Separated flow is a flow of liquid or gas, in which the flow around a body separates from its surface and causes formation of a reverse-vortex flow area. Usually, the reverse-vortex flow area is described by small pressure gradients and reduced stagnation pressure values. Most often, the separated flow appears at large Reynolds numbers, when the viscous forces impact directly the particle motion only in very thin regions with large cross variations of longitudinal velocity components. In particular, the separated flow occurs in the boundary layer. The increase of pressure near the wall along the flow direction is a necessary condition for the separated flow formation. In this case, velocity of the particles moving in the boundary layer near the body surface is small; therefore their kinetic energy is not enough to counteract the increased pressure. Kinetic energy supply into the near-wall region from the particles located farther from the body is not enough to counteract the stagnation effect of pressure gradient due to the friction forces inside the fluid at large Reynolds numbers. As a result, the particle motion in the boundary layer slows down and reverses direction. Formation of the region of reverse flow usually changes the flow pattern completely, since the free stream separates from the body to cause formation of the region filled with vortex flow.
Many flows considered within the scope of Mechanical Engineering are separated, since they include regions of strong flow stagnation and correspond to large Reynolds numbers. Attached flows take place at very small Reynolds numbers.