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Stuttgart Muhlhausen, Germany

Cyr S.,Exa GmbH | Ih K.-D.,Hyundai Motor Company | Park S.-H.,Hyundai Motor Company
SAE Technical Papers | Year: 2011

Aerodynamic simulation results are most of the time compared to wind tunnel results. It is too often simplistically believed that it suffice to take the CAD geometry of a car, prepare and run a CFD simulation to obtain results that should be comparable. With the industry requesting accuracies of a few drag counts when comparing CFD to wind tunnel results, a careful analysis of the element susceptible of creating a difference in the results is in order. In this project a detailed 1:4 scale model of the Hyundai Genesis was tested in the model wind tunnel of the FKFS. Five different underbody panel configurations of the car were tested going from a fully paneled car to a car without panels. The impact of the moving versus static ground was also tested, providing over all ten different experimental results for this car model. These ten different configurations were simulated having in mind to reproduce the testing condition in every possible aspect to be able to truly evaluate the accuracy of the CFD results. In the setup process, three aspects were considered: geometry precision, wind-tunnel environment and conditions and finally evaluation of the proper reference values. In the geometry preparation process an additional step was added to morph the geometry prepared from the CAD model. The target used was scanned data obtained from the physical model after the test session. The wind tunnel environment was reproduced in the setup to include all side effects (pressure gradient, jet expansion, etc.) and boundary conditions found in the FKFS scale model wind tunnel during the testing session. When reproducing wind tunnel environment in a simulation it is necessary to evaluate the proper values of dynamic pressure and static pressure needed to non-dimensionalize the results. A new method was used to evaluate the proper reference values. The empty wind-tunnel method was also used and results are compared to the new method. This paper shows that it is possible to achieve very accurate CFD results using PowerFLOW if all aspects of the testing condition are systematically reproduced in the simulation setup. Copyright © 2011 SAE International. Source

Mann A.,Exa Corporation | Perot F.,Exa Corporation | Kim M.-S.,Exa Corporation | Casalino D.,Exa GmbH
19th AIAA/CEAS Aeroacoustics Conference | Year: 2013

The impedance of acoustic liners is predicted using lattice-Boltzmann fluid dynamics simulations. The complete three-dimensional geometry of liners, which corresponds to the combination of micro-perforated sheets, honeycomb cavities and porous materials are directly characterized through a numerical setup that reproduces a Kundt's tube and realistic liner samples. In a first step, a mesh resolution study is performed for a One Degree of Freedom (1-DOF) liner in order to show the convergence of the deducted value of the acoustic impedance with the grid size. In a second step, the influence of all the geometric parameters of the liner is predicted for 1-DOF, Two Degree of Freedom (2-DOF) and Bulk Absorber (BA) liners. The predicted impedance is compared with two analytical impedance models available in the literature. Source

Bres G.A.,Exa Corporation | Bres G.A.,Cascade Technologies, Inc. | Freed D.,Exa Corporation | Wessels M.,Exa GmbH | And 2 more authors.
Physics of Fluids | Year: 2012

Flow and noise predictions for the tandem cylinder benchmark are performed using lattice Boltzmann and Ffowcs Williams-Hawkings methods. The numerical results are compared to experimental measurements from the Basic Aerodynamic Research Tunnel and Quiet Flow Facility (QFF) at NASA Langley Research Center. The present study focuses on two configurations: the first configuration corresponds to the typical setup with uniform inflow and spanwise periodic boundary condition. To investigate installation effects, the second configuration matches the QFF setup and geometry, including the rectangular open jet nozzle, and the two vertical side plates mounted in the span to support the test models. For both simulations, the full span of 16 cylinder diameters is simulated, matching the experimental dimensions. Overall, good agreement is obtained with the experimental surface data, flow field, and radiated noise measurements. In particular, the presence of the side plates significantly reduces the excessive spanwise coherence observed with periodic boundary conditions and improves the predictions of the tonal peak amplitude in the far-field noise spectra. Inclusion of the contributions from the side plates in the calculation of the radiated noise shows an overall increase in the predicted spectra and directivity, leading to a better match with the experimental measurements. The measured increase is about 1 to 2 dB at the main shedding frequency and harmonics, and is likely caused by reflections on the spanwise side plates. The broadband levels are also slightly higher by about 2 to 3 dB, likely due to the shear layers from the nozzle exit impacting the side plates. © 2012 American Institute of Physics. Source

Casalino D.,Exa GmbH | Hazir A.,Exa GmbH
22nd International Congress on Sound and Vibration, ICSV 2015 | Year: 2015

The unsteady jet from an axisymmetric coaxial nozzle and the associated noise are computed by using a Lattice-Boltzmann Model (LBM) for high-speed subsonic flows. This method, recently developed by Exa, consists in coupling the standard LBM formulation for iso-thermal flows with a finite-difference solution of the entropy equation. With this new formulation it is possible to accurately simulate unsteady flows past complex geometries with significantly shorter turnaround times compared to conventional CFD methods based on the solution of the Navier-Stokes equations. In the present study, the primary and secondary jet exit Mach numbers are 0.87 and 0.90, respectively, the secondary jet Reynolds number is 2.8 million, and the primary to secondary temperature ratio is 2.7. The acoustic far-field is computed through a Ffowcs-Williams and Hawkings analogy applied to a fluid surface encompassing the plume. The accuracy of the numerical solution is evaluated against literature experimental data. The time-averaged velocity field and the root-mean-square velocity and pressure fields are shown to compare quite favourably with wind-tunnel measurements. The far-field noise spectra are also in fairly good agreement with the measurements, although affected by some lack of statistical convergence. Source

Casalino D.,Exa GmbH | Ribeiro A.F.P.,Exa GmbH | Fares E.,Exa GmbH
Journal of Sound and Vibration | Year: 2014

Cavity modes taking place in the rims of two opposite wheels are investigated through Lattice-Boltzmann CFD simulations. Based on previous observations carried out by the authors during the BANC-II/LAGOON landing gear aeroacoustic study, a resonance mode can take place in the volume between the wheels of a two-wheel landing gear, involving a coupling between shear-layer vortical fluctuations and acoustic modes resulting from the combination of round cavity modes and wheel-to-wheel transversal acoustic modes. As a result, side force fluctuations and tonal noise side radiation take place. A parametric study of the cavity mode properties is carried out in the present work by varying the distance between the wheels. Moreover, the effects due to the presence of the axle are investigated by removing the axle from the two-wheel assembly. The azimuthal properties of the modes are scrutinized by filtering the unsteady flow in narrow bands around the tonal frequencies and investigating the azimuthal structure of the filtered fluctuation modes. Estimation of the tone frequencies with an ad hoc proposed analytical formula confirms the observed modal properties of the filtered unsteady flow solutions. The present study constitutes a primary step in the description of facing rim cavity modes as a possible source of landing gear tonal noise. © 2014 Elsevier Ltd. Source

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