Transition is the most complex problem of fluid mechanics, and perhaps of modern physics. A century of effort has produced very limited insight and only a few guidelines for the benefit of design engineers. Transition is the process by which a laminar flow becomes turbulent. While the definition of the word “turbulent” still leaves some uncertainty, there seems to be less doubt about what we call “transition.” In a laminar flow, it is reasonable to try to describe the motion by specifying the velocity at any point and any time. The theoretician will use analytic expressions, the numerologist will interpolate between values known at certain grid points. In a turbulent flow, this objective becomes meaningless. A full description of the flow would be of little use, since interest centers on averaged quantities, and especially on the Reynolds stress. Thus, statistical information becomes all-important.
KeywordsShear Layer Vortex Ring Velocity Fluctuation Eddy Viscosity Initial Perturbation
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