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Abstract:
In many environmental flows, an air-water free surface interacts with a turbulent flow in the
water phase. To reproduce this situation, we propose an original experimental setup, which is an
evolution of the Randomly Actuated Synthetic Jet Array (RASJA) device used to study turbulence
with a low mean flow. By using a central pump connected to jets, we generate a turbulent flow of
tunable intensity with good isotropy and horizontal homogeneity. The maximal turbulent Reynolds
number of 8800 is significantly larger than in other systems generating turbulence with low mean
flow, including RASJA experiments, for which the flow rate per jet cannot be changed. Using our
setup, we characterize the modification of the turbulence under the influence of the free surface,
which acts typically for depths smaller than the integral length measured in the bulk. We report
that the turbulent fluctuations become strongly anisotropic when approaching the free surface. The
vertical velocity fluctuations decrease close to the surface whereas the horizontal ones increase as
reported in previous theoretical predictions and numerical observations. We also observe a strong
enhancement of the amplitude of the temporal and spatial power spectra of the horizontal velocity
at large scales, showing the strengthening of these velocity fluctuations near the free surface.