@article{6486,
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 custom-shaped 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) and try to determine the optimal shape for maximal relaminarization at minimal pressure loss. The optimally modified streamwise velocity profile is found to be M-shaped, and 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 approximately 100 pipe diameters of laminar flow.},
author = {Kühnen, Jakob and Scarselli, Davide and Hof, Björn},
issn = {1528901X},
journal = {Journal of Fluids Engineering},
number = {11},
publisher = {ASME},
title = {{Relaminarization of pipe flow by means of 3D-printed shaped honeycombs}},
doi = {10.1115/1.4043494},
volume = {141},
year = {2019},
}
@article{6228,
abstract = {Following the recent observation that turbulent pipe flow can be relaminarised bya relatively simple modification of the mean velocity profile, we here carry out aquantitative experimental investigation of this phenomenon. Our study confirms thata flat velocity profile leads to a collapse of turbulence and in order to achieve theblunted profile shape, we employ a moving pipe segment that is briefly and rapidlyshifted in the streamwise direction. The relaminarisation threshold and the minimumshift length and speeds are determined as a function of Reynolds number. Althoughturbulence is still active after the acceleration phase, the modulated profile possessesa severely decreased lift-up potential as measured by transient growth. As shown,this results in an exponential decay of fluctuations and the flow relaminarises. Whilethis method can be easily applied at low to moderate flow speeds, the minimumstreamwise length over which the acceleration needs to act increases linearly with theReynolds number.},
author = {Scarselli, Davide and Kühnen, Jakob and Hof, Björn},
issn = {14697645},
journal = {Journal of Fluid Mechanics},
pages = {934--948},
publisher = {Cambridge University Press},
title = {{Relaminarising pipe flow by wall movement}},
doi = {10.1017/jfm.2019.191},
volume = {867},
year = {2019},
}
@article{422,
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.},
author = {Kühnen, Jakob and Scarselli, Davide and Schaner, Markus and Hof, Björn},
journal = {Flow Turbulence and Combustion},
number = {4},
pages = {919 -- 942},
publisher = {Springer},
title = {{Relaminarization by steady modification of the streamwise velocity profile in a pipe}},
doi = {10.1007/s10494-018-9896-4},
volume = {100},
year = {2018},
}
@article{461,
abstract = {Turbulence is the major cause of friction losses in transport processes and it is responsible for a drastic drag increase in flows over bounding surfaces. While much effort is invested into developing ways to control and reduce turbulence intensities, so far no methods exist to altogether eliminate turbulence if velocities are sufficiently large. We demonstrate for pipe flow that appropriate distortions to the velocity profile lead to a complete collapse of turbulence and subsequently friction losses are reduced by as much as 90%. Counterintuitively, the return to laminar motion is accomplished by initially increasing turbulence intensities or by transiently amplifying wall shear. Since neither the Reynolds number nor the shear stresses decrease (the latter often increase), these measures are not indicative of turbulence collapse. Instead, an amplification mechanism measuring the interaction between eddies and the mean shear is found to set a threshold below which turbulence is suppressed beyond recovery.},
author = {Kühnen, Jakob and Song, Baofang and Scarselli, Davide and Budanur, Nazmi B and Riedl, Michael and Willis, Ashley and Avila, Marc and Hof, Björn},
journal = {Nature Physics},
pages = {386--390},
publisher = {Nature Publishing Group},
title = {{Destabilizing turbulence in pipe flow}},
doi = {10.1038/s41567-017-0018-3},
volume = {14},
year = {2018},
}
@article{1837,
abstract = {Transition to turbulence in straight pipes occurs in spite of the linear stability of the laminar Hagen-Poiseuille flow if both the amplitude of flow perturbations and the Reynolds number Re exceed a minimum threshold (subcritical transition). As the pipe curvature increases, centrifugal effects become important, modifying the basic flow as well as the most unstable linear modes. If the curvature (tube-to-coiling diameter d/D) is sufficiently large, a Hopf bifurcation (supercritical instability) is encountered before turbulence can be excited (subcritical instability). We trace the instability thresholds in the Re - d/D parameter space in the range 0.01 ≤ d/D\ ≤ 0.1 by means of laser-Doppler velocimetry and determine the point where the subcritical and supercritical instabilities meet. Two different experimental set-ups are used: a closed system where the pipe forms an axisymmetric torus and an open system employing a helical pipe. Implications for the measurement of friction factors in curved pipes are discussed.},
author = {Kühnen, Jakob and Braunshier, P and Schwegel, M and Kuhlmann, Hendrik and Hof, Björn},
journal = {Journal of Fluid Mechanics},
number = {5},
publisher = {Cambridge University Press},
title = {{Subcritical versus supercritical transition to turbulence in curved pipes}},
doi = {10.1017/jfm.2015.184},
volume = {770},
year = {2015},
}
@article{2050,
abstract = {The flow instability and further transition to turbulence in a toroidal pipe (torus) with curvature ratio (tube-to-coiling diameter) 0.049 is investigated experimentally. The flow inside the toroidal pipe is driven by a steel sphere fitted to the inner pipe diameter. The sphere is moved with constant azimuthal velocity from outside the torus by a moving magnet. The experiment is designed to investigate curved pipe flow by optical measurement techniques. Using stereoscopic particle image velocimetry, laser Doppler velocimetry and pressure drop measurements, the flow is measured for Reynolds numbers ranging from 1000 to 15 000. Time- and space-resolved velocity fields are obtained and analysed. The steady axisymmetric basic flow is strongly influenced by centrifugal effects. On an increase of the Reynolds number we find a sequence of bifurcations. For Re=4075±2% a supercritical bifurcation to an oscillatory flow is found in which waves travel in the streamwise direction with a phase velocity slightly faster than the mean flow. The oscillatory flow is superseded by a presumably quasi-periodic flow at a further increase of the Reynolds number before turbulence sets in. The results are found to be compatible, in general, with earlier experimental and numerical investigations on transition to turbulence in helical and curved pipes. However, important aspects of the bifurcation scenario differ considerably.},
author = {Kühnen, Jakob and Holzner, Markus and Hof, Björn and Kuhlmann, Hendrik},
journal = {Journal of Fluid Mechanics},
pages = {463 -- 491},
publisher = {Cambridge University Press},
title = {{Experimental investigation of transitional flow in a toroidal pipe}},
doi = {10.1017/jfm.2013.603},
volume = {738},
year = {2014},
}