The propulsive characteristics of starting jets issued from tube (θ=0°), converging (0°θ90°), and orifice (θ=90°) nozzles with inlet-to-exit diameter ratios of 1≤RD≤3 are numerically investigated, where θ denotes the contraction angle. For a given RD, the total jet impulse IT increases monotonically with θ. At a fixed θ, IT initially increases with RD and then approaches a plateau, which is reached at smaller RD as θ decreases. The unsteady evolution of IT is governed by wake development associated with different θ, which alters the pressure field near nozzle exit via the leading vortex ring during the initial constant velocity stage as well as by the stopping vortex ring after jet termination. The magnitude of wall thrust generated by the pressure distribution on the nozzle outer wall increases with θ mainly by enhancing the axial projection of pressure. The enhancement of IT generation is attributed to flow contraction within converging and orifice nozzles. Achieving the maximum contraction state requires a sufficiently large RD, which is constrained by θ. Furthermore, increasing θ beyond that of orifice nozzle with θ=90° could further enhance flow contraction and promote impulse generation while being accompanied by the growth of the ineffective corner stagnation zone.
Zhu et al. (Thu,) studied this question.