Flow impinging on axisymmetric disk

Chuck Leakeas

Hello all, I am currently trying to compute the boundary layer thickness from a normally impinging flow against an axisymmetric disk, also known as Homann Flow. When looking in Schlichting's book on b.l. flows he gives the b.l. thickness as delta=2.4*sqrt(nu/a). Where delta is the b.l. thickness and nu is the kinematic viscosity of the fluid. He says that for potential flow the velocity is U=a*x, and V=-a*y where U and V are far from the plate. My question is, what is the best way to determine a? From a computed run of axisymmetric viscous flow I am very skeptical about computing a value for a. It never seems to be constant for any velocity/distance combination far from the plate. What else can I do?

Thanks, Chuck