What is the difference between wake and recirculation region?

Question

If we place a obstacle in the fluid flow, fluid will be circulating behind the obstacle. What is this called?

This might be a wake or recirculation region. What is the difference?

Answer 1

Generally the term 'wake' is used to describe any downstream disturbance to flow caused by an obstacle.

Typically the wake will expand outwards in an approximate 'V' shape until it eventually dissipates into the general 'noise' of the flow whereas recirculation occurs is a reasonably well defined region immediately downstream of the obstacle.

Recirculation is a specific condition where the flow separates from the body of the obstacle. This creates a low pressure area immediately downstream which sucks fluid back into this region from the main flow with the net result of creating a circulating vortex or pair of vortices. This condition creates marked increase in drag compared to situations where the flow remains attached to the surface.

If flow separation is likely it is often better to deliberately induce turbulence early rather than have an abrupt separation of laminar flow.

This is a major and complex subject in fluid mechanics and the above should be considered an introduction rather than a comprehensive explanation.