  We study the internal circulation within the cocoon carved out by a
relativistic jet emanating from an AGN, first developing model and then
validating it using a series of numerical simulations. We notice that a
significant increase of in this flow arises because gradients in the density
and entropy develop near the hot spot, as a consequence of Crocco vorticity
theorem. We find simple solutions for the streamlines, and we use them to
predict the mass inflow rates towards the central regions. The 2D simulations
we perform span a rather wide range of mechanical jet's input power and Black
Hole masses, and we show that the predicted nuclear mass inflows are in good
agreement with the theoretical model.
