1,721,232 research outputs found
Universality of anisotropic fluctuations from numerical simulations of turbulent flows
No full textWe present new results from a direct numerical simulation of a three-dimensional homogeneous Rayleigh-Bénard system (HRB), i.e. a convective cell with an imposed linear mean temperature profile along the vertical direction. We measure the SO(3)-decomposition of both velocity structure functions and buoyancy terms. We give a dimensional prediction for the values of the anisotropic scaling exponents in this Rayleigh-Bénard systems. Measured scaling does not follow dimensional estimate, while a better agreement can be found with the anisotropic scaling of a different system, the random-Kolmogorov-flow (RKF) (BIFERALE L., DAUMONT I., LANOTTE A. and TOSCHI P., Phys. Rev. E, 66 (2002) 056306). Our findings support the conclusion that scaling properties of anisotropic fluctuations are universal, i.e. independent of the forcing mechanism sustaining the turbulent flow
Cascades and transitions in turbulent flows
Full text linkTurbulent flows are characterized by the non-linear cascades of energy and other inviscid
invariants across a huge range of scales, from where they are injected to where they are
dissipated. Recently, new experimental, numerical and theoretical works have revealed
that many turbulent configurations deviate from the ideal three and two dimensional homogeneous and isotropic cases characterized by the presence of a strictly direct and inverse
energy cascade, respectively. New phenomena appear that alter the global and local transfer properties. In this review, we provide a critical summary of historical and recent works
from a unified point of view and we present a classification of all known transfer mechanisms. Beside the classical cases of direct and inverse energy cascades, the different scenarios include: split cascades for which an invariant flows both to small and large scales simultaneously, multiple/dual cascades of different quantities, bi-directional cascades where
direct and inverse transfers of the same invariant coexist in the same scale-range and finally
equilibrium states where no cascades are present, including the case when a large scale
condensate is formed. We classify all possible transitions from one scenario to another as
the control parameters are changed and we analyse when and why different configurations
are observed. Our discussion is based on a set of paradigmatic applications: helical turbulence, rotating and/or stratified flows, magnetohydrodynamics (MHD) turbulence, and
passive/active scalars where the transfer properties are altered as one changes the embedding dimensions, the thickness of the domain or other relevant control parameters, as, e.g.,
the Reynolds, Rossby, Froude, Peclet, or Alfv ́ en numbers. We briefly discuss the presence ́
of anomalous scaling laws in 3D hydrodynamics and in other configurations, in connection with the intermittent nature of the energy dissipation in configuration space. A quick
overview is also provided concerning the importance of cascades in other applications such
as bounded flows, quantum fluids, relativistic and compressible turbulence, and active matter, together with a discussion of the implications for turbulent modelling. Finally, we
present a series of open problems and challenges that future work needs to address
Helicoidal particles in turbulent flows with multi-scale helical injection
No full textWe present numerical and theoretical results concerning the properties of turbulent flowsunder strong multi-scale helical injection. We perform direct numerical simulations ofthe Navier-Stokes equations under a random helical stirring with power-law spectrumand with different intensities of energy and helicity injections. We show that there existsthree different regimes where the forward energy and helicity inertial transfers are: (i)both leading with respect to the external injections, (ii) energy transfer is leading andhelicity transfer is sub-leading, and (iii) both are sub-leading and helicity is maximal atall scales. As a result, the cases (ii-iii) give flows with Kolmogorov-like inertial energycascade and tunable helicity transfers/contents. We further explore regime (iii) in aLagrangian domain, by studying the kinetics of point-like isotropic helicoids, particleswhose dynamics is isotropic but breaks parity invariance. We investigate small-scalefractal clustering and preferential sampling of intense helical flow structures. Dependingon its structural parameters, the isotropic helicoids either preferentially sample co-chiralor anti-chiral flow structures. We explain these findings in limiting cases in terms of whatis known for spherical particles of different densities and inertia. Furthermore, we presenttheoretical and numerical results for a stochastic model where Lagrangian properties canbe calculated using analytical perturbation theory. Our study shows that a suitabletuning of the stirring mechanism can strongly modify the small-scale turbulent helicalproperties and demonstrates that isotropic helicoids are the simplest particles able topreferentially sense helical properties in turbulenc
Closure theory for the split energy-helicity cascades in homogeneous isotropic homochiral turbulence
No full textOn the role of the helicity in the energy transfer in three-dimensional turbulence
No full textBehavior of the turbulent flows could be changed by changing the nature of the external force or the confining geometry which essentially results in breaking some of the symmetries of the ideal homogeneous and isotropic flows. In a numerical simulation, however, it is possible to selectively break symmetries of the Navier-Stokes equations with other constraints like helicity. In a recent [1] simulation of a decimated version of the incompressible three dimensional Navier-Stokes equations, where helicity was maintained sign-definite using a helical projection, a reversal of energy cascade similar to two-dimensional Navier-Stokes equations was observed. The sign- definite helicity breaks the parity symmetry of the flow. It is one of the important symmetries of the flow that contributes to the forward energy cascade in three dimensional Navier-Stokes equations. In our study we measure the degree to which the parity symmetry controls the direction of the cascade. We introduce a mechanism in which the parity is broken stochastically but in a time frozen manner with helical constraints. We keep triadic interactions in Fourier space involving modes with definite sign of helicity and decimate the triads of other modes with opposite sign of helicity with a fixed probability. We studied the cascade of energy in three dimensional turbulence by changing the relative weight between positive and negative helicity modes. We present the results from our recent simulations
Clustering of chiral particles in flows with broken parity invariance
No full textThe dynamics of small particles suspended in turbulent flows is an important problem in Nature and in Science. Previous work has mainly focused on the motion of spherical particles, while less is known about particles with asymmetric shapes. We study particles which break parity invariance (chiral particles). Particles of different chirality may respond differently to the structures of the flow. Helicoidal-like structures in the flow affect the particles differently depending on the parity of the helicoid as well as on the chirality of the particle. For flows where one of the two parities of the helicoidal-like structures is more common suspended chiral particles experience different levels on clustering depending on their chirality. Using analytical methods and direct numerical simulations we investigate the mechanisms of preferential sampling and clustering of chiral particles in flows with local or global breaking of parity invariance
Base flow decomposition for complex moving objects in linear hydrodynamics: application to helix-shaped flagellated microswimmers
No full textThe motion of microswimmers in complex flows is ruled by the interplay between swimmer propulsion and the dynamics induced by the fluid velocity field. Here we study the motion of a chiral microswimmer whose propulsion is provided by the spinning of a helical tail with respect to its body in a simple shear flow. Thanks to an efficient computational strategy that allowed us to simulate thousands of different trajectories, we show that the tail shape dramatically affects the swimmer's motion. In the shear dominated regime, the swimmers carrying an elliptical helical tail show several different Jeffery-like (tumbling) trajectories depending on their initial configuration. As the propulsion torque increases, a progressive regularization of the motion is observed until, in the propulsion dominated regime, the swimmers converge to the same final trajectory independently on the initial configuration. Overall, our results show that elliptical helix swimmer presents a much richer variety of trajectories with respect to the usually studied circular helix tails
Intermittency in turbulence: Multiplicative random process in space and time
No full textWe present a simple stochastic algorithm for generating multiplicative processes with multiscaling both in space and in time. With this algorithm we are able to reproduce a synthetic signal with the same space and time correlation as the one coming from shell models for turbulence and the one coming from a turbulent velocity field in a quasi-Lagrangian reference frame
Homogeneous and Isotropic Turbulence: A Short Survey on Recent Developments
No full textWe present a detailed review of some of the most recent developments on Eulerian and Lagrangian turbulence in homogeneous and isotropic statistics. In particular, we review phenomenological and numerical results concerning the issue of universality with respect to the large scale forcing and the viscous dissipative physics. We discuss the state-of-the-art of numerical versus experimental comparisons and we discuss the dicotomy between phenomenology based on coherent structures or on statistical approaches. A detailed discussion of finite Reynolds effects is also presented
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