  The interaction of two isotropic turbulent fields of equal integral scale but
different kinetic energy generates the simplest kind of inhomogeneous turbulent
field. In this paper we present a numerical experiment where two time decaying
isotropic fields of kinetic energies $E_1$ and $E_2$ initially match over a
narrow region. Within this region the kinetic energy varies as a hyperbolic
tangent. The following temporal evolution produces a shearless mixing. The
anisotropy and intermittency of velocity and velocity derivative statistics is
observed. In particular the asymptotic behavior in time and as a function of
the energy ratio $E_1/E_2 \to \infty$ is discussed. This limit corresponds to
the maximum observable turbulent energy gradient for a given $E_1$ and is
obtained through the limit $E_2 \to 0$. A field with $E_1/E_2 \to \infty$
represents a mixing which could be observed near a surface subject to a very
small velocity gradient separating two turbulent fields, one of which is nearly
quiescent. In this condition the turbulent penetration is maximum and reaches a
value equal to 1.2 times the nominal mixing layer width. The experiment shows
that the presence of a turbulent energy gradient is sufficient for the
appearance of intermittency and that during the mixing process the pressure
transport is not negligible with respect to the turbulent velocity transport.
These findings may open the way to the hypothesis that the presence of a
gradient of turbulent energy is the minimal requirement for Gaussian departure
in turbulence.
