Effect of micron-sized roughness and sound receptivity on a swept wing boundary-layer transition

The effect of micron-sized roughness in the leading edge region on a swept wing boundary-layer transition is examined experimentally in detail. By applying sound waves from a loud speaker, sound receptivity to turbulent transition is also investigated. Results showed that such a minute roughness element (k=45 μm in height, 1 mm in diameter, k/δ=0.23) placed at x=40 mm resulted in early transition. In the case that no transition enhancement was observed in the presence of a single roughness element, early transition also occurred with sound excitation. Low frequency disturbance f₁ responded strongly to the sound frequency, in particular, to the direction of the sound waves. Critical spacing of roughness elements where the transition is amplified the most seems to exist in the range of λ/δ = 1.3. It is also noted that no enhancement of transition occurs when the roughness element is placed beyond 20% of the chord length.