Flow separation
Separated region
When a fluid separates from a body, it forms a separated region between the body and the fluid stream.
It depends on:
Flow speed
Reynolds number
Imagine flow over a sharp drop e.g. a waterfall:
The faster the flow the further away the water will get from the cliff edge
In a pipe sharp drops can create strong circular currents
Wake
The region of flow trailing the body where the effects of the body on velocity are felt.
Reducing pressure drag by streamlining:
Delays boundary layer separation
Reduces pressure difference between front and back