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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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite book</ref>
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.