Propulsive characteristics of single-pulsed jets with tube and orifice openings

Abstract

The effects of the nozzle exit geometry on the unsteady propulsive characteristics of single-pulsed jets are studied numerically. For both tube and orifice nozzles, the jet exit configuration is parameterized by the diameter ratio RD, which is defined as the ratio of the nozzle entrance D0 to the jet exit diameters D. It is found that the diameter ratio has significant influence on the propulsive characteristics of the single-pulsed jet during its entire ejection phase. The total impulse production is augmented considerably as the diameter ratio increases until a critical value of R D _ cir ≈ 2.0 is approached. The larger impulse production by the orifice nozzles over the tube nozzle stems from the persistent over-pressure contribution at the jet exit due largely to the fact that the flow contraction near the jet exit of the orifice nozzle results in the intensification of the radial velocity gradients and higher local pressure. By using the existing prediction of the contraction coefficient Cc to account for the flow contraction, a theoretical model has been developed with the quasi-one-dimensional flow approximation to predict the pressure thrust at the jet exit during the steady discharging stage, showing good agreement with the present numerical results. Moreover, the pressure force acting on the vertical wall of the orifice nozzle, which is proportional to the wall area, is found to be primarily responsible for the larger transient variations in the jet impulse during the onset and end of the jet ejection phase as the diameter ratio increases.