A Experimental Investigation of Entrainment and Mixing in Pulsed and Exponential Transverse Jets

Entrainment in a transverse jet is dominated by two distinct types of large-scale vortical structure. These are the shear layer vortices around the jet and the counterrotating longitudinal vortex pair. Water tunnel flow visualization experiments carried out during this study have revealed that the shear layer around the jet contains vortex loops, which are distorted and stretched due to the straining imposed on them by the crossflow. Neighboring vortex loops are forced to merge with each other in an unusual way as a result of this distortion and stretching. This merging of the shear layer vortex loops in the near field redistributes the vorticity in the jet, giving rise to a counterrotating vortex pair which dominates the flow in the far field. In two different experiments, the jet stream was externally forced in order to alter the behavior of both of these vortical structures. The resulting changes in the structure and penetration of the jet were investigated by spotlight and laser induced fluorescence flow visualization techniques. Mixing and flame length behavior was studied with chemically reactive techniques. One set of experiments have been carried out to investigate the effects of periodic forcing of the shear layer vortex loops on the flow. It was observed that the location where merging of the neighboring vortex loops takes place can be controlled, for fixed jet to crossflow velocity ratio, by varying the forcing frequency. At low frequencies noninteracting vortex rings were formed, which had large penetration and slow mixing. As the frequency was increased, the vortex rings began interacting in the far field. An optimum frequency was found where penetration and mixing were both nearly maxima. Another set of experiments was designed to affect the counterrotating vortex pair by exponentially accelerating the jet stream. It was found that, by varying the time scale of the acceleration with respect to the rotation period of the vortex pair, the size of the vortex pair could be reduced by a factor of four in the near-field. This reduction also affected the far-field flame length of the jet, increasing it by 50%.