When a fluid flows around a body, forces act between the fluid and the surface due to the viscosity. These intermolecular forces are the cause for the drag, among other things. However, due to the forces acting, the surface of the solid body also tries to bind the fluid to itself. This also leads to the fluid adhering to the surface (no-slip condition). The layer above this adherent fluid layer will not be able to simply tear itself away from it, because intermolecular attraction and pressure forces also act between the layers. As a result, any flow flowing around a body is tempted to follow the profile of the surface.
As long as the contour of a body has smooth transitions and the internal cohesion of the fluid (viscosity) is large enough to resist the inertial forces, a flow can follow the contour. A typical hydrodynamic boundary layer develops around the body, the thickness of which is largely influenced by the viscosity.
In the case of sharp transitions or when flowing around blunt bodies, however, the fluid is often no longer able to follow the profile. The boundary layer or flow begins to detach itself from the body surface. This is referred to as a boundary layer separation or flow separation. Downstream of the separation point, vortices often form, resulting in a turbulent flow. A flow separation is particularly dangerous on the wings of an aircraft, as this also causes a loss of lift and the aircraft is in danger of crashing. In aviation such a dangerous flow separation is called stall.
As already mentioned, the boundary layer separation is obvious at sharp transitions. However, a stall can also occur even with smooth transitions. This is the case if the flow slows down considerably when flowing around a body. As a result, the static pressure can increase so much that it suddenly pushes the flow in the opposite direction. This results in a recirculation area, which causes a flow separation. Since a deceleration of the fluid occurs with every body, the danger of boundary layer separation therefore also exists with every body around which the fluid flows!
The Reynolds number plays a decisive role here, since it describes the relationship between the existing inertial forces and the acting viscosity forces in a fluid. The higher the Reynolds number, the greater the inertia compared to the viscosity and the higher the risk of boundary layer separation. If the Reynolds numbers are sufficiently high, even with streamlined bodies, flow separation will eventually be unavoidable.
The flow around a body can not only be laminar, but also turbulent at increased Reynolds numbers. In general, a laminar flow around the body should be aimed for, since this reduces flow losses to a minimum. However, this is only the case if. it is ensured that no boundary layer separation occurs. If, on the other hand, this cannot be ensured, then turbulent boundary layers should be aimed for. This may sound a paradox, but a turbulent boundary layer can usually follow the profile of a body longer than a laminar flow. The reason for this is the increased transport of momentum between the fluid layers, which leads to a steeper velocity profile within the boundary layer.
With a turbulent boundary layer, the velocity in the y-direction increases faster than with a laminar boundary layer. This means a larger velocity gradient near the wall and thus higher velocity components in the boundary layer. The resulting higher kinetic energy of the boundary layer can counteract the adverse pressure gradient more, so to speak.
In the case of a turbulent boundary layer, the separation point thus shifts downstream. The turbulent wake becomes narrower with the shifting of the separation point. This reduces the flow losses and thus the pressure drag, which ultimately results in a reduction of the overall drag.
This fact is taken advantage of, for example, with golf balls. The dimples in the golf ball cause vortices and lead to a turbulent flow around the golf ball. The separation point shifts downstream and the drag is reduced to only a quarter, which makes the golf ball fly much further.
So-called turbulators (vortex generators) on the wings of aircraft act in a similar way. Often many small vanes are mounted on the wing for this purpose. These vanes create a transition from a laminar to a turbulent flow. The turbulent boundary layer, which remains longer on the wing, not only reduces drag but also the risk of hard stall. Another possibility to achieve a targeted turbulent flow around wings is the use of so-called b/ast turbulators. Air is blown out through small holes in the wing. This also causes the laminar flow to change into a turbulent flow.
Turbulators can also be found in racing cars. These are designed to move the flow separation as far back as possible so that the flow is still attached in the rear of the car for as long as possible.