Boundary Layer

In the context of the mechanical field, the Boundary Layer is identified as the flowing fluid that is exposed to a shearing force. There are found a huge amount of velocities existing across the layer of the boundary starting from highest to zero. This is seen to occur only when the fluid is in contact with the concerned surface. At the “leading edge of aircraft wings”, the boundary layers are found to be thinner. On the other hand, at the “trailing edge”, the boundary layers are found to be comparatively thicker. This leads to laminar, as well as the turbulent flow of the fluids.

Theory of Boundary Layers

This theory defines the characteristics of the flow of fluids from the perspective of fluid mechanics. This theory is found to be helpful in analyzing the nature of the liquid or gas flowing at the edge of aircraft or at the inner surface of pipes. The fluid is categorized into two parameters namely the laminar, as well as the turbulent flow of fluid. In the case of the laminar flow, the fluid is supposed to flow in the upstream direction. On the contrary, in the case of Turbulent flow, the fluid is found to flow in the downstream direction on the surface. 

“Laminar Flow”

A kind of fluid either liquid or gas, which is found to flow in a smooth manner, as well as in the regular path is termed as the “Laminar Fluid Flow”. This nature of “fluid flow” is often termed as “streamline flow”. The properties of the flow including pressure, velocity, and so many are found to remain constant in this type of fluid flow. In the case of horizontal surfaces, such as the aircraft wings, the layers of fluid are found to be thin, parallel, as well as stationary. Moreover, all the layers are found to “slide over each other”. In the perspective of straight pipes, the flow is referred to as the relative movement of a “set of concentric cylinders of fluid”. In this regard, the smoke moving in a straight path from the cigarette in the upward direction is considered to be an example of laminar flow. However, the smoke is found to change into a turbulent flow with time.

“Turbulent Flow”

This type of fluid flow is found to move with irregular fluctuations, as well as irregular mixing. Moreover, in this kind of fluid flow, the fluid speed is found to undergo continuous changes with regard to the magnitude, as well as the direction of flow. The fluids under this kind of flow are found to be maintained at low viscosity, the flow occurs through small channels. An example of such fluid flow is the river, as well as the wind flow. Moreover, the flow of blood in the arteries, transport of oil in pipelines, the flow of lava, ocean and atmosphere currents, flow through turbines, pumps, boat wakes, as well as around the tips of aircraft wings are suitable examples of Turbulent flow. 

“Hydrodynamic Boundary Layer”

This kind of boundary layer is defined as the area within which there is found a disturbance in the flow of fluid, and hence, this area is found to be “larger for highly viscous fluids”. The velocity of fluid flow is interrupted as an account of the presence of shear stress between the layers of the fluid. Hence, this kind of fluid flow is also recognized as a “Velocity Boundary Layer”. Moreover, it is found with regard to the study that there is a “Decisive Influence” of this fluid flow on the mass transport, as well as heat. There is no existence of any velocity gradient “outside the hydrodynamic boundary layer”. Hence, there is no role played by the “viscosity of the fluid”.

“Thermal Boundary Layer”

A layer of gaseous or liquid heat transferring agent between a “heat exchange surface”, as well as a free stream is referred to as the “Thermal Boundary Layer”. In this type of boundary layer, there is a continuous change in the “temperature of the heat transferring agent” with regard to the “wall of the free stream”. This layer is found to formulate at the time of rotation of the fluid, as well as the balancing of the viscous forces by the “Coriolis effect”. Moreover, there is a development of “temperatures at the free stream of the fluid”. Along with this, there is an increase in the thickness of the layer at the time of a downstream movement of the fluid, which leads to the transfer of momentum of the boundary layer. A suitable example of the “Thermal Boundary Layer” is the “turbulent flow over a flat plate”.