@article{745,
abstract = {Fluid flows in nature and applications are frequently subject to periodic velocity modulations. Surprisingly, even for the generic case of flow through a straight pipe, there is little consensus regarding the influence of pulsation on the transition threshold to turbulence: while most studies predict a monotonically increasing threshold with pulsation frequency (i.e. Womersley number, ), others observe a decreasing threshold for identical parameters and only observe an increasing threshold at low . In the present study we apply recent advances in the understanding of transition in steady shear flows to pulsating pipe flow. For moderate pulsation amplitudes we find that the first instability encountered is subcritical (i.e. requiring finite amplitude disturbances) and gives rise to localized patches of turbulence ('puffs') analogous to steady pipe flow. By monitoring the impact of pulsation on the lifetime of turbulence we map the onset of turbulence in parameter space. Transition in pulsatile flow can be separated into three regimes. At small Womersley numbers the dynamics is dominated by the decay turbulence suffers during the slower part of the cycle and hence transition is delayed significantly. As shown in this regime thresholds closely agree with estimates based on a quasi-steady flow assumption only taking puff decay rates into account. The transition point predicted in the zero limit equals to the critical point for steady pipe flow offset by the oscillation Reynolds number (i.e. the dimensionless oscillation amplitude). In the high frequency limit on the other hand, puff lifetimes are identical to those in steady pipe flow and hence the transition threshold appears to be unaffected by flow pulsation. In the intermediate frequency regime the transition threshold sharply drops (with increasing ) from the decay dominated (quasi-steady) threshold to the steady pipe flow level.},
author = {Xu, Duo and Warnecke, Sascha and Song, Baofang and Ma, Xingyu and Hof, Björn},
issn = {00221120},
journal = {Journal of Fluid Mechanics},
pages = {418 -- 432},
publisher = {Cambridge University Press},
title = {{Transition to turbulence in pulsating pipe flow}},
doi = {10.1017/jfm.2017.620},
volume = {831},
year = {2017},
}
@article{792,
abstract = {The chaotic dynamics of low-dimensional systems, such as Lorenz or Rössler flows, is guided by the infinity of periodic orbits embedded in their strange attractors. Whether this is also the case for the infinite-dimensional dynamics of Navier–Stokes equations has long been speculated, and is a topic of ongoing study. Periodic and relative periodic solutions have been shown to be involved in transitions to turbulence. Their relevance to turbulent dynamics – specifically, whether periodic orbits play the same role in high-dimensional nonlinear systems like the Navier–Stokes equations as they do in lower-dimensional systems – is the focus of the present investigation. We perform here a detailed study of pipe flow relative periodic orbits with energies and mean dissipations close to turbulent values. We outline several approaches to reduction of the translational symmetry of the system. We study pipe flow in a minimal computational cell at Re=2500, and report a library of invariant solutions found with the aid of the method of slices. Detailed study of the unstable manifolds of a sample of these solutions is consistent with the picture that relative periodic orbits are embedded in the chaotic saddle and that they guide the turbulent dynamics.},
author = {Budanur, Nazmi B and Short, Kimberly and Farazmand, Mohammad and Willis, Ashley and Cvitanović, Predrag},
issn = {00221120},
journal = {Journal of Fluid Mechanics},
pages = {274 -- 301},
publisher = {Cambridge University Press},
title = {{Relative periodic orbits form the backbone of turbulent pipe flow}},
doi = {10.1017/jfm.2017.699},
volume = {833},
year = {2017},
}
@article{463,
abstract = {We investigate transient behaviors induced by magnetic fields on the dynamics of the flow of a ferrofluid in the gap between two concentric, independently rotating cylinders. Without applying any magnetic fields, we uncover emergence of flow states constituted by a combination of a localized spiral state (SPIl) in the top and bottom of the annulus and different multi-cell flow states (SPI2v, SPI3v) with toroidally closed vortices in the interior of the bulk (SPIl+2v = SPIl + SPI2v and SPIl+3v = SPIl + SPI3v). However, when a magnetic field is presented, we observe the transient behaviors between multi-cell states passing through two critical thresholds in a strength of an axial (transverse) magnetic field. Before the first critical threshold of a magnetic field strength, multi-stable states with different number of cells could be observed. After the first critical threshold, we find the transient behavior between the three- and two-cell flow states. For more strength of magnetic field or after the second critical threshold, we discover that multi-cell states are disappeared and a localized spiral state remains to be stimulated. The studied transient behavior could be understood by the investigation of various quantities including a modal kinetic energy, a mode amplitude of the radial velocity, wavenumber, angular momentum, and torque. In addition, the emergence of new flow states and the transient behavior between their states in ferrofluidic flows indicate that richer and potentially controllable dynamics through magnetic fields could be possible in ferrofluic flow.},
author = {Altmeyer, Sebastian and Do, Younghae and Ryu, Soorok},
issn = {10541500},
journal = {Chaos},
number = {11},
publisher = {AIP},
title = {{Transient behavior between multi-cell flow states in ferrofluidic Taylor-Couette flow}},
doi = {10.1063/1.5002771},
volume = {27},
year = {2017},
}
@article{513,
abstract = {We present an experimental setup that creates a shear flow with zero mean advection velocity achieved by counterbalancing the nonzero streamwise pressure gradient by moving boundaries, which generates plane Couette-Poiseuille flow. We obtain experimental results in the transitional regime for this flow. Using flow visualization, we characterize the subcritical transition to turbulence in Couette-Poiseuille flow and show the existence of turbulent spots generated by a permanent perturbation. Due to the zero mean advection velocity of the base profile, these turbulent structures are nearly stationary. We distinguish two regions of the turbulent spot: the active turbulent core, which is characterized by waviness of the streaks similar to traveling waves, and the surrounding region, which includes in addition the weak undisturbed streaks and oblique waves at the laminar-turbulent interface. We also study the dependence of the size of these two regions on Reynolds number. Finally, we show that the traveling waves move in the downstream (Poiseuille) direction.},
author = {Klotz, Lukasz and Lemoult, Grégoire M and Frontczak, Idalia and Tuckerman, Laurette and Wesfreid, José},
journal = {Physical Review Fluids},
number = {4},
publisher = {American Physical Society},
title = {{Couette-Poiseuille flow experiment with zero mean advection velocity: Subcritical transition to turbulence}},
doi = {10.1103/PhysRevFluids.2.043904},
volume = {2},
year = {2017},
}
@article{651,
abstract = {Superhydrophobic surfaces reduce the frictional drag between water and solid materials, but this effect is often temporary. The realization of sustained drag reduction has applications for water vehicles and pipeline flows.
},
author = {Hof, Björn},
issn = {00280836},
journal = {Nature},
number = {7636},
pages = {161 -- 162},
publisher = {Nature Publishing Group},
title = {{Fluid dynamics: Water flows out of touch}},
doi = {10.1038/541161a},
volume = {541},
year = {2017},
}
@article{662,
abstract = {We report a direct-numerical-simulation study of the Taylor-Couette flow in the quasi-Keplerian regime at shear Reynolds numbers up to (105). Quasi-Keplerian rotating flow has been investigated for decades as a simplified model system to study the origin of turbulence in accretion disks that is not fully understood. The flow in this study is axially periodic and thus the experimental end-wall effects on the stability of the flow are avoided. Using optimal linear perturbations as initial conditions, our simulations find no sustained turbulence: the strong initial perturbations distort the velocity profile and trigger turbulence that eventually decays.},
author = {Shi, Liang and Hof, Björn and Rampp, Markus and Avila, Marc},
issn = {10706631},
journal = {Physics of Fluids},
number = {4},
publisher = {American Institute of Physics},
title = {{Hydrodynamic turbulence in quasi Keplerian rotating flows}},
doi = {10.1063/1.4981525},
volume = {29},
year = {2017},
}
@article{673,
abstract = {We present a numerical study of wavy supercritical cylindrical Couette flow between counter-rotating cylinders in which the wavy pattern propagates either prograde with the inner cylinder or retrograde opposite the rotation of the inner cylinder. The wave propagation reversals from prograde to retrograde and vice versa occur at distinct values of the inner cylinder Reynolds number when the associated frequency of the wavy instability vanishes. The reversal occurs for both twofold and threefold symmetric wavy vortices. Moreover, the wave propagation reversal only occurs for sufficiently strong counter-rotation. The flow pattern reversal appears to be intrinsic in the system as either periodic boundary conditions or fixed end wall boundary conditions for different system sizes always result in the wave propagation reversal. We present a detailed bifurcation sequence and parameter space diagram with respect to retrograde behavior of wavy flows. The retrograde propagation of the instability occurs when the inner Reynolds number is about two times the outer Reynolds number. The mechanism for the retrograde propagation is associated with the inviscidly unstable region near the inner cylinder and the direction of the global average azimuthal velocity. Flow dynamics, spatio-temporal behavior, global mean angular velocity, and torque of the flow with the wavy pattern are explored.},
author = {Altmeyer, Sebastian and Lueptow, Richard},
issn = {24700045},
journal = {Physical Review E - Statistical, Nonlinear, and Soft Matter Physics},
number = {5},
publisher = {American Physiological Society},
title = {{Wave propagation reversal for wavy vortices in wide gap counter rotating cylindrical Couette flow}},
doi = {10.1103/PhysRevE.95.053103},
volume = {95},
year = {2017},
}
@article{1021,
abstract = {Most flows in nature and engineering are turbulent because of their large velocities and spatial scales. Laboratory experiments on rotating quasi-Keplerian flows, for which the angular velocity decreases radially but the angular momentum increases, are however laminar at Reynolds numbers exceeding one million. This is in apparent contradiction to direct numerical simulations showing that in these experiments turbulence transition is triggered by the axial boundaries. We here show numerically that as the Reynolds number increases, turbulence becomes progressively confined to the boundary layers and the flow in the bulk fully relaminarizes. Our findings support that turbulence is unlikely to occur in isothermal constant-density quasi-Keplerian flows.},
author = {Lopez Alonso, Jose M and Avila, Marc},
issn = {00221120},
journal = {Journal of Fluid Mechanics},
pages = {21 -- 34},
publisher = {Cambridge University Press},
title = {{Boundary layer turbulence in experiments on quasi Keplerian flows}},
doi = {10.1017/jfm.2017.109},
volume = {817},
year = {2017},
}
@article{1087,
abstract = {Using extensive direct numerical simulations, the dynamics of laminar-turbulent fronts in pipe flow is investigated for Reynolds numbers between and 5500. We here investigate the physical distinction between the fronts of weak and strong slugs both by analysing the turbulent kinetic energy budget and by comparing the downstream front motion to the advection speed of bulk turbulent structures. Our study shows that weak downstream fronts travel slower than turbulent structures in the bulk and correspond to decaying turbulence at the front. At the downstream front speed becomes faster than the advection speed, marking the onset of strong fronts. In contrast to weak fronts, turbulent eddies are generated at strong fronts by feeding on the downstream laminar flow. Our study also suggests that temporal fluctuations of production and dissipation at the downstream laminar-turbulent front drive the dynamical switches between the two types of front observed up to.},
author = {Song, Baofang and Barkley, Dwight and Hof, Björn and Avila, Marc},
issn = {00221120},
journal = {Journal of Fluid Mechanics},
pages = {1045 -- 1059},
publisher = {Cambridge University Press},
title = {{Speed and structure of turbulent fronts in pipe flow}},
doi = {10.1017/jfm.2017.14},
volume = {813},
year = {2017},
}
@article{1160,
abstract = {We investigate fundamental nonlinear dynamics of ferrofluidic Taylor-Couette flow - flow confined be-tween two concentric independently rotating cylinders - consider small aspect ratio by solving the ferro-hydrodynamical equations, carrying out systematic bifurcation analysis. Without magnetic field, we find steady flow patterns, previously observed with a simple fluid, such as those containing normal one- or two vortex cells, as well as anomalous one-cell and twin-cell flow states. However, when a symmetry-breaking transverse magnetic field is present, all flow states exhibit stimulated, finite two-fold mode. Various bifurcations between steady and unsteady states can occur, corresponding to the transitions between the two-cell and one-cell states. While unsteady, axially oscillating flow states can arise, we also detect the emergence of new unsteady flow states. In particular, we uncover two new states: one contains only the azimuthally oscillating solution in the configuration of the twin-cell flow state, and an-other a rotating flow state. Topologically, these flow states are a limit cycle and a quasiperiodic solution on a two-torus, respectively. Emergence of new flow states in addition to observed ones with classical fluid, indicates that richer but potentially more controllable dynamics in ferrofluidic flows, as such flow states depend on the external magnetic field.},
author = {Altmeyer, Sebastian and Do, Younghae and Lai, Ying},
issn = {20452322},
journal = {Scientific Reports},
publisher = {Nature Publishing Group},
title = {{Dynamics of ferrofluidic flow in the Taylor-Couette system with a small aspect ratio}},
doi = {10.1038/srep40012},
volume = {7},
year = {2017},
}