@article{12165, abstract = {It may come as a surprise that a phenomenon as ubiquitous and prominent as the transition from laminar to turbulent flow has resisted combined efforts by physicists, engineers and mathematicians, and remained unresolved for almost one and a half centuries. In recent years, various studies have proposed analogies to directed percolation, a well-known universality class in statistical mechanics, which describes a non-equilibrium phase transition from a fluctuating active phase into an absorbing state. It is this unlikely relation between the multiscale, high-dimensional dynamics that signify the transition process in virtually all flows of practical relevance, and the arguably most basic non-equilibrium phase transition, that so far has mainly been the subject of model studies, which I review in this Perspective.}, author = {Hof, Björn}, issn = {2522-5820}, journal = {Nature Reviews Physics}, keywords = {General Physics and Astronomy}, pages = {62--72}, publisher = {Springer Nature}, title = {{Directed percolation and the transition to turbulence}}, doi = {10.1038/s42254-022-00539-y}, volume = {5}, year = {2023}, } @article{12105, abstract = {Data-driven dimensionality reduction methods such as proper orthogonal decomposition and dynamic mode decomposition have proven to be useful for exploring complex phenomena within fluid dynamics and beyond. A well-known challenge for these techniques is posed by the continuous symmetries, e.g. translations and rotations, of the system under consideration, as drifts in the data dominate the modal expansions without providing an insight into the dynamics of the problem. In the present study, we address this issue for fluid flows in rectangular channels by formulating a continuous symmetry reduction method that eliminates the translations in the streamwise and spanwise directions simultaneously. We demonstrate our method by computing the symmetry-reduced dynamic mode decomposition (SRDMD) of sliding windows of data obtained from the transitional plane-Couette and turbulent plane-Poiseuille flow simulations. In the former setting, SRDMD captures the dynamics in the vicinity of the invariant solutions with translation symmetries, i.e. travelling waves and relative periodic orbits, whereas in the latter, our calculations reveal episodes of turbulent time evolution that can be approximated by a low-dimensional linear expansion.}, author = {Marensi, Elena and Yalniz, Gökhan and Hof, Björn and Budanur, Nazmi B}, issn = {1469-7645}, journal = {Journal of Fluid Mechanics}, publisher = {Cambridge University Press}, title = {{Symmetry-reduced dynamic mode decomposition of near-wall turbulence}}, doi = {10.1017/jfm.2022.1001}, volume = {954}, year = {2023}, } @article{12681, abstract = {The dissolution of minute concentration of polymers in wall-bounded flows is well-known for its unparalleled ability to reduce turbulent friction drag. Another phenomenon, elasto-inertial turbulence (EIT), has been far less studied even though elastic instabilities have already been observed in dilute polymer solutions before the discovery of polymer drag reduction. EIT is a chaotic state driven by polymer dynamics that is observed across many orders of magnitude in Reynolds number. It involves energy transfer from small elastic scales to large flow scales. The investigation of the mechanisms of EIT offers the possibility to better understand other complex phenomena such as elastic turbulence and maximum drag reduction. In this review, we survey recent research efforts that are advancing the understanding of the dynamics of EIT. We highlight the fundamental differences between EIT and Newtonian/inertial turbulence from the perspective of experiments, numerical simulations, instabilities, and coherent structures. Finally, we discuss the possible links between EIT and elastic turbulence and polymer drag reduction, as well as the remaining challenges in unraveling the self-sustaining mechanism of EIT.}, author = {Dubief, Yves and Terrapon, Vincent E. and Hof, Björn}, issn = {1545-4479}, journal = {Annual Review of Fluid Mechanics}, number = {1}, pages = {675--705}, publisher = {Annual Reviews}, title = {{Elasto-inertial turbulence}}, doi = {10.1146/annurev-fluid-032822-025933}, volume = {55}, year = {2023}, } @article{12682, abstract = {Since the seminal studies by Osborne Reynolds in the nineteenth century, pipe flow has served as a primary prototype for investigating the transition to turbulence in wall-bounded flows. Despite the apparent simplicity of this flow, various facets of this problem have occupied researchers for more than a century. Here we review insights from three distinct perspectives: (a) stability and susceptibility of laminar flow, (b) phase transition and spatiotemporal dynamics, and (c) dynamical systems analysis of the Navier—Stokes equations. We show how these perspectives have led to a profound understanding of the onset of turbulence in pipe flow. Outstanding open points, applications to flows of complex fluids, and similarities with other wall-bounded flows are discussed.}, author = {Avila, Marc and Barkley, Dwight and Hof, Björn}, issn = {0066-4189}, journal = {Annual Review of Fluid Mechanics}, pages = {575--602}, publisher = {Annual Reviews}, title = {{Transition to turbulence in pipe flow}}, doi = {10.1146/annurev-fluid-120720-025957}, volume = {55}, year = {2023}, } @article{12172, abstract = {In industrial reactors and equipment, non-ideality is quite a common phenomenon rather than an exception. These deviations from ideality impact the process's overall efficiency and the effectiveness of the equipment. To recognize the associated non-ideality, one needs to have enough understanding of the formulation of the equations and in-depth knowledge of the residence time distribution (RTD) data of real reactors. In the current work, step input and pulse input were used to create RTD data for Cascade continuous stirred tank reactors (CSTRs). For the aforementioned configuration, experiments were run at various flow rates to validate the developed characteristic equations. To produce RTD data, distilled water was utilized as the flowing fluid, and NaOH was the tracer substance. The ideal behavior of tracer concentration exits age distribution, and cumulative fraction for each setup and each input was plotted and experimental results were compared with perfect behavior. Deviation of concentration exit age distribution and cumulative fractional distribution from ideal behavior is more in pulse input as compared to a step input. For ideal cases, the exit age distribution curve and cumulative fraction curves are independent of the type of input. But a significant difference was observed for the two cases, which may be due to non-measurable fluctuations in volumetric flow rate, non-achievement of instant injection of tracer in case of pulse input, and slight variations in the sampling period. Further, with increasing flow rate, concentration, exit age, and cumulative fractional curves shifted upward, and this behavior matches with the actual case.}, author = {Khatoon, Bushra and Kamil, Shoaib and Babu, Hitesh and Siraj Alam, M.}, issn = {2214-7853}, journal = {Materials Today: Proceedings}, keywords = {General Medicine}, number = {Part 1}, pages = {40--47}, publisher = {Elsevier}, title = {{Experimental analysis of Cascade CSTRs with step and pulse inputs}}, doi = {10.1016/j.matpr.2022.11.037}, volume = {78}, year = {2023}, } @article{14341, abstract = {Flows through pipes and channels are, in practice, almost always turbulent, and the multiscale eddying motion is responsible for a major part of the encountered friction losses and pumping costs1. Conversely, for pulsatile flows, in particular for aortic blood flow, turbulence levels remain low despite relatively large peak velocities. For aortic blood flow, high turbulence levels are intolerable as they would damage the shear-sensitive endothelial cell layer2,3,4,5. Here we show that turbulence in ordinary pipe flow is diminished if the flow is driven in a pulsatile mode that incorporates all the key features of the cardiac waveform. At Reynolds numbers comparable to those of aortic blood flow, turbulence is largely inhibited, whereas at much higher speeds, the turbulent drag is reduced by more than 25%. This specific operation mode is more efficient when compared with steady driving, which is the present situation for virtually all fluid transport processes ranging from heating circuits to water, gas and oil pipelines.}, author = {Scarselli, Davide and Lopez Alonso, Jose M and Varshney, Atul and Hof, Björn}, issn = {1476-4687}, journal = {Nature}, number = {7977}, pages = {71--74}, publisher = {Springer Nature}, title = {{Turbulence suppression by cardiac-cycle-inspired driving of pipe flow}}, doi = {10.1038/s41586-023-06399-5}, volume = {621}, year = {2023}, } @phdthesis{12726, abstract = {Most motions of many-body systems at any scale in nature with sufficient degrees of freedom tend to be chaotic; reaching from the orbital motion of planets, the air currents in our atmosphere, down to the water flowing through our pipelines or the movement of a population of bacteria. To the observer it is therefore intriguing when a moving collective exhibits order. Collective motion of flocks of birds, schools of fish or swarms of self-propelled particles or robots have been studied extensively over the past decades but the mechanisms involved in the transition from chaos to order remain unclear. Here, the interactions, that in most systems give rise to chaos, sustain order. In this thesis we investigate mechanisms that preserve, destabilize or lead to the ordered state. We show that endothelial cells migrating in circular confinements transition to a collective rotating state and concomitantly synchronize the frequencies of nucleating actin waves within individual cells. Consequently, the frequency dependent cell migration speed uniformizes across the population. Complementary to the WAVE dependent nucleation of traveling actin waves, we show that in leukocytes the actin polymerization depending on WASp generates pushing forces locally at stationary patches. Next, in pipe flows, we study methods to disrupt the self–sustaining cycle of turbulence and therefore relaminarize the flow. While we find in pulsating flow conditions that turbulence emerges through a helical instability during the decelerating phase. Finally, we show quantitatively in brain slices of mice that wild-type control neurons can compensate the migratory deficits of a genetically modified neuronal sub–population in the developing cortex.}, author = {Riedl, Michael}, issn = {2663-337X}, pages = {260}, publisher = {Institute of Science and Technology Austria}, title = {{Synchronization in collectively moving active matter}}, doi = {10.15479/at:ista:12726}, year = {2023}, } @article{13274, abstract = {Viscous flows through pipes and channels are steady and ordered until, with increasing velocity, the laminar motion catastrophically breaks down and gives way to turbulence. How this apparently discontinuous change from low- to high-dimensional motion can be rationalized within the framework of the Navier-Stokes equations is not well understood. Exploiting geometrical properties of transitional channel flow we trace turbulence to far lower Reynolds numbers (Re) than previously possible and identify the complete path that reversibly links fully turbulent motion to an invariant solution. This precursor of turbulence destabilizes rapidly with Re, and the accompanying explosive increase in attractor dimension effectively marks the transition between deterministic and de facto stochastic dynamics.}, author = {Paranjape, Chaitanya S and Yalniz, Gökhan and Duguet, Yohann and Budanur, Nazmi B and Hof, Björn}, issn = {1079-7114}, journal = {Physical Review Letters}, keywords = {General Physics and Astronomy}, number = {3}, publisher = {American Physical Society}, title = {{Direct path from turbulence to time-periodic solutions}}, doi = {10.1103/physrevlett.131.034002}, volume = {131}, year = {2023}, } @article{14361, abstract = {Whether one considers swarming insects, flocking birds, or bacterial colonies, collective motion arises from the coordination of individuals and entails the adjustment of their respective velocities. In particular, in close confinements, such as those encountered by dense cell populations during development or regeneration, collective migration can only arise coordinately. Yet, how individuals unify their velocities is often not understood. Focusing on a finite number of cells in circular confinements, we identify waves of polymerizing actin that function as a pacemaker governing the speed of individual cells. We show that the onset of collective motion coincides with the synchronization of the wave nucleation frequencies across the population. Employing a simpler and more readily accessible mechanical model system of active spheres, we identify the synchronization of the individuals’ internal oscillators as one of the essential requirements to reach the corresponding collective state. The mechanical ‘toy’ experiment illustrates that the global synchronous state is achieved by nearest neighbor coupling. We suggest by analogy that local coupling and the synchronization of actin waves are essential for the emergent, self-organized motion of cell collectives.}, author = {Riedl, Michael and Mayer, Isabelle D and Merrin, Jack and Sixt, Michael K and Hof, Björn}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Synchronization in collectively moving inanimate and living active matter}}, doi = {10.1038/s41467-023-41432-1}, volume = {14}, year = {2023}, } @article{14754, abstract = {The large-scale laminar/turbulent spiral patterns that appear in the linearly unstable regime of counter-rotating Taylor–Couette flow are investigated from a statistical perspective by means of direct numerical simulation. Unlike the vast majority of previous numerical studies, we analyse the flow in periodic parallelogram-annular domains, following a coordinate change that aligns one of the parallelogram sides with the spiral pattern. The domain size, shape and spatial resolution have been varied and the results compared with those in a sufficiently large computational orthogonal domain with natural axial and azimuthal periodicity. We find that a minimal parallelogram of the right tilt significantly reduces the computational cost without notably compromising the statistical properties of the supercritical turbulent spiral. Its mean structure, obtained from extremely long time integrations in a co-rotating reference frame using the method of slices, bears remarkable similarity with the turbulent stripes observed in plane Couette flow, the centrifugal instability playing only a secondary role.}, author = {Wang, B. and Mellibovsky, F. and Ayats López, Roger and Deguchi, K. and Meseguer, A.}, issn = {1471-2962}, journal = {Philosophical Transactions of the Royal Society A}, keywords = {General Physics and Astronomy, General Engineering, General Mathematics}, number = {2246}, publisher = {The Royal Society}, title = {{Mean structure of the supercritical turbulent spiral in Taylor–Couette flow}}, doi = {10.1098/rsta.2022.0112}, volume = {381}, year = {2023}, }