Direct numerical simulations of turbulent flow over mean-flow-misaligned waves *

Wind-wave misalignment is a phenomenon that occurs when waves are moving in a direction different than that of the local wind. The combination of light winds and fast-propagating waves (swell) prevails in global oceans, especially in tropical oceans such as the warm-pool area in the Pacific Ocean and has been a topic of active research for more than five decades. Better understanding of offshore wind dynamics is also key to creating optimized offshore wind farms. In this study we conduct direct numerical simulations of wind-wave misalignment at a low Reynolds number (Re=180) by considering a turbulent half-channel flow driven by a combined streamwise and spanwise pressure-gradient forcing and over idealized sinusoidal waves. For our simulations we consider wave ages corresponding to slow-, intermediate-, and fast-moving waves, two wave-steepness levels (mild and steep waves) and six different misalignment angles, from perfectly aligned wind-following waves to perfectly aligned but wind-opposing waves. The simulations are undertaken using the open-source-code Nalu-Wind from the ExaWind stack and the results show that the mean-velocity, velocity-variance, and momentum-flux vertical profiles are greatly modulated by the wave-age and wave-steepness parameters. Slow-moving waves are found to increase the wall roughness and behave like stationary bumps, whereas fast-moving waves reduce the wall roughness and act more like a partial-slip wall. Finally, the applied misalignment angle appears to affect both the first- and second-order statistics. Their effect is more pronounced for faster-moving waves and become prominent when considering perfectly aligned, wind-opposing waves.