On Reynolds Number Dependence of Micro-ramp-Induced Transition

The transitional flow features past a micro-ramp are investigated in the incompressible flow regime at critical and supercritical roughness-height based Reynolds number (Reh). Tomographic PIV is adopted as the diagnostic technique to identify the instantaneous three-dimensional flow behaviour in the domain of 73 ramp height streamwise. The instantaneous and time-averaged flow topology are characterized in detail focusing on the influence of Reynolds number. A main difference is the longer streamwise presence range of primary vortex pair in the time-averaged flow topology at critical Reh compared with supercritical condition, resulting in different type of modification to the mean flow. The transition process is significantly delayed when decreasing Reh. By performing Proper Orthogonal Decomposition (POD) analysis, two major types of eigenmodes with symmetric and asymmetric spatial distribution of velocity fluctuations in the near wake are observed, corresponding to the presence vortex shedding and sinuous wiggling motion respectively. The correlation between the former POD modes and the transition process strongly depends on Reh.