  We present laboratory experiments on turbulence in a linearly stratified
fluid driven by an ensemble of internal gravity waves which approaches
statistical homogeneity and axi-symmetry. In a way similar to several recent
experimental works, non-linearities develop through the establishment of a set
of internal wave modes at discrete frequencies, when the forcing amplitude is
increased. We show that the most energetic of these modes are resonant
eigenmodes of the fluid domain. The discretization of the energy in frequency
and wavenumber associated to the emergence of these modes prevents the flow
from approaching a regime described by the Weak/Wave Turbulence Theory, in
which a forward cascade carried by a statistical ensemble of weakly non-linear
waves in an infinite domain forms an energy continuum in the frequency and
wavenumber spaces. We then show that the introduction of slightly tilted panels
at the top and at the bottom of the fluid domain allows to inhibit the
emergence of the discrete wave modes. In this new configuration, the non-linear
regime results in a continuum of energy over one decade of frequencies which is
mainly carried by internal gravity waves verifying the dispersion relation. We
therefore achieved a turbulent flow approaching a three-dimensional internal
wave turbulence regime with no discretization of the energy in the frequency
and wavenumber domains. These results constitute a significant step forward in
the search of the laboratory observation of a fully-developed weakly-non-linear
internal-gravity-wave turbulence.
