Exploring flow transition induced by surface-mounted riblets using large eddy simulations

A well-resolved large eddy simulation is employed to study the transition of spatially developing boundary layer when excited by distributed blade-riblets that are arranged in different configurations: one is two-dimensional spanwise riblets (denoted as R1), and the other being three-dimensional herringbone pattern of alternating converging and diverging riblets (denoted as R2). The inlet Reynolds number based on momentum thickness and freestream velocity is 360. The flow characteristics vary considerably between the two configurations. A transition delay is observed over R1-riblets owing to the recirculation bubbles occupying the entire cavity space between the consecutive riblets. The appearance of T-S waves over R1-riblets is manifested through the modal analysis, which, in concurrence with streaks, leads to the development of Lambda-vortices and, thereby, exhibiting characteristics associated with the mixed-mode transition. Contradictorily, the T-S waves are bypassed when subjected to R2-riblets attributed to enhanced spanwise disturbances. The roughness-induced streaks destabilize via a lift-up mechanism and interact with background disturbances, leading to flow transition. Post-transition conditions for self-similarity are met for both configurations, albeit earlier for R2-riblets.