A Robust methodology for RANS Simulations of Highly Underexpanded Jets

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A priori analysis of subgrid-scale models for shock wave / boundary layer interaction

This study addresses the subgrid-scale modeling issue for large eddy simulation of shock wave / boundary layer interaction. By using a reference flow database, obtained by direct numerical simulation, a priori testing of the most prominent LES models is carried out. The various modelling and filtering approaches are discussed and compared, leading to suggest a priori the most appropriate closure strategy

Statistical Evaluation of the Shock Wave Boundary Layer Interaction Phenomenon

Turbulent velocity and thermal correlations from direct numerical simulation data of a spatially growing compressible turbulent boundary layer interacting with an impinging shock are discussed. The cross-stream variation of the velocity second-moments and the thermal fluxes one boundary layer thickness upstream of the shock impingement point are discussed. Other correlations are examined to further statistically quantify the effect of the oblique shock-turbulence interaction

Multigrid sequential data assimilation for the large-eddy simulation of a massively separated bluff-body flow

The potential for data-driven applications to scale-resolving simulations of turbulent flows is assessed herein. Multigrid sequential data assimilation algorithms have been used to calibrate solvers for Large Eddy Simulation for the analysis of the high-Reynolds-number flow around a rectangular cylinder of aspect ratio 5:1. This test case has been chosen because of a number of physical complexities which elude accurate representation using reduced-order numerical simulation. The results for the statistical moments of the velocity and pressure flow field show that the data-driven techniques employed, which are based on the Ensemble Kalman Filter, are able to significantly improve the predictive features of the solver for reduced grid resolution. In addition, it was observed that, despite the sparse and asymmetric distribution of observation in the data-driven process, the data augmented results exhibit perfectly symmetric statistics and a significantly improved accuracy also far from the sensor location

Dynamics of round jet impingement

International audienceThe impingement of a round jet issued from a convergent nozzle is known to produce intense tonal noise. These tones are generated by a feedback process involving a Kelvin-Helmholtz instability wave and an upstream propagating wave. The frequency structure of these tones is experimentally explored by varying the jet Mach number from 0.7 to 1.5 and we observe evidence that neutral-upstream modes of the jet are active in the feedback process. Tone frequency predictions obtained using the intrinsic jet modes are compared to the standard model involving free-stream acoustic waves: the neutral waves model provides a convincing agreement with the experimental data

Post-shock downstream evolution of turbulent boundary layer based on quadrant analysis

Quadrant analysis is one of the technique that commonly used to detect and to quantify the composition of occurrence of different turbulent events (e.g. ejections and sweeps) in a turbulent boundary layer. In the present communication the post-shock behaviour of the supersonic turbulent boundary layer will be presented on the basis of above mentioned analysis. In addition the effect of two different wall-temperature conditions will also be investigated

Near-field wavepackets and the far-field sound of a subsonic jet

This paper details the analysis of the relationship between the near-field pressure fluctuations of an unforced, subsonic free jet (0.4 ≤ M ≤ 0.6) and its low-angle, far-field sound emissions. Azimuthal rings of six microphones recorded pressure fluctuations on a conical surface in the jet near field while an azimuthal ring of three microphones recorded fluctuations in the far field at θ = 20° and R/D = 47.1. Recent measurements have shown close agreement between the velocity fluctuations up to the end of the potential core of the currently studied jet and predictions from the linear Parabolised Stability Equations (PSE), indicating the presence of linear wavepackets in the jet velocity field. Solutions of the Linearised Euler Equations (LEE) reported in the present paper also show good agreement with measurements, and provide a first step toward a time-domain description of the said wavepackets. Though the agreement for PSE in the velocity field breaks down downstream of the potential core, Proper Orthogonal Decomposition (POD) of the current results shows that the wavepackets do persist in this region and are clearly apparent in the near pressure field. Attention is then turned to establishing a relationship between these wavepackets and the radiated sound by comparing simultaneously-obtained measurements of the far-field pressure both directly to the near-field signature as well as to numerical predictions of the far-field emissions available from a recent technique using a tailored Green’s function. The direct comparisons are made by correlations between the POD modes and the far-field sound. The first POD mode captures most of the flow energy for the frequency range studied, and the correlation between this mode and the far field is nearly identical to the correlation using the full near-field signal. Higher POD modes also show significant correlation to the far field with a different space–time structure than the first mode. The Green’s function predictions are performed both statistically and in the time domain, and though they are shown to be valid for a near-field array with a long axial extent, the experimental limitation of a shorter array (0.5 ≤ x/D ≤ 8.9), which truncates the wavepacket source in the calculations, causes inaccurate predictions for the experimental data. This error is thought to be the result of a spurious source introduced by the truncation that interferes both constructively and destructively with the wavepacket source. A validation problem shows that this error would be smaller for a higher-M jet

Contribution à l'étude numérique des jets supersoniques sous-détendus

Y.D'ANGELO Professeur INSA Rouen P. BRUEL Chargé de Recherche CNRS, LMA Pau C. FABRE Responsable Projets de Recherche "Moteur et Intégration Nacelle", Airbus France, Toulouse P. MILLAN Directeur de Recherche, ONERA, Toulouse F. ROGER Professeur, Université de Poitiers D. VANDROMME Professeur, INSA de RouenThis study stems in particular from the need to control, during the integration stage of an aeronautical engine, the hazards related to the possible discharge of hot and highly pressurized gases issued from an orifice accidentally created at the combustion chamber wall. After a review of the phenomenology of such a flow, a strategy is proposed to overcome the difficulties encountered when such jets are to be dealt with numerically in order to obtain converged solutions at a moderate computing cost / accuracy ratio. The chosen method relies on i) a mixed finite volume / finite element approach to integrate the axisymmetrical formulation of the compressible Navier-Stokes equations, ii) upwind schemes (TVD) to evaluate the convective fluxes, iii) an anisotropic readaptation algorithm for unstructured meshes and iv) a modified k-ε turbulence model including compressibility effects. The analysis of the results demonstrate the efficiency of the proposed strategy which leads us to correctly forecast the overall jet structure which proved to be very sensitive to the inlet conditions prescription. Such a feature strongly suggests that the corresponding certification tests definition should be updated accordingly.Cette étude est inspirée en particulier par le besoin, lors de la phase d'intégration d'un moteur aéronautique, de maîtriser a priori les risques d'endommagements liés à la possible décharge d'un écoulement de gaz brûlés à haute pression et haute température, issu d'un orifice apparaissant accidentellement au niveau de la paroi de la chambre de combustion. Après avoir brossé un tableau exhaustif de la phénoménologie des écoulements considérés, une stratégie est proposée afin de surmonter les difficultés numériques particulières associées à la simulation de tels jets, et d'obtenir des solutions stationnaires pour un rapport coût / précision raisonnable. La procédure retenue repose sur l'utilisation i) d'une méthode mixte volumes finis / éléments finis pour intégrer la formulation axisymétrique des équations de Navier-Stokes en régime compressible, ii) de schémas décentrés (TVD) pour évaluer les flux convectifs, iii) d'une méthode de réadaptation anisotrope de maillages non-structurés et iv) d'un modèle de turbulence k-ε modifié afin de réintégrer les principaux effets de compressibilité. L'analyse des résultats obtenus démontre que la stratégie retenue nous permet de prévoir correctement la structure d'ensemble des différents jets considérés et confirme que celle-ci est très sensible aux conditions d'entrée prévalant au niveau de l'orifice, ce qui nous conduit d'ores et déjà à préconiser une évolution de la définition des tests de certification

Turbulent mixing and molecular transport in highly under-expanded hydrogen jets

International audienceHighly under-expanded hydrogen jets releasing in quiescent air atmosphere are studied using highly resolved numerical simulations accounting for complex multicomponent molecular transport phenomena. In a first step of the analysis, the main overall features of the hydrogen jet structure are described and compared to those of the classical under-expanded air jet at the same nozzle pressure ratio (NPR). Even if the global flow topology remains quite similar in both cases (i.e., hydrogen and air discharges), the modification of both mean density and mean velocity gradients leads to different relative energy levels for each velocity component. The corresponding change of fluid properties mainly leads to an enhanced mixing at the jet periphery. In comparison to the air case, the turbulence development within the internal part of the under-expanded hydrogen jet surrounding the subsonic core also yields a different structure. While a significantly higher peak of streamwise turbulent stress is observed downstream of the reflected shock, the vorticity dynamics is dampened by viscous diffusion and velocity divergence (i.e., volumetric expansion) contributions. Then, the performance of the simplified Hirschfelder and Curtiss approximation of the multicomponent molecular diffusion phenomena is evaluated with respect to the detailed multicomponent transport representation, as deduced from the EGLIB library. The detailed representation of molecular phenomena is shown to have a significant influence on the estimated local levels of hydrogen mass flux, leading to a non-negligible alteration of the global jet structure