On the boundary layer structure over standing waves

The impact of standing waves on developing boundary layers was explored through a combination of laboratory experiments and direct numerical simulations. The experimental setup consisted of an open-circuit wind tunnel featuring a test section of 305 mm x 305 mm and a length of 1.3 m. Within this test section, an 800 mm flexible wall was equipped with eight sinusoidal standing waves controlled by an Arduino-operated mechanical system. It generated sinusoidal standing waves with a 5 mm amplitude and 100 mm wavelength varied at frequencies 0.5, 1, 2, and 4 Hz, under a freestream flow of 1.2 m/s. Direct Numerical Simulations (DNS) replicating the setup were performed using fast and scalable CFD software nekRS. Scenarios with overlapping and complementary wall frequencies, including 3, 4, 4.5, 6, and 12 Hz, were also explored to validate and extend the study. Phase-locked and high-frame-rate flow measurements and numerical simulations revealed distinct spatiotemporal modulation of the wall motion, significantly impacting the turbulence structure. Specifically, the compensated velocity spectra demonstrated large changes in the flow structure at locations coinciding with the positions of the wall nodes and the points of maximum vertical variation. The results also indicate a substantial modulation of near-wall turbulent stress and memory-related behavior.