10 research outputs found
Towards practical large-eddy simulations of complex turbulent ïŹows
International audienceA Shear-Improved Smagorinsky model (SISM) allowing to address non-homogeneous and unsteady ïŹow conïŹgurations in a physically-sound manner, without adding signiïŹcant complication and computation compared to the standard Smagorinsky model, is studied and implemented. Interestingly, the SISM does not call for any adjustable parameter nor ad-hoc damping function. It makes use here of an exponential smoothing algorithm to estimate the ensemble-average of the strain from the temporal evolution of the flow. Application on a ïŹow past a circular cylinder is used as a test of the method
Zonal large-eddy simulation of a fan tip-clearance flow, with evidence of vortex wandering
The flow in a fan test-rig is studied with combined experimental and numerical methods, with a focus on the tip-leakage flow. A zonal RANS/LES approach is introduced for the simulation: the region of interest at tip is computed with full large-eddy simulation (LES), while Reynolds-averaged NavierâStokes (RANS) is used at inner radii. Detailed comparisons with the experiment show that the simulation gives a good description of the flow. In the region of interest at tip, a remarkable prediction of the velocity spectrum is achieved, over about six decades of energy. The simulation precisely captures both the tonal and broadband contents. Furthermore, a detailed analysis of the simulation allows identifying a tip-leakage vortex (TLV) wandering, whose influence onto the spectrum is also observed in the experiment. This phenomenon might be due to excitation by upstream turbulence from the casing boundary layer and/or the adjacent TLV. It may be a precursor of rotating instability. Finally, considering the outlet duct acoustic spectrum, the vortex wandering appears to be a major contribution to noise radiation
A Kalman filter adapted to the estimation of mean gradients in the large-eddy simulation of unsteady turbulent flows
A computationally-efficient method based on Kalman filtering is introduced to capture âon the flyâ the low-frequency (or very large-scale) patterns of a turbulent flow in a large-eddy simulation (LES). This method may be viewed as an adaptive exponential smoothing in time with a varying cut-off frequency that adjusts itself automatically to the local rate of turbulence of the simulated flow. It formulates as a recursive algorithm, which requires only few arithmetic operations per time step and has very low memory usage. In practice, this smoothing algorithm is used in LES to evaluate the low-frequency component of the rate of strain, and implement a shear-improved variant of the Smagrosinkyâs subgrid-scale viscosity. Such approach is primarily devoted to the simulation of turbulent flows that develop large-scale unsteadiness associated with strong shear variations. As a severe test case, the flow past a circular cylinder at Reynolds number (in the subcritical turbulent regime) is examined in details. Aerodynamic and aeroacoustic features including spectral analysis of the velocity and the far-field pressure are found in good agreement with various experimental data. The Kalman filter suitably captures the pulsating behavior of the flow and provides meaningful information about the large-scale dynamics. Finally, the robustness of the method is assessed by varying the parameters entering in the calibration of the Kalman filter
Towards Hybrid CAA with ground effects
CAA based on the Linearised Euler Equations (LEE) is applied to propagate aerodynamic sound over an extended distance including ground effects. The LEE are coupled to data from an LES via an acoustic analogy to follow-up the sound from the source to the extended far field: the complete chain is illustrated on the sound generated by a cylinder in a M ⌠0.2 and Re ⌠48000 flow. A very good agreement is found in free field between the approach based on the Ffowcs-Williams & Hawkings (FWH) analogy only and the combined FWH-LEE approach. The ability of the combined approach to handle complex boundary conditions is illustrated on the same data set with a rigid and a grassy ground
Aspects cinétiques et acoustiques en simulation numérique des grandes échelles, et application à l'étude du contrÎle de l'écoulement de jeu en turbomachines
Flows in turbomachines such as jet engines are subject to large vortical structures and strong turbulent intensities. In particular, secondary flows generated in the fan tip region result in energy losses, instabilities and noise radiation. An accurate simulation of such flows can be achieved with large-eddy simulation(LES), which reproduces the most energetic turbulent eddies. In regard of the flowphysics of highly unsteady wall-bounded flows, the SISM (shear-improved Smagorinsky model) is selected to model the sub-grid scales in the present study. This model is local and takes into account the influence of the mean shear. Two smoothing algorithms that are local in space are developed to evaluate the mean flow required by the model : an exponential averaging and an adaptative Kalman filter. These methods are first tested in a channel flow configuration. The numerical approaches are then evaluated on a relevant academic test case :the flow past a circular cylinder in the sub-critical regime (Re = 4, 7 Ă 104). This flow is dominated by large quasi-periodic vortical structures together with high intensity turbulent fluctuations; quite similarly but much simpler than those found in the tip gap flow. The aerodynamic as well as the acoustic results of the simulations are in very good agreement with the experimental data. A comparative study of the two smoothing algorithms for mean-flow extraction shows that the adaptability of the Kalman filtering leads to slightly better results. Finally, the study of a fan tip-gap flow is carried out with a zonal approach (LES in the tip region, RANS in the hub and midspan region). The reference simulation gives remarkable results in the blade-tip region, particularly for the velocity spectra. A second simulation with a control device by suction through the casing close to the blade leading edge shows two interesting features : a reduction of the turbulence level around the blade tip, and a modification of the tip-vortex trajectory (thus preventing impingement on the adjacent blade). These effects induce a notable reduction of the noise sources.Les Ă©coulements en turbomachines (et notamment en turborĂ©acteurs) sont caractĂ©risĂ©s par de larges structures tourbillonnaires et de fortes intensitĂ©s turbulentes. Ainsi, l'Ă©coulement secondaire dans la rĂ©gion du jeu, en tĂȘte d'aube, est l'origine de pertes d'Ă©nergie, d'instabilitĂ©s et de nuisances sonores. Une simulation fine de ces Ă©coulements peut ĂȘtre obtenue par l'emploi de mĂ©thodes LES (Large-Eddy Simulation), qui permettent de capturer les fluctuations turbulentes majeures. Compte tenu des phĂ©nomĂšnes rencontrĂ©s, le modĂšle de sous-maille SISM (shear-improved Smagorinsky model) est retenu ici. Ce modĂšle est local dans son Ă©criture, et prend en compte l'influence du cisaillement moyen. Nous proposons ici deux mĂ©thodes de filtrage (locales en espace elles aussi) pour obtenir une Ă©valuation du champ moyen requis par le modĂšle. Ces mĂ©thodes sont, dans un premier temps, testĂ©es sur une configuration de canal plan. L'Ă©coulement en rĂ©gime sous-critique autour d'un barreau cylindrique (Re = 4, 7 Ă104) est proposĂ© comme cas-test acadĂ©mique sĂ©lectif pour ces mĂ©thodes : cet Ă©coulement prĂ©sente de larges structures tourbillonnaires ainsi qu'une turbulence intense, tout comme l'Ă©coulement de jeu. Les simulations permettent l'obtention de rĂ©sultats trĂšs proches des donnĂ©es expĂ©rimentales. Une Ă©tude comparĂ©e des deux algorithmes d'extraction du champ moyen montre que l'adaptativitĂ© du filtrage de Kalman offre toutefois des rĂ©sultats lĂ©gĂšrement meilleurs. Enfin, l'analyse d'un Ă©coulement de jeu par une mĂ©thode zonale est rĂ©alisĂ©e (approche LES en tĂȘte d'aube, RANS en pied). La simulation de rĂ©fĂ©rence obtient des rĂ©sultats remarquables dans la zone de tĂȘte d'aube, en retrouvant notamment les spectres de vitesse expĂ©rimentaux. Une seconde simulation avec l'emploi d'un dispositif de contrĂŽle par aspiration au niveau du carter montre deux consĂ©quences principales Ă ce dispositif: une rĂ©duction des niveaux de turbulence aux environs de la tĂȘte d'aube, et une modification de la trajectoire du tourbillon de jeu. Celui-ci rencontre l'aube suivante dans la configuration de rĂ©fĂ©rence, ce qui n'est plus le cas dans la configuration avec contrĂŽle. Ces deux observations ont une importance certaine dans la rĂ©duction des sources acoustiques
Zonal Large-Eddy Simulation of a Fan Tip-Clearance Flow, With Evidence of Vortex Wandering
The flow in a fan test-rig is studied with combined experimental and numerical methods,
with a focus on the tip-leakage flow. A zonal RANS/LES approach is introduced for the simulation:
the region of interest at tip is computed with full large-eddy simulation (LES), while
Reynolds-averaged Navier-Stokes (RANS) is used at inner radii. Detailed comparisons with the
experiment show that the simulation gives a good description of the flow. In the region of interest
at tip, a remarkable prediction of the velocity spectrum is achieved, over about six decades of
energy. The simulation precisely captures both the tonal and broadband contents. Furthermore,
a detailed analysis of the simulation allows identifying a tip-leakage vortex wandering, whose influence onto the spectrum is also observed in the experiment. This phenomenon might be due
to excitation by upstream turbulence from the casing boundary layer and/or the adjacent tipleakage
vortex. It may be a precursor of rotating instability. Finally, considering the outlet duct
acoustic spectrum, the vortex wandering appears to be a major contribution to noise radiation
Smoothing algorithms for mean-flow extraction in large-eddy simulation of complex turbulent flows
The following article has been submitted to Physics of Fluids. After it is published, it will be found at http://pof.aip.orgInternational audienceBased on physical arguments, the importance of separating the mean-flow from turbulence in the modeling of the subgrid-scale eddy-viscosity is emphasized. Therefore, two distinct time-domain smoothing algorithms are proposed to estimate the mean-flow as the simulation progresses, namely, an exponentially weighted moving average (or exponential smoothing) and an adaptive low-pass Kalman filter. These algorithms highlight the longer-term evolution or cycles of the flow but erase short-term fluctuations. Indeed, it is our assumption that the mean-flow (in the statistical sense) may be approximated as the low-frequency component of the velocity field and that the turbulent part of the flow adds itself to this "unsteady mean." The cutoff frequency separating these two components is fixed according to some characteristic time-scale of the flow in the exponential smoothing, but inferred dynamically from the recent history of the flow in the Kalman filter. In practice, these two algorithms are implemented in large-eddy simulations that rely on a shear-improved Smagorinsky's model. In this model, the magnitude of mean-flow rate of strain is subtracted from the magnitude of the instantaneous rate of strain in the subgrid-scale eddy-viscosity. Two test-cases have been investigated: a turbulent plane-channel flow (Rew = 395) and the flow past a circular cylinder in the subcritical turbulent regime (ReD = 4.7Ă104). Comparisons with direct numerical simulation and experimental data demonstrate the good efficiency of the whole modeling. This allows us to address nonhomogeneous unsteady configurations without adding significant complication and computational cost to the standard Smagorinsky's model. From a computational viewpoint, this modeling deserves interest since it is entirely local in space. It is therefore adapted for parallelization and convenient for boundary conditions
Towards practical large-eddy simulations of complex turbulent ïŹows
International audienceA Shear-Improved Smagorinsky model (SISM) allowing to address non-homogeneous and unsteady ïŹow conïŹgurations in a physically-sound manner, without adding signiïŹcant complication and computation compared to the standard Smagorinsky model, is studied and implemented. Interestingly, the SISM does not call for any adjustable parameter nor ad-hoc damping function. It makes use here of an exponential smoothing algorithm to estimate the ensemble-average of the strain from the temporal evolution of the flow. Application on a ïŹow past a circular cylinder is used as a test of the method
Biens produits par l'Ă©cosystĂšme
Dans EFESE-Ă©cosystĂšmes agricoles, la production agricole de biomasse vĂ©gĂ©tale nâest pas conceptualisĂ©e comme un SE dâapprovisionnement mais comme un processus de production de biens agricoles (cf. section 3.1). Le niveau de production agricole rĂ©sulte des interactions entre les SE de rĂ©gulation dont bĂ©nĂ©ficie lâagriculteur, appelĂ©s « SE intrants » ci-aprĂšs, et des apports dâintrants exogĂšnes (ex. Ă©nergie du travail du sol, fertilisants et amendements, eau dâirrigation, produits phytosanitaires). DiffĂ©rents modes de production de biens vĂ©gĂ©taux peuvent ĂȘtre distinguĂ©s suivant la part relative de la production agricole permise par les SE de rĂ©gulation pour un niveau de production donnĂ©e. Par exemple, il est possible de distinguer les systĂšmes de production « conventionnels » dont le niveau de production agricole est fortement basĂ© sur les intrants exogĂšnes, des systĂšmes de production, souvent qualifiĂ©s dâ« agroĂ©cologiques », dont le niveau de production agricole est fortement dĂ©terminĂ© par les SE dit « intrants » (Bommarco et al., 2013; Duru et al., 2015a; Gliessman, 2007; Therond et al., 2017)
: Rapport scientifique de l'étude réalisée par l'INRA - Novembre 2017
Lâambition de lâĂ©tude Inra "EFESE-EA" est de dĂ©crire les mĂ©canismes et dĂ©terminants de la fourniture des services Ă©cosystĂ©miques par les Ă©cosystĂšmes agricoles sur la base d'une revue des connaissances existantes, et de procĂ©der Ă leur Ă©valuation Ă lâĂ©chelle nationale sur la base dâindicateurs dĂ©finis dans le cadre de lâĂ©tude. Lâorganisation du travail, telle que prĂ©vue en dĂ©but dâĂ©tude, se voulait sĂ©quentielle : (1) identification et spĂ©cification biophysiques dâune liste de biens agricoles et services Ă©cosystĂ©miques ; (2) Ă©valuation biophysique : quantification du niveau de fourniture des biens et services identifiĂ©s Ă lâĂ©tape (1) (3) Ă©valuation Ă©conomique : quantification de la valeur Ă©conomique des services (le plus souvent dans une unitĂ© monĂ©taire) Dans le temps imparti Ă lâĂ©tude, le collectif dâexperts a donnĂ© la prioritĂ© aux volets biophysiques (1) et (2) afin : - dâinstruire de façon robuste la conceptualisation des biens et services (volet 1) : ce travail constitue un front de recherche actuel, associĂ© Ă une littĂ©rature acadĂ©mique abondante mais parfois non stabilisĂ©e, que le collectif dâexperts sâest attachĂ© Ă analyser de façon Ă proposer des choix de conceptualisation argumentĂ©s ; - de pousser au maximum lâexercice dâĂ©valuation biophysique (volet 2) dans le cadre de la demande initiale formulĂ©e par le MEEM : cartographier la production dâun large panel de biens agricoles et les SE rendus par les Ă©cosystĂšmes agricoles Ă la rĂ©solution spatiale la plus fine possible, et Ă lâĂ©chelle France entiĂšre. A noter que le prĂ©sent exercice ne constituant pas un projet de recherche mais bien une Ă©tude institutionnelle Inra (au sens des procĂ©dures DEPE), lâensemble des Ă©valuations dĂ©veloppĂ©es dans le prĂ©sent rapport est rĂ©alisĂ©e Ă partir de donnĂ©es existantes, aucun travail dâexpĂ©rimentation visant Ă acquĂ©rir de nouvelles donnĂ©es de terrain nâayant Ă©tĂ© conduit. Il rĂ©sulte de ce choix de priorisation que : - le volet dâĂ©valuation Ă©conomique (3) est initiĂ© pour quelques SE mais peu dĂ©veloppĂ© en comparaison des volets (1) et (2) ; - tout en veillant Ă Ă©laborer des mĂ©thodologies dâĂ©valuation biophysiques traçables et robustes, les experts ont pris le parti de proposer des mĂ©thodologies plus exploratoires pour quelques SE pour lesquels les donnĂ©es actuelles ne permettent pas dâĂ©valuer directement le niveau de fourniture Ă lâĂ©chelle France entiĂšre : dans ces cas particuliers (signalĂ©s explicitement dans les sections du rapport dont ils font lâobjet), les mĂ©thodologies ont Ă©tĂ© mises en Ćuvre jusquâĂ la rĂ©alisation des cartographies dans le but de donner Ă voir le potentiel quâoffrent ces mĂ©thodologies et la nature des rĂ©sultats quâelles peuvent produire sous condition de leur validation France entiĂšre, plutĂŽt que dans le but dâinterprĂ©ter pour eux-mĂȘmes les rĂ©sultats obtenus. Les experts se sont alors particuliĂšrement attachĂ©s Ă relativiser les rĂ©sultats quantitatifs ainsi produits, et Ă accompagner les cartographies dâun descriptif dĂ©taillĂ© des protocoles de validation quâil faudrait mettre en Ćuvre dans les suites de lâĂ©tude pour stabiliser et valider ces mĂ©thodologies exploratoires. Ce parti pris du groupe de travail EFESE-Ă©cosystĂšmes agricoles est compatible avec lâobjectif poursuivi dans le programme EFESE, qui se donne pour objectif de produire un guide mĂ©thodologique pour lâĂ©valuation des biens et SE en en pointant les limites, difficultĂ©s, prĂ©cautions et amĂ©liorations possibles associĂ©es Ă chacune des pistes avancĂ©es