The drag crisis is an interesting physical phenomenon for flow over bluff bodies, where the drag coefficient suddenly drops as the Reynolds number increases. This behavior is caused by the flow transitioning from laminar to turbulent flow in the boundary layer. The turbulent flow remains attached to the surface longer than the laminar flow, thereby reducing the size of the wake behind the body. The phenomenon has been extensively studied experimentally for simple geometries such as flow over a sphere or a cylinder. Numerical simulations, however, have mainly been performed in either the subcritical or the supercritical region. In this study, we investigate the ability of steady-state RANS CFD models to predict the drag crisis for flow around a sphere. Experiments using oil-film visualization are performed to determine the type of transition for flow over a sphere. Simulations are performed using OpenFOAM, with the k-ω shear stress transport (SST) turbulence model. For transition modelling, the Langtry-Menter modification to the SST model is considered. The simulations show improved prediction of the drag coefficient in the subcritical regime, but the implementation of the model is found to be unstable in the critical and supercritical range.
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