  We study the propagation of monochromatic surface waves on a turbulent flow.
The flow is generated in a layer of liquid metal by an electromagnetic forcing.
This forcing creates a quasi two-dimensional (2D) turbulence with strong
vertical vorticity. The turbulent flow contains much more energy than the
surface waves. In order to focus on the surface wave, the deformations induced
by the turbulent flow are removed. This is done by performing a coherent phase
averaging. For wavelengths smaller than the forcing lengthscale, we observe a
significant increase of the wavelength of the propagating wave that has not
been reported before. We suggest that it can be explained by the random
deflection of the wave induced by the velocity gradient of the turbulent flow.
Under this assumption, the wavelength shift is an estimate of the fluctuations
of deflection angle. The local measurements of the wave frequency far from the
wavemaker do not reveal such systematic behavior, although a small shift is
measured. Finally we quantify the damping enhancement induced by the turbulent
flow. We review various theoretical scaling laws proposed previously. Most of
them propose a damping that increases as the square of Froude number. In
contrast, our experimental results show a turbulent damping increasing linearly
with the Froude number. We interpret this linear behaviour as a balance between
the time spent by a wave to cross a turbulent structure with the turbulent
mixing time. The larger is the ratio of these 2 times, the more energy is
extracted from the progressive wave. Finally, mechanisms of energy exchange and
open issues are discussed and further studies are proposed.

Fluid Dynamics