Instability and Turbulence in Jets

Publisher Summary This chapter discusses instability and turbulence in fully developed flows. In the past, the instability and turbulence were treated quite separately and differently. The former was cast into an eigen-value problem in which an unsteady, linear partial differential equation, the well-known Orr-Summerfeld equation, was solved; the point of departure of the latter one is the Reynolds equations, a set of time averaged, nonlinear partial differential equations that are incomplete. The main objective was to study the behavior of these structures in space and time and, in particular, to understand the dynamics of their motion. It was also seen that the results described were deduced using a deterministic point of view: the vortex structures generated by the unstable shear layer interact with each other through an induction mechanism. The random behavior was caused by outside and self-induced disturbances that produce phase fluctuations in the interaction process. Finally, if the description of the flow development is a realistic one, one must conclude that the turbulent transport process is actuated by a long range force field, the induced pressure field and, therefore, cannot be expressed in terms of a local gradient diffusion model.