Effect of forcing on the vorticity field in a confined wake

Several recent studies have found that when a low Reynolds number, plane wake is forced with sufficient amplitude, the normalized mixing product, measured as the amount of mixed fluid per unit width of the wake, can be increased to levels larger than those seen in high Reynolds number mixing layers. However, no studies examining the velocity and vorticity fields of this flow have been conducted. The present study examines the velocity and vorticity field of a low Reynolds number plane wake within a confining channel in order to better understand the vortex-vortex and vortex-wall interactions in order to shed light on the mechanisms which lead to increases in the amount of mixed fluid within the wake.; Molecular Tagging Velocimetry (MTV) is used to measure the velocity field in both the streamwise (u, v velocities in x, y plane) and cross-stream (v, w velocities in y, z plane) measurement planes. The spanwise and streamwise vorticity components are then computed from their respective velocity fields.; Measurements in the streamwise plane have found that a distinct spatial periodicity exists in the u_rms field that is not found in either the unforced case or in unconfined forced flows. A model was developed which relates this spatial periodicity to the phase difference between the forcing input and the rolling up of the vorticity shed from the splitter plate. From these data, it was also determined that the phase at which vorticity is shed is dependent upon the forcing amplitude.; The forced wake flow is dominated by the shedding of concentrated, spanwise vortex core rollers. As these cores develop downstream, the levels of peak vorticity within the core decrease. A very small amount of $-6w/6z$ is sufficient to generate a very large decrease in peak vorticity levels. This same quantity has also been found to be a good predictor of the spatial location where mixing enhancement will occur in the forced wake.; Mixing enhancement is accomplished by the generation of regions of streamwise vorticity from the reorientation of the primary spanwise vortex cores. A model was developed which describes how these cores develop. The multiple regions of streamwise vorticity are the result of the passage and reorientation of multiple spanwise rollers. These reoriented “legs” of streamwise vorticity interact with the regions of streamwise vorticity resulting from the passage of previous spanwise vortex rollers to generate the additional surface area necessary for mixing enhancement. (Abstract shortened by UMI.)...