Generating isotropic and homogeneous turbulence in a laboratory flow
Haitao Xu
Max-Planck-Institute
Abstract:
Most of the theoretical work on turbulence are based on the assumption of isotropy and homogeneity. Traditionally, homogeneous and isotropic turbulence can only be approximated by the flow behind a grid in a wind tunnel. However, it is difficult to conduct Lagrangian measurement in wind tunnels due to the large mean flow speeds. Homogeneous and isotropic turbulence without a mean flow can be achieved in direct numerical simulations (DNS) of cubic boxes with periodic boundary conditions. Even in DNS, long simulation times are needed to reach good degree of homogeneity and isotropy. We have constructed an apparatus, named the Lagrangian Exploration Module (LEM), which is demonstrated to generate highly homogeneous and isotropic turbulence with negligible mean flow and is suitable for experimentally studying the Lagrangian properties of turbulence. The apparatus is shaped as an icosahedron, with a propeller installed on each of its 12 vertices. The propellers are driven by independently controlled motors. We have measured the flow when the propellers were running in both "constant-driving" and "random-driving" modes and observed homogeneous and isotropic turbulence in both modes. We compare the turbulence in the LEM with the von Karman swirling flow between two counter-rotating disks and with data from DNS.