Suppression of pressure fluctuation and turbulent friction at the wall in spatially developing turbulent boundary layers via streamwise-traveling waves and wall oscillations

Turbulent boundary layers generate large wall pressure fluctuations and high skin friction drag, which in turn raise aerodynamic and acoustic penalties for aircraft, ships, and wind-energy devices. Active near-wall flow control techniques that target the dominant vortex structures have therefore attracted growing attention as a practical route to simultaneous noise and drag reduction. The impact of flow controls on zero-pressure gradient turbulent boundary layer flow at high Reynolds number was studied using Direct Numerical Simulation to investigate their influence on the altered vortex features and the resulting wall pressure fluctuations. Four distinct boundary layer flow control strategies, blowing (BW), suction (SC), spanwise wall oscillation (SWO), and streamwise-traveling wave with wall oscillation (STWO), were applied to examine their effects on enstrophy, vorticity, wall pressure, and their respective correlations. Turbulent statistics revealed that SC increased skin friction due to enhanced turbulence near the wall, while reductions were seen in BW, SWO, and STWO, with STWO showing the most significant decrease. Analysis of premultiplied energy spectra of the pressure fluctuations in SWO and STWO indicated notable suppression of wall pressure fluctuations and the associated vortex energies generated by the near-wall quasi-streamwise vortices. The correlation between wall pressure fluctuations and streamwise vorticity highlighted the effectiveness of these control methods, particularly with STWO, in redistributing energy and attenuating turbulence structures responsible for wall pressure fluctuations. Flow visualization and enstrophy analysis further confirmed that the control strategies substantially altered vortex structures, demonstrating the potential of SWO and STWO as effective approaches for reducing wall pressure fluctuations and drag mitigation.