  Recent studies have shown that entrainment effectively describes the
behaviour of natural and forced separating/reattaching flows developing behind
bluff bodies, potentially paving the way to new, scalable separation control
strategies. In this perspective, we propose a new interpretative framework for
separated flows, based on mass entrainment. The cornerstone of the approach is
an original model of the mean flow, representing it as a stationary vortex
scaling with the mean recirculation length. We test our model on a set of mean
separated topologies, obtained by forcing the flow over a descending ramp with
a rack of synthetic jets. Our results show that both the circulation of the
vortex and its characteristic size scale simply with the intensity of the
backflow (i.e. the amount of mass going through the recirculation region). This
suggests that the vortex model captures the essential functioning of mean mass
entrainment, and that it could be used to model and/or predict the mean
properties of separated flows. In addition, we use the vortex model to show
that the backflow (i.e. an integral quantity) can be estimated from a single
wall-pressure measurement (i.e. a pointwise quantity). Since the backflow also
appears to be anticorrelated to the characteristic velocity of the synthetic
jets, this finding suggests that industrially deployable, closed-loop control
systems based on mass entrainment might be within reach.
