The Turbulent Boundary Layer in a Compressible Fluid

A transformation is derived from first principles to reduce the boundary-layer equations for a general compressible two-dimensional flow to incompressible form. For the case of boundary-layer flow of a Newtonian fluid past a smooth wall, but with no other restrictions, it is shown that the combination (ρ∞μ∞/ρwμw) CfReθ is an invariant of the transformation. This result is called the law of corresponding stations. In order to apply the transformation to the problem of the turbulent boundary layer on a smooth wall, it is assumed that the sublayer Reynolds number is unaffected by compressibility or heat transfer provided the density and viscosity are evaluated at a mean sublayer temperature defined by the transformation. Explicit formulas are obtained for the effect of Mach number and heat transfer on surface friction when the fluid is a perfect gas, the pressure is constant, and the stagnation temperature is constant or linear in the velocity. An appendix contains a brief critical discussion of the mean-temperature hypothesis, the laminar-film hypothesis, and other analytical ideas related to the idea of a transformation.