Study on unsteady aerodynamic forces using a system of wind tunnels of which air streams can change speed and direction rapidly

本研究的目标是开发一种新的风洞系统，该系统可以主动产生湍流以及速度和方向可以快速变化的流动。该系统由两个风洞组成，每个风洞的风扇由计算机控制的交流伺服电机驱动，每个风洞有一个面积为40cm × 40cm的方形风道，能够在0.1秒内启动15米/秒的速度流。两个相同的风洞在地板上对齐，因为它们的气流以60度角相交。合并后的流可以获得两个水平湍流波动分量或随时间变化的速度和流方向等特征。在两种稳定速度的不同组合下，驱动两个风洞以测量合并流的稳定速度和方向。从而建立了两种风速与风速和风向之间的关系。从这些关系中，控制电压的历史产生随时间的速度和方向的流。利用该装置和控制方法生成了顺向起伏分量和横向起伏分量服从卡门谱的湍流。对于非定常气动力的测量，在合并流中垂直放置一个方形圆柱体。在速度逐级变化的气流中，阻力和负升力方向比定常气动力更强。在方向变化率恒定的流中，高变化率时的气动力与稳定变化率时的气动力差别更大。

The objectives of the present study are to develop a new wind tunnel system which can actively generate turbulent flows as well as flows of which speed and direction can change rapidly. The system consists of two wind tunnels of which fans are driven by AC servomotors controlled by computers Each wind tunnel has a square duct with 40cm x 40cm cross section and has a capacity to initiate 15 m/s speed stream only in 0.1 second. The two same wind tunnels are aligned on the floor as their air streams cross at the angle of 60 degrees. The resulting merged stream may acquire the features such as two horizontal turbulent fluctuation components, or time dependent speed and stream direction.At various combinations of two steady speeds, the two wind tunnels are driven in order to measure the steady speed and direction of the resulting merged streams. Thus, the relations between the two wind speeds and the resulting speed and direction are established. From these relations, the histories of the control voltages to generate streams of time dependent speed and direction.This facility and the control method are used to generate turbulent flows of which stream-wise fluctuation component and the lateral fluctuation component obey Karman spectra. As for the unsteady aerodynamics force measurement, a square cylinder is placed vertically in the merged streams. In streams with stepwise change of the flow speed, stronger drag forces and negative lift direction in comparison to the steady aerodynamic forces are observed. In streams with constant rate of direction change, it is observed that the aerodynamic forces at higher rate differs from the steady aerodynamic forces much more.