Boundary layer control means

TL;DR: A variable jet propulsion (VJP) is characterized by an oval cross-section of the manifold as discussed by the authors, which is reinforced against the effect of pressure tending to deform the manifold toward a more nearly circular crosssection by a hollow reinforcing ring pressurized from within the exhaust duct.

TL;DR: In this article, the authors describe a jet-propulsion nozzles, which are composed of a fixed convergent tubular nozzle portion, a series of circumferentially-spaced adjustable petals, and a fixed divergent rearwardly extending portion leading to an exhaust orifice.