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

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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.