page_title: Flow separation
section_title: Adverse pressure gradient
hierarchical_section_title: Flow separation / Adverse pressure gradient
context_page_description: Whenever there is relative movement between a fluid and a solid surface, whether externally round a body, or internally in an enclosed passage, a boundary layer exists with viscous forces present in the layer of fluid close to the surface. Boundary layers can be either laminar or turbulent. A reasonable assessment of whether the boundary layer will be laminar or turbulent can be made by calculating the Reynolds number of the local flow conditions.
  Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. Separation occurs in flow that is slowing down, with pressure increasing, after passing the thickest part of a streamline body or passing through a widening passage, for example.
  Flowing against an increasing pressure is known as flowing in an adverse pressure gradient. The boundary layer separates when it has travelled far enough in an adverse pressure gradient that the speed of the boundary layer relative to the surface has stopped and reversed direction. The flow becomes detached from the surface, and instead takes the forms of eddies and vortices.
context_section_description: The flow reversal is primarily caused by adverse pressure gradient imposed on the boundary layer by the outer potential flow. The streamwise momentum equation inside the boundary layer is approximately stated as
  where  are streamwise and normal coordinates.
  An adverse pressure gradient is when , which then can be seen to cause the velocity  to decrease along  and possibly go to zero if the adverse pressure gradient is strong enough.
caption: Graphical representation of the velocity profile in the boundary layer. The last profile represents reverse flow which shows separated flow.