TY - GEN AB - Codes and data for reproducing the results of N. B. Budanur and B. Hof "An autonomous compartmental model for accelerating epidemics" AU - Budanur, Nazmi B ID - 11711 TI - burakbudanur/autoacc-public ER - TY - CHAP AB - Streaky structures in the boundary layers are often generated by surface roughness elements and/or free-stream turbulence, and are known to have significant effects on boundary-layer instability. In this paper, we investigate the impact of two forms of streaks on the instability of supersonic boundary layers. The first concerns the streaks generated by an array of spanwise periodic and streamwise elongated surface roughness elements, and our interest is how these streaks influence the lower-branch viscous first modes, whose characteristic wavelength and frequency are on the classical triple-deck scales. By adapting the triple-deck theory in the incompressible regime to the supersonic one, we first derived a simplified system which allows for efficient calculation of the streaks. The asymptotic analysis simplifies a bi-global eigenvalue problem to a one-dimensional problem in the spanwise direction, showing that the instability is controlled at leading order solely by the spanwise-dependent wall shear. In the fundamental configuration, the streaks stabilize first modes at low frequencies but destabilize the high-frequency ones. In the subharmonic configuration, the streaks generally destabilize the first mode across the entire frequency band. Importantly, the spanwise even modes are of radiating nature, i.e. they emit acoustic waves spontaneously to the far field. Streaks of the second form are generated by low-frequency vortical disturbances representing free-stream turbulence. They alter the flow in the entire layer and their effects on instability are investigated by solving the inviscid bi-global eigenvalue problem. Different from the incompressible case, a multitude of compressible instability modes exists, of which the dominant mode is an inviscid instability associated with the spanwise shear. In addition, there exists a separate branch of instability modes that have smaller growth rates but are spontaneously radiating. AU - Liu, Jianxin AU - Marensi, Elena AU - Wu, Xuesong ED - Sherwin, Spencer ED - Schmid, Peter ED - Wu, Xuesong ID - 10820 SN - 1875-3507 T2 - IUTAM Laminar-Turbulent Transition TI - Effects of streaky structures on the instability of supersonic boundary layers VL - 38 ER - TY - JOUR AB - We investigate the local self-sustained process underlying spiral turbulence in counter-rotating Taylor–Couette flow using a periodic annular domain, shaped as a parallelogram, two of whose sides are aligned with the cylindrical helix described by the spiral pattern. The primary focus of the study is placed on the emergence of drifting–rotating waves (DRW) that capture, in a relatively small domain, the main features of coherent structures typically observed in developed turbulence. The transitional dynamics of the subcritical region, far below the first instability of the laminar circular Couette flow, is determined by the upper and lower branches of DRW solutions originated at saddle-node bifurcations. The mechanism whereby these solutions self-sustain, and the chaotic dynamics they induce, are conspicuously reminiscent of other subcritical shear flows. Remarkably, the flow properties of DRW persist even as the Reynolds number is increased beyond the linear stability threshold of the base flow. Simulations in a narrow parallelogram domain stretched in the azimuthal direction to revolve around the apparatus a full turn confirm that self-sustained vortices eventually concentrate into a localised pattern. The resulting statistical steady state satisfactorily reproduces qualitatively, and to a certain degree also quantitatively, the topology and properties of spiral turbulence as calculated in a large periodic domain of sufficient aspect ratio that is representative of the real system. AU - Wang, B. AU - Ayats López, Roger AU - Deguchi, K. AU - Mellibovsky, F. AU - Meseguer, A. ID - 12137 JF - Journal of Fluid Mechanics KW - Mechanical Engineering KW - Mechanics of Materials KW - Condensed Matter Physics KW - Applied Mathematics SN - 0022-1120 TI - Self-sustainment of coherent structures in counter-rotating Taylor–Couette flow VL - 951 ER - TY - JOUR AB - Theoretical foundations of chaos have been predominantly laid out for finite-dimensional dynamical systems, such as the three-body problem in classical mechanics and the Lorenz model in dissipative systems. In contrast, many real-world chaotic phenomena, e.g., weather, arise in systems with many (formally infinite) degrees of freedom, which limits direct quantitative analysis of such systems using chaos theory. In the present work, we demonstrate that the hydrodynamic pilot-wave systems offer a bridge between low- and high-dimensional chaotic phenomena by allowing for a systematic study of how the former connects to the latter. Specifically, we present experimental results, which show the formation of low-dimensional chaotic attractors upon destabilization of regular dynamics and a final transition to high-dimensional chaos via the merging of distinct chaotic regions through a crisis bifurcation. Moreover, we show that the post-crisis dynamics of the system can be rationalized as consecutive scatterings from the nonattracting chaotic sets with lifetimes following exponential distributions. AU - Choueiri, George H AU - Suri, Balachandra AU - Merrin, Jack AU - Serbyn, Maksym AU - Hof, Björn AU - Budanur, Nazmi B ID - 12259 IS - 9 JF - Chaos: An Interdisciplinary Journal of Nonlinear Science KW - Applied Mathematics KW - General Physics and Astronomy KW - Mathematical Physics KW - Statistical and Nonlinear Physics SN - 1054-1500 TI - Crises and chaotic scattering in hydrodynamic pilot-wave experiments VL - 32 ER - TY - JOUR AB - We report frictional drag reduction and a complete flow relaminarization of elastic turbulence (ET) at vanishing inertia in a viscoelastic channel flow past an obstacle. We show that the intensity of the observed elastic waves and wall-normal vorticity correlate well with the measured drag above the onset of ET. Moreover, we find that the elastic wave frequency grows with the Weissenberg number, and at sufficiently high frequency it causes a decay of the elastic waves, resulting in ET attenuation and drag reduction. Thus, this allows us to substantiate a physical mechanism, involving the interaction of elastic waves with wall-normal vorticity fluctuations, leading to the drag reduction and relaminarization phenomena at low Reynolds number. AU - Kumar, M. Vijay AU - Varshney, Atul AU - Li, Dongyang AU - Steinberg, Victor ID - 12279 IS - 8 JF - Physical Review Fluids KW - Fluid Flow and Transfer Processes KW - Modeling and Simulation KW - Computational Mechanics SN - 2469-990X TI - Relaminarization of elastic turbulence VL - 7 ER - TY - JOUR AB - In this paper, we explore the stability and dynamical relevance of a wide variety of steady, time-periodic, quasiperiodic, and chaotic flows arising between orthogonally stretching parallel plates. We first explore the stability of all the steady flow solution families formerly identified by Ayats et al. [“Flows between orthogonally stretching parallel plates,” Phys. Fluids 33, 024103 (2021)], concluding that only the one that originates from the Stokesian approximation is actually stable. When both plates are shrinking at identical or nearly the same deceleration rates, this Stokesian flow exhibits a Hopf bifurcation that leads to stable time-periodic regimes. The resulting time-periodic orbits or flows are tracked for different Reynolds numbers and stretching rates while monitoring their Floquet exponents to identify secondary instabilities. It is found that these time-periodic flows also exhibit Neimark–Sacker bifurcations, generating stable quasiperiodic flows (tori) that may sometimes give rise to chaotic dynamics through a Ruelle–Takens–Newhouse scenario. However, chaotic dynamics is unusually observed, as the quasiperiodic flows generally become phase-locked through a resonance mechanism before a strange attractor may arise, thus restoring the time-periodicity of the flow. In this work, we have identified and tracked four different resonance regions, also known as Arnold tongues or horns. In particular, the 1 : 4 strong resonance region is explored in great detail, where the identified scenarios are in very good agreement with normal form theory. AU - Wang, B. AU - Ayats López, Roger AU - Meseguer, A. AU - Marques, F. ID - 12146 IS - 11 JF - Physics of Fluids KW - Condensed Matter Physics KW - Fluid Flow and Transfer Processes KW - Mechanics of Materials KW - Computational Mechanics KW - Mechanical Engineering SN - 1070-6631 TI - Phase-locking flows between orthogonally stretching parallel plates VL - 34 ER - TY - JOUR AB - The mammalian neocortex is composed of diverse neuronal and glial cell classes that broadly arrange in six distinct laminae. Cortical layers emerge during development and defects in the developmental programs that orchestrate cortical lamination are associated with neurodevelopmental diseases. The developmental principle of cortical layer formation depends on concerted radial projection neuron migration, from their birthplace to their final target position. Radial migration occurs in defined sequential steps, regulated by a large array of signaling pathways. However, based on genetic loss-of-function experiments, most studies have thus far focused on the role of cell-autonomous gene function. Yet, cortical neuron migration in situ is a complex process and migrating neurons traverse along diverse cellular compartments and environments. The role of tissue-wide properties and genetic state in radial neuron migration is however not clear. Here we utilized mosaic analysis with double markers (MADM) technology to either sparsely or globally delete gene function, followed by quantitative single-cell phenotyping. The MADM-based gene ablation paradigms in combination with computational modeling demonstrated that global tissue-wide effects predominate cell-autonomous gene function albeit in a gene-specific manner. Our results thus suggest that the genetic landscape in a tissue critically affects the overall migration phenotype of individual cortical projection neurons. In a broader context, our findings imply that global tissue-wide effects represent an essential component of the underlying etiology associated with focal malformations of cortical development in particular, and neurological diseases in general. AU - Hansen, Andi H AU - Pauler, Florian AU - Riedl, Michael AU - Streicher, Carmen AU - Heger, Anna-Magdalena AU - Laukoter, Susanne AU - Sommer, Christoph M AU - Nicolas, Armel AU - Hof, Björn AU - Tsai, Li Huei AU - Rülicke, Thomas AU - Hippenmeyer, Simon ID - 10791 IS - 1 JF - Oxford Open Neuroscience TI - Tissue-wide effects override cell-intrinsic gene function in radial neuron migration VL - 1 ER - TY - JOUR AB - When crawling through the body, leukocytes often traverse tissues that are densely packed with extracellular matrix and other cells, and this raises the question: How do leukocytes overcome compressive mechanical loads? Here, we show that the actin cortex of leukocytes is mechanoresponsive and that this responsiveness requires neither force sensing via the nucleus nor adhesive interactions with a substrate. Upon global compression of the cell body as well as local indentation of the plasma membrane, Wiskott-Aldrich syndrome protein (WASp) assembles into dot-like structures, providing activation platforms for Arp2/3 nucleated actin patches. These patches locally push against the external load, which can be obstructing collagen fibers or other cells, and thereby create space to facilitate forward locomotion. We show in vitro and in vivo that this WASp function is rate limiting for ameboid leukocyte migration in dense but not in loose environments and is required for trafficking through diverse tissues such as skin and lymph nodes. AU - Gaertner, Florian AU - Reis-Rodrigues, Patricia AU - De Vries, Ingrid AU - Hons, Miroslav AU - Aguilera, Juan AU - Riedl, Michael AU - Leithner, Alexander F AU - Tasciyan, Saren AU - Kopf, Aglaja AU - Merrin, Jack AU - Zheden, Vanessa AU - Kaufmann, Walter AU - Hauschild, Robert AU - Sixt, Michael K ID - 10703 IS - 1 JF - Developmental Cell SN - 1534-5807 TI - WASp triggers mechanosensitive actin patches to facilitate immune cell migration in dense tissues VL - 57 ER - TY - JOUR AB - 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. AU - Avila, Kerstin AU - Hof, Björn ID - 8999 IS - 1 JF - Entropy TI - Second-order phase transition in counter-rotating taylor-couette flow experiment VL - 23 ER - TY - JOUR AB - 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. AU - Klotz, Lukasz AU - Pavlenko, A. M. AU - Wesfreid, J. E. ID - 9207 JF - Journal of Fluid Mechanics SN - 0022-1120 TI - Experimental measurements in plane Couette-Poiseuille flow: Dynamics of the large- and small-scale flow VL - 912 ER - TY - JOUR AB - 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. AU - Liu, T. AU - Semin, B. AU - Klotz, Lukasz AU - Godoy-Diana, R. AU - Wesfreid, J. E. AU - Mullin, T. ID - 9297 JF - Journal of Fluid Mechanics SN - 0022-1120 TI - Decay of streaks and rolls in plane Couette-Poiseuille flow VL - 915 ER - TY - JOUR AB - High impact epidemics constitute one of the largest threats humanity is facing in the 21st century. In the absence of pharmaceutical interventions, physical distancing together with testing, contact tracing and quarantining are crucial in slowing down epidemic dynamics. Yet, here we show that if testing capacities are limited, containment may fail dramatically because such combined countermeasures drastically change the rules of the epidemic transition: Instead of continuous, the response to countermeasures becomes discontinuous. Rather than following the conventional exponential growth, the outbreak that is initially strongly suppressed eventually accelerates and scales faster than exponential during an explosive growth period. As a consequence, containment measures either suffice to stop the outbreak at low total case numbers or fail catastrophically if marginally too weak, thus implying large uncertainties in reliably estimating overall epidemic dynamics, both during initial phases and during second wave scenarios. AU - Scarselli, Davide AU - Budanur, Nazmi B AU - Timme, Marc AU - Hof, Björn ID - 9407 IS - 1 JF - Nature Communications TI - Discontinuous epidemic transition due to limited testing VL - 12 ER - TY - JOUR AB - Turbulence in the flow of fluid through a pipe can be suppressed by buoyancy forces. As the suppression of turbulence leads to severe heat transfer deterioration, this is an important and undesirable phenomenon in both heating and cooling applications. Vertical flow is often considered, as the axial buoyancy force can help drive the flow. With heating measured by the buoyancy parameter 𝐶, our direct numerical simulations show that shear-driven turbulence may either be completely laminarised or it transitions to a relatively quiescent convection-driven state. Buoyancy forces cause a flattening of the base flow profile, which in isothermal pipe flow has recently been linked to complete suppression of turbulence (Kühnen et al., Nat. Phys., vol. 14, 2018, pp. 386–390), and the flattened laminar base profile has enhanced nonlinear stability (Marensi et al., J. Fluid Mech., vol. 863, 2019, pp. 50–875). In agreement with these findings, the nonlinear lower-branch travelling-wave solution analysed here, which is believed to mediate transition to turbulence in isothermal pipe flow, is shown to be suppressed by buoyancy. A linear instability of the laminar base flow is responsible for the appearance of the relatively quiescent convection driven state for 𝐶≳4 across the range of Reynolds numbers considered. In the suppression of turbulence, however, i.e. in the transition from turbulence, we find clearer association with the analysis of He et al. (J. Fluid Mech., vol. 809, 2016, pp. 31–71) than with the above dynamical systems approach, which describes better the transition to turbulence. The laminarisation criterion He et al. propose, based on an apparent Reynolds number of the flow as measured by its driving pressure gradient, is found to capture the critical 𝐶=𝐶𝑐𝑟(𝑅𝑒) above which the flow will be laminarised or switch to the convection-driven type. Our analysis suggests that it is the weakened rolls, rather than the streaks, which appear to be critical for laminarisation. AU - Marensi, Elena AU - He, Shuisheng AU - Willis, Ashley P. ID - 9467 JF - Journal of Fluid Mechanics SN - 00221120 TI - Suppression of turbulence and travelling waves in a vertical heated pipe VL - 919 ER - TY - JOUR AB - We show that turbulent dynamics that arise in simulations of the three-dimensional Navier--Stokes equations in a triply-periodic domain under sinusoidal forcing can be described as transient visits to the neighborhoods of unstable time-periodic solutions. Based on this description, we reduce the original system with more than 10^5 degrees of freedom to a 17-node Markov chain where each node corresponds to the neighborhood of a periodic orbit. The model accurately reproduces long-term averages of the system's observables as weighted sums over the periodic orbits. AU - Yalniz, Gökhan AU - Hof, Björn AU - Budanur, Nazmi B ID - 9558 IS - 24 JF - Physical Review Letters SN - 0031-9007 TI - Coarse graining the state space of a turbulent flow using periodic orbits VL - 126 ER - TY - JOUR AB - Single photon emitters in atomically-thin semiconductors can be deterministically positioned using strain induced by underlying nano-structures. Here, we couple monolayer WSe2 to high-refractive-index gallium phosphide dielectric nano-antennas providing both optical enhancement and monolayer deformation. For single photon emitters formed on such nano-antennas, we find very low (femto-Joule) saturation pulse energies and up to 104 times brighter photoluminescence than in WSe2 placed on low-refractive-index SiO2 pillars. We show that the key to these observations is the increase on average by a factor of 5 of the quantum efficiency of the emitters coupled to the nano-antennas. This further allows us to gain new insights into their photoluminescence dynamics, revealing the roles of the dark exciton reservoir and Auger processes. We also find that the coherence time of such emitters is limited by intrinsic dephasing processes. Our work establishes dielectric nano-antennas as a platform for high-efficiency quantum light generation in monolayer semiconductors. AU - Sortino, Luca AU - Zotev, Panaiot G. AU - Phillips, Catherine L. AU - Brash, Alistair J. AU - Cambiasso, Javier AU - Marensi, Elena AU - Fox, A. Mark AU - Maier, Stefan A. AU - Sapienza, Riccardo AU - Tartakovskii, Alexander I. ID - 10203 JF - Nature Communications TI - Bright single photon emitters with enhanced quantum efficiency in a two-dimensional semiconductor coupled with dielectric nano-antennas VL - 12 ER - TY - JOUR AB - Turbulence generally arises in shear flows if velocities and hence, inertial forces are sufficiently large. In striking contrast, viscoelastic fluids can exhibit disordered motion even at vanishing inertia. Intermediate between these cases, a state of chaotic motion, “elastoinertial turbulence” (EIT), has been observed in a narrow Reynolds number interval. We here determine the origin of EIT in experiments and show that characteristic EIT structures can be detected across an unexpectedly wide range of parameters. Close to onset, a pattern of chevron-shaped streaks emerges in qualitative agreement with linear and weakly nonlinear theory. However, in experiments, the dynamics remain weakly chaotic, and the instability can be traced to far lower Reynolds numbers than permitted by theory. For increasing inertia, the flow undergoes a transformation to a wall mode composed of inclined near-wall streaks and shear layers. This mode persists to what is known as the “maximum drag reduction limit,” and overall EIT is found to dominate viscoelastic flows across more than three orders of magnitude in Reynolds number. AU - Choueiri, George H AU - Lopez Alonso, Jose M AU - Varshney, Atul AU - Sankar, Sarath AU - Hof, Björn ID - 10299 IS - 45 JF - Proceedings of the National Academy of Sciences KW - multidisciplinary KW - elastoinertial turbulence KW - viscoelastic flows KW - elastic instability KW - drag reduction SN - 0027-8424 TI - Experimental observation of the origin and structure of elastoinertial turbulence VL - 118 ER - TY - THES AB - Most real-world flows are multiphase, yet we know little about them compared to their single-phase counterparts. Multiphase flows are more difficult to investigate as their dynamics occur in large parameter space and involve complex phenomena such as preferential concentration, turbulence modulation, non-Newtonian rheology, etc. Over the last few decades, experiments in particle-laden flows have taken a back seat in favour of ever-improving computational resources. However, computers are still not powerful enough to simulate a real-world fluid with millions of finite-size particles. Experiments are essential not only because they offer a reliable way to investigate real-world multiphase flows but also because they serve to validate numerical studies and steer the research in a relevant direction. In this work, we have experimentally investigated particle-laden flows in pipes, and in particular, examined the effect of particles on the laminar-turbulent transition and the drag scaling in turbulent flows. For particle-laden pipe flows, an earlier study [Matas et al., 2003] reported how the sub-critical (i.e., hysteretic) transition that occurs via localised turbulent structures called puffs is affected by the addition of particles. In this study, in addition to this known transition, we found a super-critical transition to a globally fluctuating state with increasing particle concentration. At the same time, the Newtonian-type transition via puffs is delayed to larger Reynolds numbers. At an even higher concentration, only the globally fluctuating state is found. The dynamics of particle-laden flows are hence determined by two competing instabilities that give rise to three flow regimes: Newtonian-type turbulence at low, a particle-induced globally fluctuating state at high, and a coexistence state at intermediate concentrations. The effect of particles on turbulent drag is ambiguous, with studies reporting drag reduction, no net change, and even drag increase. The ambiguity arises because, in addition to particle concentration, particle shape, size, and density also affect the net drag. Even similar particles might affect the flow dissimilarly in different Reynolds number and concentration ranges. In the present study, we explored a wide range of both Reynolds number and concentration, using spherical as well as cylindrical particles. We found that the spherical particles do not reduce drag while the cylindrical particles are drag-reducing within a specific Reynolds number interval. The interval strongly depends on the particle concentration and the relative size of the pipe and particles. Within this interval, the magnitude of drag reduction reaches a maximum. These drag reduction maxima appear to fall onto a distinct power-law curve irrespective of the pipe diameter and particle concentration, and this curve can be considered as the maximum drag reduction asymptote for a given fibre shape. Such an asymptote is well known for polymeric flows but had not been identified for particle-laden flows prior to this work. AU - Agrawal, Nishchal ID - 9728 KW - Drag Reduction KW - Transition to Turbulence KW - Multiphase Flows KW - particle Laden Flows KW - Complex Flows KW - Experiments KW - Fluid Dynamics SN - 2663-337X TI - Transition to turbulence and drag reduction in particle-laden pipe flows ER - TY - JOUR AB - 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. AU - Lopez Alonso, Jose M AU - Feldmann, Daniel AU - Rampp, Markus AU - Vela-Martín, Alberto AU - Shi, Liang AU - Avila, Marc ID - 7364 JF - SoftwareX TI - nsCouette – A high-performance code for direct numerical simulations of turbulent Taylor–Couette flow VL - 11 ER - TY - JOUR AB - 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. AU - Budanur, Nazmi B AU - Marensi, Elena AU - Willis, Ashley P. AU - Hof, Björn ID - 7534 IS - 2 JF - Physical Review Fluids SN - 2469-990X TI - Upper edge of chaos and the energetics of transition in pipe flow VL - 5 ER - TY - JOUR AB - 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. AU - Yalniz, Gökhan AU - Budanur, Nazmi B ID - 7563 IS - 3 JF - Chaos SN - 1054-1500 TI - Inferring symbolic dynamics of chaotic flows from persistence VL - 30 ER - TY - JOUR AB - 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. AU - Paranjape, Chaitanya S AU - Duguet, Yohann AU - Hof, Björn ID - 8043 JF - Journal of Fluid Mechanics SN - 00221120 TI - Oblique stripe solutions of channel flow VL - 897 ER - TY - JOUR AB - 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. AU - Suri, Balachandra AU - Kageorge, Logan AU - Grigoriev, Roman O. AU - Schatz, Michael F. ID - 8634 IS - 6 JF - Physical Review Letters KW - General Physics and Astronomy SN - 0031-9007 TI - Capturing turbulent dynamics and statistics in experiments with unstable periodic orbits VL - 125 ER - TY - JOUR AB - 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. AU - Xu, Duo AU - Varshney, Atul AU - Ma, Xingyu AU - Song, Baofang AU - Riedl, Michael AU - Avila, Marc AU - Hof, Björn ID - 7932 IS - 21 JF - Proceedings of the National Academy of Sciences of the United States of America SN - 00278424 TI - Nonlinear hydrodynamic instability and turbulence in pulsatile flow VL - 117 ER - TY - THES AB - 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. AU - Scarselli, Davide ID - 7258 SN - 2663-337X TI - New approaches to reduce friction in turbulent pipe flow ER - TY - THES AB - 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. AU - Shamipour, Shayan ID - 8350 SN - 2663-337X TI - Bulk actin dynamics drive phase segregation in zebrafish oocytes ER - TY - JOUR AB - The hairpin instability of a jet in a crossflow (JICF) for a low jet-to-crossflow velocity ratio is investigated experimentally for a velocity ratio range of R ∈ (0.14, 0.75) and crossflow Reynolds numbers ReD ∈ (260, 640). From spectral analysis we characterize the Strouhal number and amplitude of the hairpin instability as a function of R and ReD. We demonstrate that the dynamics of the hairpins is well described by the Landau model, and, hence, that the instability occurs through Hopf bifurcation, similarly to other hydrodynamical oscillators such as wake behind different bluff bodies. Using the Landau model, we determine the precise threshold values of hairpin shedding. We also study the spatial dependence of this hydrodynamical instability, which shows a global behaviour. AU - Klotz, Lukasz AU - Gumowski, Konrad AU - Wesfreid, José Eduardo ID - 5943 JF - Journal of Fluid Mechanics TI - Experiments on a jet in a crossflow in the low-velocity-ratio regime VL - 863 ER - TY - JOUR AB - We consider the motion of a droplet bouncing on a vibrating bath of the same fluid in the presence of a central potential. We formulate a rotation symmetry-reduced description of this system, which allows for the straightforward application of dynamical systems theory tools. As an illustration of the utility of the symmetry reduction, we apply it to a model of the pilot-wave system with a central harmonic force. We begin our analysis by identifying local bifurcations and the onset of chaos. We then describe the emergence of chaotic regions and their merging bifurcations, which lead to the formation of a global attractor. In this final regime, the droplet’s angular momentum spontaneously changes its sign as observed in the experiments of Perrard et al. AU - Budanur, Nazmi B AU - Fleury, Marc ID - 5878 IS - 1 JF - Chaos: An Interdisciplinary Journal of Nonlinear Science SN - 1054-1500 TI - State space geometry of the chaotic pilot-wave hydrodynamics VL - 29 ER - TY - JOUR AB - Phase-field methods have long been used to model the flow of immiscible fluids. Their ability to naturally capture interface topological changes is widely recognized, but their accuracy in simulating flows of real fluids in practical geometries is not established. We here quantitatively investigate the convergence of the phase-field method to the sharp-interface limit with simulations of two-phase pipe flow. We focus on core-annular flows, in which a highly viscous fluid is lubricated by a less viscous fluid, and validate our simulations with an analytic laminar solution, a formal linear stability analysis and also in the fully nonlinear regime. We demonstrate the ability of the phase-field method to accurately deal with non-rectangular geometry, strong advection, unsteady fluctuations and large viscosity contrast. We argue that phase-field methods are very promising for quantitatively studying moderately turbulent flows, especially at high concentrations of the disperse phase. AU - Song, Baofang AU - Plana, Carlos AU - Lopez Alonso, Jose M AU - Avila, Marc ID - 6413 JF - International Journal of Multiphase Flow SN - 03019322 TI - Phase-field simulation of core-annular pipe flow VL - 117 ER - TY - JOUR AB - In pipes and channels, the onset of turbulence is initially dominated by localizedtransients, which lead to sustained turbulence through their collective dynamics. In thepresent work, we study numerically the localized turbulence in pipe flow and elucidate astate space structure that gives rise to transient chaos. Starting from the basin boundaryseparating laminar and turbulent flow, we identify transverse homoclinic orbits, thepresence of which necessitates a homoclinic tangle and chaos. A direct consequence ofthe homoclinic tangle is the fractal nature of the laminar-turbulent boundary, which wasconjectured in various earlier studies. By mapping the transverse intersections between thestable and unstable manifold of a periodic orbit, we identify the gateways that promote anescape from turbulence. AU - Budanur, Nazmi B AU - Dogra, Akshunna AU - Hof, Björn ID - 6978 IS - 10 JF - Physical Review Fluids TI - Geometry of transient chaos in streamwise-localized pipe flow turbulence VL - 4 ER - TY - JOUR AB - Polymer additives can substantially reduce the drag of turbulent flows and the upperlimit, the so called “maximum drag reduction” (MDR) asymptote is universal, i.e. inde-pendent of the type of polymer and solvent used. Until recently, the consensus was that,in this limit, flows are in a marginal state where only a minimal level of turbulence activ-ity persists. Observations in direct numerical simulations using minimal sized channelsappeared to support this view and reported long “hibernation” periods where turbu-lence is marginalized. In simulations of pipe flow we find that, indeed, with increasingWeissenberg number (Wi), turbulence expresses long periods of hibernation if the domainsize is small. However, with increasing pipe length, the temporal hibernation continuouslyalters to spatio-temporal intermittency and here the flow consists of turbulent puffs sur-rounded by laminar flow. Moreover, upon an increase in Wi, the flow fully relaminarises,in agreement with recent experiments. At even larger Wi, a different instability is en-countered causing a drag increase towards MDR. Our findings hence link earlier minimalflow unit simulations with recent experiments and confirm that the addition of polymersinitially suppresses Newtonian turbulence and leads to a reverse transition. The MDRstate on the other hand results from a separate instability and the underlying dynamicscorresponds to the recently proposed state of elasto-inertial-turbulence (EIT). AU - Lopez Alonso, Jose M AU - Choueiri, George H AU - Hof, Björn ID - 7397 JF - Journal of Fluid Mechanics SN - 0022-1120 TI - Dynamics of viscoelastic pipe flow at low Reynolds numbers in the maximum drag reduction limit VL - 874 ER - TY - THES AB - In many shear flows like pipe flow, plane Couette flow, plane Poiseuille flow, etc. turbulence emerges subcritically. Here, when subjected to strong enough perturbations, the flow becomes turbulent in spite of the laminar base flow being linearly stable. The nature of this instability has puzzled the scientific community for decades. At onset, turbulence appears in localized patches and flows are spatio-temporally intermittent. In pipe flow the localized turbulent structures are referred to as puffs and in planar flows like plane Couette and channel flow, patches arise in the form of localized oblique bands. In this thesis, we study the onset of turbulence in channel flow in direct numerical simulations from a dynamical system theory perspective, as well as by performing experiments in a large aspect ratio channel. The aim of the experimental work is to determine the critical Reynolds number where turbulence first becomes sustained. Recently, the onset of turbulence has been described in analogy to absorbing state phase transition (i.e. directed percolation). In particular, it has been shown that the critical point can be estimated from the competition between spreading and decay processes. Here, by performing experiments, we identify the mechanisms underlying turbulence proliferation in channel flow and find the critical Reynolds number, above which turbulence becomes sustained. Above the critical point, the continuous growth at the tip of the stripes outweighs the stochastic shedding of turbulent patches at the tail and the stripes expand. For growing stripes, the probability to decay decreases while the probability of stripe splitting increases. Consequently, and unlike for the puffs in pipe flow, neither of these two processes is time-independent i.e. memoryless. Coupling between stripe expansion and creation of new stripes via splitting leads to a significantly lower critical point ($Re_c=670+/-10$) than most earlier studies suggest. While the above approach sheds light on how turbulence first becomes sustained, it provides no insight into the origin of the stripes themselves. In the numerical part of the thesis we investigate how turbulent stripes form from invariant solutions of the Navier-Stokes equations. The origin of these turbulent stripes can be identified by applying concepts from the dynamical system theory. In doing so, we identify the exact coherent structures underlying stripes and their bifurcations and how they give rise to the turbulent attractor in phase space. We first report a family of localized nonlinear traveling wave solutions of the Navier-Stokes equations in channel flow. These solutions show structural similarities with turbulent stripes in experiments like obliqueness, quasi-streamwise streaks and vortices, etc. A parametric study of these traveling wave solution is performed, with parameters like Reynolds number, stripe tilt angle and domain size, including the stability of the solutions. These solutions emerge through saddle-node bifurcations and form a phase space skeleton for the turbulent stripes observed in the experiments. The lower branches of these TW solutions at different tilt angles undergo Hopf bifurcation and new solutions branches of relative periodic orbits emerge. These RPO solutions do not belong to the same family and therefore the routes to chaos for different angles are different. In shear flows, turbulence at onset is transient in nature. Consequently,turbulence can not be tracked to lower Reynolds numbers, where the dynamics may simplify. Before this happens, turbulence becomes short-lived and laminarizes. In the last part of the thesis, we show that using numerical simulations we can continue turbulent stripes in channel flow past the 'relaminarization barrier' all the way to their origin. Here, turbulent stripe dynamics simplifies and the fluctuations are no longer stochastic and the stripe settles down to a relative periodic orbit. This relative periodic orbit originates from the aforementioned traveling wave solutions. Starting from the relative periodic orbit, a small increase in speed i.e. Reynolds number gives rise to chaos and the attractor dimension sharply increases in contrast to the classical transition scenario where the instabilities affect the flow globally and give rise to much more gradual route to turbulence. AU - Paranjape, Chaitanya S ID - 6957 KW - Instabilities KW - Turbulence KW - Nonlinear dynamics TI - Onset of turbulence in plane Poiseuille flow ER - TY - JOUR AB - During bacterial cell division, the tubulin-homolog FtsZ forms a ring-like structure at the center of the cell. This Z-ring not only organizes the division machinery, but treadmilling of FtsZ filaments was also found to play a key role in distributing proteins at the division site. What regulates the architecture, dynamics and stability of the Z-ring is currently unknown, but FtsZ-associated proteins are known to play an important role. Here, using an in vitro reconstitution approach, we studied how the well-conserved protein ZapA affects FtsZ treadmilling and filament organization into large-scale patterns. Using high-resolution fluorescence microscopy and quantitative image analysis, we found that ZapA cooperatively increases the spatial order of the filament network, but binds only transiently to FtsZ filaments and has no effect on filament length and treadmilling velocity. Together, our data provides a model for how FtsZ-associated proteins can increase the precision and stability of the bacterial cell division machinery in a switch-like manner. AU - Dos Santos Caldas, Paulo R AU - Lopez Pelegrin, Maria D AU - Pearce, Daniel J. G. AU - Budanur, Nazmi B AU - Brugués, Jan AU - Loose, Martin ID - 7197 JF - Nature Communications SN - 2041-1723 TI - Cooperative ordering of treadmilling filaments in cytoskeletal networks of FtsZ and its crosslinker ZapA VL - 10 ER - TY - JOUR AB - Electron transport in two-dimensional conducting materials such as graphene, with dominant electron–electron interaction, exhibits unusual vortex flow that leads to a nonlocal current-field relation (negative resistance), distinct from the classical Ohm’s law. The transport behavior of these materials is best described by low Reynolds number hydrodynamics, where the constitutive pressure–speed relation is Stoke’s law. Here we report evidence of such vortices observed in a viscous flow of Newtonian fluid in a microfluidic device consisting of a rectangular cavity—analogous to the electronic system. We extend our experimental observations to elliptic cavities of different eccentricities, and validate them by numerically solving bi-harmonic equation obtained for the viscous flow with no-slip boundary conditions. We verify the existence of a predicted threshold at which vortices appear. Strikingly, we find that a two-dimensional theoretical model captures the essential features of three-dimensional Stokes flow in experiments. AU - Mayzel, Jonathan AU - Steinberg, Victor AU - Varshney, Atul ID - 6069 JF - Nature Communications SN - 2041-1723 TI - Stokes flow analogous to viscous electron current in graphene VL - 10 ER - TY - JOUR AB - Speed of sound waves in gases and liquids are governed by the compressibility of the medium. There exists another type of non-dispersive wave where the wave speed depends on stress instead of elasticity of the medium. A well-known example is the Alfven wave, which propagates through plasma permeated by a magnetic field with the speed determined by magnetic tension. An elastic analogue of Alfven waves has been predicted in a flow of dilute polymer solution where the elastic stress of the stretching polymers determines the elastic wave speed. Here we present quantitative evidence of elastic Alfven waves in elastic turbulence of a viscoelastic creeping flow between two obstacles in channel flow. The key finding in the experimental proof is a nonlinear dependence of the elastic wave speed cel on the Weissenberg number Wi, which deviates from predictions based on a model of linear polymer elasticity. AU - Varshney, Atul AU - Steinberg, Victor ID - 6014 JF - Nature Communications SN - 2041-1723 TI - Elastic alfven waves in elastic turbulence VL - 10 ER - TY - JOUR AB - Recent studies suggest that unstable recurrent solutions of the Navier-Stokes equation provide new insights into dynamics of turbulent flows. In this study, we compute an extensive network of dynamical connections between such solutions in a weakly turbulent quasi-two-dimensional Kolmogorov flow that lies in the inversion symmetric subspace. In particular, we find numerous isolated heteroclinic connections between different types of solutions—equilibria, periodic, and quasiperiodic orbits—as well as continua of connections forming higher-dimensional connecting manifolds. We also compute a homoclinic connection of a periodic orbit and provide strong evidence that the associated homoclinic tangle forms the chaotic repeller that underpins transient turbulence in the symmetric subspace. AU - Suri, Balachandra AU - Pallantla, Ravi Kumar AU - Schatz, Michael F. AU - Grigoriev, Roman O. ID - 6779 IS - 1 JF - Physical Review E SN - 2470-0045 TI - Heteroclinic and homoclinic connections in a Kolmogorov-like flow VL - 100 ER - TY - JOUR AB - 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. AU - Kühnen, Jakob AU - Scarselli, Davide AU - Hof, Björn ID - 6486 IS - 11 JF - Journal of Fluids Engineering SN - 00982202 TI - Relaminarization of pipe flow by means of 3D-printed shaped honeycombs VL - 141 ER - TY - JOUR AB - 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. AU - Scarselli, Davide AU - Kühnen, Jakob AU - Hof, Björn ID - 6228 JF - Journal of Fluid Mechanics SN - 00221120 TI - Relaminarising pipe flow by wall movement VL - 867 ER - TY - JOUR AB - Segregation of maternal determinants within the oocyte constitutes the first step in embryo patterning. In zebrafish oocytes, extensive ooplasmic streaming leads to the segregation of ooplasm from yolk granules along the animal-vegetal axis of the oocyte. Here, we 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 oocyte. This wave functions in segregation by both pulling ooplasm animally and pushing yolk granules vegetally. Using biophysical experimentation and theory, we show that ooplasm pulling is mediated by bulk actin network flows exerting friction forces on the ooplasm, while yolk granule pushing is achieved by a mechanism closely resembling actin comet formation on yolk granules. Our study defines a novel role of cell-cycle-controlled bulk actin polymerization waves in oocyte polarization via ooplasmic segregation. AU - Shamipour, Shayan AU - Kardos, Roland AU - Xue, Shi-lei AU - Hof, Björn AU - Hannezo, Edouard B AU - Heisenberg, Carl-Philipp J ID - 6508 IS - 6 JF - Cell SN - 00928674 TI - Bulk actin dynamics drive phase segregation in zebrafish oocytes VL - 177 ER - TY - JOUR AU - Schwayer, Cornelia AU - Shamipour, Shayan AU - Pranjic-Ferscha, Kornelija AU - Schauer, Alexandra AU - Balda, M AU - Tada, M AU - Matter, K AU - Heisenberg, Carl-Philipp J ID - 7001 IS - 4 JF - Cell SN - 0092-8674 TI - Mechanosensation of tight junctions depends on ZO-1 phase separation and flow VL - 179 ER - TY - JOUR AB - Suspended particles can alter the properties of fluids and in particular also affect the transition fromlaminar to turbulent flow. An earlier study [Mataset al.,Phys. Rev. Lett.90, 014501 (2003)] reported howthe subcritical (i.e., hysteretic) transition to turbulent puffs is affected by the addition of particles. Here weshow that in addition to this known transition, with increasing concentration a supercritical (i.e.,continuous) transition to a globally fluctuating state is found. At the same time the Newtonian-typetransition to puffs is delayed to larger Reynolds numbers. At even higher concentration only the globallyfluctuating state is found. The dynamics of particle laden flows are hence determined by two competinginstabilities that give rise to three flow regimes: Newtonian-type turbulence at low, a particle inducedglobally fluctuating state at high, and a coexistence state at intermediate concentrations. AU - Agrawal, Nishchal AU - Choueiri, George H AU - Hof, Björn ID - 6189 IS - 11 JF - Physical Review Letters SN - 00319007 TI - Transition to turbulence in particle laden flows VL - 122 ER - TY - JOUR AB - Over the past decade, the edge of chaos has proven to be a fruitful starting point for investigations of shear flows when the laminar base flow is linearly stable. Numerous computational studies of shear flows demonstrated the existence of states that separate laminar and turbulent regions of the state space. In addition, some studies determined invariant solutions that reside on this edge. In this paper, we study the unstable manifold of one such solution with the aid of continuous symmetry reduction, which we formulate here for the simultaneous quotiening of axial and azimuthal symmetries. Upon our investigation of the unstable manifold, we discover a previously unknown traveling-wave solution on the laminar-turbulent boundary with a relatively complex structure. By means of low-dimensional projections, we visualize different dynamical paths that connect these solutions to the turbulence. Our numerical experiments demonstrate that the laminar-turbulent boundary exhibits qualitatively different regions whose properties are influenced by the nearby invariant solutions. AU - Budanur, Nazmi B AU - Hof, Björn ID - 291 IS - 5 JF - Physical Review Fluids TI - Complexity of the laminar-turbulent boundary in pipe flow VL - 3 ER - TY - JOUR AB - Creeping flow of polymeric fluid without inertia exhibits elastic instabilities and elastic turbulence accompanied by drag enhancement due to elastic stress produced by flow-stretched polymers. However, in inertia-dominated flow at high Re and low fluid elasticity El, a reduction in turbulent frictional drag is caused by an intricate competition between inertial and elastic stresses. Here we explore the effect of inertia on the stability of viscoelastic flow in a broad range of control parameters El and (Re,Wi). We present the stability diagram of observed flow regimes in Wi-Re coordinates and find that the instabilities' onsets show an unexpectedly nonmonotonic dependence on El. Further, three distinct regions in the diagram are identified based on El. Strikingly, for high-elasticity fluids we discover a complete relaminarization of flow at Reynolds number in the range of 1 to 10, different from a well-known turbulent drag reduction. These counterintuitive effects may be explained by a finite polymer extensibility and a suppression of vorticity at high Wi. Our results call for further theoretical and numerical development to uncover the role of inertial effect on elastic turbulence in a viscoelastic flow. AU - Varshney, Atul AU - Steinberg, Victor ID - 17 IS - 10 JF - Physical Review Fluids TI - Drag enhancement and drag reduction in viscoelastic flow VL - 3 ER - TY - JOUR AB - We report quantitative evidence of mixing-layer elastic instability in a viscoelastic fluid flow between two widely spaced obstacles hindering a channel flow at Re 1 and Wi 1. Two mixing layers with nonuniform shear velocity profiles are formed in the region between the obstacles. The mixing-layer instability arises in the vicinity of an inflection point on the shear velocity profile with a steep variation in the elastic stress. The instability results in an intermittent appearance of small vortices in the mixing layers and an amplification of spatiotemporal averaged vorticity in the elastic turbulence regime. The latter is characterized through scaling of friction factor with Wi and both pressure and velocity spectra. Furthermore, the observations reported provide improved understanding of the stability of the mixing layer in a viscoelastic fluid at large elasticity, i.e., Wi 1 and Re 1 and oppose the current view of suppression of vorticity solely by polymer additives. AU - Varshney, Atul AU - Steinberg, Victor ID - 16 IS - 10 JF - Physical Review Fluids TI - Mixing layer instability and vorticity amplification in a creeping viscoelastic flow VL - 3 ER - TY - JOUR AB - This study treats with the influence of a symmetry-breaking transversal magnetic field on the nonlinear dynamics of ferrofluidic Taylor-Couette flow – flow confined between two concentric independently rotating cylinders. We detected alternating ‘flip’ solutions which are flow states featuring typical characteristics of slow-fast-dynamics in dynamical systems. The flip corresponds to a temporal change in the axial wavenumber and we find them to appear either as pure 2-fold axisymmetric (due to the symmetry-breaking nature of the applied transversal magnetic field) or involving non-axisymmetric, helical modes in its interim solution. The latter ones show features of typical ribbon solutions. In any case the flip solutions have a preferential first axial wavenumber which corresponds to the more stable state (slow dynamics) and second axial wavenumber, corresponding to the short appearing more unstable state (fast dynamics). However, in both cases the flip time grows exponential with increasing the magnetic field strength before the flip solutions, living on 2-tori invariant manifolds, cease to exist, with lifetime going to infinity. Further we show that ferrofluidic flow turbulence differ from the classical, ordinary (usually at high Reynolds number) turbulence. The applied magnetic field hinders the free motion of ferrofluid partials and therefore smoothen typical turbulent quantities and features so that speaking of mildly chaotic dynamics seems to be a more appropriate expression for the observed motion. AU - Altmeyer, Sebastian ID - 519 JF - Journal of Magnetism and Magnetic Materials TI - Non-linear dynamics and alternating ‘flip’ solutions in ferrofluidic Taylor-Couette flow VL - 452 ER - TY - JOUR AB - In pipes, turbulence sets in despite the linear stability of the laminar Hagen–Poiseuille flow. The Reynolds number ( ) for which turbulence first appears in a given experiment – the ‘natural transition point’ – depends on imperfections of the set-up, or, more precisely, on the magnitude of finite amplitude perturbations. At onset, turbulence typically only occupies a certain fraction of the flow, and this fraction equally is found to differ from experiment to experiment. Despite these findings, Reynolds proposed that after sufficiently long times, flows may settle to steady conditions: below a critical velocity, flows should (regardless of initial conditions) always return to laminar, while above this velocity, eddying motion should persist. As will be shown, even in pipes several thousand diameters long, the spatio-temporal intermittent flow patterns observed at the end of the pipe strongly depend on the initial conditions, and there is no indication that different flow patterns would eventually settle to a (statistical) steady state. Exploiting the fact that turbulent puffs do not age (i.e. they are memoryless), we continuously recreate the puff sequence exiting the pipe at the pipe entrance, and in doing so introduce periodic boundary conditions for the puff pattern. This procedure allows us to study the evolution of the flow patterns for arbitrary long times, and we find that after times in excess of advective time units, indeed a statistical steady state is reached. Although the resulting flows remain spatio-temporally intermittent, puff splitting and decay rates eventually reach a balance, so that the turbulent fraction fluctuates around a well-defined level which only depends on . In accordance with Reynolds’ proposition, we find that at lower (here 2020), flows eventually always resume to laminar, while for higher ( ), turbulence persists. The critical point for pipe flow hence falls in the interval of $2020 , which is in very good agreement with the recently proposed value of . The latter estimate was based on single-puff statistics and entirely neglected puff interactions. Unlike in typical contact processes where such interactions strongly affect the percolation threshold, in pipe flow, the critical point is only marginally influenced. Interactions, on the other hand, are responsible for the approach to the statistical steady state. As shown, they strongly affect the resulting flow patterns, where they cause ‘puff clustering’, and these regions of large puff densities are observed to travel across the puff pattern in a wave-like fashion. AU - Vasudevan, Mukund AU - Hof, Björn ID - 5996 JF - Journal of Fluid Mechanics SN - 0022-1120 TI - The critical point of the transition to turbulence in pipe flow VL - 839 ER - TY - JOUR AB - The drag of turbulent flows can be drastically decreased by adding small amounts of high molecular weight polymers. While drag reduction initially increases with polymer concentration, it eventually saturates to what is known as the maximum drag reduction (MDR) asymptote; this asymptote is generally attributed to the dynamics being reduced to a marginal yet persistent state of subdued turbulent motion. Contrary to this accepted view, we show that, for an appropriate choice of parameters, polymers can reduce the drag beyond the suggested asymptotic limit, eliminating turbulence and giving way to laminar flow. At higher polymer concentrations, however, the laminar state becomes unstable, resulting in a fluctuating flow with the characteristic drag of the MDR asymptote. Our findings indicate that the asymptotic state is hence dynamically disconnected from ordinary turbulence. © 2018 American Physical Society. AU - Choueiri, George H AU - Lopez Alonso, Jose M AU - Hof, Björn ID - 328 IS - 12 JF - Physical Review Letters TI - Exceeding the asymptotic limit of polymer drag reduction VL - 120 ER - TY - JOUR AB - Recent studies suggest that unstable, nonchaotic solutions of the Navier-Stokes equation may provide deep insights into fluid turbulence. In this article, we present a combined experimental and numerical study exploring the dynamical role of unstable equilibrium solutions and their invariant manifolds in a weakly turbulent, electromagnetically driven, shallow fluid layer. Identifying instants when turbulent evolution slows down, we compute 31 unstable equilibria of a realistic two-dimensional model of the flow. We establish the dynamical relevance of these unstable equilibria by showing that they are closely visited by the turbulent flow. We also establish the dynamical relevance of unstable manifolds by verifying that they are shadowed by turbulent trajectories departing from the neighborhoods of unstable equilibria over large distances in state space. AU - Suri, Balachandra AU - Tithof, Jeffrey AU - Grigoriev, Roman AU - Schatz, Michael ID - 136 IS - 2 JF - Physical Review E TI - Unstable equilibria and invariant manifolds in quasi-two-dimensional Kolmogorov-like flow VL - 98 ER - TY - JOUR AB - 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. AU - Kühnen, Jakob AU - Scarselli, Davide AU - Schaner, Markus AU - Hof, Björn ID - 422 IS - 4 JF - Flow Turbulence and Combustion TI - Relaminarization by steady modification of the streamwise velocity profile in a pipe VL - 100 ER - TY - JOUR AB - 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. AU - Kühnen, Jakob AU - Song, Baofang AU - Scarselli, Davide AU - Budanur, Nazmi B AU - Riedl, Michael AU - Willis, Ashley AU - Avila, Marc AU - Hof, Björn ID - 461 JF - Nature Physics TI - Destabilizing turbulence in pipe flow VL - 14 ER - TY - JOUR AB - Systems such as fluid flows in channels and pipes or the complex Ginzburg–Landau system, defined over periodic domains, exhibit both continuous symmetries, translational and rotational, as well as discrete symmetries under spatial reflections or complex conjugation. The simplest, and very common symmetry of this type is the equivariance of the defining equations under the orthogonal group O(2). We formulate a novel symmetry reduction scheme for such systems by combining the method of slices with invariant polynomial methods, and show how it works by applying it to the Kuramoto–Sivashinsky system in one spatial dimension. As an example, we track a relative periodic orbit through a sequence of bifurcations to the onset of chaos. Within the symmetry-reduced state space we are able to compute and visualize the unstable manifolds of relative periodic orbits, their torus bifurcations, a transition to chaos via torus breakdown, and heteroclinic connections between various relative periodic orbits. It would be very hard to carry through such analysis in the full state space, without a symmetry reduction such as the one we present here. AU - Budanur, Nazmi B AU - Cvitanović, Predrag ID - 1211 IS - 3-4 JF - Journal of Statistical Physics TI - Unstable manifolds of relative periodic orbits in the symmetry reduced state space of the Kuramoto–Sivashinsky system VL - 167 ER -