Relaminarising pipe flow by wall movement

Abstract

Received xx; revised xx; accepted xx)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.Key words: Authors should not enter keywords on the manuscript, as these must be chosen by the author during the online submission process and will then be added during the typesetting process (see http://journals.cambridge.org/data/relatedlink/jfm-keywords.pdf for the full list)

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.…”

“…One technique was by stirring the flow with four rotors. 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.…”

Designing a minimal baffle to destabilise turbulence in pipe flows

“…Recently, there has been a series of experiments (Kühnen et al 2018 a , b ; Kühnen, Scarselli & Hof 2019; Scarselli, Kühnen & Hof 2019) showing that the flattening of a turbulent streamwise velocity profile in a pipe flow leads to a full collapse of turbulence for Reynolds numbers up to , thus reducing the frictional losses by as much as 90 %. Different experimental techniques were employed to obtain the flattened base profile – e.g.…”

Resolvent analysis of turbulent channel flow with manipulated mean velocity profile

“…Very recently, Kühnen et al (2018) and Scarselli et al (2019) numerically and experimentally demonstrated that controls aiming at decelerating the flow in the center of a channel and accelerating it in the near-wall region lead to relaminarization of turbulent flow. In other words, making the turbulent mean velocity profile 'flattened' successfully suppresses the turbulence intensities.…”