Relaminarising pipe flow by wall movement

TL;DR

This paper experimentally demonstrates that relaminarising turbulent pipe flow can be achieved by rapidly moving a pipe segment to flatten the velocity profile, reducing turbulence and promoting flow stability.

Contribution

It provides a quantitative analysis of how wall movement can induce relaminarisation, identifying thresholds and minimum lengths needed at various Reynolds numbers.

Findings

Relaminarisation occurs when the velocity profile is flattened.

Minimum shift length and speed depend on Reynolds number.

Flow fluctuations decay exponentially after profile modification.

Abstract

Following the recent observation that turbulent pipe flow can be relaminarised by a relatively simple modification of the mean velocity profile, we here carry out a quantitative experimental investigation of this phenomenon. Our study confirms that a flat velocity profile leads to a collapse of turbulence and in order to achieve the blunted profile shape, we employ a moving pipe segment that is briefly and rapidly shifted in the streamwise direction. The relaminarisation threshold and the minimum shift length and speeds are determined as a function of Reynolds number. Although turbulence is still active after the acceleration phase, the modulated profile possesses a severely decreased lift-up potential as measured by transient growth. As shown, this results in an exponential decay of fluctuations and the flow relaminarises. While this method can be easily applied at low to moderate flow speeds, the minimum streamwise length over which the acceleration needs to act increases linearly with the Reynolds number.