698 research outputs found
Lattice Boltzmann Methods for thermal flows: continuum limit and applications to compressible Rayleigh-Taylor systems
We compute the continuum thermo-hydrodynamical limit of a new formulation of
lattice kinetic equations for thermal compressible flows, recently proposed in
[Sbragaglia et al., J. Fluid Mech. 628 299 (2009)]. We show that the
hydrodynamical manifold is given by the correct compressible Fourier-
Navier-Stokes equations for a perfect fluid. We validate the numerical
algorithm by means of exact results for transition to convection in
Rayleigh-B\'enard compressible systems and against direct comparison with
finite-difference schemes. The method is stable and reliable up to temperature
jumps between top and bottom walls of the order of 50% the averaged bulk
temperature. We use this method to study Rayleigh-Taylor instability for
compressible stratified flows and we determine the growth of the mixing layer
at changing Atwood numbers up to At ~ 0.4. We highlight the role played by the
adiabatic gradient in stopping the mixing layer growth in presence of high
stratification and we quantify the asymmetric growth rate for spikes and
bubbles for two dimensional Rayleigh- Taylor systems with resolution up to Lx
\times Lz = 1664 \times 4400 and with Rayleigh numbers up to Ra ~ 2 \times
10^10.Comment: 26 pages, 13 figure
Impact of a heterogeneous stator on the rotor-stator interaction-noise: an analytical, experimental and numerical investigation
La présente étude vise à quantifier par une modélisation analytique, des essais et des simulations
numériques, l’impact d’un stator hétérogène sur le bruit d’interaction rotor-stator
dans les turbomachines axiales. Le travail développé s’appuie sur des premières observations
sur un ventilateur axial à basse vitesse à l’École Centrale de Lyon, l’étage LP3. Il
a été observé que les deux premières fréquences de passage des pales (FPP) rayonnaient
à des niveaux élevés alors qu’elles devaient être coupées par le conduit selon le critère de
Tyler & Sofrin. Une campagne expérimentale est alors réalisée sur la configuration de
ventilateur hétérogène qui permet la caractérisation des contenus spectral et modal. Afin
de s’assurer qu’aucune distorsion d’entrée d’air n’est présente, un écran pour le contrôle
de la turbulence est utilisé. Des techniques de décomposition modale sont utilisées sur des
antennes pseudo-aléatoires afin d’obtenir les modes acoustiques prédominants. Les résultats
montrent un fort rayonnement acoustique des deux premières fréquences de passage
des pales et mettent en évidence des modes dominants. La même expérience est ensuite
simulée numériquement en utilisant la méthode de Boltzmann sur réseau. Les simulations
montrent un bon comportement de la turbomachine mais prédisent une augmentation de
pression inférieure à celle de l’expérience. La comparaison entre un stator homogène et
hétérogène permet de quantifier directement l’impact de l’hétérogénéité. L’hétérogénéité
est alors responsable d’une augmentation du niveau tonal de plus de 10 dB aux deux premières
FPP. Le contenu modal mesuré sur la configuration hétérogène est bien retrouvé
par les simulations numériques. En outre, l’analyse de l’écoulement dans l’espacement
inter-rotor-stator a permis de mettre en évidence l’impact de l’hétérogénéité sur le champ
potentiel. Finalement, la modélisation analytique est axée sur deux sources dominantes :
le bruit d’interaction de sillages et le bruit d’interaction potentielle. Les résultats montrent
une contribution mineure de ce dernier. Les mêmes modes dominants sont retrouvés
dans certaines directions de propagation en accord avec ce qui est observé expérimentalement.
En dernier lieu, une étude d’optimisation de la position des bras support est
présentée. Une des configurations optimales montrant une forte atténuation du niveau
de bruit tonal est validée numériquement par des simulations numériques. Les résultats
montrent que l’optimisation du positionnement angulaire des aubes structurelles permet
d’obtenir une réduction significative des niveaux aux deux premières FPP. L’étude des différentes
composantes (analytique, expérimentale et numérique) fournit ainsi une meilleure
compréhension des mécanismes de bruit modifiés par l’hétérogénéité du stator.Abstract: The present study aims to quantify by means of analytical modelling, experiments and
numerical simulations, the impact of a heterogeneous stator on the rotor-stator noise in
axial turbomachines. This study starts with the first observations on an axial low-speed
fan at École Centrale de Lyon, the LP3 stage. It has been observed that the first two blade
passing frequencies (BPF) were radiating at high levels while they were expected to be
cut-off by the duct according to Tyler & Sofrin’s criterion. An experiment is then carried
out with the heterogeneous stator configuration which makes it possible to characterize
the spectral and modal contents. To ensure that no inflow distortion is present at the
inlet, a Turbulence Control Screen is used. Modal decomposition techniques are used
with pseudo-random antennas to obtain the predominant acoustic modes. Results show
a strong acoustic radiation of the first two BPFs and evidence some dominant modes.
The same experiment is then simulated numerically using the lattice Boltzmann method.
The simulations show a good physical behaviour of the turbomachine but predict a lower
pressure-rise compared with the experiment. The comparison between homogeneous and
heterogeneous stators allows quantifying directly the impact of the heterogeneity. The
heterogeneity is responsible for a level increase of more than 10 dB at the first two BPFs.
The modal content from the numerical simulations on the heterogeneous configuration
is also in good agreement with the experiment. In addition, the analysis of the flow
in the inter-stage made it possible to highlight the impact of the heterogeneity on the
potential field. Finally, the analytical modelling is focused on two dominant sources:
wake-interaction noise and potential-interaction noise. Results put in evidence a minor
contribution of the latter despite the short rotor-stator spacing. The same dominant
modes are found in certain propagation directions in accordance with what is measured in
the experiment. Finally, an optimisation of the modified vanes angular position is carried
out. One of the optimal configurations showing a great noise attenuation is numerically
validated by the LBM. The numerical results show that the optimisation of the azimuthal
positioning of the modified vanes makes it possible to obtain a significant reduction of
the levels at the first two BPFs. Thereby, the comparison of the analytical, experimental
and numerical investigations allows achieving a better understanding of the modification
of noise mechanisms caused by the heterogeneity of the stator
Aerodynamic and Aeroacoustic Numerical Investigation of Turbofan Engines using Lattice Boltzmann Methods
International audienceIn recent years, lattice Boltzmann methods showed promising advantages over standard Navier-Stokes equation-based solvers. In this work, the capacity to predict both self noise and interaction noise is evaluated. First, a rod-airfoil interaction case is investigated, where the turbulence wake of the rod impinges the leading edge of the airfoil. Thereafter, a semi-infinite ducted axial fan is studied, where the turbulent boundary layers on each blades generate self noise which propagates into the duct, and radiates to the far-field. Subsequently, a ducted grid simulation is performed to verify the properties of the grid-generated turbulence. Finally, the grid and the axial-fan are combined within the same configuration, which comprises both self-noise and interaction noise. For each configuration, the agreements with experiments are satisfactory, however, acoustic propagation issues have been encounters from the duct intake to the free field. Nevertheless, the implemented wall model at the solid boundaries seems to correctly predict the acoustic sources on the blades
Definition of a benchmark for low Reynolds number propeller aeroacoustics
Experimental and numerical results of a propeller of 0.3 m diameter operated at 5000 RPM and axial velocity ranging from 0 to 20 m/s and advance ratio ranging from 0 to 0.8 are presented as a preliminary step towards the definition of a benchmark configuration for low Reynolds number propeller aeroacoustics. The corresponding rotational tip Mach number is 0.23 and the Reynolds number based on the blade sectional chord and flow velocity varies from about 46000 to 106000 in the operational domain and in the 30% to 100% blade radial range. Force and noise measurements carried out in a low-speed semi-anechoic wind-tunnel are compared to scale-resolved CFD and low-fidelity numerical predictions. Results identify the experimental and numerical challenges of the benchmark and the relevance of fundamental research questions related to transition and other low Reynolds number effects
Simulation-Based Airframe Noise Prediction of a Full-Scale, Full Aircraft
A previously validated computational approach applied to an 18%-scale, semi-span Gulfstream aircraft model was extended to the full-scale, full-span aircraft in the present investigation. The full-scale flap and main landing gear geometries used in the simulations are nearly identical to those flown on the actual aircraft. The lattice Boltzmann solver PowerFLOW was used to perform time-accurate predictions of the flow field associated with this aircraft. The simulations were performed at a Mach number of 0.2 with the flap deflected 39 deg. and main landing gear deployed (landing configuration). Special attention was paid to the accurate prediction of major sources of flap tip and main landing gear noise. Computed farfield noise spectra for three selected baseline configurations (flap deflected 39 deg. with and without main gear extended, and flap deflected 0 deg. with gear deployed) are presented. The flap brackets are shown to be important contributors to the farfield noise spectra in the mid- to high-frequency range. Simulated farfield noise spectra for the baseline configurations, obtained using a Ffowcs Williams and Hawkings acoustic analogy approach, were found to be in close agreement with acoustic measurements acquired during the 2006 NASA-Gulfstream joint flight test of the same aircraft
Mixing multi-core CPUs and GPUs for scientific simulation software
Recent technological and economic developments have led to widespread availability of
multi-core CPUs and specialist accelerator processors such as graphical processing units
(GPUs). The accelerated computational performance possible from these devices can be very
high for some applications paradigms. Software languages and systems such as NVIDIA's
CUDA and Khronos consortium's open compute language (OpenCL) support a number of
individual parallel application programming paradigms. To scale up the performance of some
complex systems simulations, a hybrid of multi-core CPUs for coarse-grained parallelism and
very many core GPUs for data parallelism is necessary. We describe our use of hybrid applica-
tions using threading approaches and multi-core CPUs to control independent GPU devices.
We present speed-up data and discuss multi-threading software issues for the applications
level programmer and o er some suggested areas for language development and integration
between coarse-grained and ne-grained multi-thread systems. We discuss results from three
common simulation algorithmic areas including: partial di erential equations; graph cluster
metric calculations and random number generation. We report on programming experiences
and selected performance for these algorithms on: single and multiple GPUs; multi-core CPUs;
a CellBE; and using OpenCL. We discuss programmer usability issues and the outlook and
trends in multi-core programming for scienti c applications developers
Towards the definition of a benchmark for low Reynolds number propeller aeroacoustics
Experimental and numerical results of a propeller of 0.3 m diameter operated in
quiescent standard ambient conditions at 5000 RPM and axial velocity ranging from 0 to 20
m/s and advance ratio ranging from 0 to 0.8 are presented as a preliminary step towards the
definition of a benchmark configuration for low Reynolds number propeller aeroacoustics. The
corresponding rotational tip Mach number is 0.231 and the Reynolds number based on the blade
sectional chord and flow velocity in the whole radial and operational domain ranges from about
54000 to 106000. Force and noise measurements carried out in a low-speed semi-anechoic windtunnel are compared with scale-resolved CFD and low-fidelity numerical results. Results identify
the experimental and numerical challenges of the benchmark and the relevance of fundamental
research questions related to transition and other low Reynolds number effects
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