The response of a turbulent boundary layer to a sudden ridge-type roughness array with a sinusoidal pattern

An experimental investigation was conducted to analyze the impact of a sinusoidal ridge-type roughness patch on the response of a moderate Reynolds number turbulent boundary layer (friction Reynolds number $R{e}_{\tau }<1100$). A streamwise sinusoidal pattern with an amplitude denoted as $A$ and a wavelength $\Lambda$ was imposed on the ridges while maintaining fixed the spanwise spacing between the ridges, $S$, smaller than the boundary layer thickness, ${\delta }_{.99}$ ($S/{\delta }_{.99}<1$). Two sinusoidal ridges are tested plus the standard straight configuration; the wavelengths being $2.6{\delta }_{.99}$ and $1.3{\delta }_{.99}$. Oil droplet interferometric measurement conducted downstream the ridges, in the recessed part, revealed a local reduction in skin friction for the sinusoidal ridged configurations. Hot-wire anemometry was employed to measure the boundary layer on the smooth wall downstream of the roughness array. The measurements revealed a modification of the mean velocity profile and an increase in the shape factor. Significant modifications in spectral content between configurations with aligned and sinusoidal ridge-type roughness are shown. The presence of ridges caused a noticeable upward shift of energy and the emergence of an outer peak in the contour of the premultiplied energy spectrogram. The outer peak is located at a wall normal distance in wall units between 80 and 180 depending on the Reynolds number. Interestingly, configurations with sinusoidal patterns exhibited a more pronounced upward shift of energy in the premultiplied spectra. The energy associated with the outer peak for the configuration having shorter wavelength is doubled compared with standard straight ridges. These findings suggest that the response of the turbulent boundary layer is influenced not solely by the spanwise spacing of ridges but also by the waviness of the ridges, which further contributes to its intensification.