Drag and flow reversal in mixed convection past a heated sphere

Axisymmetric simulation of mixed convection past a heated sphere presented in a recent paper of Mograbi and Bar-Ziv [J. Aerosol Sci. 36, 387 (2005)] reveals a huge recirculation in the opposing flow configuration at very small Reynolds numbers and at Richardson numbers exceeding unity, the size of which seems to adjust to a computational domain of any size. In this paper we show that, for Reynolds numbers as low as one, the flow is nonaxisymmetric and that its three dimensionality brings the recirculation within finite bounds. At higher Reynolds numbers (Re), the cases of Re=10 and 100 are considered at a fixed Prandtl number of 0.72. The flow becomes very soon not only three dimensional but also chaotic. Especially at higher Reynolds numbers (100), the arising convective structures do not resist the opposing flow and do not travel upstream more than several sphere diameters.