Vortex-dynamics model for entrainment in jets and plumes

Abstract

Recent experimental results of Bhat and Narasimha (1996) have revealed a dramatic difference in the entrainment between jets and plumes subjected to off-source volumetric heating and their unheated counterparts. Experimental observations show that plumes entrain more rapidly than jets; the greater entrainment by the plume is typically attributed to the presence of buoyancy in the plume. In contrast, the addition of buoyancy away from the source by volumetric heating produces the opposite effect of reduced entrainment. Apart from buoyancy, other factors such as acceleration due to pressure gradients or other body forces can also affect the rate of entrainment. In this paper, we develop a model for entrainment to explain the mechanism by which buoyancy produces contrasting effects on entrainment in volumetrically heated flows in comparison to their unheated counterparts. The model highlights the role of density stratification in the process of vortex sheet roll-up in free shear flows. With this model, we are also able to explain the higher entrainment of the plume relative to the unheated jet. The model is further extended to explain entrainment behavior during acceleration due to an applied pressure gradient or other body forces.