Cfd fem-08 muffler-pre-processing_methodology_sharada_motors


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Cfd fem-08 muffler-pre-processing_methodology_sharada_motors

  1. 1. Muffler Pre-Processing Methodology and Comparative Study Using HyperMesh Dr. S. Rajadurai Suresh Natarajan N.Manikandan Head - R&D Asst Manager - CAE Senior Engineer -Product Sharda Motor Industries Ltd. Sharda Motor Industries Lt., Development Research and Development Research and Development Sharda Motor Industries Ltd. Mahindra World City, Mahindra World City, Chennai – 603 002, Tamilnadu. Research and Development Chennai – 603 002,Tamilnadu. Mahindra World City, Chennai – 603 002, Tamilnadu.Abbreviations: NVH - Noise, Vibration, Harshness, TL -Transmission lossKeywords: Exhaust System, Muffler, Resonator, Impedance tube, Glass wool AbstractThis paper summarizes the Pre-processing methodology of Exhaust system muffler using pre-processing tool HyperMesh forcalculating transmission loss in the muffler. Also pre-processing methodology from CFD tool also explained. The advantages of usingHyperMesh in reducing the pre-processing time are explained in this paper. The transmission loss for particular frequency range i.e. 50 -1000 Hz from experimental test setup i.e. Impedance tube are correlated with simulation results from 3D simulation tool for the developedFE model from HyperMesh and CFD tool are compared and explained in detail in this study.IntroductionInternal combustion engines are typically equipped with an exhaust muffler to suppress the acoustic pulsegenerated by the combustion process. A high intensity pressure wave generated by combustion in the enginecylinder propagates along the exhaust pipe and radiates from the exhaust pipe termination. The pulse repeatsat the firing frequency of the engine which is defined by f=(engine rpm x number of cylinders) /120 for afour stroke engine. Exhaust mufflers are designed to reduce sound levels at these frequencies. The purposeof an automotive muffler is to reduce the engine noise emission. If vehicles did not have a muffler, therewould be an unbearable amount of engine exhaust noise in the environment. The exhaust system contributes32% of the total noise emitted from the vehicle. Table1: Noise contributors of vehicle Contributors Engine Exhaust Intake Driveline Fan& Other structural Miscellaneous radiator components Noise 21% 32% 14% 14% 6% 10% 3%Transmission lossThe transmission loss is defined as the ratio between the sound power incident to the muffler (Wi) and thetransmitted sound power (Wt) for the case that there is a reflection free termination on the downstream side. TL = 10 log (Wi / Wt) 1Simulation Driven Innovation
  2. 2. The transmission loss and backpressure are the major considerations in thedesign of exhaust system. They are not compromised to each other. If the transmission loss increases, thebackpressure value also increases and it leads to affect the performance of the vehicle. Figure.1:Transmission loss of a mufflerPre-processing methodology using CFD toolPre-processing for CFD analysis involves the creation of surface meshes and setup of boundary conditionsfor thermal analysis and heat transfer modelling. Meshing is a key step to creating accurate model, correctmesh continuity and mesh density are needed to efficiently compute results and capture the boundary layereffects. The quality of CFD solution is depending on the quality of the underlying volume mesh.Basic volume mesh Types1. Trimmed Mesh2. Tetrahedral mesh3. Polyhedral meshCFD process flowchart Initially CFD analysis may be developed by the following steps, they are Import CAD geometry (step. file for internal parts identification) Parts specification (Specify each internal parts) Meshing Values (Base size & Tolerancing) Contact Prevention (Welded and joining parts) Surface Wrapper (Check mesh quality to repair surface) Surface Remesh HyperMesh (Development of Volume mesh i.e. (Tetra mesh) Figure 2:CFD Pre-processing methodology 2Simulation Driven Innovation
  3. 3. CAD geometry:CAD systems and their geometric representations have been around for quite some time. Almost all CADsystems have involved into similar representations for their models. One of the most important aspects tomesh Generation is accessing CAD geometry. CAD systems often use relatively large tolerances on basis toprovide model operations. This approach is referred to as variable tolerances and modelling by different CADsystems. The use of these large variable tolerances produces gaps and overlaps in the geometry andtopology of the CAD system. Figure .3: CAD geometry ModelThe major issue with CAD geometry access for mesh generation is the need to understand the analysisrequirements. An appropriate mesh and geometry is to be used for meshing. Meshing is a function of theanalysis to be performed and the desired accuracy. There does not exist an optimal mesh independent of theanalysis to be performed. The element shape quality test for good mesh independent of the analysis is to beperformed for the accuracy desired. The appropriate mesh is one, that produces the desired accuracy for theproblem to be solved.Surface Wrapping: When preparing a model for a CFD analysis, surfaces need to be closed in order to generate mesh. Theprocess of surface wrapping typically involves taking poor quality CAD surface, closing holes in geometry,joining disconnected surfaces, removing interior and overlapping surfaces. Figure 4: Surface wrapper muffler with internalsChecking the imported surface will allows us to choose a course of action required for generating a volumemesh.(e.g.) if surface is free from errors and has good quality triangulation, we can directly proceed to volumemesh generation. But if the surface has errors, then we have to use either manual repair tools or surfacewrapping to fix errors.Surface RemesherThe surface remesher is used to improve the overall quality of the surface and optimize it for the volumemesh models. Specific boundaries can also be omitted from the process so that the original triangulation fromthe imported mesh can be improved. The surface remesher is typically used for remeshing surfaces producedby the surface wrapper and STL data as well as improving the surface for the volume mesh. It also aids thesubsurface generator when the prism mesher option is selected. 3Simulation Driven Innovation
  4. 4. Figure .5: Surface Remesh muffler modelVolume mesh generation using HyperMeshSurface mesh or shell mesh developed from CFD tool is imported into HyperMesh as typical solver input file.The model is checked for free edges and if found any free edges that needs to be corrected manually tocreate the muffler with internal components as single volume. For generating tetra mesh, the model should bea single volume. The volume mesh has been developed with Tetramesh option in HyperMesh and elementquality parameters such as tetra collapse is to be checked. If Glasswool is inserted into middle chamber ofmuffler, it should be in a separate collector. Figure .6: Volume mesh model of muffler from HyperMesh for CFD surface remesh model 4Simulation Driven Innovation
  5. 5. LMS Virtual Lab CFD Muffler Model SetupThe complete volume muffler imported to the LMS Virtual Lab work bench, the inputs and measuringconditions are feed as per the procedure Figure .7: LMS Virtual Lab Work BenchTransmission Loss Result Graph(CFD Mesh Muffler) Figure.8: TL plot for CFD modelIntroduction to HyperMeshAltair Engineering HyperMesh is a commercial Pre-processing tool for most of the FEA and CFD solvercodes. It is used to develop surface, solid and hybrid mesh with 1D, weld, rigid, mass element connections fordifferent types of geometries. Also it is used to develop various solver specific input deck i.e. defining materialproperties, element properties, loads, boundary conditions and solver settings for performing different CAEanalysis such as static linear and Non-linear, modal, thermal, dynamic, buckling, crash, NVH and impactanalysis. With the use of batch meshing, in-built templates and macros, our pre-processing time has beenreduced in order to quickly analyze the products for design modification and optimization. This will reduce theproduct development time to minimize the cost and effort. 5Simulation Driven Innovation
  6. 6. Pre-processing methodology using HyperMesh Import CAD geometry Geometry cleanup and Mid surface Extraction Merging of baffle surface and pipe weld surface by trimming operation Organizing of component collector for perforation surface on pipe and baffle plate, pipe outer surface and baffle surface Shell meshing of muffler components Solid meshing of internal pipes with perforations Solid meshing of chamber 1, 2 and 3 i.e. endplate1, baffle plate1 with internal Node set creation for baffle plates and internal pipes without perforation 3D detaching of pipe outer surface and baffle surface without holes from solid mesh Exporting as solver specific input file (.bdf) for acoustic simulation Figure.9: HyperMesh Pre-processing methodologyGeometry simplificationCAD model of exhaust muffler is imported into HyperMesh. Geometry cleanup activity such as removing ofunwanted lines, duplicate surfaces, pinholes and small holes has been done. Midsurface extraction of eachcomponents has been done and baffle plate is merged with muffler shell by trim operation. Also all the weldgap between pipe to end plate, baffle plate are merged to have node-to-node connectivity at the time of shell 6Simulation Driven Innovation
  7. 7. meshing. Surface to be created at both ends of the inlet pipe, outlet pipe andmiddle pipe to have enclosed volume. Figure .10: Muffler Geometry simplification With InternalsPerforated Tube;Perforated tubes in the muffler as used to escape are deliver the hot exhaust gases through the perforationholes in the tube.The hole diameter and numbr of perforations are calculated as per the muffler volume andrequirment. The perforation need have to proper spacing between the each hole for better performance.Hereperforations are closed with the creation of surfaces and it should be in separate collector.After geometrysimplification,perforation surfaces are to be meshed with shell elements with minimum element length to havemore number of nodes on the surface to achieve proper flow of gases and minimize noise level. Perforated tube 7Simulation Driven Innovation
  8. 8. Perforations Figure.11: Perforated Tube and PerforationsPerforated Baffle Figure.12: Perforated Baffle and PerforationsBaffle plates have many design uses in general in different applications. However, If baffles are created to thepressurized conditions required within our systems design for proper distribution, and flow rate throughout oursystem. Many baffles are adjustable and some are fixed. Air noise is something that baffles are used to helpreduces the backpressure during the hot gas passing through the muffler. Perforated baffles are used toexpand the exhaust gases from one room to another room through the use of their perforations. Perforatedand non-perforated baffle plates are merged with muffler shell with the use of trim operation.Shell meshingShell meshing of muffler components such as muffler shell, end plates, baffle plates and internal perforatedpipes are done with shell elements. Perforation surface are meshed and kept as a separate collector. Themuffler assembly meshed model is checked for free edges to achieve enclosed volume. This enclosedvolume is necessary to create the solid or volume mesh. 8Simulation Driven Innovation
  9. 9. Figure.13: Shell mesh of muffler with internals and detaching of nodes Once we complete the shell mesh, the internals and different rooms are split as per the requirements, theeach room should be created as single volume with their respective internals for that the inlet and outletopenings must be closed for creating tetra mesh.Solid meshingInitially internal perforated pipes surfaces are kept as a separate collector in order to have three chambers.Solid mesh (Tetrahedral elements) is created for each separated internal pipes up to the weld region of baffleplates and end plates inside the muffler. Once the volume mesh for each internal pipes are completed, themodel is carefully selected for developing the solid mesh for each chamber. Here chamber1 components areconsidered as endplate1,internal perforated pipes and baffle plate1 and taken for developing solid mesh.Similarly chamber2 components as baffle plate1and 2 and internal pipes between baffles considered fordeveloping solid mesh. Likewise chamber3 components are baffle plate2 and plate2 and internal pipes inbetween baffle plate and end plate are taken for creating solid mesh. Figure.14: Muffler Solid Mesh from HyperMesh with Wireframe Model 9Simulation Driven Innovation
  10. 10. Development of FE model for 3D Acoustic solverAfter creating solid mesh, node sets are created for baffle plates shell mesh without considering perforationsurface and internal pipes without perforations.3D Detaching of created node sets for baffle plates andperforated pipes from tetra mesh of each chambers are to be done to have flow of gases in all perforations inbaffle plate and internal pipes. Then the model is finally checked for whether the connectivity has beenremoved for baffle plates and internal pipes outer surfaces to verify the process steps.Finally, the finished model is updated with the required element types and exported as Nastran input file forimporting it into 3D acoustic simulation solver. The cut section view of the model is verified for correction orproceeds for acoustic simulation to determine the transmission loss inside the muffler.HyperMesh Muffler Setup in LMS Virtual Lab Figure15: LMS Virtual Lab Work Bench for HyperMesh model Figure.16: Transmission Loss Result Graph (HyperMesh Model) 10Simulation Driven Innovation
  11. 11. Transmission Loss Measurement from Test LabNVH test lab setup used to measure the transmission loss for Muffler/Resonators. the equipment which ishaving 2 quarter inch Microphones and 2 half inch microphones and Up stream, Downstream impedancetube. The transmission loss setup to measure TL by 2 load conditions are i.e.1.Rigid load and 2.AnechoicTermination. The test set up is shown below, Figure.17: Schematic Representation of Test Lab Setup Figure.18: Impedance tube test Setup Figure.19: Example muffler for TL calculation 11Simulation Driven Innovation
  12. 12. Transmission Loss Result Graph (Test Lab) Figure.20: TL curve for Experimental methodCFD Muffler TL Vs Test Lab TL Comparison Result Figure.21: TL comparison curve for CFD model Vs Test Lab 12Simulation Driven Innovation
  13. 13. HyperMesh TL Vs NVH TL Comparison Result Figure.22: TL comparison curve for HyperMesh model Vs Test LabHyperMesh TL Vs CFD Muffler TL Comparison Result Figure.23: TL comparison plot for HyperMesh model Vs CFD model 13Simulation Driven Innovation
  14. 14. HyperMesh, CFD and NVH Muffler TL Comparison Graph Figure.24: TL comparison plot for CFD, HyperMesh model Vs Test labResults and Discussion1) In this paper, FE model development of muffler from Altairs Pre-processing tool HyperMesh and CFD tool for calculating the transmission loss using 3D Acoustic simulation solver has been explained in detail.2) The transmission loss parameter for muffler assembly calculated from 3D acoustic solver for HyperMesh model and CFD model are shown in figure23.3) It has been observed from Transmission loss curve for HyperMesh model is well correlated with CFD model and also with NVH Lab measurement i.e. Impedance tube. as shown in figure 24.Benefits SummaryAltairs Pre-processing tool HyperMesh is effectively used to develop FE model of the muffler assembly forcalculating transmission loss in the muffler. Also it is used to develop the volume mesh for the developed CFDshell model .HyperMesh is also used for surface repair and trimming operation to have nodal connectivity atthe time of meshing.HyperMesh pre-processing methodology helps in reducing the pre-processing time for muffler assembly withgood quality mesh control and element quality parameters such as minimum element length, tria elementmaximum and minimum angles and tetra collapse are achieved properly to get better correlated results oftransmission loss using 3D acoustic simulation solver. 14Simulation Driven Innovation
  15. 15. ChallengesThe key challenges faced by us that, at the time of volume mesh development for each chamber of muffler,the components such as internal pipes passing throughout the end plate and baffle plate are considered asthrough hole and suppose if any internal pipes are passing through half of the chamber has been consideredas cavity i.e. outer surface of pipe is taken for volume mesh creation.After the development of tetrameshing of whole muffler assembly, the volume mesh connection for eachperforation on baffle plate and internal pipes has been done by 3D detaching of volume mesh from alreadycreated node sets for baffle plate and perforated pipes without considering holes.Sometimes, the developed 3D FE model of muffler from HyperMesh is imported into 3D acoustic simulationtool, internal components of muffler are not visible if we are viewing in wireframe mode or cut section view.Future PlansAltairs HyperMesh pre-processing methodology has to be implemented for different kind of muffler assemblysuch as glass wool packed muffler ,integrated muffler (in-built catalytic converter),in-built resonator typemufflers to determine transmission loss in the muffler using 3D acoustic simulation tool to achieve correlatedresults with experimental test setup i.e. Impedance tube.ConclusionIt has been concluded that Transmission loss of muffler for developed FE model from HyperMesh and CFDtool has been correlated with experimental testing for the particular frequency range i.e. 0 - 1000 Hz.HyperMesh Pre-processing tool has been effectively utilized to develop the FE model for TL calculation in 3Dacoustic simulation solver and also method for reducing the meshing time has been explained in this paper. AcknowledgementThe authors would like to thank Jose bright, Gokulraj, Madhan and Ashwini and Sharda Motor R&D teammembers for providing constant support through the project. Reference1) Steven J.Owen(1996),A Survey of Unstructured Mesh Generation Technology2) Senesh.K, Balasubramaniam.P, sudhir Kumar.B(1998) HyperMesh Customization to Manage FEM data For Custom Hybrid Thermal-Deflection Solver3) CFD software users manual4) Sandeep Palaksha, Abhijith Balakrishanan(1997) Modelling CFD specific Domain Using HyperMesh for Display Case used in the Retail Industry5) Z.Tao and A.F.Seybert " A review at current Techniques for Muffler Transmission loss measurement". 15Simulation Driven Innovation