Vortex Dynamics and Instability Mechanisms in a Radially Lobed Nozzle

Abstract

The application of radially lobed nozzles has seen renewed challenges in the recent past with their roles in combustion chambers and passive flow control. The free jet flow from such nozzles has been studied for different flow conditions and compared to jets from round nozzles, verifying their improved mixing abilities. The precise mixing mechanisms of these nozzles are, however, not entirely understood and yet to be analyzed for typical jet parameters and excitation modes. This study carries out three-dimensional large eddy simulations (LESs) of the flow from a tubular radially lobed nozzle to identify instability mechanisms and vortex dynamics that lead to enhanced mixing. The flow is studied at two Reynolds numbers of around 6000 and 75,000, based on the effective jet diameter. The low Reynolds number jet is compared to that from a round nozzle and experimental data to demonstrate changes in mixing mechanisms. The present simulations confirmed the presence of Kelvin–Helmholtz (K–H)-like modes and their evolution. The analysis also confirms the evolution of three distinct types of structures—the large-scale streamwise modes at the lobe crests, corresponding K–H structures at the troughs, and an additional set of structures generated from the lobe walls. The higher Reynolds number simulations indicate changes in the mechanics with a subdued role of the lobe walls.