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.

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