Influence of riblet geometry and freestream turbulence on boundary layer transition

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High-fidelity large eddy simulations are used to investigate the influence of distributed surface-mounted riblets on spatially developing laminar boundary layer under varying freestream turbulence (FST). The spanwise homogenous riblets are employed in the study, with varying cross-sectional shapes: Square and semi-circular represented as SQ and SC, respectively. The inlet Reynolds number based on momentum thickness and freestream velocity is 360. The flow features exhibit considerable differences between the two riblet geometries under the influence of FST. Instantaneous results reveal the development of Λ-vortices as a genesis of flow transition over SQ-riblets across all the FST levels. While the streamwise streaks are evident over SC-riblets, where spot-like perturbations are observed at an FST of 6.0%. Here, the streaks are susceptible to sinuous secondary instability with subsequent breakdown into small-scale eddies. Thus, the novelty of this study lies in the identification of the modes of transition and flow structures, altered substantially by the riblet geometry and flow environments. Moreover, the turbulent kinetic energy budget is discussed, where an imbalance between production and dissipation is apparent in the transitional region, while a balance between them is established in the equilibrium turbulent flow.