## Chapter 8. Boundary-Layer And Other Nearly Unidirectional Flows

### 8.1 Introduction

ALTHOUGH the Navier-Stokes equations apply to the flow of a Newtonian fluid, they are—in general—very difficult to solve exactly. There is also the problem that an apparently correct solution may be physically unrealistic for high Reynolds numbers, because of turbulence or other instabilities.

Two particular types of simplifying approximations may be made, depending on the Reynolds number (the ratio of inertial to viscous effects), in order to make the situation more tractable:

**1.** Omit the inertial terms, as was done in the examples of Chapter 6. The resulting equations are typically appropriate for the flow near a fixed boundary, where viscous action is particularly ...