Boundary layer equations pressure change across

It is assumed that the thickness of the layer, i.e., the boundary layer, ih which there are significant temperature differences and velocities parallel to the (surface is small, i.e., that 8/L pressure changes across the boundary layer are negl ible and the momentum equation for the jc-direction, i.e., the vertical direction, is... 

Since the forcing terms in equation (34) all vanish in this problem, we obtain equation (39), in which for simplicity we shall introduce the further assumption that C = 1—that is, p/t = [see equation (30)]. This assumption (that pp does not vary across the boundary layer) often is reasonable for gases if changes in the average molecular weight are negligible, then— because of the constancy of the pressure—the ideal-gas law implies that p 1/T, in which case constancy of pp corresponds to p T, a dependence close to the kinetic-theory predictions discussed in Appendix E. With C = 1, equation (39) is the Blasius equation [4], F " -F FF" = 0, and in view of equation (28), the boundary conditions implied by equations (48) and (49) are F co) = 1 and F (0) = 0. Use may be made of the present formula for p, C = 1, F (0) = 0, and equations (27) and (29) to ascertain the boundary condition implied by equation (50) the calculation results in... 

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