Numerical investigation of flows with three-dimensional separation

When low aspect ratio geometries such as submarines, torpedoes, or missiles are operated at large angles of attack three-dimensional separation will occur on the leeward side. Separation increases losses and can result in undesirable unsteady forces. An improved understanding of three-dimensional separation is desirable as it may open the door to new methods for the control or prevention of separation. Numerical simulations of three-dimensional separation can provide detailed insight into instability mechanisms and the resultant flow structures. For most technical applications the Reynolds numbers are too high for direct numerical simulations and lower-fidelity approaches such as hybrid turbulence models become attractive. In this paper a new hybrid turbulence model blending strategy is proposed that adjusts the model contribution according to the local grid resolution. The strategy is validated for two-dimensional plane channel flow at Re_τ= 395 and for the Stanford asymmetric diffuser which features a turbulent three-dimensional separation. The model is then employed for simulations of a hemisphere-cylinder geometry at 10 and 30 degrees angle of attack. The simulations demonstrate satisfactory model performance over a wide range of Reynolds numbers (5×10³< Re_D< 5×10⁶). A nose separation bubble is captured for the lower Reynolds numbers and leeward vortices are observed for 30deg angle of attack regardless of Reynolds number. Different from, e.g., hemisphere-cylinder geometries asymmetric separation and roll-instability were reported for non-body-of-revolution geometries. The paper concludes with a brief discussion of simulations that were carried out for the Virginia Tech ellipsoid model at Re_L= 20, 000.