Rapid temporal acceleration of a turbulent pipe flow

Measurements were made of fully developed turbulent pipe flows subjected to temporal pressure gradients larger than those considered previously. Integral profile parameters exhibited similar qualitative trends when time was scaled with the flow rise time and this differed from corresponding spatial development of flows subjected to steady streamwise pressure gradients. Velocity profiles were initially characterized by significant reduction of their wake component. This was accompanied by turbulence intensity freezing in the core region of the pipe and a rapid turbulence increase in the near-wall region that propagated towards the pipe centre. The final phase of the acceleration was characterized by reconstitution of the wake, producing a velocity profile inflection and the generation of turbulence in that vicinity. These features were not observed in previous investigations, possibly due to the present use of higher ramp rates and initial Reynolds numbers.