Large-eddy simulation of tripping effects on the flow over a 6 : 1 prolate spheroid at angle of attack

Abstract

Large-eddy simulation is used to simulate the flow around a 6 : 1 prolate spheroid at $10^\circ$ and $20^\circ$ angles of attack, and Reynolds number $4.2 \times 10^6$ . Flows with and without trip are compared to understand the relative effect of the trip on the state of the boundary layer and separation. For the tripped case, the geometry of the trip is resolved to better predict its effect on the downstream flow. The simulations employ overset grids that allow adequate resolution of the trips without significant increase in the overall computational cost. Results suggest that while the trip accelerates transition to turbulence at $10^\circ$ , it does not induce a fully developed turbulent boundary layer as intended at $20^\circ$ . Rather, the influence of the trip is localized, and the near-wall flow converges towards a solution similar to that of the non-tripped case upstream of separation. This is due to two distinct phenomena: directly downstream of the trip, favourable pressure gradient and streamline curvature effects suppress the disturbance on the windward side. Further along the spheroid, the boundary layer receives a small fraction of the initial perturbation due to spanwise and wall-normal streamline curvatures inducing a secondary flow that advects the low-momentum trip wake to the leeward side. The locations of transition and separation are insensitive to the presence of the trip. The simulation results are used to construct a regime map that identifies different regions characterized by distinct boundary layer properties and flow features. The present results underscore the difficulty associated with tripping smooth bodies at angle of attack, and the importance of accounting for transition in simulations of such flows, even on tripped geometries.