Near-wall turbulence and transitional behavior on the rib-roughened surfaces

This study utilizes Large-Eddy Simulation to investigate the impact of longitudinal triangular riblets on the laminar-to-turbulent transition in the 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 the 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) (inner-scaled dimensions are 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, though increasing riblet spacing (S+=75) delays the transition by 17.5%. Both cases exhibited increased drag compared to the flat plate. For h+=12.5 and w+=S+=25, transition was delayed by 37%, with a modest shear-drag reduction of 8.8%. The most significant result from considered cases, h+=w+=S+=12.5, showed a 47% delay in transition and a 13.69% reduction in shear 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.