LAUSR

Starting jets emanating from a straight nozzle and orifices of different orifice-to-tube diameter ratios are investigated using time-resolved particle image velocimetry. The invariants of the motion, namely, the circulation, the hydrodynamic impulse, and the kinetic energy, are measured and compared to the classic slug-flow model, and this for both fixed exhaust speed and fixed diameter-based Reynolds numbers. An extension to the slug-flow model is proposed to account for the contraction the fluid is experiencing when being pushed out through orifice geometries. The contraction coefficients obtained for two-dimensional jets formed through a slit in a channel are applied to the axisymmetric problem. This modified slug-flow model is shown to better predict the invariants of the motion with discrepancies of the order of 10% compared to underpredictions of 130%, 50%, and 120% for the circulation, the hydrodynamic impulse, and the kinetic energy, respectively, using the classic slug-flow model. Moreover, for a fixed target exhaust speed, the model suggests the existence of a maximum in the production of impulse and energy at an orifice-to-tube diameter ratio of about 0.9, which was also observed experimentally for the kinetic energy. Practically speaking, this suggests that the most efficient way of producing a starting jet is using an orifice plate of ratio close to 1, but different from a straight nozzle. Finally, the overpressure correction of Krueger [“The significance of vortex ring formation and nozzle exit overpressure to pulsatile jet propulsion,” Ph.D. thesis (California Institute of Technology, 2001)] is applied and revisited to account for the orifice-to-tube diameter ratio. Overall, good agreement with the present experimental data is found.