When wind blows over the surface of a viscous liquid, a clear transition from irregular small-amplitude streamwise-oriented wrinkles to well-defined nearly two-dimensional regular waves is observed at a critical wind velocity. We examine how free-stream turbulence in the air influences the growth of wrinkles and regular waves, as well as the transition between these two regimes. Experiments are carried out in a wind tunnel, in which air is blown over a tank filled with silicone oil whose viscosity is 50 times higher than that of water. The free-stream turbulence is enhanced using upstream grids, achieving relative turbulence intensities up to $8\%$. Surface deformations are measured using free-surface synthetic Schlieren with micrometer accuracy. Velocity measurements are performed using hot-wire anemometry above the interface and particle image velocimetry in the liquid. Results reveal two primary effects of grid-enhanced free-stream turbulence: an increase in the wrinkle amplitude and a reduction in the critical wind speed at the onset of regular waves. Nevertheless, the wrinkle-wave transition still corresponds to an approximately constant friction velocity. We also observe a nonmonotonic evolution of the wave amplitude with fetch, which we rationalize using a wave energy balance model that accounts for the downstream decrease in friction velocity.

Close

PrevNext

View figure in article

• Shear flows
• Turbulence
• Waves and free surface flows