If you are interested in the historical development of fluid dynamics and aerodynamics, then you must already know the importance of the term ‘boundary layer’. The concept of boundary layer is something that resolved a century long D’Alembert’s Paradox and revolutionized the field of aerodynamics. Introduction of thin boundary layer theory made Ludwig Prandtl known as the father of modern aerodynamics. Today, every fluid flow problem revolves around the concept of boundary layer.
Learn in detail about d’alembert’s paradox here https://www.geniuserc.com/dalemberts-paradox-and-its-resolution-explained/
What is a Boundary layer?
A boundary layer is the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant. The origin of the boundary layer theory can be traced to an investigation conducted by Prandtl (1904) concerning the motion of a fluid with very small viscosity. Boundary layer is a region in flow field inside which fluid flows with a higher relative velocity or a layer inside which a relative motion between fluid particle may exist. The distance from surface of the body to a place where the velocity of flow is 0.99 times of the maximum velocity of flow is called as thickness of boundary layer. It is usually denoted by delta.
Formation of Boundary Layer
When a fluid or air passes over any body then it follows no slip condition at point of contact between fluid and the body. In other words, the relative velocity between fluid and the body is zero at point of contact. The molecules just above the surface are slowed down in their collisions with the molecules sticking to the surface. These molecules in turn slow down the flow just above them.
The farther one moves away from the surface, the fewer the collisions affected by the object surface. This creates a thin layer of fluid near the surface in which the velocity changes from zero at the surface to the free stream value away from the surface. Engineers call this layer the boundary layer because it occurs on the boundary of the fluid.
The velocity of flow will go on increasing rapidly till at the extreme layer. The portion which is outside the boundary layer has a high value of Reynolds’s Number because of high velocity of flow. The Reynolds’s Number is the ratio of inertial forces to viscous forces and is a convenient parameter for predicting if a flow condition will be laminar or turbulent. Re=inertial forces/viscous forces. There are two types of boundary layer.
Laminar Boundary Layer:
Laminar boundary layer is one in which each liquid particle has a definite path. The paths of individual particles do not cross each other. All the molecules in the fluid move in the same direction and at the same speed. It is also called as stream line flow. Laminar flow is common only in cases in which the flow channel is relatively small, the fluid is moving slowly, and its viscosity is relatively high. Oil flow through a thin tube or blood flow through capillaries is laminar. Most other kinds of fluid flow are turbulent except near solid boundaries, where the flow is often laminar, especially in a thin layer just adjacent to the surface.
Turbulent Boundary layer:
In turbulent boundary layer, each liquid particle does not have a definite path. The path of individual particle also cross each other. The molecules in the fluid move in different directions and at different speed. At some distance back from the leading edge, the smooth laminar flow breaks down and transitions to a turbulent flow. The velocity at which the flow changes from the laminar flow to turbulent flow is called critical velocity. It has two types lower critical velocity and upper critical velocity.
Common examples of turbulent flow are blood flow in arteries, oil transport in pipelines, lava flow, atmosphere and ocean currents, the flow through pumps and turbines, and the flow in boat wakes and around aircraft-wing tips.
Significance of Boundary layer:
The boundary layers are so thin and yet so significant. The details of the flow within the boundary layer are very important for many problems in aerodynamics, including wing stall, the skin friction drag on an object, and the heat transfer that occurs in high speed flight. Fluid boundary layer is of extreme importance when it comes to studying the aerodynamics. Fluid boundary layer also needs to be considered when the tennis balls, golf balls, cricket balls are designed. The fuzz on the tennis balls, the spotted design on golf balls, the sim on cricket balls, all affect the formation and characteristics of fluid boundary layer.
Also check out this link to learn more about boundary layer https://en.wikipedia.org/wiki/Boundary_layer#Types_of_boundary_layer