Coherent structures in fully-developed pipe turbulence

A turbulent mean profile for pipe flow is prescribed which closely matches experimental observations. The nature of perturbations superimposed upon this profile is then considered. Optimal growth calculations predict two distinct classes of structures, clearly associated with near-wall and large-scale structures. Quantitative correspondence of the spanwise wavelength of wall-structures with experimental observations is very good. The response to harmonic forcing is also considered, and the linear growth tested with direct numerical simulation of forced turbulence. Despite the very simple eddy viscosity assumption, this linear approach predicts well the surprisingly large growth of outer-scale modes in the bulk flow. Un profil moyen turbulent est prescrit dans une conduite cylindrique, en adequation avec les observations experimentales. Nous considerons ensuite la nature des perturbations a cet ecoulement synthetique. Le calcul des croissances optimales predit deux types de structures, associees respectivement aux structures de proche-paroi et de grande echelle. Un excellent accord quantitatif est trouve avec les resultats experimentaux quant a la longueur d'onde transversale. La reponse harmonique est egalement etudiee, et la croissance lineaire observee comparee a des simulations numeriques directes de turbulence forcee. Malgre de l'hypothese simple de type `Eddy viscosity', cette approche lineaire predit efficacement la croissance spectaculaire des modes de grande echelle au coeur de l'ecoulement.Comment: 5 pages; Congres Francais de Mecanique, Marseille (2009

PNJL model for adjoint fermions

Recent work on QCD-like theories has shown that the addition of adjoint fermions obeying periodic boundary conditions to gauge theories on R^3 X S^1 can lead to a restoration of center symmetry and confinement for sufficiently small circumference L of S^1. At small L, perturbation theory may be used reliably to compute the effective potential for the Polyakov loop P in the compact direction. Periodic adjoint fermions act in opposition to the gauge fields, which by themselves would lead to a deconfined phase at small L. In order for the fermionic effects to dominate gauge field effects in the effective potential, the fermion mass must be sufficiently small. This indicates that chiral symmetry breaking effects are potentially important. We develop a Polyakov-Nambu-Jona Lasinio (PNJL) model which combines the known perturbative behavior of adjoint QCD models at small L with chiral symmetry breaking effects to produce an effective potential for the Polyakov loop P and the chiral order parameter psi-bar psi. A rich phase structure emerges from the effective potential. Our results are consistent with the recent lattice simulations of Cossu and D'Elia, which found no evidence for a direct connection between the small-L and large-L confining regions. Nevertheless, the two confined regions are connected indirectly if an extended field theory model with an irrelevant four-fermion interaction is considered. Thus the small-L and large-L regions are part of a single confined phase.Comment: 6 pages, 4 figures; presented at INPC 201

A test of first order scaling in Nf=2 QCD

We complete our analysis of Nf=2 QCD based on the lattice staggered fermion formulation. Using a series of Monte Carlo simulations at fixed (amq*Ls^yh) one is able to test the universality class with given critical exponent yh. This strategy has been used to test the O(4) universality class and it has been presented at the previous Lattice conferences. No agreement was found with simulations in the mass range amq=[0.01335,0.15] using lattices with Ls=16 up to 32 and Lt=4. With the same strategy, we now investigate the possibility of a first order transition using a new set of Monte Carlo data corresponding to yh=3 in the same mass and volume range as the one used for O(4). A substantial agreement is observed both in the specific heat scaling and in the scaling of the chiral condensate, while the chiral susceptibilities still presents visible deviation from scaling in the mass range explored.Comment: 5 pages, 6 figures, Presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg, German

Two flavor QCD and confinement - II

This paper is part of a program of investigation of the chiral transition in Nf=2 QCD, started in Phys.Rev.D72:114510,2005. Progress is reported on the understanding of some possible systematic errors. A direct test of first order scaling is presented.Comment: 7 pages, 6 figure

A test of first order scaling in Nf =2 QCD: a progress report

We present the status of our analysis on the order of the finite temperature transition in QCD with two flavors of degenerate fermions. Our new simulations on large lattices support the hypothesis of the first order nature of the transition, showing a preliminary two state signal. We will discuss the implications and the next steps in our analysis.Comment: 6 pages, 4 figures. Talk presented at The XXVI International Symposium on Lattice Field Theory, July 14 - 19, 2008 - Williamsburg, Virginia, US

On the phase diagram of the Higgs SU(2) model

The Higgs SU(2) model with fixed Higgs length is usually believed to have two different phases at high gauge coupling (\beta), separated by a line of first order transitions but not distinuguished by any typical symmetry associated with a local order parameter, as first proved by Fradkin and Shenker. We show that in regions of the parameter space where it is usually supposed to be a first order phase transition only a smooth crossover is in fact present.Comment: 6 pages, 6 figures. Talk presented at The XXVI International Symposium on Lattice Field Theory, July 14 - 19, 2008 - Williamsburg, Virginia, US

Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow

The magnetorotational (MRI) dynamo has long been considered one of the possible drivers of turbulent angular momentum transport in astrophysical accretion disks. However, various numerical results suggest that this dynamo may be difficult to excite in the astrophysically relevant regime of magnetic Prandtl number (Pm) significantly smaller than unity, for reasons currently not well understood. The aim of this article is to present the first results of an ongoing numerical investigation of the role of both linear and nonlinear dissipative effects in this problem. Combining a parametric exploration and an energy analysis of incompressible nonlinear MRI dynamo cycles representative of the transitional dynamics in large aspect ratio shearing boxes, we find that turbulent magnetic diffusion makes the excitation and sustainment of this dynamo at moderate magnetic Reynolds number (Rm) increasingly difficult for decreasing Pm. This results in an increase in the critical Rm of the dynamo for increasing kinematic Reynolds number (Re), in agreement with earlier numerical results. Given its very generic nature, we argue that turbulent magnetic diffusion could be an important determinant of MRI dynamo excitation in disks, and may also limit the efficiency of angular momentum transport by MRI turbulence in low Pm regimes.Comment: 7 pages, 6 figure

Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?

In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo process involving the magnetorotational instability (MRI). High-resolution simulations exhibit a tendency towards statistical self-organization of MRI dynamo turbulence into large-scale cyclic dynamics. Understanding the physical origin of these structures, and whether they can be sustained and transport angular momentum efficiently in astrophysical conditions, represents a significant theoretical challenge. The discovery of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has notably served to highlight the role of turbulent magnetic diffusion in the seeming decay of the dynamics at low magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a common regime in accretion disks. The connection between these simple structures and the statistical organization reported in turbulent simulations remained elusive, though. Here, we report the numerical discovery in moderate aspect ratio Keplerian shearing boxes of new periodic, incompressible, three-dimensional nonlinear MRI dynamo solutions with a larger dynamical complexity reminiscent of such simulations. These "chimera" cycles are characterized by multiple MRI-unstable dynamical stages, but their basic physical principles of self-sustainment are nevertheless identical to those of simpler cycles found in azimuthally elongated boxes. In particular, we find that they are not sustained at low Pm either due to subcritical turbulent magnetic diffusion. These solutions offer a new perspective into the transition from laminar to turbulent instability-driven dynamos, and may prove useful to devise improved statistical models of turbulent accretion disk dynamos.Comment: 12 pages, 8 figures, submitted to A&