Experiments on Transition to Turbulence in a Constant-Acceleration Pipe Flow

Experiments were conducted to study transition to turbulence in pipe flows started from rest with a linear increase in mean velocity. The data were taken at the Unsteady Flow Loop Facility at the Naval Underwater System Center, using a 5-cm diameter pipe 30 meters long. Instrumentation included static pressure, wall pressure, and wall shear stress sensors, as well as a laser Doppler velocimeter and a transient flowmeter. A downstream control valve was programmed to produce nearly constant mean flow accelerations, a, from 2 to 12 m/s². In each of 37 runs, the time of transition to turbulence was the same throughout the pipe to within ± 30 ms, indicating a global instability. As acceleration increased, the transition Reynolds number Re_D increased monotonically from 2 × 10⁵ to 5 × 10⁵. Other dimensionless transition parameters are also presented, the simplest and most effective of which is T* ≈ 400 ± 10 percent for the present experiments, where T* = t_tr(a²/ν)^1/3 and ν is kinematic viscosity.