# Turbulent oscillating channel flow subjected to a free-surface stress

W. Kramer, H.J.H. Clercx, V. Armenio

The channel ﬂow subjected to a wind stress at the free surface and an oscillating pressure gradient is investigated using large-eddy simulations. The orientation of the surface stress is parallel with the oscillating pressure gradient and a purely pulsating mean ﬂow develops. The Reynolds number is typically Re$_\omega$ = 10^6 and the Keulegan–Carpenter number—the ratio between the oscillation period and advection time scale—is KC=80. Results compare favorably to the data from direct numerical simulations obtained over a single period. A slowly pulsating mean ﬂow occurs with the turbulent ﬂow essentially being statistically steady. Logarithmic boundary layers are present at both the bottom wall and the free surface. Turbulent streaks are observed in the bottom and free-surface layer. The viscous sublayer below the free surface is, however, much thinner. This observation is veri﬿ed by simulations we performed for a purely wind-driven channel ﬂow. For the oscillating ﬂow, additional low-speed splats localized regions of upwelling occur at the free surface when the mean velocity and stress are in the same direction.