@phdthesis{7258,
  abstract     = {Many flows encountered in nature and applications are characterized by a chaotic motion known as turbulence. Turbulent flows generate intense friction with pipe walls and are responsible for considerable amounts of energy losses at world scale. The nature of turbulent friction and techniques aimed at reducing it have been subject of extensive research over the last century, but no definite answer has been found yet. In this thesis we show that in pipes at moderate turbulent Reynolds numbers friction is better described by the power law first introduced by Blasius and not by the Prandtl–von Kármán formula. At higher Reynolds numbers, large scale motions gradually become more important in the flow and can be related to the change in scaling of friction. Next, we present a series of new techniques that can relaminarize turbulence by suppressing a key mechanism that regenerates it at walls, the lift–up effect. In addition, we investigate the process of turbulence decay in several experiments and discuss the drag reduction potential. Finally, we examine the behavior of friction under pulsating conditions inspired by the human heart cycle and we show that under such circumstances turbulent friction can be reduced to produce energy savings.},
  author       = {Scarselli, Davide},
  issn         = {2663-337X},
  pages        = {174},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{New approaches to reduce friction in turbulent pipe flow}},
  doi          = {10.15479/AT:ISTA:7258},
  year         = {2020},
}

@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         = {1469-7645},
  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{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},
}