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 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

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

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

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

Conception d'un amortisseur de vibrations magnétique à raideurs ajustables

National audienceThis study deals with the design of a magnetic vibration absorber. The vibrating magnetic mass is placed in a magnetic field created by fixed permanent magnets. The resulting linear and nonlinear stiffnesses can be controlled by adjusting the relative positions of the static magnets. The equations of the model of the damper are obtained using a multipole expansion. The proposed magnetic absorber can be used as a tuned mass damper, a nonlinear energy sink or a bi-stable vibration absorber. These 3 cases are experimentally observed using static force measurements.Cette étude porte sur la réalisation d'un amortisseur de vibrations constitué d'une masse oscillante magnétique placée dans un champ créé par des aimants statiques. En ajustant les positions de ces derniers, il est possible de contrôler les valeurs des raideurs linéaires et non linéaires afin de couvrir les cas d'un amortisseur à masse accordée, d'un puits d'énergie non linéaire et d'un amortisseur bi-stable. Une décomposition multipolaire permet d'établir une équation modèle pour le comportement de l'oscillateur magnétique. Une réalisation expérimentale permet d'observer les 3 configurations recherchées grâce à des mesures d'efforts statique

Fluctuation-Dissipation Relations and statistical temperatures in a turbulent von K\'arm\'an flow

We experimentally characterize the fluctuations of the non-homogeneous non-isotropic turbulence in an axisymmetric von K\'arm\'an flow. We show that these fluctuations satisfy relations analogous to classical Fluctuation-Dissipation Relations (FDRs) in statistical mechanics. We use these relations to measure statistical temperatures of turbulence. The values of these temperatures are found to be dependent on the considered observable as already evidenced in other far from equilibrium systems.Comment: four pages 2 figures one tabl

Conception d'un amortisseur de vibrations magnétiques à raideurs ajustables

Cette étude porte sur la réalisation d'un amortisseur de vibrations constitué d'une masse oscillante magnétique placée dans un champ créé par des aimants statiques. En ajustant les positions de ces derniers, il est possible de contrôler les valeurs des raideurs linéaires et non linéaires afin de couvrir les cas d'un amortisseur à masse accordée, d'un puits d'énergie non linéaire et d'un amortisseur bi-stable. Une décomposition multipolaire permet d'établir une équation modèle pour le comportement de l'oscillateur magnétique. Une réalisation expérimentale permet d'observer les 3 configurations recherchées grâce à des mesures d'efforts statique