Non-Equilibrium and Equilibrium Boundary Layers without Pressure Gradient

Abstract

The effect of the friction parameter ω, defined as the ratio of the friction velocity to the free stream velocity, has been investigated on mean velocity fields for non-equilibrium and equilibrium boundary layers developing under zero pressure gradient. The wall shear stress was measured by a drag balance using a floating element device with a zero displacement mechanism. For the equilibrium boundary layer, the local skin friction coefficient is independent of two parameters, both the streamwise distance and the Reynolds number, based on the momentum thickness, and the boundary layer thickness is proportional to the streamwise distance. On the other hand, for the non-equilibrium boundary layer, the local skin friction coefficient depends on the above two parameters. The wake parameters for both boundary layers approach constant values, which depend on the surface condition, for high Reynolds numbers. From analysis using both the momentum integral equation and Coles’s wake law, the wake parameter for the equilibrium boundary layer is uniquely expressed as a function of the friction parameter. However, for the non-equilibrium boundary layer, the wake parameter depends on the friction parameter as well as the growth rate of the boundary layer thickness.