An immersed boundary method for geometrical shock dynamics

International audienceA new algorithm is presented for the resolution of the Geometrical Shock Dynamics model in presence of obstacles in an Eulerian framework. The numerical method relies on a Fast-Marching like algorithm on a Cartesian grid, which allows dealing with complex geometrical configurations and shock waves interactions at a reduced computational cost. The application of an homogeneous Neumann condition at the border of rigid obstacles, not aligned with the mesh, is based on the Immersed Boundary Method. For a given obstacle, a set of ghost points is defined in the solid area and the corresponding unknown flow parameters are updated using a compatibility condition. A good agreement is observed between numerical results, experimental data and CFD simulations, both in 2D and 3D, which demonstrates the validity and the capabilities of this method

Étude théorique et numérique des écoulements cisaillés libres à masse volumique fortement variable

The subject of this work concerns the application of large-eddy simulation to the mixing of two fluids with different thermodynamical properties.Numerical errors in the discretisation of Navier-Stokes equations and their interaction with subgrid models are investigated on a self decaying isotropic homogeneous turbulence. A high resolution numerical code is then developed for the simulation of binary mixing layers. Reduction of early acoustic waves' amplitude is achieved by use of a temporal self-similar initial condition.The relative magnitude of subgrid terms arising from filtered equations is investigated on explicit filtering of direct numerical simulation results of temporal $N_2/O_2$ and $H_2/O_2$ mixing layers.Implicit closure (MILES) is then evocated on the basis of WENO schemes.L'objet de ce travail concerne l'application de la méthode de simulation des grandes échelles au mélange de deux fluides à propriétés thermodynamiques différentes.L'origine des erreurs numériques liées à la discrétisation des équations de Navier-Stokes ainsi que leur interaction avec un modèle sous-maille sont étudiées pour une turbulence homogène et isotrope en auto-amortissement. Un code de calcul à haut pouvoir de solution est alors développé pour la simulation de couches de mélange bi-espèces. La réduction de l'amplitude des ondes acoustiques initiales est assurée par l'utilisation d'un champ en similitude temporelle.L'importance relative des termes sous-maille issus des équations filtrées est mesurée à partir du filtrage explicite des champs de simulations numériques directes des couches de mélange temporelles $N_2/O_2$ et $H_2/O_2$.L'utilisation d'une fermeture implicite autour d'un schéma numérique dissipatif est ensuite évoquée

Étude théorique et numérique des écoulements cisaillés libres à masse volumique fortement variable

The subject of this work concerns the application of large-eddy simulation to the mixing of two fluids with different thermodynamical properties.Numerical errors in the discretisation of Navier-Stokes equations and their interaction with subgrid models are investigated on a self decaying isotropic homogeneous turbulence. A high resolution numerical code is then developed for the simulation of binary mixing layers. Reduction of early acoustic waves' amplitude is achieved by use of a temporal self-similar initial condition.The relative magnitude of subgrid terms arising from filtered equations is investigated on explicit filtering of direct numerical simulation results of temporal $N_2/O_2$ and $H_2/O_2$ mixing layers.Implicit closure (MILES) is then evocated on the basis of WENO schemes.L'objet de ce travail concerne l'application de la méthode de simulation des grandes échelles au mélange de deux fluides à propriétés thermodynamiques différentes.L'origine des erreurs numériques liées à la discrétisation des équations de Navier-Stokes ainsi que leur interaction avec un modèle sous-maille sont étudiées pour une turbulence homogène et isotrope en auto-amortissement. Un code de calcul à haut pouvoir de solution est alors développé pour la simulation de couches de mélange bi-espèces. La réduction de l'amplitude des ondes acoustiques initiales est assurée par l'utilisation d'un champ en similitude temporelle.L'importance relative des termes sous-maille issus des équations filtrées est mesurée à partir du filtrage explicite des champs de simulations numériques directes des couches de mélange temporelles $N_2/O_2$ et $H_2/O_2$.L'utilisation d'une fermeture implicite autour d'un schéma numérique dissipatif est ensuite évoquée

Etude théorique et numérique des écoulements cisaillés libres à masse volumique fortement variable

ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

Extension of Geometrical Shock Dynamics for blast waves propagation

International audienceThe direct numerical simulation of blast waves (accidental or industrial explosions) is a challenging task due to the wide range of spatial and temporal scales involved. Moreover, in a real environment (topography, urban area. . .), the blast wave interacts with the geometrical obstacles resulting in reflection, diffraction and waves recombination phenomena. The shape of the front becomes complex, which limits the efficiency of simple empirical methods. This work aims at contributing to the development of a fast running metho

Beyond the limitation of Geometrical Shock Dynamics for diffraction over wedges

Geometrical Shock Dynamics (GSD) is a simplified model for nonlinear shock wave propagation for which the front evolution is governed by a local relation between the geometry of the shock and its velocity, so-called A − M rule. Numerous studies have proven the ability of the GSD model to estimate correctly the leading shock front in interaction with obstacles. Nevertheless, a solution for the problem of diffraction over a convex corner does not always exist, especially for weak shocks. To overcome this limitation, we propose an ad-hoc modification of the A−M relation for two-dimensional configurations: an extra term based on the transverse variation of the Mach number is added. This new closure is fitted against experimental observations, which ensures, by construction, a correct behaviour for expansive shocks. A Lagrangian numerical solver is developed, for which this new term is activated only on specific parts of the front. Results of this new model are compared with the original GSD model, experiments, and Eulerian simulations for several cases of increasing complexity. A noticeable improvement of the solution is observed

Implicit large eddy simulation of vitiation effects in supersonic air/H2 combustion

International audienceThis paper presents and discusses Implicit Large Eddy Simulation (ILES) results of vitiation effects in ground tests of supersonic air/H2 combustion. This work is useful for realistic extrapolation of ground test data to flight conditions. The high-enthalpy flow configuration retained, typical of scramjet engines, is the Mach 2 LAERTE combustion chamber of the French aerospace lab ONERA. The supersonic air co-flow is preheated by burning a small amount of hydrogen in oxygen-enriched air. Stable numerical simulations of such high-speed turbulent shocked flows require dissipative numerics that interact with molecular diffusion. The premature ignition in the case of vitiation by oxygen atom can be observed using reduced finite rate chemistry in the quasi-laminar approximation, i.e. without any subgrid combustion model, because the Damköhler number (ratio of turbulent to chemical time scale) is less than unity. Turbulent time scales are only moderately affected by vitiation, but chemical time scales are significantly reduced

Comparison of Geometrical Shock Dynamics and Kinematic models for shock wave propagation

International audienceGeometrical Shock Dynamics (GSD) is a simplified model for nonlinear shock wave propagation. It is based on the decomposition of the shock front into elementary ray tubes with a simple relation linking its local curvature and velocity. This relation is obtained under the assumption of strong shock in order to neglect the effect of the post-shock flow on the front. More recently, a new simplified model, referenced as the Kinematic model, was proposed. This model is obtained by combining the three-dimensional Euler equations and the Rankine-Hugoniot relations at the front, which leads to an equation for the normal variation of the shock Mach number at the wave front. In the same way as GSD, the Kinematic model is closed by neglecting the post-shock flow effects. Although each model's approach is different, we prove here their structural equivalence: the Kinematic model can be rewritten under the form of GSD with a specific A − M relation. Both models are thus compared through a wide variety of examples including experimental data or Eulerian simulations results when available. Attention is drawn to the simple cases of compression ramps and convex corners' diffraction. The analysis is completed by the more complex cases of the diffraction over a cylinder, a sphere, a mound and a trough

Structural, electrical and magnetic characterization of in-situ crystallized ZnO:Co thin films synthesized by reactive magnetron sputtering

Équipe 101 : Nanomagnétisme et électronique de spinInternational audienceZn1-xCoxO (0 < x < 0.146) conductive thin films have been deposited by reactive magnetron sputtering of metallic Zn and Co targets at high pressure and temperature. The structural properties have been investigated by using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It has been observed that all as-deposited films are crystallized in pure hcp ZnO structure and neither traces of metallic nor oxide Co-rich clusters were detected. The average grain size estimated from full width at half maximum of XRD results varied between 65 and 83 nm. XPS analyses exhibit that Co ions are successfully entered into ZnO lattice as Co. The electrical properties including conductivity, carrier density and carrier mobility were determined by Hall effect measurements in a temperature range from 300 K to 475 K. The conductivity of the films decreases from sigma(300K) = 2.2 x 10(4) to 2.3 x 10(-1) Sm-1 as the Co content changes from 0 to 0.146. Magnetic measurements reveal the absence of ferromagnetism even at 3 K and a paramagnetic Curie-Weiss behavior associated to magnetic clusters