Mechanisms of boundary layer transition induced by isolated roughnes

Boundary layer transition is a relevant phenomenon in many aerodynamic and aero-thermodynamic problems and has been extensively investigated from the past century till recent times. Among the factors affecting the transition process, surface roughness plays a key role. When a roughness element with sufficiently large height (h) compared to the boundary layer thickness (δ) is immersed in a laminar boundary layer, it will produce spanwise varying disturbances with the potential to accelerate the transition process. In the thesis, a fundamental study is carried out to understand the physical mechanism of isolated roughness element induced transition. Experiments are performed in incompressible flow regime covering both critical and supercritical conditions. Tomographic particle image velocimetry (PIV) is employed as the main experimental diagnostic technique, returning the three-dimensional velocity and vorticity field of the flow.

The three-dimensional wake flow behaviour is firstly identified behind roughness element of micro-ramp geometry. The micro-ramp produces a pair of counter-rotating streamwise vortices in the wake, transporting low momentum fluid away from the wall by the central upwash motion, and sweeping the high momentum flow towards the near-wall region sideward. The shear layer around the central low-speed region is related to the growth of Kelvin-Helmholtz (K-H) instability. The active range of the primary vortices and the central low-speed region in the streamwise direction is associated to the selection of the dominant instability mechanism, which decreases with the increase of roughness-height based Reynolds number (Re_h). 
The instantaneous flow field reveals that the earliest unstable structures featuring hairpin shape are caused by the K-H instability at the separated shear layer. The evolution of K-H vortices is strongly influenced by Re_h. At Re_h = 1170, the K-H vortices are lift up under the upwash motion effect of the quasi-streamwise vortices, following by paring, distortion and finally breakdown. The active region of K-H vortices is separated from the inception of turbulent wedge, where early stage transition occurs. When Re_h decreases approaching the critical value, the K-H vortices progressed gradually until the overall shear layer is destabilized, indicating the correlation between K-H instability and transition. The POD analysis yields the symmetric (K-H) and asymmetric mode. The disturbance energy associated to the symmetric modes changes with Re_h. At higher Re_h, the disturbance energy of the symmetric modes quickly decays, having a comparable contribution as the asymmetric modes. When Re_h < 1000, the symmetric modes produce a remarkably higher level of disturbance energy until the onset of transition, indicating its dominance. 
The effectiveness of roughness element on promoting transition is strongly influenced by its geometry. The bluff-front roughness elements induce horseshoe vortices due to upstream separation. The different rotation direction of these vortices compared to the micro-ramp leads to early inception of sideward growth of fluctuations, and more rapid transition process. While for the slender micro-ramp, significant longer distance is required to for the onset of transition.