CHAPTER IV - BOUNDARY LAYERS

Abstract

This chapter discusses the formation of boundary layers for large Reynolds number fluids. For large Reynolds numbers (R), the decrease of the velocity to zero occurs almost exclusively in a thin layer adjoining the wall. This is called the boundary layer, and it is characterized by the presence of considerable velocity gradients in it. The flow in the boundary layer may be either laminar or turbulent. The rapid decrease of the velocity in the boundary layer is because of the viscosity, which cannot be neglected even if R is large. The flow is rotational throughout the turbulent region; in the absence of separation, it would be rotational only in the boundary layer where the viscosity is important. The vorticity would be zero in the main stream. The flow in the boundary layer must be separated from the surface of the body, with the streamlines consequently leaving the surface layer and entering the interior of the fluid. The decrease in the mean velocity, both in the turbulent and in the laminar boundary layer, is because of the viscosity of the fluid. The manner of variation of the mean velocity in the layer does not depend directly on the viscosity; the viscosity appears in the expression for the velocity gradient only in the viscous sublayer. The total thickness of the boundary layer, however, is determined by the viscosity, and it vanishes when the viscosity is zero.