  It has been widely accepted that nozzle exit over-pressure is responsible for
thrust augmentation in starting jets over steady jets and a few pulsed jet
propulsion systems have been developed based on this finding. However, no other
study has been conducted to confirm the nozzle exit over-pressure effect. In
this paper, thrust and nozzle exit pressure are numerically investigated under
different jet velocity programs (negative slope (NS), positive slope (PS) and
impulsive). Starting jets are generated for fluid slug length to diameter (L/D)
ratios of 2-5 at each jet velocity program. In contradiction to the findings
presented in [P. Krueger and M. Gharib, J. Phys. Fluids, 15, 1271 (2003)] the
results in this paper reveal that the time-averaged thrust due to nozzle exit
pressure in starting jets from a tube nozzle is negligible or negative
resulting in no thrust augmentation when compared to steady jets with
hypothetical identical velocity programs. Also, nozzle exit experiences both
over-pressure and under-pressure in accelerating and decelerating portions of
jet velocity programs respectively in both NS and PS velocity programs.
Furthermore, it is found that secondary counter-rotating vortex is generated
during decelerating portion of jet velocity programs which may be linked to the
nozzle exit underpressure explaining the reduced thrust during starting jets
generated by different velocity programs.
