced aerodynamic and aeroelastic effects of large cooling towers

Zhao Lin, Zhan Yanyan, Wang Zhinan, Liang Yuwen, Liu Xiaopeng, Cheng Xiaoxiang, Zhang Junfeng, Ke Shitang, Wang Xiaosong, Cao Shuyang, Ge Yaojun

2017, 31(3): 1-15. doi: 10.11729/syltlx20160201

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Abstract:
As typical thin-wall flexible structures, large cooling towers are of critical sensitivity to wind loads during its life-cycle. It's introduced systematically the research development of large cooling towers made by Tongji wind-resistance group during the past decades, including the study on statistic characteristics of the wind load from the full-scale observation, experiment and analysis of wind-induced structural responses, multi-objective structural optimization, the study on wind-induced loads and effects under disaster climates, the development of the self-developed whole process integration software platform, and so on. Some key problems in engineering application and fundamental investigation discovered during the development of large cooling towers in China have been resolved successfully.

Discussionon several important issues in measurement and indirect verification of nonlinear galloping self-excited forceson rectangular cylinders

Zhu Ledong, Zhuang Wanlyu, Gao Guangzhong

2017, 31(3): 16-31. doi: 10.11729/syltlx20170024

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Abstract:
The nonlinear galloping self-excited forces on a 3:2 rectangular cylinder were measured via wind tunnel tests of a spring-suspended sectional model with synchronous measurements on dynamic force and vibration displacement by using miniature dynamic force balances elaborately developed. The measurement accuracy of the self-excited force was verified indirectly through comparing the time histories of the nonlinear galloping displacement of the sectional model reconstructed by using the measured time histories of the self-excited force with the corresponding measured ones. The importance of considering the nonlinearities of the effective damping and stiffness parameters of the sectional model system in such verification was discussed. The percentages of both the non-wind-induced and wind-induced self-excited forces in the total measured dynamic forces were also evaluated as well as the influences of neglecting the nonlinearities of the non-wind-induced additional aerodynamic damping and inertial forces on the measurement accuracy of galloping self-excited force. It can then be found that for the 3:2 rectangular cylinder the portion of the non-wind-induced self-excited force in the measured total dynamic force exceeds that of the wind-induced self-excited force, and therefore, the non-wind-induced self-excited force should be deducted when extracting the wind-induced self-excited force from the measured total dynamic force. The nonlinearities of the non-wind-induced damping and inertial forces exert some influence on the measurement accuracy of the galloping self-excited force, and deserve to be considered. The nonlinearities of the equivalent damping and stiffness parameters of the sectional model system result in a significant influence on the reconstruction accuracy of the galloping displacement time histories of the sectional model system, and thus, it should also be taken into account in the indirect verification of the measurement accuracy of the galloping self-excited force.

Influence of vibration amplitude on motion-induced aerodynamic force of a streamline box girder

Xiong Long, Wang Qi, Liao Haili, Li Mingshui

2017, 31(3): 32-37. doi: 10.11729/syltlx20160204

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Abstract:
The influence of the vibration amplitude on the motion-induced aerodynamic force of the streamline box girder has been studied. Based on a 1:70 rigid sectional model and electronic pressure scanner valves, the aerodynamic force of the model has been obtained by wind tunnel tests. The range of the torsional amplitude is from 2° to 16°, the range of the vertical amplitude is from 5mm to 23mm, and the angles of attack are 0° and ±5°. At 0° angle of attack, when the torsional amplitude is less than 8° and the vertical reduced amplitude is less than 0.46, the proportions of the linear motion-induced aerodynamic force remain stable and are more than 95%. When the amplitude of the torsional vibration is greater than 8° and the angle of attack is +5°, the linear harmonic components decrease and the high-order harmonic ones increase with the increase of the amplitude. The change of flutter derivatives shows that the torsional amplitude affects A₂^*, A₃^* and H₂^* dramatically, which implies the nonlinear change of the aerodynamic force. However, the vertical amplitude only affects H₄^*, which is not the control parameter of flutter analysis, and has no influence on the other flutter derivatives. The brief conclusion is that although the force-amplitude behavior of a streamline box girder is nonlinear, there is no obvious high-order component in the motion-induced aerodynamic force under a certain amplitude, especially under 8° torsional amplitude.

Effects of end plates on aerodynamic force of rectangular prisms in wind tunnel test

Zheng Yunfei, Liu Qingkuan, Ma Wenyong, Liu Xiaobing

2017, 31(3): 38-45. doi: 10.11729/syltlx20170015

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Abstract:
It is commonly accepted that end plates can reduce the influence of flow on both ends to ensure nominally two-dimensional flow in the section-rigid model tests in wind tunnels. One of the governing parameters is the size of the end plate. In order to reveal the effects of end plates on the aerodynamic force distribution on the rectangular prisms which corresponds to the bridge cross-section with three side ratios of 1, 5 and 10, the variations of aerodynamic force, wind pressure coefficients and St with the size of end plates are discussed based on the data from rigid model pressure tests. The results indicate that the free ends of a cylinder have obvious impact on the wind pressure distribution not only near the ends but also in the middle. The influenced region of the cylinder becomes smaller with increasing side ratio. End plates can strengthen the suction on the leeward side. As for the larger angle of attack, the larger end plate is needed to keep the nominally two-dimensional flow. The size of the end plate also has important effect on the Strouhal number of all three models.

Wind pressure on flat roof building in heterogeneous terrain

Chen Bo, Du Kun, Yang Qingshan

2017, 31(3): 46-51, 59. doi: 10.11729/syltlx20160213

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Abstract:
With pressure measurement experiments in wind tunnel, wind pressure distribution on the flat roof of a building in two kinds of uniform terrains and one kind of heterogeneous terrain was investigated. The results demonstrate: the mean and fluctuating wind pressure coefficients on the roof in a uniform rough terrain are larger than those in a uniform smooth terrain, and this difference of the fluctuating pressure is more significant; When the pressure coefficients are normalized by the dynamic wind pressure of the building location, fluctuating wind pressure coefficients decrease significantly and mean pressure coefficients change slowly with an increase of the distance from the terrain change point to the building location in a rough to smooth heterogeneous terrain; When the pressure coefficients are normalized by the dynamic wind pressure of upstream terrain, mean pressure coefficients at the separation area change slowly, the amplitudes at the downstream roof area increase, and fluctuating wind pressure coefficients decrease slightly with an increase of the distance from the terrain change point to the building location, but these changes are small in the range of transition boundary layer; The main factor affecting total mean wind loads on the roof is the incoming dynamic pressure.

Analytical model for structure design of floating element wall shear stress micro-sensor with capacitive sensing

Ding Guanghui, Ma Binghe, Deng Jinjun, Yuan Weizheng

2017, 31(3): 53-59. doi: 10.11729/syltlx20170004

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Abstract:
The capacitive sensor with a micro floating element can measure the wall shear stress in the flow boundary layer directly. The sensor output voltage is linear to the applied wall shear stress and it is much more precise than conventional methods. We built up the analytical model for the sensor design by studying how structural parameters influence the sensor performances. The sensor consists of a floating element, folded tethers, movable comb fingers, fixed comb fingers and anchors, etc. And the sensor performances include measurement range, natural frequency, nonlinearity, sensitivity and resolution. The relationships between structural parameters and sensor performances are clarified explicitly by the analytical model. And the micro-sensor design becomes much easier and more efficient. Static and dynamic calibration were carried out and the results indicate a good agreement with predictions of the analytical model.

Preliminary application of hot-film wall shear stress sensor under breaking waves

Hao Siyu, Xia Yunfeng, Xu Hua, Cai Zhewei

2017, 31(3): 60-65. doi: 10.11729/syltlx20170037

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Abstract:
The knowledge of the bed shear stress under breaking waves is essential for understanding the sediment transport and beach morphology in the surf zone. The turbulence and vortexes generated by breaking waves have a significant effect on the bed shear stress. Application tests of the bed shear stress measurement under breaking waves by the MEMS flexible hot-film shear stress sensor are conducted in a wave flume. The experimental results show that the sensor can be applied to the measurement of the bed shear stress under breaking waves. Before the breaking point, the direction of the bed shear stress can be determined by the near-bed velocity. The change of the bed shear stress is gentle before wave breaking. The fluctuations and the peak value of the bed shear stress increase after wave breaking. The extreme value of the average maximum bed shear stress along the slope appears after the plunging point.

Calibration and uncertainty analysis of MEMS wall shear stress sensor

Shen Xue, Tian Yukui, Zhang Xuan, Sun Hailang, Xie Hua, Zhang Nan

2017, 31(3): 66-71. doi: 10.11729/syltlx20170019

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Abstract:
In this paper, the calibration and uncertainty analysis of the MEMS shear stress sensor are undertaken. Shear stress is generated by the pressure gradient method, the main test equipment is the flat verification water channel. Different voltage outputs are measured under different wall shear stress conditions, and the calibration coefficient can be got through matching the shear stress and voltage with the least square method. The shear stress uncertainty and calibration coefficient uncertainty can be got through measuring the pressure and voltage repeatedly and checking the related product manual. As the result indicated, the uncertainty of the shear stress measurement is within 7%, it tends to be smaller as the free-stream velocity gets higher. The uncertainty of the voltage measurement is within 7%, and it also tends to be smaller as the free-stream velocity gets higher, so that the sensor can be used to wall shear stress measurement reliably. The shape of calibration curve is reasonable, its correlation coefficient of fitting is large enough, it means that the calibration equation is reliably.

Research on the calibration device with temperature control for underwater wall shear stress sensor

Xia Yunfeng, Hao Siyu, Xu Hua, Cai Zhewei, Zhang Shizhao, Yan Jiechao

2017, 31(3): 72-77. doi: 10.11729/syltlx20170025

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Abstract:
The appearance of new wall shear stress sensors provides new methods for measuring the underwater bed shear stress in estuarine and coastal engineering. The thermal wall shear stress sensor is affected by the ambient temperature significantly. The relevant research and application of these sensors are inseparable from the accurate calibration. Based on the relationship between the wall shear stress and the pressure gradient along the duct with high aspect ratio, a static calibration device for underwater wall shear stress sensor with temperature control is developed. This device can produce wall shear stress for calibration at different water temperature. The calibration device can provide a maximum water temperature of 35℃. A static calibration experiment of the MEMS flexible hot-film wall shear stress sensor is conducted using the device, and the calibration coefficients under different water temperature conditions are determined. The linear relation between the calibration coefficient B and the water temperature is found.

Application of thermal shear stress gauge in study on wave-current dynamics

Xu Hua, Xia Yunfeng, Cai Zhewei, Hao Siyu, Zhang Shizhao

2017, 31(3): 78-81, 93. doi: 10.11729/syltlx20170034

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Abstract:
The movement of sediment at estuary and coast is directly restricted by the bed shear stress. Therefore, the basic research on the influence of the bed shear stress on the sediment movement is important. However, there is no available method for measuring and computing the bed shear stress under a complicated dynamic condition like the wave current. This paper conducts the measurement and test research on the bed shear stress in a long launder of direct current by the new thermal shearometer based on micro-nanotechnology. As the research results show, the thermal shearometer has high response frequency and strong stability. The measuring result reflects the basic law of the bed shear stress with the wave and wave-current effect, and confirms that the method of measuring bed shear stress under wave-current condition with the thermal shearometer is feasible. Meanwhile, a preliminary method to compute the shear stress compounded by wave-current is put forward. It benefits further research on the basic theory of the sediment movement with complicated dynamic effects.

Research on underwater flat plate wall shear stress measurement with MEMS sensors array

Tian Yukui, Zhang Xuan, Shen Xue, Sun Hailang, Xie Hua, Zhang Nan

2017, 31(3): 82-87. doi: 10.11729/syltlx20170031

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Abstract:
Aiming at underwater wall shear stress measurement, researches have been conducted which include MEMS wall shear stress sensors calibration, flat plate model design, weak signal detection system development, flat plate boundary parameter estimation and CFD analysis, and near-wall LDV boundary layer profile measurement. The local free stream velocities ranging from 0.2m/s to 0.7m/s are adopted for the underwater flat plate test in a precision water flume. And the wall shear stress is directly measured by MEMS sensors array. Also the boundary layer profiles of the investigated area upon the plate surface are detected with LDV and mean wall shear stress values are extrapolated. The results from the MEMS sensors match well with those from the LDV velocity profile fittings and are in good agreement with empirical estimations with an overall bias less than 5%.

Experimental study of the effects of contact deformation on drop coalescence scenario

Wei Cunju, Li Yingjie, Wang Luhai, Yang Jiming

2017, 31(3): 88-93. doi: 10.11729/syltlx20160146

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Abstract:
The evolution of cross section of the liquid bridge during drop coalescence is captured with a new type of experimental setup which contains the generation of large drops, top-view observation and high-speed shadowgraph. The optical results support the previous findings obtained with electrical measurements in the initial stage of the coalescence. Thanks to the unique advantages of the top-view observation, the shape and position of the liquid bridge connecting the drops is clearly demonstrated in this paper. Two coalescence scenarios with different approaching speed of the drops, v_a, are distinguished as the center scenario and the off-center scenario based on the location of the starting point of coalescence. The critical speed, v_cross, which divides the scenarios, is noticed and measured with the present device. It is found that the approaching speed has little influence on the coalescence process in the center scenario when v <v_cross. On the contrary, the onset of coalescence switches to the edge of the contacting film formed by the approaching drops and the off-center scenario appears consequently when v> v_cross.

2017 Vol. 31, No. 3