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Nature offers us clear signs that the phenomenon of transition is associated with a fundamental property of fluid flow. Any laminar flows is characterized by a critical number that serves as a limit line for the transition of flow from laminar to turbulent .for example flows like Free-jet flow (axisymmetric) is limited to stay laminar by Re < 10–30, where Re is based on nozzle diameter and mean velocity through the nozzle〔J. Reynolds, Observations of a liquid-into-liquid jet, J. Fluid Mech., Vol. 14,1962, pp. 552–556.〕 Natural convection boundary layer flow on Isothermal wall is limited by conditions Gr < 1.5 × 109 (Pr = 0.71) and Gr < 1.3 × 109 (Pr = 6.7) where The Grashof number Gr is based on wall height and wall–ambient temperature difference.〔R. L. Mahajan and B. Gebhart, An experimental determination of transition limits in a vertical natural convection flow adjacent to a surface, J. Fluid Mech., Vol. 91,1979, pp. 131–154.〕 These empirical critical limits have been observed to be universal, despite reported numerical variations from one experimental reporting of transition observations to another. The real challenge is to predict these numbers. although the critical transition numbers differ in orders of magnitude from one flow class to another, they all seem to suggest that an appropriate Reynolds number based on the relevant velocity and transversal dimension of the flow has in all cases the same order of magnitude, O(102).〔W. M. Kays and M. E. Crawford, Convective Heat and Mass Transfer, 2nd ed.,McGraw-Hill, New York, 1980, p. 163.〕 This observation is true not only for forced boundary layer flow, as shown in Ref. 11, but for wall jet flows and free-jet flows encountered in natural convection phenomena, as well as for jet and wake flows (Table 6.1). It is important to keep in mind that this seemingly universal transition Reynolds number is a number considerably greater than O(1). The flow has the natural property to meander with a characteristic wavelength during transition, regardless of the nature of the disturbing agent. This observation is important because it illustrates the conflict between hydrodynamic stability thinking, to which the postulate of disturbances is a necessity, and the natural meandering∗ tendency of real-life flows during transition〔Bejan A., Convection HeatTransfer , Wiley, New York,Chapter 6.〕 ==The Laws== Transition from laminar flow to turbulent flow is characterized by two scaling laws: # A universal proportionality between longitudinal wavelength and stream thickness that is, by a meander or buckling phenomenon # A local Reynolds number of order 102, where the Reynolds number is based on the local stream velocity scale and the local stream thickness scale These features can be seen in many flows that undergo transition in nature; an extensive compilation.〔Bejan A., Entropy Generation through Heat and Fluid Flow, Wiley, New York, Chapter 4.〕〔Bejan A., Buckling flows: A new frontier in fluid mechanics, in T. C. Chawla, ed., Annual Review of Numerical Fluid Mechanics and Heat Transfer, Vol. 1, Hemisphere, Washington, DC, 1987, pp. 262–304.〕 Perhaps, the most striking transition phenomenon that confirms conclusions 1 and 2 is the highly regular (buckled) vortex street formed in the wake of a solid obstacle. There, the universal proportionality between wake wavelength and wake thickness is obvious, and the local Reynolds number is certainly of order 102 . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Scaling laws of transition」の詳細全文を読む スポンサード リンク
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