Relaminarization by steady modification of the streamwise velocity profile in a pipe

TL;DR

This study demonstrates that steady modifications to the streamwise velocity profile in a pipe can fully relaminarize turbulent flow, significantly reducing drag and pressure drop, with potential applications in flow control.

Contribution

It introduces simple, steady flow control devices that modify the velocity profile to achieve complete relaminarization at high Reynolds numbers, a novel approach in flow management.

Findings

Total relaminarization achieved up to Re=6000

Pressure drop reduced by a factor of 3.4 after relaminarization

Transient relaminarization reduces drag at higher Reynolds numbers

Abstract

We show that a rather simple, steady modification of the streamwise velocity profile in a pipe can lead to a complete collapse of turbulence and the flow fully relaminarizes. Two different devices, a stationary obstacle (inset) and a device to inject additional fluid through an annular gap close to the wall, are used to control the flow. Both devices modify the streamwise velocity profile such that the flow in the center of the pipe is decelerated and the flow in the near wall region is accelerated. We present measurements with stereoscopic particle image velocimetry to investigate and capture the development of the relaminarizing flow downstream these devices and the specific circumstances responsible for relaminarization. We find total relaminarization up to Reynolds numbers of 6000, where the pressure drop in the downstream distance is reduced by a factor of 3.4 due to relaminarization. In a smooth straight pipe the flow remains completely laminar downstream of the control. Furthermore, we show that transient (temporary) relaminarization in a spatially confined region right downstream the devices occurs also at much higher Reynolds numbers, accompanied by a significant drag reduction. The underlying physical mechanism of relaminarization is attributed to a weakening of the near-wall turbulence production cycle.