FEM/BEM Studies on the Interior Acoustics of a Car Contents

Introduction! Objectives:Investigation of the effectiveness and accuracy ofVirtual.Lab in the field of interior car acoustics– Focus on:• preprocessinghandling of “real life” models, large amount of data• postprocessingAcoustic Transfer Functions (ATF), Grid Contribution• the entire workflow from the <strong>FEM</strong> structural modelto the resulting sound field! Test case: Mercedes station wagonLMS Conference Europe, 22. & 23. March 2006www.novicos.de

Mesh preparation (2)! Removal of unnecessary elements– Removal of groups of elements• working with groups imported to VL from Nastran input dataproperty-ID• working with groups generated directly in VLelement type, material ID, feature angle, …Nastran element groups (PID)Groups of elementsof the same typeLMS Conference Europe, 22. & 23. March 2006www.novicos.de

Mesh preparation (3)! Closing of holes– different Methods availableto generate elements inside the holesand gaps– closing of all/only selected holesinside a group of elements– efficient procedureLMS Conference Europe, 22. & 23. March 2006www.novicos.de

LMS Conference Europe, 22. & 23. March 2006www.novicos.deMesh Wrapping (1)! „Wrapping“ the FE meshinto a new boundary meshselectFE meshuser definedelement sizeuser defined relationbetween old&new mesheswith respect to a specificacoustic fluidapplicable to a FE meshon the boundary

Mesh Wrapping (2)! Leads to BE meshes (QUAD4-Elements)for a whole variety of applicationstwo examples starting from the same FE mesh:fine boundary meshing(46319 nodes and 46334 elements)coarse boundary meshing(8962 nodes and 8981 elements)LMS Conference Europe, 22. & 23. March 2006www.novicos.de

LMS Conference Europe, 22. & 23. March 2006www.novicos.de<strong>BEM</strong> acoustic model! BE mesh used in acoustic calculationsCoarse mesh, but with satisfying accuracy to represent details(details of the car body, seats …)Boundary mesh with ca. 12300nodes and 12400 elements4 field points (passengers ears) definevisualisation surface

Transfer of structural eigenmodes! Data transfer between two modelsFE model (Nastran) -> simplified FE model (in VL)! Data interpolation between two meshesFE structural mesh (Nastran) -> simplified FE model (in VL)! High efficiency:– several mapping algorithmsimplemented in VL– preliminary statisticsa reliable estimate of the qualityof the data transferLMS Conference Europe, 22. & 23. March 2006www.novicos.de

Coupled <strong>FEM</strong>/<strong>FEM</strong> computations! Work flowFE structureNASTRAN filesFE meshmesh preparationremoving of unnecessaryelementsclosing of holes…FE mesh„mesh wrapping“passenger compartmentseats…„cavity meshing“BE meshmodesloadsacoustic modelmodel solutionsound fieldLMS Conference Europe, 22. & 23. March 2006www.novicos.de

LMS Conference Europe, 22. & 23. March 2006www.novicos.deCavity Meshing (1)! Generating an acoustic FE mesh (HEXA8 elements)inside a given boundary requires 2 stepsSTEP 1: scan for volumesinside the boundarySTEP 2: generate elements inside the volumesfound in STEP 1 and selected by the user

LMS Conference Europe, 22. & 23. March 2006www.novicos.deCavity meshing (2)! Options„smooth“ and „sharp“options controldetails/accuracyof the cavity meshSharp featureSmooth feature

LMS Conference Europe, 22. & 23. March 2006www.novicos.de<strong>FEM</strong> acoustic model! FE mesh used in acoustic calculationsCoarse mesh but with satisfying accuracy to represent details(details of the car body,seats …)HEXA8 mesh with ca. 310000 elementscross section thru driver seat cross section thru cockpit tunnel

LMS Conference Europe, 22. & 23. March 2006www.novicos.deResults (1)! Sound pressure levels at driver's earSound Pressure [dB]current Akusmod/Nastranmesh & solverVL LMS mesh Virtual.Lab & solverVL Virtual.Lab/Nastranmesh & current solverFrequency [Hz]

LMS Conference Europe, 22. & 23. March 2006www.novicos.deResults (2)! Sound pressure levels at the occupants` ears140Sound Pressure [dB]Schaldruck [dB]135Fahrer Driver130Beifahrer Occupant 1125Hinterm Occupant Fahrer 2120Hinterm Occupant Beifahrer 3115110105100959085807570656040 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120Frequency Frequenz [Hz]

LMS Conference Europe, 22. & 23. March 2006www.novicos.deResults (3)! Colour maps of sound pressure levels120 Hz 150 Hz

LMS Conference Europe, 22. & 23. March 2006www.novicos.deResults <strong>FEM</strong> acoustics (4)! Eigenmodes of the passenger compartmentca. 43 Hzca. 83 Hzca. 92 Hzca. 125 Hz

LMS Conference Europe, 22. & 23. March 2006www.novicos.deAcoustic Transfer Functions (1)! ATF is an acoustic transfer function, which gives the relationbetween the normal displacements/velocities at a surface nodeand the sound pressure at a given field point.node 1{v 1 (f)}{ATF 1 (f)}{p (f)} = ?{ATF N (f)}field pointsurface meshnode N{v N (f)}The matrix of all ATF i (f)characterizes the acousticbehavior of a BE model.

LMS Conference Europe, 22. & 23. March 2006www.novicos.deAcoustic Transfer Functions (3)! Interest: vibroacoustic optimizationVIBRATIONConcentrates on structural vibrationsusing maxima of surface velocities vACOUSTICSConcentrates on sound radiationusing maxima of ATFsVIBROACOUSTICSTakes both the structural vibrationsand the sound radiation into considerationusing product v*ATF (Grid contribution)

LMS Conference Europe, 22. & 23. March 2006www.novicos.deResults (5)! ATF's for the driver's ear40 Hz 90 Hz92 Hz 120 Hz

LMS Conference Europe, 22. & 23. March 2006www.novicos.deResults (7)! Analysis at 40 Hz using Grid ContributionsVelocity v ATFGridcontributionv*ATF

LMS Conference Europe, 22. & 23. March 2006www.novicos.deResults (6)! Analysis at 120Hz using Grid ContributionsVelocity v ATFGridcontributionv*ATF

LMS Conference Europe, 22. & 23. March 2006www.novicos.deConclusions! Efficient simulations of interior car acousticsin LMS Virtual.Lab– safe handling of large amount of data– all steps needed to perform a coupled calculation of sound(model preparation, solving, examination of results)carried out in a single CAE tool– many useful commands dedicated to <strong>FEM</strong>/<strong>BEM</strong> acoustics:mesh wrapping, cavity meshing, fluid-structure coupling, …! Valuable insights into the vibro-acoustics of a carcompartment due to unique approaches in VL– Acoustic Transfer Functions (ATF)– Grid Contribution