Influence of Varying Freestream Turbulence on Flow Transition Over Distributed Surface Roughness

High-fidelity Large Eddy Simulations (LES) are used to investigate the influence of distributed surface roughness on the spatially developing laminar boundary layer under varying freestream turbulence (FST) levels of 0.07 to 6%. The inlet Reynolds number based on the momentum thickness and freestream velocity is 360. The square-shaped (SQ) riblets that are employed in the present study are homogeneous in the spanwise direction and distributed in a series along the streamwise length. The aspect ratio (s/k) considered here is 3, where k is the riblet height, and s is the longitudinal spacing between the consecutive rows of riblets. The instantaneous flow structures reveal a Λ vortex-induced breakdown for FST levels below 3.2%, attributed to the near wall streamwise vortices arising from embedded recirculation regions between riblets. On the contrary, at a high FST level of 6%, the flow is characterized by low- and high-speed streaks generated via shear sheltering, resulting in the turbulent flow downstream. Moreover, an exponential growth of velocity fluctuations is apparent for a lower FST, which becomes algebraic for a higher FST. The onset and end of transition shift progressively upstream with an increase in FST. Interestingly, the flow features exhibit self-similarity in the turbulent region irrespective of FST levels.