(399b) Cross-Gradient Transport and Mixing in Turbulent Shear Flows

Experiments have demonstrated that in some turbulent shear flows turbulent diffusivity is not a scalar, but significant turbulent transport can occur in a direction not aligned with that of the mean concentration gradient. The experiments took place in a rectangular turbulent channel flow of water with a coflowing spanwise ducted rectangular jet in the channel. A dye was added to the jet so that mixing with ambient fluid occurred downstream of the jet exit. Simultaneous measurements of velocity and concentration were made using PIV and PLIF, which allowed determination of velocity and concentration profiles and various other turbulent statistical quantities. In all cases the downstream turbulent transport was greater than the cross-stream transport in the direction of the mean concentration gradient, in agreement with the measurements of Tavoularis and Corrsin. This was true both for the ducted jet experiments and for cases where the velocities in each stream were the same, the transport occurring in the wakes behind the splitter plates. Additional confirmation showed the same results determined by LES. The cross-gradient transport has been used in modeling turbulent transport and mixing in a conditional moment closure. A simple model and argument supporting this phenomenon has been developed by Blaisdell.

A second experiment using measurements made with stereo PIV and PLIF involved the transport and mixing in a multi-inlet vortex reactor (MIVR). This is a device scaled up from microreactors used to determine rates of fast reactions in order to determine the suitability for large-scale nanoprecipitation applications. A rather flat cylindrical reaction chamber has four horizontal tangential inlets and a vertical withdrawal tube on the cylinder’s axis, with fluid dye in alternate inlets. The fluid entering the chamber undergoes a helical path on its way from the inlets until it reaches the vertical outlet with a 5-fold increase in velocity. There appears to be no net shear associated with the transport and mixing, but rather considerable stretching of fluid elements occurs with concomitant increase in local scalar gradients along directions of compressive strain, resulting in rapid mixing, as also suggested by previous DNS results in a study of chemical reactions.