Chapter 9 : Laminar Boundary Layers
Lecture 29 :

Momentum-Integral Equations For The Boundary Layer

  • To employ boundary layer concepts in real engineering designs, we need approximate methods that would quickly lead to an answer even if the accuracy is somewhat less. 
  • Karman and Pohlhausen devised a simplified method by satisfying only the boundary conditions of the boundary layer flow rather than satisfying Prandtl's differential equations for each and every particle within the boundary layer. We shall discuss this method herein.
  • Consider the case of  steady, two-dimensional and incompressible flow, i.e. we shall refer to Eqs (28.10) to (28.14). Upon integrating the dimensional form of Eq. (28.10) with respect to y = 0 (wall) to y = δ (where δ signifies the interface of the free stream and the boundary layer), we obtain   

  • The second term of the left hand side can be expanded as

or,   by continuity equation


  • Substituting Eq. (29.11) in Eq. (29.10) we obtain
  • Substituting the relation between and the free stream velocity for the inviscid zone in Eq. (29.12) we get


which is reduced to            

  • Since the integrals vanish outside the boundary layer, we are allowed to increase the integration limit to infinity (i.e $ \delta=\infty$. )

  • Substituting Eq. (29.6) and (29.7) in Eq. (29.13) we obtain

  • (29.14)
    where     is the displacement thickness  
    is momentum thickness

Equation (29.14) is known as momentum integral equation for two dimensional incompressible laminar boundary layer. The same remains valid for turbulent boundary layers as well.
Needless to say, the wall shear stress will be different for laminar and turbulent flows.

  • The term signifies space-wise acceleration of the free stream. Existence of this term means that free stream pressure gradient is present  in the flow direction.
  • For example,  we get finite value of outside the boundary layer in the entrance region of a pipe or a channel. For external flows, the existence of depends on the shape of the body.
  • During the flow over a flat plate, and the momentum integral equation is reduced to