Transitional and Near-Wall Turbulence Dynamics over Rib-Roughened Surfaces

This study utilizes Large Eddy Simulation (LES) to investigate the impact of longitudinal triangular riblets on the laminar-to-turbulent transition in boundary layer flow. Five cases are examined: one involving a flat plate and four with ribbed plates. Among the ribbed cases, three use a riblet aspect ratio of two, whereas one has an aspect ratio of one. Arrays of longitudinal triangular riblets are positioned on a flat plate, and the transition to turbulence is initiated by controlled excitation of a Tollmien-Schlichting (TS) wave imposed on a Blasius velocity profile in a stable region. The longitudinal triangular riblets attenuate the TS wave, leading to a lower growth rate of turbulence. For higher riblet height ($h$) and width ($w$), with inner-scaled dimensions $h^+ = 25$, $w^+ = S^+ = 50$ (where $S$ is the spacing between two riblets), an early transition is triggered by high-frequency disturbances generated at the leading edge of the roughness elements. However, increasing riblet spacing to $S^+ = 75$ delays the transition by 17.5 percent. Both cases exhibited increased drag compared to the flat plate. For $h^+ = 12.5$ and $w^+ = S^+ = 25$, transition was delayed by 37 percent, with a modest overall drag reduction of 8.8 percent. The most significant result from the considered cases, $h^+ = w^+ = S^+ = 12.5$, showed a 47 percent delay in transition and a 13.69 percent reduction in overall drag. Smaller riblets cause minimal disturbance at the leading edge of roughness, resulting in a transition mechanism similar to a flat plate, while also reducing pressure loss, secondary flows, and velocity fluctuations.