D113 円管内流の層流乱流遷移の最小臨界レイノルズ数

概要

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A conceptual model was constructed for the problem of determining in circular pipes the conditions under which the transition from laminar to turbulent flow occurs. From many previous experimental investigations, it became clear that (i) plots of the transition length versus the Reynolds number (Re) show that the transition occurs in the entrance region under the condition of a natural disturbance, and (ii) plots of the critical Re (Rc) versus the ratio of bellmouth diameter to the pipe diameter show that for the case of a straight pipe, Rc takes a minimum value of approximately 2000. In the entrance region, the velocity profile changes from uniform at the inlet to parabolic at the fully developed region. The normal wall strength works as an acceleration force and decreases inversely as Re increases. Hence, the onset of the transition should depend on whether or not the acceleration power provided by the normal wall strength exceeds a required value, which is a constant of 0.785 regardless of Re. The minimum Rc of 2040 was thus obtained when using J0=101 radial grid points. Furthermore, it was considered why Rc takes the minimum value under the condition of natural disturbances.
2000-07-25