Geometry effect of isolated roughness on boundary layer transition investigated by tomographic PIV

- The transition of boundary layer induced by isolated roughness elements is analyzed by using tomographic PIV.
- Roughness geometry greatly modifies the wake flow topology and the evolution process towards transition.
- The bluff front elements induce a horseshoe vortex, leading to more rapid transition than the slender micro-ramp.
- The lateral spreading of the turbulent wedge is caused by a destabilizing mechanism involving convection and regeneration of hairpin vortices at the turbulent-non-turbulent interface.

Boundary layer transition over isolated roughness elements is investigated in the incompressible flow regime using tomographic PIV. Four different geometries (cylinder, square, hemisphere and micro-ramp) are considered maintaining constant height and span of the element. The main target is to compare the different flow topologies and study the effect of the element shape on accelerating boundary layer transition. The measurement domain encompasses the full transition process until the turbulent regime is established. The flow behavior is described by means of vortex topology and by statistical analysis of the velocity fluctuations. The instantaneous flow topology elucidates the mechanism of transition along its stages. A main distinction is observed between the bluff front elements that induce a horseshoe vortex due to upstream flow separation, leading to more rapid transition and the slender micro-ramp. The later geometry requires significant longer distance for transition onset. The mechanism of sideward propagation of the turbulent non-turbulent interface features a continuous convection and generation of hairpin-like vortices and remains the common denominator among all elements considered.