Chapter 9 - Boundary Layers and Related Topics

Abstract

This chapter focuses on a boundary-layer hypothesis and examines its consequences. The equations of fluid motion within the boundary layer can be simplified because of the layer's thinness, and exact or approximate solutions can be obtained in many cases. The Boundary-layer phenomena provide explanations for the lift and drag characteristics of bodies of various shapes in high Reynolds number flows, including turbulent flows. The fluid mechanics of curved sports-ball trajectories is described in the chapter. The simplifications of the boundary-layer assumption are: diffusion in the stream-wise direction is negligible compared to that in the wall normal direction and the pressure field can be found from the outer flow, so that it is regarded as a known quantity within the boundary layer. The boundary layer is so thin that the pressure does not change across it. Since the fluid velocity in the boundary layer smoothly joins that of the outer flow, there is no obvious demarcation of the boundary layer's edge. The three most common thickness definitions are described in the chapter. The process of changing from laminar to turbulent flow is called transition, and it occurs in a wide variety of flows as the Reynolds number increases. The complicated phenomenon known as boundary-layer transition is described in general terms. Several features of the description of flow over a circular cylinder qualitatively apply to flows over other two-dimensional blunt bodies.