Boundary layer

In flowing fluids, the boundary layer is the area in the immediate vicinity of a solid wall where the velocity rises asymptotically (i.e. it approaches but never joins a given curve) from the value at the wall (no-slip conditions) to the value of the main flow which is not influenced by friction (free stream flow).

The boundary layer thickness is usually defined as the distance from the wall to the point where the flow velocity reaches 99 % of the free stream value. In the very thin boundary layer associated with free stream flows with high Reynolds numbers, a steep rise in velocity occurs perpendicular to the wall.

In contrast to the free stream flow which is practically frictionless (see Potential flow) the friction caused within the boundary layer by inertia and friction forces of the same order of magnitude must not be neglected because the. The friction forces also act on the wall causing frictional resistance.

The free stream and boundary flows influence each other: on the one hand, the free stream flow is deflected from the wall by the boundary layer's displacement effect and on the other, the free stream flow imposes a pressure pattern on the boundary layer which largely determines the boundary layer's development.

The flow in the boundary layer can be laminar als auch turbulent.  However, a laminar boundary layer is thinner than a turbulent boundary layer given the same free stream velocity. In the case of a turbulent boundary layer flow, the velocity profile is broader with a steep velocity gradient towards the wall, resulting in a much higher frictional resistance than in the case of a laminar boundary layer.

In the immediate vicinity of the wall, even a turbulent boundary layer always possesses a laminar sub-layer as all transverse movements, including turbulent fluctuations, must necessarily disappear at the wall itself.

In a flow around a body, a laminar boundary layer initially develops and grows in flow direction before becoming unstable after travelling a certain distance, and developing into turbulent flow under the influence of disturbances, e.g. wall roughness or turbulent fluctuations in the free stream flow. See Fig. 1 Head loss (refer to the Annex for an enlarged presentation) 

The boundary layer may become detached from the body (boundary layer separation). This phenomenon arises in flow regions where the static pressure which is imposed on the boundary layer by the free stream flow rises in flow direction.

The free stream flow is then deflected away from the wall by the boundary layer separation. A dead water zone characterised by eddies and vortices develops downstream of the point of separation. The flow velocities in this dead water zone are erratic in terms of magnitude and direction; part of this dead water flows backwards (recirculation effect).

There is no significant frictional resistance in the separation path downstream of the separation point. However, as a result of the dead water, an increase in the pressure resistance occurs which is far more significant than the decrease in frictional resistance. This means that the body's total flow resistance increases considerably in the event of boundary layer separation. Such flow separations should be avoided as far as possible by design measures and hydrodynamic streamlining equipment such as fittings, nozzles or diffuser elements. 

A particular type of flow separation involves so-called separation bubbles which develop in flow profiles where the boundary layer becomes turbulent immediately downstream of a laminar separation and re-attaches to the wall.

In curved ducts and rotating systems, the equilibrium in the free stream flow between the pressure forces on the one hand and the forces of inertia on the other hand is disturbed by the lower flow velocities in the boundary layer. The result is three-dimensional secondary flows.

The boundary layer plays an important role in flow through pipes. At the pipe inlet there is often a constant velocity distribution. A boundary layer is formed at the wall and its thickness increases with increased distance downstream from the pipe inlet. The core flow which has not yet been affected by friction is accelerated until, after a sufficient distance, the boundary layer has grown to its full width. From this point downstream, the pipe flow's velocity profile remains unchanged.