In fluid flow, a solid wall near the boundary layer region, the speed increases asymptotically (i.e. the method but never joins a given curve) the value of the wall (no slip condition) the value of the main flow is not subject to friction (free Flow).
The thickness of the boundary layer is usually defined as the distance from the wall to the point where the velocity reaches 99% of the free flow value. In the extremely thin boundary layer associated with high Reynolds number free flow, the velocity perpendicular to the wall rises sharply.
Contrary to the practically frictionless free flow (see potential flow), the friction in the boundary layer caused by inertia and friction of the same order of magnitude cannot be ignored, because. Friction also acts on the wall, causing frictional resistance.
Free flow and boundary flow affect each other: On the one hand, the free flow deflects from the wall's boundary layer displacement effect, on the other hand, the pressure mode of the free flow on the boundary layer determines the development of the boundary layer to a large extent.
The flow in the boundary layer can be laminar or turbulent. However, at the same free flow velocity, the laminar boundary layer is thinner than the turbulent boundary layer. In the case of a turbulent boundary layer, the velocity profile is wider and the velocity gradient to the wall is larger, resulting in a much larger frictional resistance than the laminar boundary layer.
Close to the wall, even the turbulent boundary layer always has a laminar flow sublayer, because all lateral motion including turbulent fluctuations will inevitably disappear at the wall.
In the flow around the body, the laminar boundary layer first develops and grows in the flow direction, becomes unstable after a certain distance, and develops into turbulence under the influence of disturbance, such as wall roughness or turbulent fluctuations in free flow. See figure 1 head loss (see attachment for enlarged explanation)
The boundary layer may be separated from the body (boundary layer separation). This phenomenon occurs in the flow region where the static pressure exerted by the free flow on the boundary layer rises in the flow direction.
Then, due to the separation of the boundary layer, the free-flowing airflow is deflected from the wall surface. The dead water zone characterized by vortices and eddies develops downstream of the separation point. The flow velocity in the dead water zone is unstable in size and direction; part of the dead water flows backwards (recirculation effect).
There is no obvious frictional resistance on the separation path downstream of the separation point. However, due to the presence of stagnant water, the increase in pressure resistance is far greater than the decrease in friction resistance. This means that in the case of boundary layer separation, the total flow resistance of the body increases significantly. This flow separation should be avoided as much as possible through design measures and fluid dynamic streamlined equipment such as fittings, nozzles or diffuser elements.
A special type of flow separation involves so-called separation bubbles, which immediately become turbulent at the downstream boundary layer of the laminar separation and reattach to the wall.
In curved pipes and rotating systems, the balance between pressure and inertial forces in free flow is disturbed by the lower flow velocity in the boundary layer. The result is a three-dimensional secondary flow.
The boundary layer plays an important role in the flow of the pipeline. There is usually a constant velocity distribution at the pipe entrance. A boundary layer is formed on the pipe wall, and the thickness of the boundary layer increases with the increase of the downstream distance from the pipe inlet. The core flow unaffected by friction accelerates until after a sufficient distance, the boundary layer grows to its full width. From this point downstream, the velocity distribution of the pipeline flow remains unchanged.