Investigation of asymmetric flow past a slender body at high angles of attack

Abstract

This paper presents an investigation of flow asymmetry around a slender body at high angles of attack. The paper investigated the numerical aspect of simulating such flows. The impact of three simulation parameters, including grid resolution, discretization scheme, and turbulent flow modeling, was assessed. It was shown that insufficient grid density resulted in highly dissipated solution. At high angles, where flow asymmetry is expected to develop around the body, the dissipation from poor grid resolution prevented the flow asymmetry. At higher grid resolution, the solution demonstrated a switch between two bistable states. Two spatial discretization schemes, namely central and bounded, were tested in the course of this study. The results illustrated the necessity to use non-dissipative unbiased discretization schemes. Large eddy simulation was performed using two sub-grid-scale models in addition to a run without a model. The sub-grid-scale models generated similar results except for switching of asymmetry direction and the axial location of separation foci. The study shows that grid resolution and solution scheme have a profound effect on the validity of the simulation of flow around slender bodies at high angles of attack. The study also showed that stringent grid requirements marginalized the effect of the sub-grid-scale model. Computations were then carried out at seven angles of attack $$\alpha = 30^{\circ }$$ , $$40^\circ $$ , $$50^\circ $$ , $$52.5^\circ $$ , $$55^\circ $$ , $$57.5^\circ $$ , and $$60^\circ $$ . Analysis was performed on mean and unsteady flow fields. The total normal force increased with increasing angle of attack. On the other hand, the total side force started to increase rapidly for angles of attack $$\alpha > 50^{\circ }$$ and reached a maximum at $$\alpha =57.5^{\circ }$$ before decreasing at $$\alpha =60^{\circ }$$ . A bistable mode was observed for $$\alpha > 50^{\circ }$$ in which the orientation of resultant forces switches with angle of attack. Comparison of computed dominant frequencies with experiment showed an acceptable agreement. Several dominant modes were identified: very low-frequency mode, low-frequency mode, intermediate-frequency mode, and high-frequency mode. The modes were shown to develop with increasing angle of attack. Surface flow pathlines revealed the existence of separation foci at $$\alpha =57.5^{\circ }$$ and $$60^{\circ }$$ , and a high-frequency tonal mode was observed to accompany the formation of separation foci.