Fluid Redistribution in the Turbulent Boundary Layer Under the Microramp Control

Abstract

Recent studies on boundary-layer control with microvortex generators suggest that the microramp should not be simply treated as a traditional streamwise vortex-inducing device. Here, the wake organization downstream of the microramp and its function of fluid redistribution are investigated with a large-eddy simulation at Mach 2.7. It is confirmed by passive scalar convection and streamline tracing that the fluid composing the wake is basically collected from a thin layer close to the wall, particularly beneath 20% of the boundary-layer thickness here, which is responsible for the momentum deficit and low-density features of the wake, as well as the later complex wake evolution. Thus, the microramp has actually worked as a novel near-wall fluid collector, depriving the inner part of the incoming boundary layer, and assembles it into the wake. When developing downstream, the shear induced Kelvin–Helmholtz vortex grows and pairs quickly upon the wake, giving birth to a train of large-scale hairpin vortices with a typical frequency of . The similarity between the local density and passive scalar distribution patterns suggests the advection of the collected low-momentum fluid along with the large-scale vortices is the main reason for the intermittent wake structures presented in the previous studies.