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

Abstract

Based on a novel control scheme, where a steady modification of the streamwise velocity profile leads to complete relaminarization of initially fully turbulent pipe flow, we investigate the applicability and usefulness of customshaped honeycombs for such control. The custom-shaped honeycombs are used as stationary flow management devices which generate specific modifications of the streamwise velocity profile. Stereoscopic particle image velocimetry and pressure drop measurements are used to investigate and capture the development of the relaminarizing flow downstream these devices. We compare the performance of straight (constant length across the radius of the pipe) honeycombs with custom-shaped ones (variable length across the radius). An attempt is made to find the optimal shape for maximal relaminarization at minimal pressure loss. The maximum attainable Reynolds number for total relaminarization is found to be of the order of 10.000. Consequently the respective reduction in skin friction downstream of the device is almost by a factor of 5. The break-even point, where the additional pressure drop caused by the device is balanced by the savings due to relaminarization and a net gain is obtained, corresponds to a downstream stretch of distances as low as approx. 100 pipe diameters of laminar flow.

Stabilising pipe flow by a baffle designed using energy stability

“…This is in agreement with the experimental observations and is assumed to make the flow more stable (Kühnen et al. 2018 a , b , 2019). At moderate , the optimal velocity profile resembles the mean axial velocity profile around in Kühnen et al.…”

“…The former effect is particularly prominent for a = 0.1 and α = 1.01 where the baffle actually causes a valley in the velocity profile, although for larger a only a flattening is seen. This is in agreement with the experimental observations and is assumed to make the flow more stable (Kühnen et al 2018a(Kühnen et al ,b, 2019. At moderate a, the optimal velocity profile resembles the mean axial velocity profile around Re = 3800 in Kühnen et al (2018a) (see their figure 7a), e.g.…”

“…The baffle is modelled by a linear drag force as in Marensi et al (2019) and for convenience is rescaled here by the Reynolds number: f = −F(r, θ, z)u/Re (F ≥ 0) so that F/Re represents 1/K, where K is the permeability of the porous media and U is the laminar flow consistent with the baffle. Kühnen et al (2019) discuss how baffles with variable K can be made using 3-D printing techniques.…”

“…In contrast to these active control approaches, Kühnen et al. (2018 a ) and Kühnen, Scarselli & Hof (2019) found that the passive approach of inserting a localised baffle in a pipe could similarly flatten the flow profile and thereby kill pipe flow turbulence completely up to so that the reduction in skin friction is . To explain this effect, Kühnen et al.…”

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.…”