A computational investigation was carried out on low Reynolds number laminar inflow starting annular jets using multiple blocking ratios and atmospheric ambient conditions. The jet exit velocity conditions are imposed as steady, unit pulsed, and sinusoidal pulsed while the jet surroundings and the far-field jet inlet upstream conditions are left atmospheric. The reason is to examine the flow behavior in and around the jet inlet under these conditions. The pulsation mode behavior is analyzed based on the resultant of the momentum and pressure forces at the entry of the annulus, the circulation and vortex formation, and the propulsion efficiency of the inflow jets. The results show that under certain conditions, the net force of inflow jets (sinusoidal pulsed jets in particular) could point opposite to the flow direction due to the adverse pressure drops in the flow. The propulsion efficiency is also found to increase with pulsation frequency and the sinusoidal pulsed inflow jets are more efficient than the unit pulsed inflow jets. In addition, steady inflow jets did not trigger the formation of vortices, while unit and sinusoidal pulsed inflow jets triggered the formation of vortices under a certain range of frequencies.

The effects of the blocking ratio and atmospheric altitude on starting annular air jets at a low Reynolds number are investigated by examining the velocity profiles, jet entrainment and vortex formation. A 2D axisymmetric numerical model is developed to perform the analysis. The numerical model is validated with theoretical and experimental results from other studies. In order to achieve a comprehensive analysis, the annular jet is tested for blocking ratios with the values of 0.00 (i.e. a round jet), 0.50 and 0.75. Air properties at altitudes of 0.00 km, 18.90 km and 33.75 km are tested to simulate atmospheric environments at sea level, edge of near space and near space, respectively. The results showed that the starting jet velocity profile, entrainment and vortex formation is a strong function of blocking ratio. On the other hand, the same parameters are shown to be nearly independent, strong and weak functions of atmospheric altitude, respectively.