On Reynolds number dependence of micro-ramp-induced transition

The variation of transitional flow features past a micro-ramp is investigated when the Reynolds number is decreased approaching the critical regime. Experiments are conducted in the incompressible flow spanning from supercritical to subcritical roughness-height-based Reynolds number (Re-h = 1170, 730, 460 and 320) with tomographic particle image velocimetry. The effect of Reh on three-dimensional flow behaviour is analysed in a domain encompassing 73 ramp heights in the streamwise direction. Above the critical Reh, the primary vortex pair and induced central low-speed region in the mean flow field are active over longer range when decreasing Reh. In the instantaneous flow, at Re-h < 1000, the hairpin vortices induced by Kelvin-Helmholtz (K-H) instability progress gradually from close to the micro-ramp into the region where the overall shear layer is destabilized, indicating the correlation between the K-H instability and the onset of transition. The breakdown of K-H vortices as observed at Re-h = 1170, does not occur at lower Re-h. Decreasing Reh, the secondary vortex structures make their first appearance significantly downstream, postponing the formation of sideward disturbances, which destabilize the local shear layer by ejection events. Two major types of eigenmodes with symmetric and asymmetric spatial distribution of velocity fluctuations in the near wake are clearly identified by proper orthogonal decomposition. The symmetric and asymmetric modes correspond to the presence of vortex shedding and a sinuous wiggling motion respectively. It is found that Reh is the key factor determining the importance of the symmetric mode. At Re-h = 1170, the disturbance energy of the symmetric mode decays before the onset of transition, suggesting that it is relatively insignificant in the process. However, decreasing Re-h to 730 and 460, the symmetric mode produces continuous growth of high level disturbance energy, leading to transition.