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1
Alert, Ricard, Jaume Casademunt, and Jean-François Joanny. "Active Turbulence." Annual Review of Condensed Matter Physics 13, no. 1 (2022): 143–70. http://dx.doi.org/10.1146/annurev-conmatphys-082321-035957.
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Active fluids exhibit spontaneous flows with complex spatiotemporal structure, which have been observed in bacterial suspensions, sperm cells, cytoskeletal suspensions, self-propelled colloids, and cell tissues. Despite occurring in the absence of inertia, chaotic active flows are reminiscent of inertial turbulence, and hence they are known as active turbulence. Here, we survey the field, providing a unified perspective over different classes of active turbulence. To this end, we divide our review into sections for systems with either polar or nematic order, and with or without momentum conservation (wet or dry). Comparing to inertial turbulence, we highlight the emergence of power-law scaling with either universal or nonuniversal exponents. We also contrast scenarios for the transition from steady to chaotic flows, and we discuss the absence of energy cascades. We link this feature to both the existence of intrinsic length scales and the self-organized nature of energy injection in active turbulence, which are fundamental differences from inertial turbulence. We close by outlining the emerging picture, remaining challenges, and future directions.
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Thampi, S. P., and J. M. Yeomans. "Active turbulence in active nematics." European Physical Journal Special Topics 225, no. 4 (2016): 651–62. http://dx.doi.org/10.1140/epjst/e2015-50324-3.
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Alhumairi, Mohammed, and Özgür Ertunç. "Active-grid turbulence effect on the topology and the flame location of a lean premixed combustion." Thermal Science 22, no. 6 Part A (2018): 2425–38. http://dx.doi.org/10.2298/tsci170503100a.
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Lean premixed combustion under the influence of active-grid turbulence was computationally investigated, and the results were compared with experimental data. The experiments were carried out to generate a premixed flame at a thermal load of 9 kW from a single jet flow combustor. Turbulent combustion models, such as the coherent flame model and turbulent flame speed closure model were implemented for the simulations performed under different turbulent flow conditions, which were specified by the Reynolds number based on Taylor?s microscale, the dissipation rate of turbulence, and turbulent kinetic energy. This study shows that the applied turbulent combustion models differently predict the flame topology and location. However, similar to the experiments, simulations with both models revealed that the flame moves toward the inlet when turbulence becomes strong at the inlet, that is, when Re? at the inlet increases. The results indicated that the flame topology and location in the coherent flame model were more sensitive to turbulence than those in the turbulent flame speed closure model. The flame location behavior on the jet flow combustor significantly changed with the increase of Re?.
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Xi, Li, and Michael D. Graham. "Intermittent dynamics of turbulence hibernation in Newtonian and viscoelastic minimal channel flows." Journal of Fluid Mechanics 693 (January 17, 2012): 433–72. http://dx.doi.org/10.1017/jfm.2011.541.
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AbstractMaximum drag reduction (MDR), the asymptotic upper limit of reduction in turbulent friction drag by polymer additives, is the most important unsolved problem in viscoelastic turbulence. Recent studies of turbulence in minimal flow units have identified time intervals showing key features of MDR. These intervals, denoted ‘hibernating turbulence’ are found in both Newtonian and viscoelastic flows. The present study provides a comprehensive examination of this turbulence hibernation phenomenon in the minimal channel geometry, and discusses its impact on the turbulent dynamics and drag reduction. Similarities between hibernating turbulence and MDR are established in terms of both flow statistics (an intermittency factor, mean and fluctuating components of velocity) and flow structure (weak vortices and nearly streamwise-invariant kinematics). Hibernation occurs more frequently at high levels of viscoelasticity, leading to flows that increasingly resemble MDR. Viscoelasticity facilitates the occurrence of hibernation by suppressing the conventional ‘active’ turbulence, but has little influence on hibernation itself. At low Weissenberg number $\mathit{Wi}$, the average duration of active turbulence intervals is constant, but above a critical value of $\mathit{Wi}$, the duration decreases dramatically, and accordingly, the fraction of time spent in hibernation increases. This observation can be explained with a simple mathematical model that posits that the lifetime of an active turbulence interval is the time that it takes for the turbulence to stretch polymer molecules to a certain threshold value; once the molecules exceed this threshold, they exert a large enough stress on the flow to suppress the active turbulence. This model predicts an explicit form for the duration as a function of $\mathit{Wi}$ and the simulation results match this prediction very closely. The critical point where hibernation frequency becomes substantially increased coincides with the point where qualitative changes are observed in overall flow statistics – the transition between ‘low-drag-reduction’ and ‘high-drag-reduction’ regimes. Probability density functions of important variables reveal a much higher level of intermittency in the turbulent dynamics after this transition. It is further confirmed that hibernating turbulence is a Newtonian structure during which polymer extension is small. Based on these results, a framework is proposed that explains key transitions in viscoelastic turbulence, especially the convergence toward MDR.
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Michalec, François-Gaël, Itzhak Fouxon, Sami Souissi, and Markus Holzner. "Zooplankton can actively adjust their motility to turbulent flow." Proceedings of the National Academy of Sciences 114, no. 52 (2017): E11199—E11207. http://dx.doi.org/10.1073/pnas.1708888114.
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Calanoid copepods are among the most abundant metazoans in the ocean and constitute a vital trophic link within marine food webs. They possess relatively narrow swimming capabilities, yet are capable of significant self-locomotion under strong hydrodynamic conditions. Here we provide evidence for an active adaptation that allows these small organisms to adjust their motility in response to background flow. We track simultaneously and in three dimensions the motion of flow tracers and planktonic copepods swimming freely at several intensities of quasi-homogeneous, isotropic turbulence. We show that copepods synchronize the frequency of their relocation jumps with the frequency of small-scale turbulence by performing frequent relocation jumps of low amplitude that seem unrelated to localized hydrodynamic signals. We develop a model of plankton motion in turbulence that shows excellent quantitative agreement with our measurements when turbulence is significant. We find that, compared with passive tracers, active motion enhances the diffusion of organisms at low turbulence intensity whereas it dampens diffusion at higher turbulence levels. The existence of frequent jumps in a motion that is otherwise dominated by turbulent transport allows for the possibility of active locomotion and hence to transition from being passively advected to being capable of controlling diffusion. This behavioral response provides zooplankton with the capability to retain the benefits of self-locomotion despite turbulence advection and may help these organisms to actively control their distribution in dynamic environments. Our study reveals an active adaptation that carries strong fitness advantages and provides a realistic model of plankton motion in turbulence.
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Cunningham, Andrew J., Adam Frank, Eric G. Blackman, and Alice Quillen. "Hypersonic swizzle sticks: jets, fossil cavities and turbulence in molecular clouds." Proceedings of the International Astronomical Union 2, S237 (2006): 172–76. http://dx.doi.org/10.1017/s174392130700141x.
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AbstractThe ubiquity and high density of outflows from young stars in clusters make them an intriguing candidate for the source of turbulence energy in molecular clouds. In this contribution we discuss new studies, both observational and theoretical, which address the issue of jet/outflow interactions and their ability to drive turbulent flows in molecular clouds. Our results are surprising in that they show that fossil cavities, rather than bow shocks from active outflows, constitute the mechanism of re-energizing turbulence. We first present simulations which show that collisions between active jets are ineffective at converting directed momentum and energy in outflows into turbulence. This effect comes from the ability of radiative cooling to constrain the surface area through which colliding outflows entrain ambient gas. We next discuss observational results which demonstrate that fossil cavities from “extinct” outflows are abundant in molecular material surrounding clusters such as NGC 1333. These structures, rather than the bow shocks of active outflows, comprise the missing link between outflow energy input and re-energizing turbulence. In a separate theoretical/simulation study we confirm that the evolution of cavities from decaying outflow sources leads to structures which match the observations of fossil cavities. Finally we present new results of outflow propagation in a fully turbulent medium exploring the explicit mechanisms for the transfer of energy and momentum between the driving wind and the turbulent environment.
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Nika, Kralj, Ravnik Miha, and Kos Žiga. "Defect Line Coarsening and Refinement in Active Nematics." Physical Review Letters 130 (March 21, 2023): 128101. https://doi.org/10.1103/PhysRevLett.130.128101.
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Active matter is naturally out of equilibrium which results in the emergence of diverse dynamic steady states, including the omnipresent chaotic state known as the active turbulence. However, much less is known how active systems dynamically depart out of these configurations, such as get excited or damped to a different dynamic steady state. In this Letter, we demonstrate the coarsening and refinement dynamics of topological defect lines in three-dimensional active nematic turbulence. Specifically, using theory and numerical modeling, we are able to predict the evolution of the active defect density away from the steady state due to time-dependent activity or viscoelastic material properties, establishing a single length scale phenomenological description of defect line coarsening and refinement in a three-dimensional active nematic. The approach is first applied to growth dynamics of a single active defect loop, and then to a full three-dimensional active defect network. More generally, this Letter provides insight into the general coarsening phenomena between dynamical regimes in 3D active matter, with a possible analogy in other physical systems.
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Farrell, Brian F., and Petros J. Ioannou. "Turbulence suppression by active control." Physics of Fluids 8, no. 5 (1996): 1257–68. http://dx.doi.org/10.1063/1.868897.
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Krishnamoorthy, V., and S. P. Sukhatme. "The Effect of Free-Stream Turbulence on Gas Turbine Blade Heat Transfer." Journal of Turbomachinery 111, no. 4 (1989): 497–501. http://dx.doi.org/10.1115/1.3262299.
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This paper describes the results of systematic investigations undertaken to study the effect of free-stream turbulence on the heat transfer coefficient distribution around gas turbine rotor blades and nozzle guide vanes. The heat transfer coefficient distribution around the blade surface was obtained under a uniform heat flux boundary condition. Experiments were conducted in the Reynolds number range 2.0–8.1 × 105 (exit Mach number range 0.182 to 0.600) with the free-stream turbulence level in the range 1.0–21.3 percent. A new type of active turbulence generator was used for generating high turbulence levels. Correlations were obtained for the effect of free-stream turbulence on the local heat transfer coefficient in the laminar, transitional, and turbulent boundary layer regions.
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Franks, Peter J. S. "Has Sverdrup's critical depth hypothesis been tested? Mixed layers vs. turbulent layers." ICES Journal of Marine Science 72, no. 6 (2014): 1897–907. http://dx.doi.org/10.1093/icesjms/fsu175.
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Abstract Sverdrup (1953. On conditions for the vernal blooming of phytoplankton. Journal du Conseil International pour l'Exploration de la Mer, 18: 287–295) was quite careful in formulating his critical depth hypothesis, specifying a “thoroughly mixed top layer” with mixing “strong enough to distribute the plankton organisms evenly through the layer”. With a few notable exceptions, most subsequent tests of the critical depth hypothesis have ignored those assumptions, using estimates of a hydrographically defined mixed-layer depth as a proxy for the actual turbulence-driven movement of the phytoplankton. However, a closer examination of the sources of turbulence and stratification in turbulent layers shows that active turbulence is highly variable over time scales of hours, vertical scales of metres, and horizontal scales of kilometres. Furthermore, the mixed layer as defined by temperature or density gradients is a poor indicator of the depth or intensity of active turbulence. Without time series of coincident, in situ measurements of turbulence and phytoplankton rates, it is not possible to properly test Sverdrup's critical depth hypothesis.
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Priambudi Setyo Pratomo, Hariyo, Fandi Dwiputra Suprianto, and Teng Sutrisno. "Hybrid Turbulence Models: Recent Progresses and Further Researches." E3S Web of Conferences 130 (2019): 01013. http://dx.doi.org/10.1051/e3sconf/201913001013.
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Turbulence simulation remains one of the active research activities in computational engineering. Along with the increase in computing power and the prime motivation of improving the accuracy of statistical turbulence modeling approaches and reducing the expensive computational cost of both direct numerical and large turbulence scale- resolving simulations, various hybrid turbulence models being capable of capturing unsteadiness in the turbulence are now accessible. Nevertheless this introduces the daunting task to select an appropriate method for different cases as one can not know a priori the inherent nature of the turbulence. It is the aim of this paper to address recent progresses and further researches within a branch of the hybrid RANS-LES models examined by the first author as simple test cases but generating complex turbulent flows are available from experimentation. In particular, failure of a seamless hybrid formulation not explicitly dependent on the grid scale is discussed. From the literature, it is practical that at least one can go on with confidence when choosing a potential hybrid model by intuitively distinguishing between strongly and weakly unstable turbulent flows.
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Umlauf, Lars. "The Description of Mixing in Stratified Layers without Shear in Large-Scale Ocean Models." Journal of Physical Oceanography 39, no. 11 (2009): 3032–39. http://dx.doi.org/10.1175/2009jpo4006.1.
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Abstract Large-scale geophysical flows often exhibit layers with negligible vertical shear and infinite gradient Richardson number Ri. It is well known that these layers may be regions of active mixing, even in the absence of local shear production of turbulence because, among other processes, turbulence may be supplied by vertical turbulent transport from neighboring regions. This observation is contrasted by the behavior of most turbulence parameterizations used in ocean climate modeling, predicting the collapse of mixing of mass and matter if the Richardson number exceeds a critical threshold. Here, the performance of a simple model without critical Richardson number is evaluated, taking into account the diffusion of turbulence into layers without shear production and therefore avoiding the suppression of mixing at large values of Ri. The model is based on the framework of second-moment turbulence closures, focusing on the consistent modeling of the turbulent length scale for strongly stratified turbulence. Results are compared to eddy-resolving simulations of stratified shear flows that have recently become available. The model is simple enough for inclusion in ocean climate models.
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Faragó, Dávid, and Péter Bencs. "Measurement of turbulence properties." Analecta Technica Szegedinensia 14, no. 1 (2020): 67–75. http://dx.doi.org/10.14232/analecta.2020.1.67-75.
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The aim of the research is to investigate anisotropic turbulence intensities, id est to investigate the distribution of Reynolds stresses and energy spectra in a square cross-section channel, downstream of a semi-active jet turbulence grid generating anisotropic turbulent airflow. In addition to the semi-active jet turbulence grid, another type of turbulence grid was developed and experimentally investigated. This grid contains vertical, flexible strips of aluminum (in this case, there are no perpendicular (horizontal) grid elements), which vibrate at a frequency depending on the velocity of the main airflow. Besides the investigation of the velocity- and turbulence intensity distributions, another main objective of the research is to measure the von Kármán energy spectrum when the turbulence cannot be considered isotropic. This aspiration of ours is justified by the knowledge gap present in the literature in this specific field. Monin has carried out a theoretical study to extend and generalize the von Kármán – Howarth isotropic principal stress equation to the anisotropic regime. The proposed new experimental work aims to provide a solid experimental background for verifying and validating the physical correctness of the Monin equation, which may result in a new theoretical understanding and perception of the major issues and the nature of anisotropic turbulence. Since the anisotropic energy spectra are expected to exhibit different characteristics from the isotropic Kolmogorov spectra, these new experimental results may contribute to the development of new anisotropic and engineering turbulence models that can be used in industrial applications.
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Bratanov, Vasil, Frank Jenko, and Erwin Frey. "New class of turbulence in active fluids." Proceedings of the National Academy of Sciences 112, no. 49 (2015): 15048–53. http://dx.doi.org/10.1073/pnas.1509304112.
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Turbulence is a fundamental and ubiquitous phenomenon in nature, occurring from astrophysical to biophysical scales. At the same time, it is widely recognized as one of the key unsolved problems in modern physics, representing a paradigmatic example of nonlinear dynamics far from thermodynamic equilibrium. Whereas in the past, most theoretical work in this area has been devoted to Navier–Stokes flows, there is now a growing awareness of the need to extend the research focus to systems with more general patterns of energy injection and dissipation. These include various types of complex fluids and plasmas, as well as active systems consisting of self-propelled particles, like dense bacterial suspensions. Recently, a continuum model has been proposed for such “living fluids” that is based on the Navier–Stokes equations, but extends them to include some of the most general terms admitted by the symmetry of the problem [Wensink HH, et al. (2012) Proc Natl Acad Sci USA 109:14308–14313]. This introduces a cubic nonlinearity, related to the Toner–Tu theory of flocking, which can interact with the quadratic Navier–Stokes nonlinearity. We show that as a result of the subtle interaction between these two terms, the energy spectra at large spatial scales exhibit power laws that are not universal, but depend on both finite-size effects and physical parameters. Our combined numerical and analytical analysis reveals the origin of this effect and even provides a way to understand it quantitatively. Turbulence in active fluids, characterized by this kind of nonlinear self-organization, defines a new class of turbulent flows.
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Hearst, R. Jason, Guillaume Gomit, and Bharathram Ganapathisubramani. "Effect of turbulence on the wake of a wall-mounted cube." Journal of Fluid Mechanics 804 (September 9, 2016): 513–30. http://dx.doi.org/10.1017/jfm.2016.565.
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The influence of turbulence on the flow around a wall-mounted cube immersed in a turbulent boundary layer is investigated experimentally with particle image velocimetry and hot-wire anemometry. Free-stream turbulence is used to generate turbulent boundary layer profiles where the normalised shear at the cube height is fixed, but the turbulence intensity at the cube height is adjustable. The free-stream turbulence is generated with an active grid and the turbulent boundary layer is formed on an artificial floor in a wind tunnel. The boundary layer development Reynolds number ($Re_{x}$) and the ratio of the cube height ($h$) to the boundary layer thickness ($\unicode[STIX]{x1D6FF}$) are held constant at $Re_{x}=1.8\times 10^{6}$ and $h/\unicode[STIX]{x1D6FF}=0.47$. It is demonstrated that the stagnation point on the upstream side of the cube and the reattachment length in the wake of the cube are independent of the incoming profile for the conditions investigated here. In contrast, the wake length monotonically decreases for increasing turbulence intensity but fixed normalised shear – both quantities measured at the cube height. The wake shortening is a result of heightened turbulence levels promoting wake recovery from high local velocities and the reduction in strength of a dominant shedding frequency.
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Neuhaus, Lars, Daniel Ribnitzky, Michael Hölling, et al. "Model wind turbine performance in turbulent–non-turbulent boundary layer flow." Journal of Physics: Conference Series 2767, no. 4 (2024): 042018. http://dx.doi.org/10.1088/1742-6596/2767/4/042018.
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Abstract With increasing distance from the coast and greater hub heights, wind turbines expand into unknown, hardly researched environmental conditions. As height increases, laminar flow conditions become more likely. With the simultaneous increase in rotor diameter, very different flow conditions, from laminar to turbulent, occur over the rotor area. It is crucial to understand the effects of these different flow conditions on wind turbines. We approach this through wind tunnel experiments, presenting a setup with two different active grids. This setup enables the generation of four different flows – homogeneous, shear, turbulent–non-turbulent, and turbulent–non-turbulent shear flow – each with four different turbulence levels. The turbulent–non-turbulent flows exhibit a turbulence intensity gradient between the quasi-laminar flow at the upper and turbulent flow at the lower rotor half, establishing a turbulent–non-turbulent interface between the two rotor halves. In a second step, we investigate the Model Wind Turbine Oldenburg with a rotor diameter of 1.8 m (MoWiTO 1.8) under these conditions and analyze their effects on power output and blade loads. While the power fluctuations depend directly on the turbulence intensity, an additional turbulence intensity gradient shows no significant effect. A stronger effect can be observed for the blade root bending moments, the fluctuations of which increase with shear and also in turbulent–non-turbulent flow.
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Baungaard, Mads, Maarten Paul van der Laan, and Mark Kelly. "RANS modeling of a single wind turbine wake in the unstable surface layer." Wind Energy Science 7, no. 2 (2022): 783–800. http://dx.doi.org/10.5194/wes-7-783-2022.
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Abstract. Unstable atmospheric conditions are often observed during the daytime over land and for significant periods offshore and are hence relevant for wake studies. A simple k–ε RANS turbulence model for simulation of wind turbine wakes in the unstable surface layer is presented, which is based on Monin–Obukhov similarity theory (MOST). The turbulence model parametrizes buoyant production of turbulent kinetic energy (TKE) without the use of an active temperature equation, and flow balance is ensured throughout the domain by modifications of the turbulence transport equations. Large eddy simulations and experimental data from the literature are used for validation of the model.
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Urzay, J., A. Doostmohammadi, and J. M. Yeomans. "Multi-scale statistics of turbulence motorized by active matter." Journal of Fluid Mechanics 822 (June 8, 2017): 762–73. http://dx.doi.org/10.1017/jfm.2017.311.
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A number of micro-scale biological flows are characterized by spatio-temporal chaos. These include dense suspensions of swimming bacteria, microtubule bundles driven by motor proteins and dividing and migrating confluent layers of cells. A characteristic common to all of these systems is that they are laden with active matter, which transforms free energy in the fluid into kinetic energy. Because of collective effects, the active matter induces multi-scale flow motions that bear strong visual resemblance to turbulence. In this study, multi-scale statistical tools are employed to analyse direct numerical simulations (DNS) of periodic two-dimensional (2-D) and three-dimensional (3-D) active flows and to compare the results to classic turbulent flows. Statistical descriptions of the flows and their variations with activity levels are provided in physical and spectral spaces. A scale-dependent intermittency analysis is performed using wavelets. The results demonstrate fundamental differences between active and high-Reynolds-number turbulence; for instance, the intermittency is smaller and less energetic in active flows, and the work of the active stress is spectrally exerted near the integral scales and dissipated mostly locally by viscosity, with convection playing a minor role in momentum transport across scales.
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Vinuesa, Ricardo, Oriol Lehmkuhl, Adrian Lozano-Durán, and Jean Rabault. "Flow Control in Wings and Discovery of Novel Approaches via Deep Reinforcement Learning." Fluids 7, no. 2 (2022): 62. http://dx.doi.org/10.3390/fluids7020062.
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In this review, we summarize existing trends of flow control used to improve the aerodynamic efficiency of wings. We first discuss active methods to control turbulence, starting with flat-plate geometries and building towards the more complicated flow around wings. Then, we discuss active approaches to control separation, a crucial aspect towards achieving a high aerodynamic efficiency. Furthermore, we highlight methods relying on turbulence simulation, and discuss various levels of modeling. Finally, we thoroughly revise data-driven methods and their application to flow control, and focus on deep reinforcement learning (DRL). We conclude that this methodology has the potential to discover novel control strategies in complex turbulent flows of aerodynamic relevance.
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Enciu, D., and I. Ursu. "Towards improving passengers safety and comfort based on turbulence tests in aerodynamic tunnel." IOP Conference Series: Earth and Environmental Science 1185, no. 1 (2023): 012007. http://dx.doi.org/10.1088/1755-1315/1185/1/012007.
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Abstract First, the article outlines the elements of an ongoing research project, aiming to demonstrate that the basic ICAO standard EDR (Eddy Dissipation Rate) index of atmospheric turbulence intensity, defined as the cubic root of the turbulence energy per unit time and mass, can be measured and calculated in Wind Tunnel (WT) according to the basic Kolmogorov concept. This involves showing that EDR index calculated in WT, taking a sufficient number of velocity measuring points, can be an objective measure of turbulence intensity, independent of the body immersed in the fluid. Secondly, it is shown that these measurements are accompanied by intermediate active vibration control tests. For this purpose, a Turbulence Generator was built and installed in the WT and an intelligent wing model with implemented LQG active control was introduced in the WT. These two procedures, complementary in substance, aim to demonstrate the consistency of the flight approach based on the objectivity of this ICAO standard EDR index and the vibration reduction methodology in a turbulent atmospheric environment.
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Alert, Ricard, Jean-François Joanny, and Jaume Casademunt. "Universal scaling of active nematic turbulence." Nature Physics 16, no. 6 (2020): 682–88. http://dx.doi.org/10.1038/s41567-020-0854-4.
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Cekli, Hakki Ergun, and Willem van de Water. "Tailoring turbulence with an active grid." Experiments in Fluids 49, no. 2 (2010): 409–16. http://dx.doi.org/10.1007/s00348-009-0812-5.
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Zhang, Qi, and Robert A. Handler. "Active suppression of buoyancy driven turbulence." International Journal of Heat and Mass Transfer 75 (August 2014): 207–12. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.03.012.
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Fiorillo, Damiano F. G., Luca Comisso, Enrico Peretti, Maria Petropoulou, and Lorenzo Sironi. "A Magnetized Strongly Turbulent Corona as the Source of Neutrinos from NGC 1068." Astrophysical Journal 974, no. 1 (2024): 75. http://dx.doi.org/10.3847/1538-4357/ad7021.
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Abstract The cores of active galactic nuclei are potential accelerators of 10–100 TeV cosmic rays, in turn producing high-energy neutrinos. This picture was confirmed by the compelling evidence of a TeV neutrino signal from the nearby active galaxy NGC 1068, leaving open the question of what is the site and mechanism of cosmic-ray acceleration. One candidate is the magnetized turbulence surrounding the central supermassive black hole. Recent particle-in-cell simulations of magnetized turbulence indicate that stochastic cosmic-ray acceleration is nonresonant, in contrast to the assumptions of previous studies. We show that this has important consequences on a self-consistent theory of neutrino production in the corona, leading to a more rapid cosmic-ray acceleration than previously considered. The turbulent magnetic-field fluctuations needed to explain the neutrino signal are consistent with a magnetically powered corona. We find that strong turbulence, with turbulent magnetic energy density higher than 1% of the rest-mass energy density, naturally explains the normalization of the IceCube neutrino flux, in addition to the neutrino spectral shape. Only a fraction of the protons in the corona, which can be directly inferred from the neutrino signal, are accelerated to high energies. Thus, in this framework, the neutrino signal from NGC 1068 provides a testbed for particle acceleration in magnetized turbulence.
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Thampi, Sumesh P., Amin Doostmohammadi, Tyler N. Shendruk, Ramin Golestanian, and Julia M. Yeomans. "Active micromachines: Microfluidics powered by mesoscale turbulence." Science Advances 2, no. 7 (2016): e1501854. http://dx.doi.org/10.1126/sciadv.1501854.
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Dense active matter, from bacterial suspensions and microtubule bundles driven by motor proteins to cellular monolayers and synthetic Janus particles, is characterized by mesoscale turbulence, which is the emergence of chaotic flow structures. By immersing an ordered array of symmetric rotors in an active fluid, we introduce a microfluidic system that exploits spontaneous symmetry breaking in mesoscale turbulence to generate work. The lattice of rotors self-organizes into a spin state where neighboring discs continuously rotate in permanent alternating directions due to combined hydrodynamic and elastic effects. Our virtual prototype demonstrates a new research direction for the design of micromachines powered by the nematohydrodynamic properties of active turbulence.
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Basevich, Valentin Y., Andrey A. Belyaev, Fedor S. Frolov, and Sergey M. Frolov. "Turbulent Flame Propagation in Hydrogen-Air and Methane-Air Mixtures in the Field of Synthetic Turbulence: Direct Numerical Simulation." Eng 4, no. 1 (2023): 748–60. http://dx.doi.org/10.3390/eng4010045.
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A technique alternative to the direct numerical simulation of turbulent combustion of gas mixtures is proposed. It is based on the solution of the three-dimensional transport equations for species concentrations and the energy conservation equation in the “synthetic” field of constant-pressure homogeneous, isotropic and statistically stationary (forced) turbulence using the detailed reaction mechanism. The synthetic turbulence with given spatial and temporal correlation functions is generated using the Monte Carlo method, assuming that the components of the vector of fluctuation velocity obey the normal Gaussian distribution. The technique is applied to the problem of turbulent combustion of fuel-lean and stoichiometric mixtures of hydrogen and methane with air at a turbulence intensity up to 10 m/s. The calculated turbulent flame propagation velocities agree satisfactorily with the values measured in the fan-stirred bomb. The predicted volume fractions of active reaction centers H, O, and OH in a turbulent flame are shown to be less than in a laminar flame up to an order of magnitude, which also agrees with the experiment. In general, calculations indicate that the “wrinkled flame” model is applicable to fuel-lean and stoichiometric mixtures of hydrogen and methane with air at turbulence intensities up to 10 m/s
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Czwielong, Felix, and Stefan Becker. "Active Turbulence Grid-Controlled Inflow Turbulence and Replication of Heat Exchanger Flow Fields in Fan Applications." International Journal of Turbomachinery, Propulsion and Power 8, no. 1 (2023): 1. http://dx.doi.org/10.3390/ijtpp8010001.
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A novel active turbulence grid of the Institute of Fluid Mechanics at FAU Erlangen-Nuremberg is introduced. The focus of this grid is not on basic investigations of fluid mechanics, as is usually the case with active turbulence grids, but the generation of defined inflow conditions for axial fans. Thus, by means of the active turbulence grid, individual turbulence characteristics in the flow to the fan can be changed; therefore, fundamental interactions between the flow mechanics at the axial fan and the sound radiation can be analyzed. In addition, the replication of the flow fields of heat exchangers by the active turbulence grid is the focus of the investigations. The investigations showed that it is possible to use the active turbulence grid to generate defined inflow conditions for axial fans. It was also possible to reproduce the heat exchanger flow fields both for the mean turbulence values and for the spatial distributions. It was found that the grid induces tonal components due to the drive motors, but also that the inherent noise has no significant influence on the spectrum of the fans under investigation. Based on selected turbulence characteristics, direct correlations were found between the spatial distribution of the turbulence level and sound radiation at the first blade passing frequency of the axial fan. As the variance of the turbulence level increases, the sound radiation of the tonal components becomes more pronounced. The total sound pressure level, however, is mainly determined by the low-frequency broadband sound. A linear relationship between the spatial mean value of the turbulence level and the total sound pressure level was found for the investigated axial fan.
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Brandenburg, Axel, Petri J. Käpylä, Igor Rogachevskii, and Nobumitsu Yokoi. "Helicity Effect on Turbulent Passive and Active Scalar Diffusivities." Astrophysical Journal 984, no. 1 (2025): 88. https://doi.org/10.3847/1538-4357/adc691.
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Abstract Turbulent flows are known to produce enhanced effective magnetic and passive scalar diffusivities, which can fairly accurately be determined with numerical methods. It is now known that, if the flow is also helical, the effective magnetic diffusivity is reduced relative to the nonhelical value. Neither the usual second-order correlation approximation nor the various τ approaches have been able to capture this. Here we show that the helicity effect on the turbulent passive scalar diffusivity works in the opposite sense and leads to an enhancement. We have also demonstrated that the correlation time of the turbulent velocity field increases with the kinetic helicity. This is a key point in the theoretical interpretation of the obtained numerical results. Simulations in which helicity is being produced self-consistently by stratified rotating turbulence resulted in a turbulent passive scalar diffusivity that was found to be decreasing with increasing rotation rate.
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SAVELSBERG, RALPH, and WILLEM VAN DE WATER. "Experiments on free-surface turbulence." Journal of Fluid Mechanics 619 (January 25, 2009): 95–125. http://dx.doi.org/10.1017/s0022112008004369.
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We study the free surface of a turbulent flow, in particular the relation between the statistical properties of the wrinkled surface and those of the velocity field beneath it. Channel flow turbulence is generated using an active grid. Through a judicial choice of the stirring protocol the anisotropy of the subsurface turbulence can be controlled. The largest Taylor Reynolds number obtained is Reλ = 258. We characterize the homogeneity and isotropy of the flow and discuss Taylor's frozen turbulence hypothesis, which applies to the subsurface turbulence but not to the surface. The surface gradient field is measured using a novel laser-scanning device. Simultaneously, the velocity field in planes just below the surface is measured using particle image velocimetry (PIV). Several intuitively appealing relations between the surface gradient field and functionals of the subsurface velocity field are tested. For an irregular flow shed off a vertical cylinder, we find that surface indentations are strongly correlated with both vortical and strain events in the velocity field. For fully developed turbulence this correlation is dramatically reduced. This is because the large eddies of the subsurface turbulent flow excite random capillary–gravity waves that travel in all directions across the surface. Therefore, the turbulent surface has dynamics of its own. Nonetheless, it does inherit both the integral scale, which determines the predominant wavelength of the capillary–gravity surface waves, and the (an)isotropy from the subsurface turbulence. The kinematical aspects of the surface–turbulence connection are illustrated by a simple model in which the surface is described in terms of waves originating from Gaussian wave sources that are randomly sprinkled on the moving surface.
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Ware, Ryan M., and Julie L. Zilles. "Tracing Discursive Turbulence as Intra-active Pedagogical Change and Becoming." Written Communication 41, no. 1 (2023): 138–66. http://dx.doi.org/10.1177/07410883231207105.
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This article reports on a mentoring case from a transdisciplinary, longitudinal writing-across-the-curriculum (WAC) initiative in which the situated complexities of integrating new writing pedagogies were observed and supported. Considering this case through an agential realist lens, we introduce the concept of “discursive turbulence”: an emergent quality of situated semiotic activity produced from the continual mixing of discourses. Discursive turbulence can emerge in myriad and complex ways, including fits-and-starts of pedagogical development, mismatched discursive alignments, affective signs of struggle and intensity, and nonlinear patterns of change. Through a series of four vignettes, we illustrate discursive turbulence as it emerged while pedagogical changes around writing were being implemented by an environmental sciences professor. We suggest that discursive turbulence is to be expected in heterodisciplinary spaces, and we argue that attention to discursive turbulence will lead to more robust accounts of learning, becoming, and literate activity, as well as new ways of supporting pedagogical becoming.
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Zhang, Xian Kui, and Tian Yi Liu. "Structure Design and Turbulence Simulation of Disaggregation Pipe in Dry Powder Inhaler." Applied Mechanics and Materials 220-223 (November 2012): 1727–31. http://dx.doi.org/10.4028/www.scientific.net/amm.220-223.1727.
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This paper provides structure design of the disaggregation pipe in active dry powder inhaler, to achieve the maximization of active pharmaceutical ingredient diffusing and disaggregating from its carriers, uses computational fluid dynamics method in CFX to simulate the turbulence in the structure, observes airflow velocity, turbulent kinetic energy and shear strain rate to evaluate the effect of diffusion and disaggregation and operate Lagrange particle tracking method in the structure to obtain particles’ superficial velocity, volume fraction and shear stress, providing a theoretical basis for structure design of the disaggregation pipe in active dry powder inhaler.
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Wall, Dylan, and Eric Paterson. "Anisotropic RANS Turbulence Modeling for Wakes in an Active Ocean Environment." Fluids 5, no. 4 (2020): 248. http://dx.doi.org/10.3390/fluids5040248.
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The problem of simulating wakes in a stratified oceanic environment with active background turbulence is considered. Anisotropic RANS turbulence models are tested against laboratory and eddy-resolving models of the problem. An important aspect of our work is to acknowledge that the environment is not quiescent; therefore, additional sources are included in the models to provide a non-zero background turbulence. The RANS models are found to reproduce some key features from the eddy-resolving and laboratory descriptions of the problem. Tests using the freestream sources show the intuitive result that background turbulence causes more rapid wake growth and decay.
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Gómez, Daniel O., and Pablo Dmitruk. "Turbulent heating of coronal active regions." Proceedings of the International Astronomical Union 3, S247 (2007): 269–78. http://dx.doi.org/10.1017/s1743921308014968.
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AbstractMagnetohydrodynamic turbulence has been proposed as a mechanism for the heating of coronal active regions, and has therefore been actively investigated in recent years. According to this scenario, a turbulent regime is driven by footpoint motions. The energy being pumped this way into active region loops, is efficiently transferred to small scales due to a direct energy cascade. The ensuing generation of fine scale structures, which is a natural outcome of turbulent regimes, helps to enhance the dissipation of either waves or DC currents.We present an updated overview of recent results on turbulent coronal heating. To illustrate this theoretical scenario, we simulate the internal dynamics of a coronal loop within the reduced MHD approximation. The application of a stationary velocity field at the photospheric boundary leads to a turbulent stationary regime after several photospheric turnover times. This regime is characterized by a broadband power spectrum and energy dissipation rate levels compatible with the heating requirements of active region loops. Also, the energy dissipation rate displays a complex superposition of impulsive events, which we associate to the so-called nanoflares. A statistical analysis yields a power law distribution as a function of their energies, which is consistent with those obtained from observations. We also study the distributions of peak dissipation rate and duration of these events.
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Mokhov, I. I., O. G. Chkhetiani, and I. A. Repina. "Turbulence, Atmosphere and Climate Dynamics." IOP Conference Series: Earth and Environmental Science 1040, no. 1 (2022): 011001. http://dx.doi.org/10.1088/1755-1315/1040/1/011001.
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Abstract The conference "Turbulence, Atmosphere and Climate Dynamics" dedicated to the memory of the Alexander M. Obukhov was held in Moscow from November 10 to 12, 2020. The topics of the conference covered the following scientific areas: turbulence; geophysical hydrodynamics; atmospheric and climate system dynamics; physics and composition of the atmosphere; air-sea interaction; wave propagation. The conference showed a high scientific level of almost all the presentations. Studies of turbulent, climatic and atmospheric processes are traditionally conducted in our country at the highest level, as evidenced by the publication in high-ranking scientific journals and the active participation of Russian scientists in international programs. List of Program committee, Organizing committee are avilable in this pdf.
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POORTE, R. E. G., and A. BIESHEUVEL. "Experiments on the motion of gas bubbles in turbulence generated by an active grid." Journal of Fluid Mechanics 461 (June 25, 2002): 127–54. http://dx.doi.org/10.1017/s0022112002008273.
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The random motion of nearly spherical bubbles in the turbulent flow behind a grid is studied experimentally. In quiescent water these bubbles rise at high Reynolds number. The turbulence is generated by an active grid of the design of Makita (1991), and can have turbulence Reynolds number Rλ of up to 200. Minor changes in the geometry of the grid and in its mode of operation improves the isotropy of the turbulence, compared with that reported by Makita (1991) and Mydlarski & Warhaft (1996). The trajectory of each bubble is measured with high spatial and temporal resolution with a specially developed technique that makes use of a position-sensitive detector. Bubble statistics such as the mean rise velocity and the root-mean-square velocity fluctuations are obtained by ensemble averaging over many identical bubbles. The resulting bubble mean rise velocity is significantly reduced (up to 35%) compared with the quiescent conditions. The vertical bubble velocity fluctuations are found to be non-Gaussian, whereas the horizontal displacements are Gaussian for all times. The diffusivity of bubbles is considerably less than that of fluid particles. These findings are qualitatively consistent with results obtained through theoretical analysis and numerical simulations by Spelt & Biesheuvel (1997).
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Quinn, Daniel B., Anthony Watts, Tony Nagle, and David Lentink. "A new low-turbulence wind tunnel for animal and small vehicle flight experiments." Royal Society Open Science 4, no. 3 (2017): 160960. http://dx.doi.org/10.1098/rsos.160960.
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Our understanding of animal flight benefits greatly from specialized wind tunnels designed for flying animals. Existing facilities can simulate laminar flow during straight, ascending and descending flight, as well as at different altitudes. However, the atmosphere in which animals fly is even more complex. Flow can be laminar and quiet at high altitudes but highly turbulent near the ground, and gusts can rapidly change wind speed. To study flight in both laminar and turbulent environments, a multi-purpose wind tunnel for studying animal and small vehicle flight was built at Stanford University. The tunnel is closed-circuit and can produce airspeeds up to 50 m s −1 in a rectangular test section that is 1.0 m wide, 0.82 m tall and 1.73 m long. Seamless honeycomb and screens in the airline together with a carefully designed contraction reduce centreline turbulence intensities to less than or equal to 0.030% at all operating speeds. A large diameter fan and specialized acoustic treatment allow the tunnel to operate at low noise levels of 76.4 dB at 20 m s −1 . To simulate high turbulence, an active turbulence grid can increase turbulence intensities up to 45%. Finally, an open jet configuration enables stereo high-speed fluoroscopy for studying musculoskeletal control in turbulent flow.
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Dogan, Eda, Ronald E. Hanson, and Bharathram Ganapathisubramani. "Interactions of large-scale free-stream turbulence with turbulent boundary layers." Journal of Fluid Mechanics 802 (August 1, 2016): 79–107. http://dx.doi.org/10.1017/jfm.2016.435.
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The scale interactions occurring within a turbulent boundary layer are investigated in the presence of free-stream turbulence. The free-stream turbulence is generated by an active grid. The free stream is monitored by a single-component hot-wire probe, while a second probe is roved across the height of the boundary layer at the same streamwise location. Large-scale structures occurring in the free stream are shown to penetrate the boundary layer and increase the streamwise velocity fluctuations throughout. It is speculated that, depending on the extent of the penetration, i.e. based on the level of free-stream turbulence, the near-wall turbulence production peaks at different wall-normal locations than the expected location of $y^{+}\approx 15$ for a canonical turbulent boundary layer. It is shown that the large scales dominating the log region have a modulating effect on the small scales in the near-wall region; this effect becomes more significant with increasing turbulence in the free stream, i.e. similarly increasing $Re_{\unicode[STIX]{x1D706}_{0}}$. This modulating interaction and its Reynolds-number trend have similarities with canonical turbulent boundary layers at high Reynolds numbers where the interaction between the large scales and the envelope of the small scales exhibits a pure amplitude modulation (Hutchins & Marusic, Phil. Trans. R. Soc. Lond. A, vol. 365 (1852), 2007, pp. 647–664; Mathis et al., J. Fluid Mech., vol. 628, 2009, pp. 311–337). This similarity has encouraging implications towards generalising scale interactions in turbulent boundary layers.
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Peng, Yi, Zhengyang Liu, and Xiang Cheng. "Imaging the emergence of bacterial turbulence: Phase diagram and transition kinetics." Science Advances 7, no. 17 (2021): eabd1240. http://dx.doi.org/10.1126/sciadv.abd1240.
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We experimentally study the emergence of collective bacterial swimming, a phenomenon often referred to as bacterial turbulence. A phase diagram of the flow of 3D Escherichia coli suspensions spanned by bacterial concentration, the swimming speed of bacteria, and the number fraction of active swimmers is systematically mapped, which shows quantitative agreement with kinetic theories and demonstrates the dominant role of hydrodynamic interactions in bacterial collective swimming. We trigger bacterial turbulence by suddenly increasing the swimming speed of light-powered bacteria and image the transition to the turbulence in real time. Our experiments identify two unusual kinetic pathways, i.e., the one-step transition with long incubation periods near the phase boundary and the two-step transition driven by long-wavelength instabilities deep inside the turbulent phase. Our study provides not only a quantitative verification of existing theories but also insights into interparticle interactions and transition kinetics of bacterial turbulence.
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Baring, Matthew G. "X-ray Synchrotron Polarization from Turbulent Plasmas in Supernova Remnants." Proceedings of the International Astronomical Union 12, S331 (2017): 242–47. http://dx.doi.org/10.1017/s174392131700480x.
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AbstractAs supernova remnants (SNRs) age, they become efficient cosmic ray accelerators at their outer shell shocks. The current paradigm for shock acceleration theory favors turbulent field environs in the proximity of these shocks, turbulence driven by current instabilities involving energetic ions. With the imminent prospect of dedicated X-ray polarimeters becoming a reality, the possibility looms of probing turbulence on scales that couple to the super-TeV electrons that emit X-rays. This paper presents model X-ray polarization signatures from energetic electrons moving in simulated MHD turbulence of varying levels of “chaos.” The emission volumes are finite slabs that represent the active regions of young SNR shells. We find that the turbulent field energy must be quite limited relative to that of the total field in order for the X-ray polarization degree to be as strong as the radio measures obtained in some remnants. Results presented are pertinent to the planned IXPE and XIPE polarimeters.
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Worlitzer, Vasco M., Gil Ariel, Avraham Be'er, Holger Stark, Markus Bär, and Sebastian Heidenreich. "Turbulence-induced clustering in compressible active fluids." Soft Matter 17, no. 46 (2021): 10447–57. http://dx.doi.org/10.1039/d1sm01276b.
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A continuum model of compressible active polar fluids, incorporating typical characteristics of bacterial swarms, is analyzed. We identify a novel phase in which self-sustained turbulence continuously creates and destroys dense clusters.
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Thampi, Sumesh P., Ramin Golestanian, and Julia M. Yeomans. "Vorticity, defects and correlations in active turbulence." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 372, no. 2029 (2014): 20130366. http://dx.doi.org/10.1098/rsta.2013.0366.
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We describe a numerical investigation of a continuum model of an active nematic, concentrating on the regime of active turbulence. Results are presented for the effect of three parameters, activity, elastic constant and rotational diffusion constant, on the order parameter and flow fields. Defects and distortions in the director field act as sources of vorticity, and thus vorticity is strongly correlated to the director field. In particular, the characteristic length of decay of vorticity and order parameter correlations is controlled by the defect density. By contrast, the decay of velocity correlations is determined by a balance between activity and dissipation. We highlight the role of microscopic flow generation mechanisms in determining the flow patterns and characteristic scales of active turbulence and contrast the behaviour of extensile and contractile active nematics.
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Cekli, Hakki Ergun, and Willem van de Water. "Stirring anisotropic turbulence with an active grid." Physics of Fluids 32, no. 7 (2020): 075119. http://dx.doi.org/10.1063/5.0008021.
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Rumsey, C. L., and R. C. Swanson. "Turbulence modelling for active flow control applications." International Journal of Computational Fluid Dynamics 23, no. 4 (2009): 317–26. http://dx.doi.org/10.1080/10618560902776794.
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WISNIEWSKI, MARTINA, RALF KISSMANN, and FELIX SPANIER. "Turbulence evolution in MHD plasmas." Journal of Plasma Physics 79, no. 5 (2013): 597–612. http://dx.doi.org/10.1017/s0022377813000147.
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AbstractTurbulence in the interstellar medium has been an active field of research in the last decade. Numerical simulations are the tool of choice in most cases. However, while there are a number of simulations on the market, some questions have not been answered finally. In this paper, we examine the influence of compressible and incompressible driving on the evolution of turbulent spectra in a number of possible interstellar medium scenarios. We conclude that the driving has an influence not only on the ratio of compressible to incompressible component but also on the anisotropy of turbulence.
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Moin, Parviz, and Thomas Bewley. "Feedback Control of Turbulence." Applied Mechanics Reviews 47, no. 6S (1994): S3—S13. http://dx.doi.org/10.1115/1.3124438.
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A brief review of current approaches to active feedback control of the fluctuations arising in turbulent flows is presented, emphasizing the mathematical techniques involved. Active feedback control schemes are categorized and compared by examining the extent to which they are based on the governing flow equations. These schemes are broken down into the following categories: adaptive schemes, schemes based on heuristic physical arguments, schemes based on a dynamical systems approach, and schemes based on optimal control theory applied directly to the Navier-Stokes equations. Recent advances in methods of implementing small scale flow control ideas are also reviewed.
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Li, Tak Chu, Yi-Hsin Liu, and Yi Qi. "Magnetic Flux Transport Signatures of Electron-only and Ion-coupled Magnetic Reconnection in Plasma Turbulence." Astrophysical Journal 986, no. 2 (2025): 154. https://doi.org/10.3847/1538-4357/add882.
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Abstract Electron-only magnetic reconnection was first detected by the Magnetospheric Multiscale (MMS) mission in Earth’s turbulent magnetosheath. Its prevalence in kinetic-scale turbulence has attracted great interest in heliophysics, but also revealed a great challenge in identifying it in turbulence, where electron flows are often complex. The magnetic flux transport (MFT) method is an innovative method to identify active reconnection in numerical simulations and in situ observations of turbulent plasmas. Here we extend this method to distinguish between electron-only and ion-coupled reconnection. The coupling of magnetic field motion with plasma flows in the diffusion regions sets distinct scales in the MFT velocity. While both forms of reconnection satisfy the MFT signature for active reconnection as MFT inflows and outflows at an X-line, the specific electron-only MFT signature is only an electron-scale MFT outflow along the current sheet normal direction, whereas the specific ion-coupled signature is a two-scale, outer-ion-and-inner-electron-scale MFT outflow in the electron diffusion region, which evolves into a single ion-scale in the ion diffusion region. These signatures are verified in a simulation of gyrokinetic turbulence. The dependence of the MFT outflow on the distance downstream from the X-lines also agrees well with the framework of magnetic field–plasma flow coupling. The new MFT signatures provide a clear and reliable tool for investigating electron-only reconnection in turbulence, independent of the development of electron outflows. They are directly applicable to kinetic and fluid simulations, and have potential application to observations of diffusion region crossings by spacecraft missions such as MMS.
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Mashayek, A., C. P. Caulfield, and W. R. Peltier. "Role of overturns in optimal mixing in stratified mixing layers." Journal of Fluid Mechanics 826 (August 8, 2017): 522–52. http://dx.doi.org/10.1017/jfm.2017.374.
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Turbulent mixing plays a major role in enabling the large-scale ocean circulation. The accuracy of mixing rates estimated from observations depends on our understanding of basic fluid mechanical processes underlying the nature of turbulence in a stratified fluid. Several of the key assumptions made in conventional mixing parameterizations have been increasingly scrutinized in recent years, primarily on the basis of adequately high resolution numerical simulations. We add to this evidence by compiling results from a suite of numerical simulations of the turbulence generated through stratified shear instability processes. We study the inherently intermittent and time-dependent nature of wave-induced turbulent life cycles and more specifically the tight coupling between inherently anisotropic scales upon which small-scale isotropic turbulence grows. The anisotropic scales stir and stretch fluid filaments enhancing irreversible diffusive mixing at smaller scales. We show that the characteristics of turbulent mixing depend on the relative time evolution of the Ozmidov length scale $L_{O}$ compared to the so-called Thorpe overturning scale $L_{T}$ which represents the scale containing available potential energy upon which turbulence feeds and grows. We find that when $L_{T}\sim L_{O}$, the mixing is most active and efficient since stirring by the largest overturns becomes ‘optimal’ in the sense that it is not suppressed by ambient stratification. We argue that the high mixing efficiency associated with this phase, along with observations of $L_{O}/L_{T}\sim 1$ in oceanic turbulent patches, together point to the potential for systematically underestimating mixing in the ocean if the role of overturns is neglected. This neglect, arising through the assumption of a clear separation of scales between the background mean flow and small-scale quasi-isotropic turbulence, leads to the exclusion of an highly efficient mixing phase from conventional parameterizations of the vertical transport of density. Such an exclusion may well be significant if the mechanism of shear-induced turbulence is assumed to be representative of at least some turbulent events in the ocean. While our results are based upon simulations of shear instability, we show that they are potentially more generic by making direct comparisons with $L_{T}-L_{O}$ data from ocean and lake observations which represent a much wider range of turbulence-inducing physical processes.
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Bellenger, H., R. Wilson, J. L. Davison, et al. "Tropospheric Turbulence over the Tropical Open Ocean: Role of Gravity Waves." Journal of the Atmospheric Sciences 74, no. 4 (2017): 1249–71. http://dx.doi.org/10.1175/jas-d-16-0135.1.
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Abstract A large set of soundings obtained in the Indian Ocean during three field campaigns is used to provide statistical characteristics of tropospheric turbulence and its link with gravity wave (GW) activity. The Thorpe method is used to diagnose turbulent regions of a few hundred meters depth. Above the mixed layer, turbulence frequency varies from ~10% in the lower troposphere up to ~30% around 12-km height. GWs are captured by their signature in horizontal wind, normalized temperature, and balloon vertical ascent rate. These parameters emphasize different parts of the wave spectrum from longer to shorter vertical wavelengths. Composites are constructed in order to reveal the vertical structure of the waves and their link with turbulence. The relatively longer-wavelength GWs described by their signature in temperature (GWTs) are more active in the lower troposphere, where they are associated with clear variations in moisture. Turbulence is then associated with minimum static stability and vertical shear, stressing the importance of the former and the possibility of convective instability. Conversely, the short waves described by their signature in balloon ascent rate (GWws) are detected primarily in the upper troposphere, and their turbulence is associated with a vertical shear maximum, suggesting the importance of dynamic instability. Furthermore, GWws appear to be linked with local convection, whereas GWTs are more active in suppressed and dry phases in particular of the Madden–Julian oscillation. These waves may be associated with remote sources, such as organized convection or local fronts, such as those associated with dry-air intrusions.
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Chian, Abraham C. L. "Order and Chaos in Accretion Disks of Active Galactic Nuclei." International Astronomical Union Colloquium 163 (1997): 663–66. http://dx.doi.org/10.1017/s0252921100043359.
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AbstractLangmuir turbulence plays an important role in electron heating and generation of plasma emission in accretion disks of active galactic nuclei. The nonlinear dynamical behavior of Langmuir turbulence and its relevance in the interpretation of AGN variability is discussed. In particular, we study nonlinear saturation of the Langmuir stimulated modulational instability, for which the low-frequency mode is a resonant ion-acoustic wave. The nonlinear system of coupled wave equations is shown to undergo transition from order to chaos via the route of quasiperiodicity. The periodic, quasiperiodic and chaotic variabilities in AGN emissions may be the electromagnetic signatures of the ordered and chaotic states of Langmuir turbulence in accretion disks or jets of AGN.
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van Buel, Reinier, and Holger Stark. "Active open-loop control of elastic turbulence." Scientific Reports 10, no. 1 (2020). http://dx.doi.org/10.1038/s41598-020-72402-y.
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Abstract We demonstrate through numerical solutions of the Oldroyd-B model in a two-dimensional Taylor–Couette geometry that the onset of elastic turbulence in a viscoelastic fluid can be controlled by imposed shear-rate modulations, one form of active open-loop control. Slow modulations display rich and complex behavior where elastic turbulence is still present, while it vanishes for fast modulations and a laminar response with the Taylor–Couette base flow is recovered. We find that the transition from the laminar to the turbulent state is supercritical and occurs at a critical Deborah number. In the state diagram of both control parameters, Weissenberg versus Deborah number, we identify the region of elastic turbulence. We also quantify the transition by the flow resistance, for which we derive an analytic expression in the laminar regime within the linear Oldroyd-B model. Finally, we provide an approximation for the transition line in the state diagram introducing an effective critical Weissenberg number in comparison to constant shear. Deviations from the numerical result indicate that the physics behind the observed laminar-to-turbulent transition is more complex under time-modulated shear flow.