Effects of discharge nozzle geometry on the propulsive characteristics of a round-headed axisymmetric body

The effects of discharge nozzle geometry on the impulse generation of starting jets have been investigated using a round-headed axisymmetric body. Six nozzle geometries are designed by varying the contraction angle θ and the contraction profile. In the absence of background co-flow, adjusting the nozzle geometry increases the total impulse IT by more than 40%. This is mainly achieved through the increased pressure-impulse generation induced by internal flow contraction in the converging section. Streamline analysis reveals that the nozzle geometry near the exit plays a dominant role in governing the efficiency of flow contraction. When the body is immersed in background co-flow without jet ejection, the reduction in pressure drag caused by weakened wake vortices is insufficient to compensate for the additional friction drag introduced by longer converging sections at smaller θ[jls-end-space/]. Therefore, nozzles with larger θ exhibit smaller total drag. With pulsed jets activated under co-flow, the pressure field governing pressure drag becomes dominated by the stronger leading vortex rings. Converging nozzles with larger θ[jls-end-space/], which generate larger impulses, also experience higher total drag. The co-flow does not affect the enhancement of impulse generation induced by internal flow contraction, and this enhancement substantially exceeds the associated increase in drag.