MIXING DUE TO GRID-GENERATED TURBULENCE OF A 2-LAYER SCALAR PROFILE

In this experimental study the mixing of passive scalars that arises f rom shear-free decaying grid-generated turbulence is examined. The flo w configuration consists of two homogeneous layers of equal density se parated by a sharp interface between different mean concentrations of passive scalar. Both layers flow through a turbulence-generating grid. To determine the effect of diffusivity the scalar was heat in one exp eriment and salt in a second. The Schmidt number - the ratio of moment um to species diffusivity - was 7 and 700 for heat and salt respective ly. Velocity, scalar and flux fields were mapped and flow visualizatio n was undertaken to study the flows. Integral lengthscales and (scalar ) flux were determined to be independent of diffusivity, whereas the s calar Taylor microscales varied with Schmidt number, S-0, thus illustr ating the disparate effects of diffusivity on large and small scales. The time series of signals showed a correspondence between locations o f we extrema and uw extrema. Flux w theta arose from intermittent even ts; and the magnitude of the time-averaged flux ($) over bar w ($) ove r bar theta was found to depend on the frequency, rather than on varia tions in the amplitude of w theta events.