8. Forces exerted on obstacles by a moving fluid

It is equivalent to consider a stationary obstacle placed in a moving fluid or a moving obstacle in a fluid at rest, as long as the law of relative motion is respected. This equivalence is exploited in aerodynamic wind tunnels, which are designed to avoid unpleasant distortions due to the presence of walls. We restrict ourselves to the case of obstacles with uniform translational motion of speed V in an immobile fluid that fills a domain very large compared to the largest dimension of the obstacle.

On each surface element of the obstacle, a force