@article{8999,
abstract = {In many basic shear flows, such as pipe, Couette, and channel flow, turbulence does not
arise from an instability of the laminar state, and both dynamical states co-exist. With decreasing flow speed (i.e., decreasing Reynolds number) the fraction of fluid in laminar motion increases while turbulence recedes and eventually the entire flow relaminarizes. The first step towards understanding the nature of this transition is to determine if the phase change is of either first or second order. In the former case, the turbulent fraction would drop discontinuously to zero as the Reynolds number decreases while in the latter the process would be continuous. For Couette flow, the flow between two parallel plates, earlier studies suggest a discontinuous scenario. In the present study we realize a Couette flow between two concentric cylinders which allows studies to be carried out in large aspect ratios and for extensive observation times. The presented measurements show that the transition in this circular Couette geometry is continuous suggesting that former studies were limited by finite size effects. A further characterization of this transition, in particular its relation to the directed percolation universality class, requires even larger system sizes than presently available. },
author = {Avila, Kerstin and Hof, Björn},
issn = {1099-4300},
journal = {Entropy},
number = {1},
publisher = {MDPI},
title = {{Second-order phase transition in counter-rotating taylor-couette flow experiment}},
doi = {10.3390/e23010058},
volume = {23},
year = {2021},
}
@article{9207,
abstract = {In this paper we experimentally study the transitional range of Reynolds numbers in
plane Couette–Poiseuille flow, focusing our attention on the localized turbulent structures
triggered by a strong impulsive jet and the large-scale flow generated around these
structures. We present a detailed investigation of the large-scale flow and show how
its amplitude depends on Reynolds number and amplitude perturbation. In addition,
we characterize the initial dynamics of the localized turbulent spot, which includes the
coupling between the small and large scales, as well as the dependence of the advection
speed on the large-scale flow generated around the spot. Finally, we provide the first
experimental measurements of the large-scale flow around an oblique turbulent band.},
author = {Klotz, Lukasz and Pavlenko, A. M. and Wesfreid, J. E.},
issn = {1469-7645},
journal = {Journal of Fluid Mechanics},
publisher = {Cambridge University Press},
title = {{Experimental measurements in plane Couette-Poiseuille flow: Dynamics of the large- and small-scale flow}},
doi = {10.1017/jfm.2020.1089},
volume = {912},
year = {2021},
}
@article{9297,
abstract = {We report the results of an experimental investigation into the decay of turbulence in plane Couette–Poiseuille flow using ‘quench’ experiments where the flow laminarises after a sudden reduction in Reynolds number Re. Specifically, we study the velocity field in the streamwise–spanwise plane. We show that the spanwise velocity containing rolls decays faster than the streamwise velocity, which displays elongated regions of higher or lower velocity called streaks. At final Reynolds numbers above 425, the decay of streaks displays two stages: first a slow decay when rolls are present and secondly a more rapid decay of streaks alone. The difference in behaviour results from the regeneration of streaks by rolls, called the lift-up effect. We define the turbulent fraction as the portion of the flow containing turbulence and this is estimated by thresholding the spanwise velocity component. It decreases linearly with time in the whole range of final Re. The corresponding decay slope increases linearly with final Re. The extrapolated value at which this decay slope vanishes is Reaz≈656±10, close to Reg≈670 at which turbulence is self-sustained. The decay of the energy computed from the spanwise velocity component is found to be exponential. The corresponding decay rate increases linearly with Re, with an extrapolated vanishing value at ReAz≈688±10. This value is also close to the value at which the turbulence is self-sustained, showing that valuable information on the transition can be obtained over a wide range of Re.},
author = {Liu, T. and Semin, B. and Klotz, Lukasz and Godoy-Diana, R. and Wesfreid, J. E. and Mullin, T.},
issn = {1469-7645},
journal = {Journal of Fluid Mechanics},
publisher = {Cambridge University Press},
title = {{Decay of streaks and rolls in plane Couette-Poiseuille flow}},
doi = {10.1017/jfm.2021.89},
volume = {915},
year = {2021},
}
@article{8043,
abstract = {With decreasing Reynolds number, Re, turbulence in channel flow becomes spatio-temporally intermittent and self-organises into solitary stripes oblique to the mean flow direction. We report here the existence of localised nonlinear travelling wave solutions of the Navier–Stokes equations possessing this obliqueness property. Such solutions are identified numerically using edge tracking coupled with arclength continuation. All solutions emerge in saddle-node bifurcations at values of Re lower than the non-localised solutions. Relative periodic orbit solutions bifurcating from branches of travelling waves have also been computed. A complete parametric study is performed, including their stability, the investigation of their large-scale flow, and the robustness to changes of the numerical domain.},
author = {Paranjape, Chaitanya S and Duguet, Yohann and Hof, Björn},
issn = {14697645},
journal = {Journal of Fluid Mechanics},
publisher = {Cambridge University Press},
title = {{Oblique stripe solutions of channel flow}},
doi = {10.1017/jfm.2020.322},
volume = {897},
year = {2020},
}
@phdthesis{8350,
abstract = {Cytoplasm is a gel-like crowded environment composed of tens of thousands of macromolecules, organelles, cytoskeletal networks and cytosol. The structure of the cytoplasm is thought to be highly organized and heterogeneous due to the crowding of its constituents and their effective compartmentalization. In such an environment, the diffusive dynamics of the molecules is very restricted, an effect that is further amplified by clustering and anchoring of molecules. Despite the jammed nature of the cytoplasm at the microscopic scale, large-scale reorganization of cytoplasm is essential for important cellular functions, such as nuclear positioning and cell division. How such mesoscale reorganization of the cytoplasm is achieved, especially for very large cells such as oocytes or syncytial tissues that can span hundreds of micrometers in size, has only begun to be understood.
In this thesis, I focus on the recent advances in elucidating the molecular, cellular and biophysical principles underlying cytoplasmic organization across different scales, structures and species. First, I outline which of these principles have been identified by reductionist approaches, such as in vitro reconstitution assays, where boundary conditions and components can be modulated at ease. I then describe how the theoretical and experimental framework established in these reduced systems have been applied to their more complex in vivo counterparts, in particular oocytes and embryonic syncytial structures, and discuss how such complex biological systems can initiate symmetry breaking and establish patterning.
Specifically, I examine an example of large-scale reorganizations taking place in zebrafish embryos, where extensive cytoplasmic streaming leads to the segregation of cytoplasm from yolk granules along the animal-vegetal axis of the embryo. Using biophysical experimentation and theory, I investigate the forces underlying this process, to show that this process does not rely on cortical actin reorganization, as previously thought, but instead on a cell-cycle-dependent bulk actin polymerization wave traveling from the animal to the vegetal pole of the embryo. This wave functions in segregation by both pulling cytoplasm animally and pushing yolk granules vegetally. Cytoplasm pulling is mediated by bulk actin network flows exerting friction forces on the cytoplasm, while yolk granule pushing is achieved by a mechanism closely resembling actin comet formation on yolk granules. This study defines a novel role of bulk actin polymerization waves in embryo polarization via cytoplasmic segregation. Lastly, I describe the cytoplasmic reorganizations taking place during zebrafish oocyte maturation, where the initial segregation of the cytoplasm and yolk granules occurs. Here, I demonstrate a previously uncharacterized wave of microtubule aster formation, traveling the oocyte along the animal-vegetal axis. Further research is required to determine the role of such microtubule structures in cytoplasmic reorganizations therein.
Collectively, these studies provide further evidence for the coupling between cell cytoskeleton and cell cycle machinery, which can underlie a core self-organizing mechanism for orchestrating large-scale reorganizations in a cell-cycle-tunable manner, where the modulations of the force-generating machinery and cytoplasmic mechanics can be harbored to fulfill cellular functions.},
author = {Shamipour, Shayan},
issn = {2663-337X},
pages = {107},
publisher = {IST Austria},
title = {{Bulk actin dynamics drive phase segregation in zebrafish oocytes }},
doi = {10.15479/AT:ISTA:8350},
year = {2020},
}
@article{8634,
abstract = {In laboratory studies and numerical simulations, we observe clear signatures of unstable time-periodic solutions in a moderately turbulent quasi-two-dimensional flow. We validate the dynamical relevance of such solutions by demonstrating that turbulent flows in both experiment and numerics transiently display time-periodic dynamics when they shadow unstable periodic orbits (UPOs). We show that UPOs we computed are also statistically significant, with turbulent flows spending a sizable fraction of the total time near these solutions. As a result, the average rates of energy input and dissipation for the turbulent flow and frequently visited UPOs differ only by a few percent.},
author = {Suri, Balachandra and Kageorge, Logan and Grigoriev, Roman O. and Schatz, Michael F.},
issn = {0031-9007},
journal = {Physical Review Letters},
keywords = {General Physics and Astronomy},
number = {6},
publisher = {American Physical Society},
title = {{Capturing turbulent dynamics and statistics in experiments with unstable periodic orbits}},
doi = {10.1103/physrevlett.125.064501},
volume = {125},
year = {2020},
}
@article{7364,
abstract = {We present nsCouette, a highly scalable software tool to solve the Navier–Stokes equations for incompressible fluid flow between differentially heated and independently rotating, concentric cylinders. It is based on a pseudospectral spatial discretization and dynamic time-stepping. It is implemented in modern Fortran with a hybrid MPI-OpenMP parallelization scheme and thus designed to compute turbulent flows at high Reynolds and Rayleigh numbers. An additional GPU implementation (C-CUDA) for intermediate problem sizes and a version for pipe flow (nsPipe) are also provided.},
author = {Lopez Alonso, Jose M and Feldmann, Daniel and Rampp, Markus and Vela-Martín, Alberto and Shi, Liang and Avila, Marc},
issn = {23527110},
journal = {SoftwareX},
publisher = {Elsevier},
title = {{nsCouette – A high-performance code for direct numerical simulations of turbulent Taylor–Couette flow}},
doi = {10.1016/j.softx.2019.100395},
volume = {11},
year = {2020},
}
@article{7534,
abstract = {In the past two decades, our understanding of the transition to turbulence in shear flows with linearly stable laminar solutions has greatly improved. Regarding the susceptibility of the laminar flow, two concepts have been particularly useful: the edge states and the minimal seeds. In this nonlinear picture of the transition, the basin boundary of turbulence is set by the edge state's stable manifold and this manifold comes closest in energy to the laminar equilibrium at the minimal seed. We begin this paper by presenting numerical experiments in which three-dimensional perturbations are too energetic to trigger turbulence in pipe flow but they do lead to turbulence when their amplitude is reduced. We show that this seemingly counterintuitive observation is in fact consistent with the fully nonlinear description of the transition mediated by the edge state. In order to understand the physical mechanisms behind this process, we measure the turbulent kinetic energy production and dissipation rates as a function of the radial coordinate. Our main observation is that the transition to turbulence relies on the energy amplification away from the wall, as opposed to the turbulence itself, whose energy is predominantly produced near the wall. This observation is further supported by the similar analyses on the minimal seeds and the edge states. Furthermore, we show that the time evolution of production-over-dissipation curves provides a clear distinction between the different initial amplification stages of the transition to turbulence from the minimal seed.},
author = {Budanur, Nazmi B and Marensi, Elena and Willis, Ashley P. and Hof, Björn},
issn = {2469-990X},
journal = {Physical Review Fluids},
number = {2},
publisher = {American Physical Society},
title = {{Upper edge of chaos and the energetics of transition in pipe flow}},
doi = {10.1103/physrevfluids.5.023903},
volume = {5},
year = {2020},
}
@article{7563,
abstract = {We introduce “state space persistence analysis” for deducing the symbolic dynamics of time series data obtained from high-dimensional chaotic attractors. To this end, we adapt a topological data analysis technique known as persistent homology for the characterization of state space projections of chaotic trajectories and periodic orbits. By comparing the shapes along a chaotic trajectory to those of the periodic orbits, state space persistence analysis quantifies the shape similarity of chaotic trajectory segments and periodic orbits. We demonstrate the method by applying it to the three-dimensional Rössler system and a 30-dimensional discretization of the Kuramoto–Sivashinsky partial differential equation in (1+1) dimensions.
One way of studying chaotic attractors systematically is through their symbolic dynamics, in which one partitions the state space into qualitatively different regions and assigns a symbol to each such region.1–3 This yields a “coarse-grained” state space of the system, which can then be reduced to a Markov chain encoding all possible transitions between the states of the system. While it is possible to obtain the symbolic dynamics of low-dimensional chaotic systems with standard tools such as Poincaré maps, when applied to high-dimensional systems such as turbulent flows, these tools alone are not sufficient to determine symbolic dynamics.4,5 In this paper, we develop “state space persistence analysis” and demonstrate that it can be utilized to infer the symbolic dynamics in very high-dimensional settings.},
author = {Yalniz, Gökhan and Budanur, Nazmi B},
issn = {1089-7682},
journal = {Chaos},
number = {3},
publisher = {AIP Publishing},
title = {{Inferring symbolic dynamics of chaotic flows from persistence}},
doi = {10.1063/1.5122969},
volume = {30},
year = {2020},
}
@article{7932,
abstract = {Pulsating flows through tubular geometries are laminar provided that velocities are moderate. This in particular is also believed to apply to cardiovascular flows where inertial forces are typically too low to sustain turbulence. On the other hand, flow instabilities and fluctuating shear stresses are held responsible for a variety of cardiovascular diseases. Here we report a nonlinear instability mechanism for pulsating pipe flow that gives rise to bursts of turbulence at low flow rates. Geometrical distortions of small, yet finite, amplitude are found to excite a state consisting of helical vortices during flow deceleration. The resulting flow pattern grows rapidly in magnitude, breaks down into turbulence, and eventually returns to laminar when the flow accelerates. This scenario causes shear stress fluctuations and flow reversal during each pulsation cycle. Such unsteady conditions can adversely affect blood vessels and have been shown to promote inflammation and dysfunction of the shear stress-sensitive endothelial cell layer.},
author = {Xu, Duo and Varshney, Atul and Ma, Xingyu and Song, Baofang and Riedl, Michael and Avila, Marc and Hof, Björn},
issn = {10916490},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = {21},
pages = {11233--11239},
publisher = {National Academy of Sciences},
title = {{Nonlinear hydrodynamic instability and turbulence in pulsatile flow}},
doi = {10.1073/pnas.1913716117},
volume = {117},
year = {2020},
}