Evaluation of Vortex Generators for Separation Control in a Transcritical Cylinder Flow

A numerical study is presented, aimed at evaluating the capability of vortex generators to produce a significant delay of separation and drag reduction in flows past smooth bluff bodies in the transcritical flow regime, with turbulent boundary layers ahead of separation. A fairly large series of steady Reynolds-averaged Navier-Stokes computations is carried out of the incompressible flow past a circular cylinder, with a simple and generic geometry, with and without vortex generators. Their effect is emulated with a body force, applied in a refined-grid region, instead of gridding the actual geometry. The flow is treated as steady, for reasons that are discussed. The parametric study includes 10 different vortex generator designs, as well as varying positions, spacings, and heights. Drag reductions of up to 60% are obtained, including those with sub-boundary-layer designs, with a height equal to half the boundary-layer thickness. Design recommendations are given, with the tolerance to vortex shedding and other variations in the flow conditions taken into account.