Separation of Flow

Flow separation occurs when a stream of gas or liquid flowing past a body separates from the body’s surface and forms a region of eddies. For subsonic flow, a typical case of a separated flow occurs at the surface of a body with curvilinear generatrices (Figure 1), such as a wing section or a sphere. The necessary conditions for the appearance of a separated flow here are the existence of a viscous boundary layer at the surface of the body and an increase in pressure in the direction of the flow. Within the boundary-layer thickness δ the flow velocity decreases from the value ν₀—the velocity of the outer stream—at the outermost limit of the layer to ν = 0 at the surface of the body, but the pressure remains the same as in the external outer stream. In the immediate vicinity of the body’s surface, where the velocity of the particles is nearly zero, the particles’ kinetic energy is not sufficient to overcome the increasing pressure. As a result, this velocity becomes equal to zero and then reverses its direction. The onset of a reversed flow causes a considerable thickening of the boundary layer and the separation of the flow from the wall.

Figure 1. Schematic of the formation of a separated flow during subsonic flow past a body with curvilinear generatrices

The formation of separated-flow regions has a marked effect on the aerodynamic or hydrodynamic characteristics of a body. For example, the aerodynamic drag of a sphere flying at a subsonic speed is mainly determined by the separated flow on the surface of the rear hemisphere. The transition of the laminar boundary layer to turbulence decreases the size of the separatedflow region and reduces considerably the aerodynamic drag of the sphere.

Separation of flow occurs on the upper surface of an airplane wing at a certain angle of incidence, and this region of separated flow grows as the angle of incidence increases. In the process, the lift of the wing first stops increasing and then begins to decrease.

At supersonic flow velocities, shock waves appear, in which the pressure rises abruptly. When a shock wave is formed at a surface having a viscous boundary layer, a separated flow appears. With supersonic flows, the formation of a separated layer has an important effect on both the aerodynamic characteristics and thermal conditions of the body.

In the majority of cases, the occurrence of a separated flow is undesirable. The methods used to prevent separation include boundary-layer suction, the injection of a gas having a higher kinetic energy into the boundary layer, and a suitable choice of the surface shape of the body in order to reduce the pressure rise in the direction of the flow. At supersonic flow velocities, however, separation may be useful. For example, a needle located in front of a blunt body helps reduce the aerodynamic drag by creating a separated flow.