![]() To investigate the influence of the divergent ratio of the upstream/downstream pipes on the reverse transition, two sets of connected pipes were tested. In this situation, the Reynolds number was smaller in the downstream pipe than in the upstream pipe. This paper shows the experimental and analytical results. The condition for a reverse transition and the laminar-to-turbulent transition in pipe flow undergoing a reverse transition were examined using entropy change and momentum balance models. In the present study, the reverse transition in pipe flow was examined experimentally and theoretically. These relationships are fundamental in physics even when flow is laminar or turbulent, regardless of whether there is disturbance. 11 revealed that the inflow turbulence from the development region into the transition region affected the downstream transition condition by entropy change, not in terms of disturbance. ![]() Kanda 10 studied a typical laminar-to-turbulent transition in straight-pipe flow by momentum balance in the transition region. However, there has been no answer to the question whether the reverse transition appears to violate the second law of thermodynamics. The reverse transition has been discussed in terms of the dissipation of disturbance. ![]() Below the critical Reynolds number, there is no transition from laminar to turbulent flow. These studies realized a reverse transition by decreasing the Reynolds number to less than the critical Reynolds number, which was reported to range from 1400 to 1700. Seki and Matsubara 8 discussed the critical Reynolds number in the case of relaminarization. Narayanan 7 reported the distance required for the reverse transition. Sibulkin 6 reported that the relaminarizing transition occurred more rapidly at smaller Reynolds numbers. Narasimha and Sreenivasan 2 reported that “a common reaction when the subject was mentioned used to be that the implied transition from disorder to order was thermodynamically impossible!” Patel and Head 3 examined the similarities and differences in reverse transitions in pipe flows and boundary layers. Consequently, the entropy of the flow appears to decrease. ![]() In this phenomenon, disturbed turbulent flow changes to less disturbed laminar flow. One of the problems is the occurrence of “relaminarization”, also known as a reverse transition 2, 3, 4, 5, 6, 7, 8, 9. Although the transition phenomenon is common and apparently simple, several problems remain to be solved. The laminar-to-turbulent transition was first described by Reynolds in the nineteenth century 1, and since that time it has been studied in pipe and duct flows. ![]()
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