Laminar-to-Turbulent Transition of Pipe Flows Triggered by Wall-Mounted, Ring-Type Obstacles

Abstract

The literature stresses the inherent stability of laminar pipe flows with parabolic velocity profiles and this paper refers to the relevant publications summarizing this work. To cause such flows to turn into their turbulent state requires laminar pipe flows to be triggered externally. Ring-type, wall-mounted obstacles can be used for this purpose, and investigations in this area are of particular interest to the authors’ work, summarized in this paper. In the investigations presented here, however, a special triggering technique was employed that allowed laminar pipe flows to be exposed to obstacle disturbances for only about 30 ms. Individual puffs and slugs could be produced in this way. Comparisons with fixed wall-mounted obstacles showed that the properties of both types of turbulent slugs were the same. Theoretical derivations are described to provide the required obstacle height as a function of the Reynolds number, to trigger fully developed laminar pipe flows to turn into their turbulent state. Corresponding experimental investigations were also performed as described. Very good agreement between the theoretical and experimental results was obtained. All this demonstrates that a relatively simple ‘ad hoc theory’ can derive the required height of ring-type, wall-inserted obstacles to trigger laminar pipe flows with parabolic velocity profiles to turn turbulent. Other ways to trigger laminar pipe flows to turn turbulent were also investigated by employing blowers and plenum chambers and varying the lengths and diameters of pipes. It is demonstrated, in a somewhat qualitative way, that the maintenance of laminar pipe flows requires all components of a test rig to be matched to each other to maintain pipe flows laminar.