In fluid mechanics a Riabouchinsky solid is a technique used for approximating boundary layer separation from a bluff body using potential flow. It is named after Dimitri Pavlovitch Riabouchinsky.^[1][2]

Riabouchinsky solids are typically used for analysing the behaviour of bodies moving through otherwise quiescent fluid (examples would include moving cars, or buildings in a windfield).

Typically the streamline that touches the edge of the body is modelled as having no transverse pressure gradient and thus may be styled as a free surface after separation.

The use of Riabouchinsky solids renders d'Alembert's paradox void; the technique typically gives reasonable estimates for the drag offered by bluff bodies moving through inviscid fluids.