Large Reynolds Number Turbulence Produced by a One Cell Grid.

This research is an exploratory analysis of the turbulence produced in a wind tunnel by a one cell grid consisting of four half cylinders placed on the inside walls of the tunnel. This arrangement allows the largest possible grid Reynolds number, and produces the largest scales of turbulent motion for a square duct. Because of the large scales of motion, turbulent processes of both practical and scientific interest develop in the downstream flow. Four main flow regimes are described: (1) A region of initial mixing where the turbulence is produced and fills the tunnel. (2) An outer wall region dominated by the interaction of the large eddies with the constraints and confinement imposed by the tunnel walls, which is shown by inertial damping of the velocity component normal to the wall far into the flow. Previously, this process was studied using a moving wall to eliminate the wall shear layer. (3) The turbulent shear boundary layer developing in the presence of large scale turbulence, a process that occurs in many engineering and geophysical situations. and (4) A region of decaying turbulence, farther downstream in the center of the tunnel, which is found to be homogeneous in second-order statistics but not third. There is a net turbulent diffusion or spatial energy transfer of turbulent kinetic energy away from the center of the tunnel toward the tunnel walls which is carried out by the third order statistics of the large scales of motion. In spite of this spatial energy transfer the decay of energy can be described by a power law, the usual form for grid turbulence; however, the spatial energy transfer results in a uniquely large exponent of decay. As usual in grid turbulence, both the energy and dissipation spectra maintain separate self-similarities. Consequences of self-similarity of the energy spectrum in conjunction with power law decay of total energy are explored and used to confirm a simple empirical hypothesis for the decay spectrum and hence for the spectral energy transfer function.