A case study of multi-structure turbulence: Uniformly sheared flow distorted by a grid

摘要

The fine and large scale structures of spatially developing uniformly sheared flow (USF) and USF distorted by a square mesh grid were examined experimentally using one and two point, two component hot wire anemometry. In agreement with previous findings, it was confirmed that USF has a far downstream region, where the dissipation parameter C-epsilon is approximately constant, and an intermediate region, where it scales with the local turbulent Reynolds number as C-epsilon proportional to Re-lambda(-0.6). The insertion of a grid across the USF resulted in the creation of multi structure turbulence (a flow in which at least two kinds of dominant eddies exist and contribute significantly to the local averages at the energy-containing level) and a permanent reduction of the kinetic energy and the integral length scales of the turbulence. Near the grid the dissipation parameter scaled as C-epsilon proportional to Re-lambda(-1), as in decaying grid turbulence, despite the opposite evolution rates of Re-lambda in the two flows. The turbulence fine structure within this multi structure region was unaffected significantly by the grid, whereas the large scale anisotropy was markedly changed. Second order structure functions, normalised by Kolmogorov scales, collapsed in the viscous sub range, whereas within the inertial sub range the effects of grid insertion were measurable.