Laminar and Turbulent Boundary Layer

The layer of the fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in brief is called boundary layer. On the aircraft wing boundary layer is the part of the flow close to the wings. The boundary layer will deform the surrounding non viscous flow and it is the phenomenon of the viscous forces. This effect is closely related to Reynolds numbers.  The laminar and turbulent boundary layer will be discussed in the upcoming paragraphs.

Laminar boundary layers come in various forms and it can be loosely classified, that depends on their structure and the circumstances under which they are created. In an oscillating body, the thin shear layer is developed is an example of a strokes boundary layer. When the fluid rotates, Coriolis Effect will balance the viscous forces, rather than convective inertia which are leading to the formation of Ekman layer. During heat transfer, thermal boundary layer will exist. In the laminar flow, the fluid moves in smooth layers or lamina. There is relatively the shear stress are low and velocity gradients are small. The distance from the start of the boundary layer and decreases with Reynolds number increases the thickness of the laminar boundary layer.

The turbulent boundary layer is described by the unsteady mixing due to vortex and eddies at many scales. The result is the higher shear stress at the wall, a fuller velocity profile and a greater boundary layer thickness. Due to the greater velocity gradient near the wall, the wall shear stress is higher. This is due to the more effective mixing associated with the turbulent flow. However the lower velocity fluid is also transported outward with the result that the distance to the edge of the layer is larger.  This is the function of laminar and turbulent boundary layer.

Aerodynamics

The much attention is paid to control the boundary layer to minimize drag in the aircraft during flight. Two things are considered, first boundary layer adds to the effective thickness of the body through the displacement thickness and increasing the pressure. Secondly, skin friction drag is created due to the shear forces at the surface of the wing. We can see the function of laminar and turbulent boundary layer in the aerodynamics. If the Reynolds numbers are high it is difficult to maintain laminar flow in the aircraft. This results in the lower skin friction due to the characteristic velocity profile of the laminar flow. If the Reynolds number is low, it is easy to desire the laminar flow. The region between this said to be boundary layer transition.

Questions:

• What is laminar flow?
• What is turbulent flow?