Relaminarization by Steady Modification of the Streamwise Velocity Profile in a Pipe

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 which injects 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 skin friction in the far 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 local skin friction drag reduction. The underlying physical mechanism of relaminarization is attributed to a weakening of the near-wall turbulence production cycle.Electronic supplementary materialThe online version of this article (10.1007/s10494-018-9896-4) contains supplementary material, which is available to authorized users.

Relaminarization of Pipe Flow by Means of 3D-Printed Shaped Honeycombs

“…Although those absolute values should be treated with caution and might vary for different FMDs (with differently sized honeycombs etc.) and different pipe diameters, the trend that the M-shape is advantageous compared to a mere flattening of the profile is consistent with observations of [2] and [3].…”

“…The velocity profiles of all successfully controlled, i.e. relaminarizing flows considered by [1], [2] and [3] are shown to have a substantially reduced transient growth. We applied the same procedure here to the exemplary velocity profiles shown in fig.…”

“…Another approach, elaborated in [2], was by means of a movable pipe segment which was used to locally accelerate the flow at the wall. A further approach, elaborated in [3], was by injecting fluid through an annular gap at the wall to accelerate the flow close to the wall or by inserting an obstacle partially blocking the pipe. All approaches force the streamwise velocity profile in a similar manner and can modify the streamwise velocity profile appropriately to achieve relaminarization.…”

“…In the present investigation we want to explore a further possibility of specifically modifying the streamwise velocity profile employing passive flow management devices. The aim is to modify the streamwise velocity profile in a pipe similar to [3] by solely passive means to achieve complete relaminarization of initially fully turbulent pipe flow. To homogeneously force the streamwise velocity profile in a pipe a very fine meshed screen with radially varying mesh size may seem the simplest choice, however unwanted apart from modifications of the streamwise velocity such screens also caused cross stream components which can prevent relaminarization.…”

Stabilisation and drag reduction of pipe flows by flattening the base profile

“…Our study is motivated by a recent experimental observation (Kühnen et al 2018b) that manipulation of the flow in a pipe with a baffle can lead to full relaminarisation for flow rates up to 3 times (Re = 6000) that for which turbulence typically appears in the presence of ambient perturbations (Re > 2000). Our focus is on theoretically capturing the phenomenon observed in experiments so that the process can then be optimised.…”