VORTICITY FIELD EVOLUTION IN A FORCED WAKE

Abstract

Abstract : The purpose of this work is to quantify the vorticity evolution in the flow field of the forced wake of a splitter plate inside a confining geometry. The interest in this flow stems from the fact that forcing a low Reynolds number 2-D wake can lead to a highly three-dimensional flow and a large increase in mixing. The authors' recent estimates, based on chemically reacting laser induced fluorescence (LIF) measurements, report the amount of molecularly mixed fluid in terms of mixed-fluid fraction to be 2.5 to 3 times larger than that in high Reynolds number natural two-stream mixing layers. Both reacting and non-reacting LIF data connect this increase in mixing to the downstream evolution of the streamwise vorticity, which is generated by the reorientation and stretching of spanwise vorticity near the side walls of the flow facility. It is believed that understanding the vorticity interaction with walls, its dynamics, and downstream evolution will be helpful to an overall strategy for mixing enhancement and control. The measurements were carried out by Molecular Tagging Velocimetry (MTV), a technique that takes advantage of molecules with long-lived excited states for nonintrusive, multi-point measurements of various fluid dynamical quantities. Small regions of the flow were tagged by a laser and their subsequent evolution was monitored over the luminescence lifetime of the molecule. A two-detector imaging system was used to acquire an image of the initially tagged regions and a subsequent image of these regions convected by the flow over a prescribed time delay. The Lagrangian displacement vectors from such image pairs were computed using a spatial correlation technique. The particular flow investigated here was highly three dimensional. This application highlights the capability of MTV to make measurements when strong out-of-plane motions are present. (5 refs.)