Chasseneuil-du-Poitou, France
Chasseneuil-du-Poitou, France

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Two-dimensional numerical simulations are carried out to examine the problem of transient electroconvection stability of dielectric liquids subjected to unipolar injection. The entire set of electrohydrodynamics equations associated with the electroconvective phenomena that occur in a layer of a dielectric liquid between two parallel electrodes subjected to a potential difference are solved numerically. We first validate the numerical simulation by comparing our linear stability electroconvection criteria with those obtained by other authors with a stability approach. In this paper, we restrict the study to the strong injection case, which corresponds to values of the non-dimensional injection parameter C greater than or equal to 10. The numerical solution of the electroconvective problem is then presented for rigid lateral boundary conditions. A detailed analysis of the scenario that occurs for different characteristic values of the stability parameter T is provided. The flow structure and its behaviour highlight the existence of different regimes, from laminar to chaotic. The development of charged plumes has been observed in particular. We compute the electrical Nusselt number for different values of the stability parameter and ion mobility. The electrical Nusselt number saturates with increasing T, a fact that it is in agreement with available experimental data. Finally, a spectral analysis is conducted for different aspect ratios of the computational domain. The spectral analysis gives an insight into the physical origin of the velocity and current oscillations. © 2012 American Institute of Physics.

Tala-Ighil N.,Institute Pprime | Fillon M.,Institute Pprime | Maspeyrot P.,Institute Pprime
Tribology International | Year: 2011

A growing interest is given to the textured hydrodynamic lubricated contacts. The use of textured surfaces with different shapes of microcavities (textures) and at different locations of the texture zone can be an effective approach to improve the performance of bearings. The present study examines the texture location influence on the hydrodynamic journal bearing performance. A numerical modelling is used to analyze the cylindrical texture shape effect on the characteristics of a hydrodynamic journal bearing. The theoretical results show that the most important characteristics can be improved through an appropriate arrangement of the textured area on the contact surface. © 2010 Elsevier Ltd. All rights reserved.

Wu J.,Institute PPRIME | Traore P.,Institute PPRIME | Vazquez P.A.,University of Seville | Perez A.T.,University of Seville
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2013

This work addresses the stability of a two-dimensional plane layer of a dielectric liquid enclosed in wall bounded cavities of different aspect ratios and subjected to unipolar injection of ions. Numerical simulations have been conducted to investigate the effect of lateral walls, especially in the development of the electroconvective instability. It is found that an unexpected change of the bifurcation nature occurs for certain cavity aspect ratios. We show that above the linear stability threshold for the rest state a supercritical bifurcation arises. This bifurcation takes place at a given value Tc 1 of the parameter T (the electric Rayleigh number). Then, a second subcritical bifurcation occurs at a second threshold T c 2, featuring a typical hysteresis loop with an associated nonlinear criterion Tf, which is very characteristic of the Coulomb-driven convection. This behavior has been confirmed by different numerical codes based on different numerical methods. The physical mechanism which leads to this situation is analyzed and discussed. The evolution of the bifurcation diagrams with the aspect ratio of the cavity is also provided and analyzed. © 2013 American Physical Society.

Mourzenko V.V.,Institute PPRIME | Thovert J.-F.,Institute PPRIME | Adler P.M.,University Pierre and Marie Curie
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2011

Since only intersections with lines or planes are usually available to quantify the properties of real fracture networks, a stereological analysis of these intersections is a crucial issue. This article-the second of a series-is devoted to the derivation of the direct relations between the properties and the observable quantities. First, this derivation is achieved for anisotropic networks whose orientations obey a Fisher probability distribution function; second, it is extended to networks which are heterogeneous in space, i.e., whose density decays according to an exponential law. Five major quantities are determined: the excluded volume, the average number of intersections with a line and with a plane, the average trace length and the surface density of trace intersections. Some of these relations are valid for any convex fracture shape and some only for circular disks; however, numerical simulations show that excellent approximations are obtained by considering disks with the same area as the noncircular fractures. All the results are summarized in Table. © 2011 American Physical Society.

Mourzenko V.V.,Institute PPRIME | Thovert J.-F.,Institute PPRIME | Adler P.M.,University Pierre and Marie Curie
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2011

The asymptotic behaviors of the permeability of isotropic fracture networks at small and large densities are characterized, and a general heuristic formula is obtained which complies with the limiting behaviors and accurately predicts the permeability of these networks over the whole density range. Theses developments are based on extensive numerical calculations and on theoretical arguments inspired by the examination of the flow distribution in the fractures at large densities. Then, the results are extended to anisotropic networks with a Fisher distribution of the fracture orientations, to polydisperse networks, and to fractured porous media. Finally, guidelines are provided for the practical evaluation of the required parameters from typical field data. A summary of the results is given in Table 3. © 2011 American Physical Society.

Deng H.-B.,Beijing Institute of Technology | Xu Y.-Q.,Beijing Institute of Technology | Chen D.-D.,Beijing Institute of Technology | Dai H.,Vanderbilt University | And 2 more authors.
Computational Mechanics | Year: 2013

Aquatic and aerial animals have developed their superior and complete mechanisms of swimming and flight. These mechanisms bring excellent locomotion performances to natural creatures, including high efficiency, long endurance ability, high maneuverability and low noise, and can potentially provide inspiration for the design of the man-made vehicles. As an efficient research approach, numerical modeling becomes more and more important in studying the mechanisms of swimming and flight. This review is focused on assessing the recent progress in numerical techniques of solving animal swimming and flight problems. According to the complexity of the problems considered, numerical studies are classified into five stages, of which the main characteristics and the numerical strategies are described and discussed. In addition, the body-conformal mesh, Cartesian-mesh, overset-grid, and meshfree methods are briefly introduced. Finally, several open issues in numerical modeling in this field are highlighted. © 2013 Springer-Verlag Berlin Heidelberg.

Benard N.,Institute PPRIME | Moreau E.,Institute PPRIME
Experiments in Fluids | Year: 2013

In this study, the flow past a circular cylinder is manipulated by two plasma discharges placed on both sides of the model (at ±50). A parametric investigation by force balance is conducted to define the sensitivity of the flow field to unsteady perturbations imparted by plasma actuators (dielectric barrier discharge) at 15.6 m/s (Re D = 40,000). Effects of simple sinusoidal waveform, burst modulation and amplitude modulation are compared for low-frequency excitations. Regardless of the excitation mode, the cylinder experiences a large increase in the drag coefficient. The larger drag increase is observed for excitation related to the lock-on regime. Fast PIV measurements and triple decomposition by proper orthogonal decomposition are performed to extract the dynamical changes in the cylinder wake and to discriminate the control effects on the coherent and fluctuating turbulence. As expected, the control principally acts on the coherent flow structures. When forced, the vortices form closer to the base of the cylinder regardless of the actuation mode. This results in the drag increase observed by force measurements. The effectiveness of burst modulation is also due to the suppression of irregular shedding that is observed in the natural flow sequence and to a high level of correlation between the upper and lower vortex shedding. Finally, flow visualizations indicate that similar results can be obtained at higher Reynolds number (Re D = 128,000, 50 m/s). © 2013 Springer-Verlag Berlin Heidelberg.

Danlos A.,CNRS Fluid Dynamics Laboratory | Lalizel G.,Institute Pprime | Patte-Rouland B.,CNRS Complex Interprofessional Research in Aerothermochemistry
Experiments in Fluids | Year: 2013

This article presents an experimental investigation of a large diameter ratio annular air jet by particle image velocimetry, Laser Doppler Anemometry, hot-wire anemometry and time-resolved tomography. Annular jets consist of a round nozzle with an obstacle placed in its center. These jets are thus defined by an external and an internal diameter corresponding to the round nozzle lips and the diameter of the obstacle, respectively. The ratio between these two diameters defines the behavior of the flow across a characteristic diameter called diameter ratio. In most industrial applications these jets have large diameter ratios, superior to 0.7. © Springer-Verlag Berlin Heidelberg 2013.

Traore P.,Institute PPRIME | Wu J.,Institute PPRIME
Journal of Fluid Mechanics | Year: 2013

This study refers to the article of Chicón, Castellanos & Martion (J. Fluid Mech., vol. 344, 1997, pp. 43-66), who presented a numerical study of electroconvection in a layer of dielectric liquid induced by unipolar injection. An important characteristic of the numerical strategy proposed by Chicón et al. lies in the fact that the Navier-Stokes equations are never solved to obtain the velocity field, which is subsequently needed in the charge density transport equation. Instead, the velocity field is explicitly provided by an expression obtained with some assumptions about the flow structure and related to the electric field (the imposed velocity field approach; IVF). The validity of the above simplification is examined through a direct comparison of the solutions obtained by solving the Navier-Stokes equations (the Navier-Stokes computation approach; NSC). It is clearly demonstrated that, even in the strong injection regime (C= 10), the results look very similar for a given range of the mobility parameter M; however, in the weak injection regime (C= 0. 1), significant discrepancies are observed. The rich flow structures obtained with the NSC approach invalidate the use of the IVF approach in the weak injection regime. © 2013 Cambridge University Press.

Wu J.,Institute PPRIME | Traore P.,Institute PPRIME | Louste C.,Institute PPRIME
Journal of Electrostatics | Year: 2013

The goal of this paper is to introduce some recently developed finite volume schemes to enable numerical simulation of electric field-space charge coupled problems. The key features of this methodology are the possibility of handling problems with complex geometries and accurately capturing the charge density distribution. The total variation diminishing (TVD) scheme and the improved deferred correction (IDC) scheme are used to compute the convective and diffusive fluxes respectively. Our technique is firstly verified with the computation of hydrostatic solutions in a two coaxial cylinders configuration. The homogeneous and autonomous injection from the inner or outer electrode is considered. Comparison has been made with the analytical solution. The numerical technique is also applied to the problem of corona discharge in a blade-plane configuration. The good agreement between our numerical solution and the one obtained with a combination approach of Finite Element Method (FEM) and Method of Characteristics (MoC) is shown. © 2012 Elsevier B.V.

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