| Popis: |
In this work, we characterize the early stages of transition in a flat-plate boundary layer caused by two types of surface roughness: distributed roughness (a strip of 24-grit emery) and isolated roughness (a cylindrical element). Towards this, we carry out extensive particle image velocimetry (PIV) measurements. The distributed roughness consists of a range of grit heights and introduces a broad range of disturbances downstream of it. In the pre-transitional region, both isolated and distributed roughness cause steady streaks, which for the latter case show a lack of spanwise symmetry. This is in contrast to the freestream turbulence (FST), which introduces unsteady streaks in the pre-transitional boundary layer. We propose tilting of spanwise vortices downstream of the distributed roughness to be the mechanism for generation of streamwise vortices, which are likely precursors to the onset of transition. For both the roughness configurations, the steady streaks develop instability resulting in localized streak breakdowns. The wall-normal PIV visualizations show streak instability features qualitatively similar to those for the FST-induced transition. For the distributed roughness, both the outer instability of lifted-up streaks and the inner instability due to streak interaction are present, whereas for the isolated roughness, the inner instability is dominant while the outer instability is much weaker. Conditional sampling of fluctuating velocity shows an asymmetry in the positive and negative fluctuations after the onset of transition, in a manner similar to that observed for the FST-induced transition. These observations suggest that the wall-normal distribution of unsteady velocity fluctuations in the early stages of transition is qualitatively similar irrespective of the source of disturbance (roughness/FST) causing transition. We expect that this commonality of features among different types of transition will be helpful in modelling flow over aerodynamic surfaces such as turbine blades and aircraft wings. |
