Mixing due to grid-generated turbulence of a two-layer scalar profile

Abstract

In this experimental study the mixing of passive scalars that arises from shear-free decaying grid-generated turbulence is examined. The flow configuration consists of two homogeneous layers of equal density separated 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 experiment and salt in a second. The Schmidt number – the ratio of momentum to species diffusivity – was 7 and 700 for heat and salt respectively. Velocity, scalar and flux fields were mapped and flow visualization was undertaken to study the flows. Integral lengthscales and (scalar) flux were determined to be independent of diffusivity, whereas the scalar Taylor microscales varied with Schmidt number, Sc, thus illustrating the disparate effects of diffusivity on large and small scales. The time series of signals showed a correspondence between locations of wθ extrema and uw extrema. Flux wθ arose from intermittent events; and the magnitude of the time-averaged flux $\overline{w\theta}$ was found to depend on the frequency, rather than on variations in the amplitude of wθ events.