This paper complements an earlier paper by Karnitz, Potter, and Smith [1] (1974) in which the mechanism of the transition of a plane Poiseuille flow between parallel plates was examined. In the present investigation an experimental critical Reynolds number of 7500 (based on average velocity and channel height) was achieved at which the flow became unstable and transition to turbulence occurred. The linear theoretical Reynolds number of 7700 for instability appears to be a simple extrapolation of the present data as the disturbance intensity is allowed to shrink to zero. Bursting (an alternating turbulent to laminar flow) was observed at transition. The transient changes in the velocity profile when the flow is intermittent between a turbulent burst and a laminar flow were observed. The major portion of the burst profile is characteristic of the one-seventh power law profile common to fully turbulent flow. Disturbances were observed to amplify to turbulent bursts in the wall boundary layers in the entrance region of the channel in high Reynolds number flows (the Reynolds number must exceed the critical Reynolds number by a sufficient amount). Thus, the wall boundary layer becomes unstable, resulting in a transition to turbulence before the flow becomes fully developed at sufficiently high Reynolds numbers.

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