Standing-wave wall forcing modulates boundary-layer transition

Laboratory experiments and direct numerical simulations (DNSs) were conducted to examine boundary-layer transition over a standing-wave wall composed of eight sinusoidal waves of wavelength lamda equals 100 λ = 100 λ =100 mm and amplitude a equals 5 a = 5 $a=5$ mm, placed downstream of a short flat-plate leading edge in a low-turbulence free stream. The DNSs reproduced two lower-frequency experimental cases for validation and extended the investigation to higher frequencies at an amplitude-based Reynolds number italic Re Subscript a Baseline equals 388 Re a = 388 Reₐ=388 , showing strong agreement in overlapping cases. The forcing is characterised by the reduced frequency italic St equals f Subscript w Baseline lamda divided by upper U Subscript c italic comma m Baseline St = f w λ / U c , m St=fwλ /Uc,m , where f Subscript w f w fw is the wall oscillation frequency, upper U Subscript c italic comma m U c , m Uc,m is the mean streamwise convection velocity, which represents the number of wall-oscillation cycles during the mean convection of a coherent structure over one wall wavelength. The transition location shifts downstream on average as italic St