Boundary Layer Coordinates
 The boundary layer equations derived are in
Cartesian coordinates.
 The Velocity components u and v represent x and y direction velocities respectively.
 For objects with small curvature, these equations can be used with 
 x coordinate : streamwise direction
 y coordinate : normal component
 They are called Boundary Layer Coordinates.
Application of Boundary Layer Theory
 The BoundaryLayer Theory is not valid beyond the point of separation.
 At the point of separation, boundary layer thickness becomes quite large for the thin layer approximation to be valid.
 It is important to note that boundary layer theory can be used to locate the point of seperation itself.
 In applying the boundary layer theory although U is the freestream velocity at the outer edge of the boundary layer, it is interpreted as the fluid velocity at the wall calculated from inviscid flow considerations ( known as Potential Wall Velocity)
 Mathematically, application of the boundary  layer theory converts the character of governing NavierStroke equations from elliptic to parabolic
 This allows the marching in flow direction, as the solution at any location is independent of the conditions farther downstream.
