Basic Physics of Laminar-Turbulent Transition
The phenomena of stability of laminar flows, transition, and turbulence were systematically studied first by O. Reynolds  in the eighties of the eighteenth century. H. Schlichting  and  and in his classical textbook Boundary Layer Theory  gives an excellent treatment of these complex flow phenomena and critically reviews the contributions up to 1979, where the seventh and last edition of his book appeared. In this chapter, we first treat the fundamental issues pertaining to the subject matter followed by original contributions recently made in the area of steady and unsteady boundary layer transition. The flow in a turbine or compressor component is characterized by a threedimensional, highly unsteady motion with random fluctuations due to the existing freestream turbulence and the interactions between the stator and rotor rows.
Considering the flows within the blade boundary layer, based on the blade geometry and pressure gradient, three distinctly different flow patterns can be identified: 1) laminar flow (or non-turbulent flow) characterized by the absence of stochastic motions, 2) turbulent flow, where flow pattern is determined by a fully stochastic motion of fluid particles, and 3) transitional flow characterized by intermittently switching from laminar to turbulent at the same spatial position. Of the three patterns, that is predominant in turbomchinery components is the transitional flow pattern. The Navier-Stokes equations presented in Chapter 3 generally describe the unsteady flow through a turbomachinery component. Using a direct numerical simulation (DNS) approach delivers the most accurate results. However, the application of DNS, for the time being, is restricted to simple flows at low Reynolds numbers. For calculating the complex turbomachinery flow field within a reasonable time frame, Reynolds averaged Navier-Stokes (RANS) equations with appropriate turbulence models are used. A detailed discussion of this topic is found in Chapter 20.
KeywordsBoundary Layer Transition Turbulent Spot Reynolds Stress Tensor Curve Plate Turbine Cascade
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