Showing papers by "Rex Britter published in 1995"

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 was found to depend on the frequency, rather than on variations in the amplitude of wθ events.

Turbulence evolution and mixing in a two-layer stably stratified fluid

Abstract: The results of an experimental study of shear-free decaying grid-generated turbulence on both sides of a sharp interface between two homogeneous layers of different densities are presented. The evolution of turbulence and mixing were examined by simultaneously mapping the velocity (u, w) and density fields @) and the vertical mass flux F(= pW/p’w’) together with flow visualization in a low-noise water tunnel. Buoyancy was induced by salinity differences so the value of the Schmidt number S, = 700. Density stratification altered the inertial-buoyancy force balance (most simply expressed by Nt, the product of the buoyancy frequency N and turbulent timescale t) so as to attenuate turbulent velocity fluctuations, vertical motions and interfacial convolutions, normalized density fluctuations, vertical flux mass, and mean interfacial thickness. Vertical velocity fluctuations w’ were found to increase with distance from the interface, whereas the u’-distribution can be non-monotonic. The maximum value of the mass flux, F, was found to be about 0.5 which was less than the typical value of 0.7 for thermally stratified wind tunnel experiments for which S, = 0.7. The vertical mass flux can be a combination of down-gradient and counter-gradient transport with the ratio varying with Nt (e.g. at Nt z 5, the flux is counter-gradient). The flux Richardson number Rf was found to increase monotonically to values of approximately 0.05.