  The surface area of turbulent/non-turbulent interfaces (TNTIs) is
continuously produced and destroyed via stretching and curvature/propagation
effects. Here, the mechanisms responsible for TNTI area growth and destruction
are investigated in a turbulent flow with and without stable stratification
through the time evolution equation of the TNTI area. We show that both terms
have broad distributions and may locally contribute to either production or
destruction. On average, however, the area growth is driven by stretching,
which is approximately balanced by destruction by the curvature/propagation
term. To investigate the contribution of different length scales to these
processes, we apply spatial filtering to the data. In doing so, we find that
the averages of the stretching and the curvature/propagation terms balance out
across spatial scales of TNTI wrinkles and this scale-by-scale balance is
consistent with an observed scale invariance of the nearby coherent vortices.
Through a conditional analysis, we demonstrate that the TNTI area production
(destruction) localizes at the front (lee) edge of the vortical structures in
the interface proximity. Finally, we show that while basic mechanisms remain
the same, increasing stratification reduces the rates at which TNTI surface
area is produced as well as destroyed. We provide evidence that this reduction
is largely connected to a change in the multiscale geometry of the interface,
which tends to flatten in the wall-normal direction at all active length scales
of the TNTI.
