Near the entrance fig 63a and b there is an irregular

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Unformatted text preview: regular laminar-turbulent interface, and vortex roll-up is visible. Further downstream (Fig. 6.3c) the puff becomes fully turbulent and very active, with helical motions visible. Far downstream (Fig. 6.3d), the puff is cone-shaped and less active, with a fuzzy ill-defined interface, sometimes called the “relaminarization” region. A complete description of the statistical aspects of turbulence is given in Ref. 1, while theory and data on transition effects are given in Refs. 2 and 3. At this introductory level we merely point out that the primary parameter affecting transition is the Reynolds number. If Re UL/ , where U is the average stream velocity and L is the “width,” or transverse thickness, of the shear layer, the following approximate ranges occur: 0 1 100 103 104 106 Re Re Re Re Re Re 1: 100: 103: 104: 106: : highly viscous laminar “creeping” motion laminar, strong Reynolds-number dependence laminar, boundary-layer theory useful transition to turbulence turbulent, moderate Reynolds-number dependence turbulent, slight Reynolds-number dependence These are representative ranges which vary somewhat with flow geometry, surface roughness, and the level of fluctuations in the inlet stream. The great majority of our analyse...
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