The effect of riblets on laminar to turbulent transition

Experiments conducted on the effect of riblets on the laminar-to-turbulent transition of a flat plate in a water tunnel are reported. Transition was determined using a Laser Doppler Velocimeter (LDV). A smooth reference surface was compared to five riblet surfaces for a range of Reynolds numbers. Smooth surface transition Reynolds number was about 2.75 × 10⁶. All of the five tested riblet surfaces had lower transition Reynolds numbers. A critical roughness Reynolds number of about 6 was determined for one of the riblet surfaces. This is much lower than the generally accepted value of 25, considered safe for distributed roughness.