Question

Explain flow separation and why it causes increase or decrease in drag

Answer 1

The reason for formation of boundary layer is viscosity and inertia; viscosity takes care of reduction of speed at the body surface, and inertia is the reason for transition of velocity between layers until it gets to the free stream.

So, the balance between inertia and viscosity is not perfect. In classical examples of boundary layer, inertia is higher and helps the flow move forward. In separation, the inertia is not strong and, at exactly the separation point, it becomes equal to the viscous forces.

The drag force, brought about by the pressure difference is known as form drag whereas the shear stress at the wall gives rise to skin friction drag. Generally, these two drag forces together are responsible for resultant drag on a body