The effect of mean flow pulsation on grid turbulence and passive scalar diffusion*

The characteristics of pulsatile, grid-generated turbulence and turbulent diffusion of a passive scalar injected from a line source downstream of a grid were investigated in a specially designed non-stationary flow wind tunnel. Flow pulsation was generated by oscillating a hinged flap downstream of the blower, which diverted part of the flow through a bypass section. Velocity measurements were taken with hot wire probes, calibrated against a laser Doppler velocimeter, in grid turbulence behind two grids with mesh sizes 51 and 19 mm in both stationary flow with a mean speed of 2.9 m/s and in roughly sinusoidally pulsating flow with a time-averaged velocity of 2.9 m/s, a phase-averaged amplitude of about 0.6 m/s and a frequency of 0.33 Hz.  Reported measurements include values of the Reynolds stresses and turbulent kinetic energy, integral length scale, Taylor and Kolmogorov microscales, energy dissipation rate, dissipation parameter and velocity derivative skewness and flatness factors. It was found that pulsation increased the ratio of the time-averaged turbulent kinetic energy and its value in stationary flow and that this ratio increased with downstream distance. Flow pulsation was also found to increase the spread of a passive scalar plume within the grid turbulence test section core, while diminishing scalar diffusion in the boundary layer.