Effects of Boundary Layer Separation
When the boundary layer separates, its displacement thickness increases sharply, which modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag, and if severe enough will also result in loss of lift and stall, all of which are undesirable. For internal flows, flow separation produces an increase in the flow losses, and stall-type phenomena such as compressor surge, both undesirable phenomena.
Another effect of boundary layer separation is shedding vortices, known as Kármán vortex street. When the vortices begin to shed off the bounded surface they do so at a certain frequency. The shedding of the vortices then could cause vibrations in the structure that they are shedding off. When the frequency of the shedding vortices reaches the resonance frequency of the structure, it could cause serious structural failures.
Read more about this topic: Flow Separation
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