The tendency of a boundary layer to separate depends mainly on the distribution of the adverse or negative effects of the velocity gradient of the boundary layer on the surface, which in turn is directly related to the pressure and its gradient by the differential form of Bernoulli's principle, which is the same as the momentum equation for external inviscid flow.

But the overall magnitudes required for separation are much larger for turbulent flow than for laminar flow. The first can tolerate a flow deceleration almost an order of magnitude stronger. A secondary influence is the Reynolds number. For a given adverse distribution, the resistance to separation of a turbulent boundary layer increases slightly with increasing Reynolds number. In contrast, the separation resistance of a laminar boundary layer is independent of the Reynolds number, a somewhat counterintuitive fact.

Boundary layer separation can occur for flows inside a conduit. It can result from causes such as a rapidly expanding pipe duct. Separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculation flow and the flow through the central region of the duct is called the dividing streamline.[6]​ The point where the dividing line joins the wall again is called the reinsertion point. As the flow progresses downstream, it eventually reaches a state of equilibrium and no reverse flow occurs.

When the boundary layer separates, its remnants form a shear layer[7]​ and the presence of a separate flow region between the shear layer and the surface modifies the exterior potential flow theory and the pressure field. In the case of airfoils, the modification of the pressure field results in an increase in parasitic drag and, if severe enough, will also result in stalling and reduced lift, both undesirable phenomena. For ducts, the separation of the flow produces an increase in losses and retention phenomena such as cavitation, also harmful phenomena.[8] Another effect of the separation of the boundary layer is the regular shedding vortices, known as Von Kármán vortex street. Vortices are shed from the breaking surface of a downstream structure at a frequency that depends on the flow velocity. Vortex shedding produces an oscillating force that can cause vibrations in the structure. If the detachment frequency coincides with a resonance frequency of the structure, it can cause structural failure. These vibrations could be produced and reflected at different frequencies depending on their origin in solid bodies or in the fluids that surround them, being able to dampen or amplify resonance phenomena.

• - Theory of the three layers of coating.

• - Aerodynamics.

• - D'Alembert's paradox.

• - Magnus Effect.

• - Anderson, John D. (2004), Introduction to Flight, McGraw-Hill. ISBN 0-07-282569-3.

• - L. J. Clancy") (1975), Aerodynamics, Pitman Publishing Limited, London ISBN 0-273-01120-0.

• - Wikimedia Commons hosts a multimedia category on Boundary Layer Separation.

• - Aerospaceweb - Golf ball dimples and drag.

• - 01.html Aerodynamics in sports equipment, recreation and machines - Golf - Instructor.

• - Marie Curie Network on Advances in Numerical and Analytical Tools for Independent Flow Prediction Archived November 2, 2018 at the Wayback Machine.

The flow reversal is mainly caused by the adverse pressure gradient imposed on the boundary layer by the exterior potential flow theory. The streamwise momentum equation within the boundary layer is approximately stated as.

where e are respectively the coordinates according to the direction of the flow and the coordinates normal to these.

An adverse pressure gradient occurs when , causing the velocity to decrease along and to reduce to zero or change direction if the adverse pressure gradient is strong enough.[5]​.

The tendency of a boundary layer to separate depends mainly on the distribution of the adverse or negative effects of the velocity gradient of the boundary layer on the surface, which in turn is directly related to the pressure and its gradient by the differential form of Bernoulli's principle, which is the same as the momentum equation for external inviscid flow.

But the overall magnitudes required for separation are much larger for turbulent flow than for laminar flow. The first can tolerate a flow deceleration almost an order of magnitude stronger. A secondary influence is the Reynolds number. For a given adverse distribution, the resistance to separation of a turbulent boundary layer increases slightly with increasing Reynolds number. In contrast, the separation resistance of a laminar boundary layer is independent of the Reynolds number, a somewhat counterintuitive fact.

Boundary layer separation can occur for flows inside a conduit. It can result from causes such as a rapidly expanding pipe duct. Separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculation flow and the flow through the central region of the duct is called the dividing streamline.[6]​ The point where the dividing line joins the wall again is called the reinsertion point. As the flow progresses downstream, it eventually reaches a state of equilibrium and no reverse flow occurs.

When the boundary layer separates, its remnants form a shear layer[7]​ and the presence of a separate flow region between the shear layer and the surface modifies the exterior potential flow theory and the pressure field. In the case of airfoils, the modification of the pressure field results in an increase in parasitic drag and, if severe enough, will also result in stalling and reduced lift, both undesirable phenomena. For ducts, the separation of the flow produces an increase in losses and retention phenomena such as cavitation, also harmful phenomena.[8] Another effect of the separation of the boundary layer is the regular shedding vortices, known as Von Kármán vortex street. Vortices are shed from the breaking surface of a downstream structure at a frequency that depends on the flow velocity. Vortex shedding produces an oscillating force that can cause vibrations in the structure. If the detachment frequency coincides with a resonance frequency of the structure, it can cause structural failure. These vibrations could be produced and reflected at different frequencies depending on their origin in solid bodies or in the fluids that surround them, being able to dampen or amplify resonance phenomena.

• - Theory of the three layers of coating.

• - Aerodynamics.

• - D'Alembert's paradox.

• - Magnus Effect.

• - Anderson, John D. (2004), Introduction to Flight, McGraw-Hill. ISBN 0-07-282569-3.

• - L. J. Clancy") (1975), Aerodynamics, Pitman Publishing Limited, London ISBN 0-273-01120-0.

• - Wikimedia Commons hosts a multimedia category on Boundary Layer Separation.

• - Aerospaceweb - Golf ball dimples and drag.

• - 01.html Aerodynamics in sports equipment, recreation and machines - Golf - Instructor.

• - Marie Curie Network on Advances in Numerical and Analytical Tools for Independent Flow Prediction Archived November 2, 2018 at the Wayback Machine.