We investigate the effect of buoyancy on the small-scale aspe
cts of
turbulent entrainment by performing direct numerical simu
lation of a gravity cur-
rent and a wall jet. In both flows, we detect the turbulent/non
turbulent interface
separating turbulent from irrotational ambient flow region
s using a range of en-
strophy iso-levels spanning many orders of magnitude. Conf
orm to expectation,
the relative enstrophy isosurface velocity
v
n
in the viscous superlayer scales with
the Kolmogorov velocity for both flow cases. We connect the in
tegral entrainment
coefficient
E
to the small-scale entrainment and observe excellent agree
ment be-
tween the two estimates throughout the viscous superlayer.
The contribution of
baroclinic torque to
v
n
is negligible, and we show that the primary reason for
reduced entrainment in the gravity current as compared to th
e wall-jet is the
reduction in the surface area of the isosurfaces.