What is the Boundary Layer?

INTRO

In fluid dynamics, Boundary Layer is a THIN (less than an inch), SLOW MOVING (nearly zero velocity) layer of moving fluid which is in contact with the surface of any object (car, boat, aircraft, golf ball etc). It STICKS and tends to wrap around the object due to the VISCOSITY of the fluid.





Yes! You guessed it right ZERO VISCOSITY (ideal) means NO BOUNDARY LAYER!

Being Aircraft Mechanics we are naturally more concerned about Aircraft!
Let's see how Boundary Layer affects our airfoils and how we control it to change overall lift and controllability characteristics

AERODYNAMICS
When the free air stream comes into contact with the airfoil, the flow closer or in contact with the airfoil tends to slow down owing to it's viscosity and surface friction with the surface of the object. This thin layer of airflow is termed as the BOUNDARY LAYER. The boundary layer consists of several layers flowing over each other, each layer is a bit faster than the layer below it, hence there is a gradual increase in velocity from airfoil surface to the free stream (see Fig 1)

Figure 1 - Gradual Increase of Layer Velocity from Surface to the Free Stream


As the boundary layer is slow-moving in nature it has low Kinetic Energy. This lower energy results in it's separation from the airfoil (Fig 2).

Figure 2

NOTE: The static pressure increases from the thickest point of the airfoil to the trailing edge (See Fig 3), thus the boundary layer needs more energy to flow to the high pressure area or it will separate from the airfoil.

Figure 3

MORE KINETIC ENERGY means FARTHER SEPARATION POINT

Thus to delay the separation point, or to get the maximum positive effects from the airfoil (lift, controllability), we try to provide more energy the boundary layer.
To energize the boundary layer devices like SLOTS, VORTEX GENERATORS are used.


P.S. I have kept the post simple, there is much more to be added to this article, but this one is to give just some general concept of the Boundary Layer. You can always suggest improvements and help others!