Fluid Redistribution in the Turbulent Boundary Layer Under the Microramp Control

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 St = 0.57. 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.