The wall shear stress w in a boundary layer is assumed to be a function of stream velocity U, boundary layer thickness , local turbulence

The wall shear stress τw in a boundary layer is assumed to be a function of stream velocity U, boundary layer thickness δ, local turbulence velocity u′, density ρ, and local pressure gradient dp/dx. Using (ρ, U, δ) as repeating variables, rewrite this relationship as a dimensionless function.