Flow separation

Flow separation occurs when a boundary layer detaches from a surface, often under an adverse pressure gradient. The detached flow can form a wake, increase drag, and cause unsteady forces.

Boundary layer: A thin region next to a surface where viscous effects make fluid velocity vary sharply from the wall to the outer flow. Separation is the detachment of this near-wall region from the surface.
Wake: A region of disturbed flow downstream of an object moving through or immersed in a fluid. Separated shear layers often merge into the wake behind a body.
Boundary-layer control: Methods that alter boundary-layer behavior to manage drag, heat transfer, or separation. Suction, blowing, and surface design can postpone or deliberately induce detachment.
Attached flow: Flow that remains in contact with a surface without a separated region. It is the direct opposite of a detached boundary layer.
Adverse pressure gradient: A pressure increase in the direction of flow that decelerates fluid moving along a surface. It removes momentum from near-wall fluid and commonly triggers detachment.
Pressure drag: The component of fluid resistance caused by pressure differences across a body. Separation can leave low pressure over the rear surface and sharply raise drag.
Vortex generator: A small surface-mounted device that creates streamwise vortices in a boundary layer. Its vortices carry faster outer fluid toward the wall and delay separation.
Laminar flow: Fluid motion in orderly layers with limited mixing between them. Laminar boundary layers generally have less near-wall momentum and separate more readily.
Wall shear stress: The tangential force per unit area exerted by a fluid on a solid boundary. Its reduction to zero marks incipient separation in many boundary-layer flows.
Vortex shedding: The alternating release of vortices from opposite sides of a body in a flow. Unsteady separated wakes can shed vortices and impose oscillating loads.

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