Une Nouvelle Pédagogie pour des Travaux Pratiques en Mécanique des Fluides

National audienceThe Fluid Mechanics Teaching Group of Ecole Centrale de Lyon introduced new teaching methods for the lab works of 2nd year students (eq. Master 1). Instead of a classical linear experiment on a dedicated rig, the students choose a scientific ant technical theme they illustrate on a panel of rigs and numerical simulations. During the first session, the students choose a theme and define objectives and concepts to illustrate, in conjunction with the theoretical classes. They also select appropriate experimental rigs and numerical simulations. During the next two sessions, they have to carry out the experiments and simulations, over the same configurations. Data are analyzed according to the objectives, combining experimental and numerical approaches. Finally, the work is synthesized through a report and a presentation during the last session. This talk is also a way to present the results to the other groups that studied different themes.L'Equipe d'Enseignement de Mécanique des Fluides de l'Ecole Centrale de Lyon pratique une nouvelle pédagogie pour les Travaux Pratiques dédiés aux élèves de 2ème année. Plutôt que de réaliser classiquement une expérience formatée sur une installation dédiée, les étudiants définissent eux-mêmes un thème scientifique et technique qu'ils illustrent grâce à un panel d'installations expérimentales et de simulations numériques. Lors d'une première séance, les étudiants choisissent leur thème d'étude et définissent des objectifs et des concepts à illustrer, en lien avec les enseignements théoriques dispensés. Ils identifient les installations expérimentales et les simulations utiles. Lors des deux séances intermédiaires, ils réalisent leurs expériences et des simulations numériques, menées sur les mêmes configurations. Les données traitées sont analysées en cohérence avec les objectifs prédéfinis, et en confrontant les approches numériques et expérimentales. La dernière séance est consacrée à la restitution. Un rapport écrit et un exposé oral constituent un exercice de synthèse. Par ailleurs, l'exposé oral est un vecteur de transmission des connaissances acquises, à l'adresse des étudiants ayant choisi un autre thème d'étude

Towards practical large-eddy simulations of complex turbulent flows

International audienceA Shear-Improved Smagorinsky model (SISM) allowing to address non-homogeneous and unsteady flow configurations in a physically-sound manner, without adding significant 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 flow past a circular cylinder is used as a test of the method

Zonal large-eddy simulation of a tip leakage flow

The flow induced by the clearance between the tip of an isolated airfoil and an end-plate is investigated numerically, using a zonal approach with large-eddy simulation in the region of interest. The results are analyzed in comparison with available experimental data, presented in a companion paper. The incoming boundary layer and the pressure distribution around the blade are evaluated. The description of the inflow-jet deviation, with an averaged approach, enables to represent the proper loading on the airfoil. Also, particular attention is paid to the powerful tipleakage vortex. The vortex characteristics are investigated using specific functions to locate its center and quantify its width. Overall, good results are obtained for the flow statistics and spectra. Furthermore, a very good description of the far-field pressure is achieved using the acoustic analogy, and the results confirm that the tip-flow essentially radiates in the central frequency range (0.7 kHz, 7 kHz)

Wavelet Analysis of a Blade Tip-Leakage Flow

The secondary flow generated by the clearance between an isolated airfoil tip and an end-plate is analyzed by means of a zonal large-eddy simulation, in comparison with available experimental data. The flow around the tip clearance is described with full large-eddy simulation, while Reynolds-averaged Navier-Stokes is employed in the rest of the computational domain in order to limit the computational cost. The various analyses of the flow characteristics (mean velocities, Reynolds stresses, spectra) show a very good agreement between the experiment and the simulation. Furthermore, a detailed analysis is carried out from the numerical results. The flow separations on the blade tip are related with the leakage distribution along the chord, which generates an intense tip-leakage vortex on the suction side. Finally, a hump in the pressure spectra at tip is investigated by means of a wavelet conditional average, and related to the unsteadiness of the aft tip separatio

Les échanges inter-échelles en simulation des grandes échelles

La simulation numérique des grandes échelles (LES) est actuellement un outil prometteur pour la prédiction des écoulements turbulents industriels. Or de nombreux points d'interrogation, tant sur le plan pratique que théorique, n'ont pas encore de réponse. L'objet de ce papier est d'examiner les échanges inter-échelles en LES, pour une application en écoulements turbulents complexes. Ces échanges entre différentes échelles résolues sont cruciaux pour la qualité de l'ecoulement simulé. Une des grandeurs caractérisant les échanges entre différentes tailles des tourbillons est le coefficient de di-symétrie des dériveées spatiales de vitesse. Dans cette communication, nous nous intéressons à cette grandeur et à son utilisation dans les modèles de sous-maille, dans le cas d'un écoulement en canal plan, simulé à l'aide d'un code industriel dont la précision de résolution des termes convectifs est d'ordre 4. Pour comparaison, un code spectral est aussi utilisé. Enfin, cette étude académique permet d'appuyer des résultats obtenus en géométrie complexe

Large Eddy Simulation of an axial compressor rotor passage: Preliminary comparison with experimental measurements

Ce résumé étendu présente les résultats préliminaires issus de la comparaison entre la Simulation aux Grandes Echelles et la mesure expérimentale de l'écoulement dans un passage inter-aubes de rotor axial.This extended abstract presents preliminary results of the comparison between Large Eddy Simulation and experimental measurements of the flow in an axial rotor passage

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 (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 “on the fly” 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 Smagrosinky’s 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

Numerical investigation of the unsteadiness of a fan tip-leakage vortex

The movement of the tip-leakage vortex in a fan flow is investigated, from data of an existing zonal large-eddy simulation. A methodology is presented to analyze the motion of the vortex, using low-pass filtering and integral-based vortex identification functions. This approach permits to focus on the largest vortices of the flow at each instant, through normalized criteria. In the present fan flow, the velocity spectra are characterized by a peak frequency associated with a natural un-steadiness, in the casing region downstream of the rotor blades. Using the present methodology, in a plane just downstream of the blade trailing edge, a significant wandering motion of the vortex is shown. Further downstream, where the vortex interacts with the adjacent blade wake, the vortex movement is much more disordered. This is interpreted as a vortex splitting phenomenon. But when only the mean flow and the peak frequency are retained in the analysis, a wandering motion is observed again