Experimental study on effect of turbulent structure on small falling air-borne particle.

为了研究湍流对小颗粒运动影响不大的情况下，湍流对小颗粒运动的影响，在实验室研制的箱式湍流发生器中，测量了颗粒在均匀和各向同性湍流中的下落速度。四个反向旋转的栅极安装在一个矩形盒中。通过快速旋转，在网格间实现了各向同性、均匀的湍流，且流场为零平均。利用高速摄像机记录颗粒在湍流流场中的下落过程，并利用粒子跟踪测速仪（PTV）对图像进行分析，得到颗粒下落的速度，通过改变湍流强度，测量颗粒下落的平均速度，并与颗粒的终端速度进行比较，研究湍流对颗粒下落速度的影响。研究中使用了两种类型的颗粒（PMMA和固体玻璃）。颗粒的尺寸为100 μm，远小于Kolmogorov尺度或湍流的最小尺度。因此，假设绕颗粒的流动是均匀的。通过改变湍流强度，控制了颗粒对湍流时间尺度的气动响应时间，考察了颗粒下落速度的平均速度的增加/减小。过去的研究报道了下降速度的最大增加的时间比的阶数单位。而固体玻璃颗粒的运动趋势相反（最大减少2%），而PMMA颗粒的运动趋势相同（最大增加15%），发现时间尺度比不是决定湍流对颗粒运动影响的唯一因素。因此，从过去的DNS研究湍流对粒子运动的影响建模需要一些谨慎。我们将继续目前的研究，寻找另一个因素来控制颗粒在湍流中的运动。

Velocity of falling particle through homogenous and isotropic turbulence was measured to study the effect of turbulence on motion of small particle, which has little influence on turbulence.Isotropic and homogeneous turbulence was generated in box-turbulence generator that had been developed in our laboratory. Four counter-rotating grids are installed in a rectangular box. By rapid rotation, isotropic and homogeneous turbulence with zero-mean flow was realized between grids. Particles are dropped in a turbulent velocity field and were recorded by high-speed camera and the image was analyzed by Particle Tracking Velocimetry (PTV) to estimate the velocity of the falling particles.Varying the intensity of turbulence, we measured the mean velocity of falling particles, which are compared with the terminal velocity of particle, and investigate the effect of the turbulence. Two types of particle (PMMA and solid glass) are used in the study. The size of particle is 100 μm and much smaller than the Kolmogorov scale or the smallest scale of the turbulence. It is, therefore, assumed that the flow around particle is uniform. By varying the intensity of the turbulence, the aerodynamic response time of particle to the turbulent time scale are controlled and the increase/decrease in the mean velocity of falling velocity are examined. The past research reported the maximum increase of the falling velocity for the time ratio of order unity. However, the reversed trend is observed for the solid glass particles (2% decrease at maximum) while the same trend (15% increase at maximum) for the PMMA particles.It is found that the time scale ratio is not the unique factor to determine the effect of turbulence on particle motion. Therefore, modelling of the effect of turbulence on particle motion from the past DNS research needs some caution. We shall continue the current research, seeking for another factor to control the particle motion in a turbulent flow.