This chapter continues the treatment initiated in Chapter 3 on the evolution of vorticity in flows. We now focus on specific flow fields and include the effects of heat release and external forcing. Hydrodynamic flow stability is a large, rich field; this chapter can provide only a brief introduction to the many fascinating instabilities that arise [1]. For these reasons, attention is specifically focused on high Reynolds number flows and several specific flow configurations of particular significance in combustor systems, including shear layers, wakes, jets, and backward-facing steps.

The vorticity that controls the hydrodynamic stability features of the flow originates largely from the boundary layers in approach flow passages or other walls. The separating boundary layer characteristics serve, then, as an important initial condition for the flows of interest to this chapter. Discussion of the stability and coherent structures present in boundary layers is outside the scope of this book, but several important characteristics of boundary layers are summarized in Aside 4.1.

This chapter starts with a discussion of free shear layers in Section 4.1, the most fundamental hydrodynamic instability of interest to practical combustors. It then considersmore complex flows that largely involve interactions of multiple free shear layers or of shear layers with walls. For example, two-dimensional wakes (Section 4.2) and jets (Section 4.3) are equivalent to two free shear layers separated by some distance, a, of oppositely signed vorticity. It is recommended that readers using this book as a text focus on Section 4.1, and then use the remaining sections as references for other specific flow configurations as required.