Jérôme Boudet

Numerical studies towards practical large-eddy simulation

Large-eddy simulation developments and validations are presented for an improved simulation of turbulent internal flows. Numerical methods are proposed according to two competing criteria: numerical qualities (precision and spectral characteristics), and adaptability to complex configurations. First, methods are tested on academic test-cases, in order to abridge with fundamental studies. Consistent results are obtained using adaptable finite volume method, with higher order advection fluxes, implicit grid filtering and “low-cost” shear-improved Smagorinsky model. This analysis particularly focuses on mean flow, fluctuations, two-point correlations and spectra. Moreover, it is shown that exponential averaging is a promising tool for LES implementation in complex geometry with deterministic unsteadiness. Finally, adaptability of the method is demonstrated by application to a configuration representative of blade-tip clearance flow in a turbomachine.

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.

Large-eddy simulation of 3-D corner separation in a linear compressor cascade

The increase of the thrust/weight ratio of aircraft engines is extremely restricted by different 3-D flow loss mechanisms. One of them is the corner separation that can form at the junction between a blade suction side and a hub or shroud. In this paper, in order to further investigate the turbulent characteristics of corner separation, large-eddy simulation (LES) is conducted on a compressor cascade configuration using NACA65 blade profiles (chord based Reynolds number: 3.82 × 105), in comparison with the previous obtained experimental data. Using the shear-improved Smagorinsky model as subgrid-scale model, the LES gives a good description of the mean aerodynamics of the corner separation, especially for the blade surface static pressure coefficient and the total pressure losses. The turbulent dynamics is then analyzed in detail, in consideration of the turbulent structures, the one-point velocity spectra, and the turbulence anisotropy. Within the recirculation region, the energy appears to concentrate around the largest turbulent eddies, with fairly isotropic characteristics. Concerning the dynamics, an aperiodic shedding of hairpin vortices seems to induce an unsteadiness of the separation envelope.

Unsteady behavior of corner separation in a compressor cascade: Large eddy simulation and experimental study

The present study carried out a pure large eddy simulation (LES) on a NACA65 linear compressor cascade (chord-based Reynolds number: 382 000), at an incidence angle of 4°. In this configuration, a corner separation at the juncture of the blade suction side and the end-wall is clearly observed experimentally. For the simulation, 852 CPUs are used in parallel and particular attention is paid to the inflow conditions, coupling LES with the calculation of the inlet boundary layer and associated velocity fluctuations. The shear-improved Smagorinsky model, which has been proved to be competent for turbomachines, is used in this study. Numerical results are carefully compared with pressure measurements, two-dimensional and stereo particle image velocimetry. LES is found to be superior to Reynolds-averaged Navier–Stokes approach in predicting the three-dimensional separated flows in the compressor cascade since pressure coefficients and losses are in very good agreement with the experiment. LES has been proven to simulate also the unsteady behavior of the separation. Finally, the experimental and numerical analysis of the inflow angle reveals high fluctuations of incidence upstream the leading edge of the blade near the end-wall, mostly associated with the incoming wall boundary layer.

Large-eddy simulation of a single airfoil tip-clear ance flow

The tip-clearance flow induced by a static blade loc ated above a plane is investigated by large-eddy simulation (LES), in comparison with a recent experiment. A shear-improved subgrid-scale model is employed in the computation, paying particular attention to the influence of mean flow structures. Indeed, the comp utation is shown to capture the main flow topology, governed by three major vortices: the tip-leakage vortex, the induced counter-rotating vortex, and the tip-separation vor tex. Mean and fluctuating quantities are correctly simulated within the gap and the tip-leak age vortex. However, it is shown that a particular care should be put on the incoming boundary layer calibration in order to represent more precisely the turbulence dragged by the counter vortex. Finally, fluctuating pressure spectra are analyzed, in good comparison between the computation and the experiment.

Large-Eddy Simulation of a rotor tip-clearance flow

Tip-leakage ows constitute a major concern for turbomachine performances, stability and broadband noise generation. A large-eddy simulation (LES) of a rotor blade is here presented and compared to experimental data from DLR. A particular attention is paid to the ow conguration of the simulation. The simulation of the inlet duct allows to recover accurately the ow conditions at the fan face. Downstream of the rotor, a detailed analysis of the mean velocity and turbulence intensity is carried out: 2D ow maps, radial proles of azimuth-averaged variables and wake parameters are plotted. The focus point of the present study is the complex tip-gap ow, and LES is shown to reproduce remarkably the experimental data in this region. These results are achieved in spite of a spurious separation that occurs on the blade suction-side and aects the lower radii (mid-span and hub regions). Finally, the velocity spectra computed by LES in the tip region are in very good agreement with the experimental data. The broadband content is accurately simulated over the whole experimental range of frequencies.

Dynamic Kalman filtering to separate low-frequency instabilities from turbulent fluctuations: Application to the Large-Eddy Simulation of unsteady turbulent flows

A dynamic method based on Kalman filtering is presented to isolate low-frequency unsteadiness from turbulent fluctuations in the large-eddy simulation (LES) of unsteady turbulent flows. The method can be viewed as an adaptive exponential smoothing, in which the smoothing factor adapts itself dynamically to the local behavior of the flow. Interestingly, the proposed method does not require any empirical tuning. In practice, it is used to estimate a shear-improved Smagorinsky viscosity, in which the low-frequency component of the velocity field is used to estimate a correction term to the Smagorinsky viscosity. The LES of the flow past a circular cylinder at Reynolds number ReD = 4.7 × 104 is examined as a challenging test case. Good comparisons are obtained with the experimental results, indicating the relevance of the shear-improved Smagorinsky model and the efficiency of the Kalman filtering. Finally, the adaptive cut-off of the Kalman filter is investigated, and shown to adapt locally and instantaneously to the complex flow around the cylinder.

Tip Leakage Flow: Advanced Measurements and Analysis

Advanced measurements of a tip clearance flow are carried out in the large subsonic anechoic wind tunnel of the LMFA in Lyon. The experimental set-up is obtained by placing the airfoil between two plates into the potential core of a turbulent rectangular jet, the upper plate being the hub and the lower the casing. The jet that exits a converging nozzle has a 450 mm cross-stream width and a 200 mm height in the span-wise direction. An h=10 mm gap is maintained between the blade and the casing plate. The incoming free stream turbulence u’/U0 is 0.5%, and the flat plate boundary layer half a chord upstream of the airfoil leading edge is about 8 mm thick. Except for the thickness being halved, the configuration is the same as the one discussed by Jacob et al.1, I.J.A, 9(3), (2010) , and Camussi et al.2, J.F.M., 660, (2010): a M ~ 0.2 and Rec ~ 950000 jet flow past a NACA5510 airfoil with 15° angle of attack. Measurements are carried out in the region of the Tip Leakage Vortex (TLV) in order to characterise its unsteady velocity. Both 2D–2 Component and stereo Time Resolved PIV techniques are successfully applied in order to provide an insight into the low frequency content of the velocity field, that are compared to LDV measurements. This experiment corresponds to a novel type of configuration that has only been studied by Jacob et al. Moreover, the TR-PIV and specifically the stereo-TR PIV are quite new techniques that have not often been applied to such complex configurations at such high speeds. The benefits of the time resolution are promising for the understanding of such broadband noise sources. Among the findings, there is a low frequency oscillation of the TLV, whose mechanism is yet unclear but which does not seem to radiate into the far field. Additionally, a hump at medium and high frequencies (0.7 – 7 kHz) is found in the far field. It can also be related to time decay double space-time correlations.

Numerical Analysis of Three-Dimensional Corner Separation in a Linear Compressor Cascade

(1)Laboratoire de Mecanique des Fluides et d’Acoustique, Ecole Centrale de Lyo´ n, 69130 Ecully, France(2)School of Jet Propulsion, Beijing University of Aeronautics and Astronautics, 100191 Beijing, China∗ Feng.Gao@ec-lyon.frABSTRACTNowadays, the internal flow in aircraft engine compressorscan be quite accurately reproduced at design condition by theCFD tools. However, CFD generally fails to simulate some sin-gular 3D phenomena, near off-design conditions, such as thecorner separation. Studies have pointed out that the separationregions are often over-estimated when the flow state is far fromdesign condition, owing to the turbulence model. Much workis devoted to improving the capability of the turbulence modelin capturing the onset and the extent of the corner separation,which is desired in the designing procedures.In this paper, steady RANS simulations are carried out in thesame configuration as an experiment of Ma et al. These simula-tions are obtained with a high-precision in-house Navier-Stokessolver (Turb’Flow). With the same mesh, an unsteady simulation(URANS) is subsequently presented, in order to investigate theinfluence of a fluctuating inflow.Attention is focused on a specific angle of attack of 4 de-grees, for which the three-dimensional corner separation isclearly observed. For the unsteady simulation, unsteadiness isimposed through perturbations of the angle of attack at the inlet.The results ofthe steady andunsteadycomputationsareanalyzedand compared with those of the experiment. The time-averagedURANS results agree well with the RANS results. The fluctuati nginflow does not show much influence on the mean performanceof the compressor cascade. The onset of the corner separationoccurs earlier in the simulations than in the experiment, consid-ering the blade surface pressure and the passage velocity pro-files. However, the cross-stream extent of the corner separationappears slightly under-estimated by CFD, according to the out-let total pressure losses andthe passagevelocityprofiles. Finally,the URANS simulation allows to recover bi-modal PDFs, as ob-served in the experiment.NOMENCLATUREc Chord lengthc

Tip-Leakage Flow: a Detailed Simulation with a Zonal Approach

The secondary flow generated by the clearance between an isolated airfoil tip and anend-plate is analyzed by means of a zonal large-eddy simulation, in comparison with avail-able experimental data. The flow around the tip clearance is described with full large-eddysimulation, while Reynolds-averaged Navier-Stokes is employed in the rest of the compu-tational domain in order to limit computational cost. The various analyses of the flowcharacteristics (mean velocities, Reynolds stresses, spectra) show a very good agreementbetween the experiment and the simulation. Looking at the mean velocities, an intensetip-leakage vortex is observed on the suction side. The Reynolds stresses are used toevaluate the anisotropy of the vortex. Finally, the spectral content is investigated in thenear-field and the far-field, and the leakage flow is shown to be characterized by a dominantcontribution in the range [0.7 kHz; 7 kH z].