DOI: 10.1615/TSFP9
GEOMETRY EFFECT OF ISOLATED ROUGHNESS ON BOUNDARY LAYER TRANSITION INVESTIGATED BY TOMOGRAPHIC PIV
SINOPSIS
Transitional flow over isolated roughness elements is investigated in the incompressible flow regime using Tomographic PIV. Three different geometries are considered (micro-ramp, cylinder and square) with same height and span. Their effect on accelerating boundary layer transition is compared and discussed. The measurement domain encompasses the full transition process and the flow development until the turbulent regime is established. The mean flow topology reveals a single pair of streamwise vortices for the micro-ramp as opposed to the additional pair associated to the horseshoe vortex around cylindrical and square elements. The statistical analysis of velocity fluctuations indicates a transition process induced from the point where the streamwise vortices induce an inflexional velocity profile. The cascade progresses downstream with a localized fluctuations increase at the turbulent-non-turbulent interface. The instantaneous flow topology contributes in explaining the transition mechanism, which appears to be dominated by hairpin-like vortices concentrating at the laminar-turbulent interface.