Propagation of Swirl in Transonic Airflow

Abstract

The propagation of swirl and the choking criteria of swirling transonic airflow are examined both numerically and experimentally in underexpanded airflow at matched nozzle reservoir pressure as well as matched mass flow. It was found that the swirl number is not conserved in the expanding subsonic flowfield inside nozzle but decreases as the throat is approached. It was also found that the throat velocity itself (not any of its components) is choked in a swirling flowfield. Therefore, the application of swirl always results in a reduction in axial Mach number component. Since the mass flow through nozzle is a function of throat density and axial Mach number, the reduction in the latter with swirl explains the observed reduction in mass flow (discharge coefficient) at matched nozzle reservoir pressure. Greater reservoir pressures, on the other hand, result in higher throat densities, which compensates for the reduced axial Mach number, and the mass flow rate can be matched its non-swirling value. It was also found that the distribution of subsonic Mach number (not any of its components) in a swirling flow is solely dependent on cross-sectional area, similar to non-swirling flows, i.e., non-swirling and swirling flows have the same subsonic Mach number profile. The supersonic swirl number was found to experience a slight local increase within shock structure, where the axial momentum undergoes significant reduction, whereas the tangential one is preserved and experiences no change.