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

通过测量均匀湍流和各向同性湍流中粒子的落体速度，研究了湍流对小粒子运动的影响。在本实验室研制的箱式湍流发生器中，产生了各向同性均匀湍流。四个反向旋转的网格安装在一个矩形框中。通过快速旋转，实现了网格间各向同性、均匀的零平均流湍流。在紊流速度场中，用高速摄像机记录粒子的下落，用粒子跟踪测速仪（PTV）对图像进行分析，估计粒子的下落速度。在不同的湍流强度下，测量了粒子的平均下落速度，并与粒子的终端速度进行了比较，研究了湍流的影响。研究中使用了两种类型的颗粒（PMMA和固体玻璃）。颗粒尺寸为100 μm，远远小于Kolmogorov尺度或湍流的最小尺度。因此，假定颗粒周围的流动是均匀的。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，并考察了平均下降速度的增减规律。以往的研究报道了顺序统一的时间比对下落速度的最大增加。然而，在固体玻璃颗粒中观察到相反的趋势（最大减少2%），而在PMMA颗粒中观察到相同的趋势（最大增加15%）。研究发现，时间尺度比并不是决定湍流对粒子运动影响的唯一因素。因此，从过去的DNS研究中对湍流对粒子运动影响的建模需要一些谨慎。我们将继续目前的研究，寻找另一个控制湍流中粒子运动的因素。

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.