Influence of test-rigs on the laminar-to-turbulent transition of pipe flows

Theoretical studies of the laminar-to-turbulent transition of pipe flow, based on linear instability considerations, suggest an inherent stability of the flow to small disturbances. Different to this finding, even in most carefully carried out experiments, transition of pipe flows occurs at finite Reynolds numbers. However, the reported critical Reynolds number range differs by nearly two orders of magnitude (1.9 * 10³ ≤ Re_cr ≤ 1 * 10⁵). No reason for this extended range is given in the literature. A closer look at the existing data shows, however, that there is a clear dependence of the critical Reynolds number on the employed pipe diameter. The authors work aimed at a closer understanding of the dependence of Re_cr on the pipe diameter of the employed test rig. It is shown that the existing data show an increase of the critical Reynolds number with increasing pipe diameter. With this understanding a new pipe flow test facility was designed, manufactured and employed for experiments to achieve an increase of the critical Reynolds number with decreasing pipe diameter. New test facility yielded much higher critical Reynolds numbers for pipe flows than those reported in literature up to date for the investigated pipe diameters. The paper also suggests that the developments of turbulent velocity fluctuations in nozzles, i.e. the decrease of longitudinal velocity fluctuations and the increase of the cross flow fluctuations, is one of the mechanisms that cause transition in pipe flows.