Clarification of Diffusion Phenomena in the Turbulent Field Including Large-Scale Eddies (Particle Diffusion in Turbulent Shear Flows)

为了在小型风洞中模拟大气湍流，研制了湍流发生器和速度分布发生器。研究了颗粒在强湍流场中的扩散行为。结果表明：（1）在湍流场中，湍流强度大于10%，纵向积分长度尺度大于发生器网格尺寸的5倍。在大尺度湍流谱中观察到了两个以上波数的惯性子区。虽然Re = <10^4>，但大尺度湍流的特征与Re =<10^6>的湍流相似。(2)大尺度湍流使二维物体下游形成的相干涡不规则地减弱或增强，使涡列弯曲。大尺度湍流中尾迹的最大速度亏损值小于网格湍流中尾迹的最大速度亏损值。在大尺度湍流中，速度缺陷的半值宽度增大。(3)通过在湍流发生器下游安装狭缝和蜂窝结构，产生了三维各向异性湍流和轴对称湍流。用理论模型描述了各向同性的恢复过程。(4)速度分布发生器能够在试验段的下游区域形成任意的垂直速度分布剖面。湍流的水平分布几乎是均匀的。(5)在大尺度湍流场中，颗粒迅速扩散，浓度分布宽度增大。湍流扩散系数在弱湍流时为270 mm 2/s，在大湍流时为7300 mm 2/s。在湍流剪切流中，浓度分布偏向低速侧。

A turbulence generator and a velocity distribution generator were developed in order to enable to simulate the atmospheric turbulence in a small wind tunnel. The particle diffusion was investigated in a strong turbulence fields generated by the equipments. The results were as follows,(1): In the turbulence field, the turbulence intensities exceeded 10 % and the longitudinal integral length scale was more than five times as large as the mesh size of the generator. Inertial subrange of more than two orders in wave number was observed in the spectrum of the large scale turbulence. The characteristic features of the large scale turbulence were similar to those of a turbulence of Re = <10^6> though Re = <10^4> .(2): The coherent vortices formed downstream of the two-dimensional bodies were irregularly weakened or intensified by the large scale turbulence and the vortex streets meandered. The magnitude of the maximum velocity defect of the wake in the large scale turbulence was smaller than that in the grid turbulence. And the half-value width of the velocity defects was enlarged in the large scale turbulence.(3): A three-dimensional anisotropic turbulence and an axisymmetric turbulence were generated by installing a slit and a honeycomb downstream of the turbulence generator. The process of the return to isotropy was expressed by the theoretical model.(4): The velocity distribution generator was able to form arbitrary profiles of vertical velocity distributions in its downstream region in the test section. The horizontal distribution of the turbulence was almost homogeneous.(5): The particles rapidly diffused and the width of distribution of these concentration was enlarged in the large scale turbulence field. The eddy diffusivity was 270 <mm^2> /s in the weak turbulence and 7300 <mm^2> /s in the large scale turbulence. In the turbulent shear flow, the concentration distribution leaned to the side of lower velocity.