Particle-resolved study of the onset of turbulence (original) (raw)

Particle trapping in three-dimensional fully developed turbulence

Physics of Fluids, 2005

The statistical properties of fluid particles transported by a three-dimensional fully developed turbulent flow are investigated by means of high resolution direct numerical simulations. Single trajectory statistics are investigated in a time range spanning more than three decades, from less than a tenth of the Kolmogorov time scale, , up to one large-eddy turnover time. Our analysis reveals the existence of relatively rare trapping events in vortex filaments which give rise to enhanced intermittency on Lagrangian observables up to 10 . Lagrangian velocity structure function attain scaling properties in agreement with the multifractal prediction only for time lags larger than those affected by trapping.

Understanding turbulence through numerical simulations

2004

Turbulence is one of the important problems in classical physics that still remain unsolved. The Navier-Stokes equations have been studied for almost two centuries now, and although they seem to properly describe the dynamics of uids, we still do not have a clear understanding of even the simplest turbulent ows.

Interaction between fast particles and turbulence

2008

A systematic study of high energetic alpha particle interaction with microinstability driven turbulence (ITG) is presented. The alpha particles are considered to be passive, thus not modifying the fine structure of the turbulence, and modelled as Maxwellian distributed. Both the turbulent fields and the evolution of the alpha distribution are computed by means of an Eulerian, flux tube code. It is shown how significant transport of high pressure distributions can occur, and how the direction and the intensity of the particle flux is sensitive to the choice of the temperature scale length of the alpha distribution, due to thermodiffusive phenomena. The diffusivity of an ITER-like case is studied, starting from an analytical treatment of the density and temperature profiles, which we show to be significant. New interpretative tools, by means of a single particle following code, are also presented.

Special issue of the “Turbulence and Interaction-TI2006” conference

Computers & Fluids, 2008

During the conference, many contributions showed substantial video animations giving some deep insight into the very different interaction phenomena. The conference also provided strong evidence that the three pillars of science, namely theory, experiments and computing, through their interplay come to achieving progress in understanding and predicting the physics of complicated flows and engineering problems.

Onset of Turbulence from the Receptivity Stage of Fluid Flows

Physical Review Letters, 2011

The traditional viewpoint of fluid flow considers the transition to turbulence to occur by the secondary and nonlinear instability of wave packets, which have been created experimentally by localized harmonic excitation. The boundary layer has been shown theoretically to support spatiotemporal growing wave fronts by Sengupta, Rao, and Venkatasubbaiah [Phys. Rev. Lett. 96, 224504 (2006)] by a linear mechanism, which is shown here to grow continuously, causing the transition to turbulence. Here, we track spatiotemporal wave fronts to a nonlinear turbulent state by solving the full 2D Navier-Stokes equation, without any limiting assumptions. Thus, this is the only demonstration of deterministic disturbances evolving from a receptivity stage to the full turbulent flow. This is despite the prevalent competing conjectures of the event being three-dimensional and/or stochastic in nature.

A kinetic model of plasma turbulence

Journal of Plasma Physics, 2014

A Hybrid Vlasov-Maxwell (HVM) model is presented and recent results about the link between kinetic effects and turbulence are reviewed. Using five-dimensional (2D in space and 3D in the velocity space) simulations of plasma turbulence, it is found that kinetic effects (or non-fluid effects) manifest through the deformation of the proton velocity distribution function (DF), with patterns of non-Maxwellian features being concentrated near regions of strong magnetic gradients. The direction of the proper temperature anisotropy, calculated in the main reference frame of the distribution itself, has a finite probability of being along or across the ambient magnetic field, in general agreement with the classical definition of anisotropy T ⊥ /T (where subscripts refer to the magnetic field direction). Adopting the latter conventional definition, by varying the global plasma beta (β) and fluctuation level, simulations explore distinct regions of the space given by T ⊥ /T and β || , recovering solar wind observations. Moreover, as in the solar wind, HVM simulations suggest that proton anisotropy is not only associated with magnetic intermittent events, but also with gradient-type structures in the flow and in the density. The role of alpha particles is reviewed using multi-ion kinetic simulations, revealing a similarity between proton and helium non-Maxwellian effects. The techniques presented here are applied to 1D spacecraft-like analysis, establishing a link between non-fluid phenomena and solar wind magnetic discontinuities. Finally, the dimensionality of turbulence is investigated, for the first time, via 6D HVM simulations (3D in both spaces). These preliminary results provide support for several previously reported studies based on 2.5D simulations, confirming several basic conclusions. This connection between kinetic features and turbulence open a new path on the study of processes such as heating, particle acceleration, and temperature-anisotropy, commonly observed in space plasmas.

Whistler turbulence forward cascade: Three-dimensional particle-in-cell simulations

Geophysical Research Letters, 2011

This manuscript describes the first ensemble of three-dimensional (3D) particle-in-cell (PIC) plasma simulations of whistler turbulence. The computational model represents a collisionless, homogeneous, magnetized plasma on which an initial spectrum of relatively long wavelength whistler fluctuations is imposed. The simulations represent a range of initial fluctuation amplitudes and follow the temporal evolution of the system as it decays into a broadband, anisotropic, turbulent spectrum at shorter wavelengths via a forward cascade. The resulting 3D turbulence is similar in many ways to whistler turbulence from previous two-dimensional (2D) PIC simulations, although the anisotropies in 3D are stronger than in comparable 2D runs. The most important difference is that reduced magnetic fluctuation spectra from the 3D simulations show a clear break in the perpendicular wavenumber (k ⊥ ) spectra. Spectra at small k ⊥ are relatively steep, but spectra at larger k ⊥ are even steeper, similar in character to magnetic spectra at electron scales recently measured in the solar wind.

Barriers for Transport in Turbulent Plasmas

Physical Review Letters, 2003

The control of transport due to electrostatic turbulence is investigated using test-particle simulations. We show that a barrier for the transport, that is, a region where transport is reduced, can be generated through the randomization of phases of the turbulent field. This corresponds to the annihilation of coherent structures which are present at all scales, without actually suppressing turbulence. When the barrier is active, a flux of particles towards the center of the simulation box is present inside the region where the barrier is located.

Transition to Turbulence in Particle Laden Flows

Physical Review Letters

Suspended particles can alter the properties of fluids and in particular also affect the transition from laminar to turbulent flow. An earlier study [Matas et al., Phys. Rev. Lett. 90, 014501 (2003)] reported how the subcritical (i.e., hysteretic) transition to turbulent puffs is affected by the addition of particles. Here we show that in addition to this known transition, with increasing concentration a supercritical (i.e., continuous) transition to a globally fluctuating state is found. At the same time the Newtonian-type transition to puffs is delayed to larger Reynolds numbers. At even higher concentration only the globally fluctuating state is found. The dynamics of particle laden flows are hence determined by two competing instabilities that give rise to three flow regimes: Newtonian-type turbulence at low, a particle induced globally fluctuating state at high, and a coexistence state at intermediate concentrations.