Effects of contraction angle on the propulsive characteristics of starting jets from converging nozzles

The propulsive characteristics of starting jets issued from tube (θ = 0 °), converging (0 ° < θ < 90 °), and orifice (θ = 90 °) nozzles with inletto-exit diameter ratios of 1 ≤ R D ≤ 3 are numerically investigated, where θ denotes the contraction angle. For a given R D, the total jet impulse I T increases monotonically with θ. At a fixed θ, I T initially increases with R D and then approaches a plateau, which is reached at smaller R D as θ decreases. The unsteady evolution of I T 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 I T generation is attributed to flow contraction within converging and orifice nozzles. Achieving the maximum contraction state requires a sufficiently large R D, 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.