Local Code: Real Estates

Popular images of entropy—a breaking glass, the lowering heap of compost, even our own descent into dust, provide a familiar, but subtly inaccurate thermodynamic picture. Viewed through an informational as well as physical lens, entropy is not a consistent movement towards flatness and uniformity, but something else as well

Statistical mechanics of Beltrami flows in axisymmetric geometry: Theory reexamined

A simplified thermodynamic approach of the incompressible axisymmetric Euler equations is considered based on the conservation of helicity, angular momentum and microscopic energy. Statistical equilibrium states are obtained by maximizing the Boltzmann entropy under these sole constraints. We assume that these constraints are selected by the properties of forcing and dissipation. The fluctuations are found to be Gaussian while the mean flow is in a Beltrami state. Furthermore, we show that the maximization of entropy at fixed helicity, angular momentum and microscopic energy is equivalent to the minimization of macroscopic energy at fixed helicity and angular momentum. This provides a justification of this selective decay principle from statistical mechanics. These theoretical predictions are in good agreement with experiments of a von Karman turbulent flow and provide a way to measure the temperature of turbulence and check Fluctuation-Dissipation Relations (FDR). Relaxation equations are derived that could provide an effective description of the dynamics towards the Beltrami state and the progressive emergence of a Gaussian distribution. They can also provide a numerical algorithm to determine maximum entropy states or minimum energy states.Comment: 25 pages, 2 figure

Do finite size neutrally buoyant particles cluster?

We investigate the preferential concentration of particles which are neutrally buoyant but with a diameter significantly larger than the dissipation scale of the carrier flow. Such particles are known not to behave as flow tracers (Qureshi et al., Phys. Re. Lett. 2007) but whether they do cluster or not remains an open question. For this purpose, we take advantage of a new turbulence generating apparatus, the Lagrangian Exploration Module which produces homogeneous and isotropic turbulence in a closed water flow. The flow is seeded with neutrally buoyant particles with diameter 700\mum, corresponding to 4.4 to 17 times the turbulent dissipation scale when the rotation frequency of the impellers driving the flow goes from 2 Hz to 12 Hz, and spanning a range of Stokes numbers from 1.6 to 24.2. The spatial structuration of these inclusions is then investigated by a Voronoi tesselation analysis, as recently proposed by Monchaux et al. (Phys. Fluids 2010), from images of particle concentration field taken in a laser sheet at the center of the flow. No matter the rotating frequency and subsequently the Reynolds and Stokes numbers, the particles are found not to cluster. The Stokes number by itself is therefore shown to be an insufficient indicator of the clustering trend in particles laden flows

Turbulent Von Karman Swirling Flows

International audienceWe investigate the turbulent Von Karman flow generated by two counter-rotating flat or bladed disks. Numerical predictions based on a Reynolds Stress Model (RSM) are compared to velocity measurements performed at CEA (Ravelet, 2005). This flow is of practical importance in many industrial devices such as in gas-turbine aeroengines. From an academic point of view, this configuration is often used for studying fundamental aspects of developed turbulence and especially of magneto-hydrodynamic turbulence

Turbulent Von Kármán flow between two counter-rotating disks

National audienceThe present work considers the turbulent Von Kármán flow generated by two coaxial counter-rotating smooth (viscous stirring) or bladed (inertial stirring) disks enclosed by a cylindrical vessel. Numerical predictions based on one-point statistical modeling using a low Reynolds number second-order full stress transport closure (RSM) are compared to velocity measurements performed at CEA. An efficient way to model the rule of straight blades is proposed. The influences of the rotational Reynolds number, the aspect ratio of the cavity, the rotating disk speed ratio and of the presence or not of impellers are investigated to get a precise knowledge of the dynamics and the turbulence properties in the Von Kármán configuration. In particular, we highlighted the transition be-tween the Batchelor and the Stewartson flow structures and the one between the merged and separated boundary layer regimes in the smooth disk case. We determined also the transition between the one cell and the two cell regimes for both viscous and inertial stirrings

Turbulence modeling of the Von Karman flow: viscous and inertial stirrings

International audienceThe present work considers the turbulent Von Karman flow generated by two counter-rotating smooth flat (viscous stirring) or bladed (inertial stirring) disks. Numerical predictions based on one-point statistical modeling using a low Reynolds number second-order full stress transport closure (RSM model) are compared to velocity measurements performed at CEA (Commissariat à l'Energie Atomique, France). The main and significant novelty of this paper is the use of a drag force in the momentum equations to reproduce the effects of inertial stirring instead of modelling the blades themselves. The influences of the rotational Reynolds number, the aspect ratio of the cavity, the rotating disk speed ratio and of the presence or not of impellers are investigated to get a precise knowledge of both the dynamics and the turbulence properties in the Von Karman configuration. In particular, we highlighted the transition between the merged and separated boundary layer regimes and the one between the Batchelor (1951) and the Stewartson (1953) flow structures in the smooth disk case. We determined also the transition between the one cell and the two cell regimes for both viscous and inertial stirrings

Turbulent Von Karman Swirling Flows

International audienceWe investigate the turbulent Von Karman flow generated by two counter-rotating flat or bladed disks. Numerical predictions based on a Reynolds Stress Model (RSM) are compared to velocity measurements performed at CEA (Ravelet, 2005). This flow is of practical importance in many industrial devices such as in gas-turbine aeroengines. From an academic point of view, this configuration is often used for studying fundamental aspects of developed turbulence and especially of magneto-hydrodynamic turbulence

Critical exponents in zero dimensions

In the vicinity of the onset of an instability, we investigate the effect of colored multiplicative noise on the scaling of the moments of the unstable mode amplitude. We introduce a family of zero dimensional models for which we can calculate the exact value of the critical exponents $\beta_m$ for all the moments. The results are obtained through asymptotic expansions that use the distance to onset as a small parameter. The examined family displays a variety of behaviors of the critical exponents that includes anomalous exponents: exponents that differ from the deterministic (mean-field) prediction, and multiscaling: non-linear dependence of the exponents on the order of the moment