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Boundary Layer Analysis
This chapter will treat the physics of aerodynamic boundary layers flows. The objectives include identification of relevant boundary layer parameters, derivation of their governing equations, and formulation of solution methods. Additional objectives are to obtain insight into boundary layer behavior and how it determines overall viscous losses and profile drag.
4.1 Boundary Layer Flow Features and Overview
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In general, a boundary layer flow is either laminar with smooth and nearly parallel streamlines, or turbulent with chaotic motion and significant fluid mixing. Most aerodynamic flows over streamline shapes, such as the airfoil flow shown in Figure 4.1, have laminar boundary layers on each side starting from the leading edge stagnation point, which eventually undergo transition and become turbulent. The two boundary layers then merge at the trailing edge into a wake which is almost invariably turbulent. The airfoil’s profile drag is related to the properties of the far-downstream wake, as derived in Appendix C.
The goals of this chapter are description and prediction of the important aspects and parameters of the boundary layer flow shown in Figure 4.1. Examples are quantities such as the mass defect m(s) and displacement thickness 5*(s) distributions, already identified in Chapter 3 as being required to model the boundary layer’s effects on the overall potential flow. Both laminar and turbulent boundary layers as well as the transition locations will be considered.
Other goals of this chapter include prediction of profile drag, and prediction of boundary layer behavior in general, in particular its response to pressure gradients. A major motivation is the fact that much of aerodynamic design can be viewed as “boundary layer management,” in that boundary layers determine profile drag, and their separation also determines the maximum attainable lift, as discussed in Chapter 3. Hence, boundary layer behavior ultimately sets fundamental limits on most aspects of aerodynamic performance.
The focus here will be on 2D flows, which is sufficient to investigate the majority of the important features of boundary layer behavior. Basic 3D effects will also be briefly considered.
