Two questions in one day! What?
Okay, so the power equation for air flow is as follows: $$ P = 1/2 \cdot \rho \cdot A \cdot V^3 $$ In a wind tunnel due to the Bernoulli principle as the area ($A$) decreases, the velocity ($V$) proportionally increases; but if the flow is assumed to be in-compressible (talking low speed wind tunnels here) then the density does not change. This means that the $V^3$ term dominates in the power equation and the power is increased. So here is my question, what is that actual physical mechanism that adds the power? I mean I get that there's a pressure gradient, but what causes that? I've always imagined it was the shape of the convergent cone that takes the drag force and by Newtons third law it directs some of the drag force back into the air and increasing the pressure, but I want to know for sure.