Cfd fem-08 muffler-pre-processing_methodology_sharada_motors

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Simulation Driven Innovation

Muffler Pre-Processing Methodology and Comparative Study

Using HyperMesh

Dr. S. Rajadurai Head - R&D

Sharda Motor Industries Ltd. Research and Development

Mahindra World City, Chennai – 603 002,Tamilnadu.

Suresh Natarajan Asst Manager - CAE

Sharda Motor Industries Lt., Research and Development

Mahindra World City, Chennai – 603 002, Tamilnadu.

N.Manikandan Senior Engineer -Product

Development

Sharda Motor Industries Ltd. Research and Development

Mahindra World City, Chennai – 603 002, Tamilnadu.

Abbreviations: NVH - Noise, Vibration, Harshness, TL -Transmission loss

Keywords: Exhaust System, Muffler, Resonator, Impedance tube, Glass wool

Abstract

This paper summarizes the Pre-processing methodology of Exhaust system muffler using pre-processing tool HyperMesh for calculating transmission loss in the muffler. Also pre-processing methodology from CFD tool also explained. The advantages of using HyperMesh 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 developed FE model from HyperMesh and CFD tool are compared and explained in detail in this study.

Introduction

Internal combustion engines are typically equipped with an exhaust muffler to suppress the acoustic pulse generated by the combustion process. A high intensity pressure wave generated by combustion in the engine cylinder propagates along the exhaust pipe and radiates from the exhaust pipe termination. The pulse repeats at the firing frequency of the engine which is defined by f=(engine rpm x number of cylinders) /120 for a four stroke engine. Exhaust mufflers are designed to reduce sound levels at these frequencies. The purpose of an automotive muffler is to reduce the engine noise emission. If vehicles did not have a muffler, there would be an unbearable amount of engine exhaust noise in the environment. The exhaust system contributes 32% of the total noise emitted from the vehicle. Table1: Noise contributors of vehicle

Contributors Engine Exhaust Intake Driveline Fan&

radiator

Other structural

components

Miscellaneous

Noise 21% 32% 14% 14% 6% 10% 3%

Transmission loss The transmission loss is defined as the ratio between the sound power incident to the muffler (Wi) and the transmitted sound power (Wt) for the case that there is a reflection free termination on the downstream side.

TL = 10 log (Wi / Wt)

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The transmission loss and backpressure are the major considerations in the design of exhaust system. They are not compromised to each other. If the transmission loss increases, the backpressure value also increases and it leads to affect the performance of the vehicle.

Figure.1:Transmission loss of a muffler

Pre-processing methodology using CFD tool

Pre-processing for CFD analysis involves the creation of surface meshes and setup of boundary conditions for thermal analysis and heat transfer modelling. Meshing is a key step to creating accurate model, correct mesh continuity and mesh density are needed to efficiently compute results and capture the boundary layer effects. The quality of CFD solution is depending on the quality of the underlying volume mesh.

Basic volume mesh Types

1. Trimmed Mesh 2. Tetrahedral mesh 3. Polyhedral mesh

CFD 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

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CAD geometry:

CAD systems and their geometric representations have been around for quite some time. Almost all CAD systems have involved into similar representations for their models. One of the most important aspects to mesh Generation is accessing CAD geometry. CAD systems often use relatively large tolerances on basis to provide model operations. This approach is referred to as variable tolerances and modelling by different CAD systems. The use of these large variable tolerances produces gaps and overlaps in the geometry and topology of the CAD system.

Figure .3: CAD geometry Model

The major issue with CAD geometry access for mesh generation is the need to understand the analysis requirements. An appropriate mesh and geometry is to be used for meshing. Meshing is a function of the analysis to be performed and the desired accuracy. There does not exist an optimal mesh independent of the analysis to be performed. The element shape quality test for good mesh independent of the analysis is to be performed for the accuracy desired. The appropriate mesh is one, that produces the desired accuracy for the problem to be solved. Surface Wrapping:

When preparing a model for a CFD analysis, surfaces need to be closed in order to generate mesh. The process 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 internals

Checking the imported surface will allows us to choose a course of action required for generating a volume mesh.(e.g.) if surface is free from errors and has good quality triangulation, we can directly proceed to volume mesh generation. But if the surface has errors, then we have to use either manual repair tools or surface wrapping to fix errors.

Surface Remesher

The surface remesher is used to improve the overall quality of the surface and optimize it for the volume mesh models. Specific boundaries can also be omitted from the process so that the original triangulation from the imported mesh can be improved. The surface remesher is typically used for remeshing surfaces produced by the surface wrapper and STL data as well as improving the surface for the volume mesh. It also aids the subsurface generator when the prism mesher option is selected.

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Figure .5: Surface Remesh muffler model

Volume mesh generation using HyperMesh

Surface 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 to create the muffler with internal components as single volume. For generating tetra mesh, the model should be a single volume. The volume mesh has been developed with Tetramesh option in HyperMesh and element quality parameters such as tetra collapse is to be checked. If Glasswool is inserted into middle chamber of muffler, it should be in a separate collector.

Figure .6: Volume mesh model of muffler from HyperMesh for CFD surface remesh model

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LMS Virtual Lab CFD Muffler Model Setup

The complete volume muffler imported to the LMS Virtual Lab work bench, the inputs and measuring conditions are feed as per the procedure

Figure .7: LMS Virtual Lab Work Bench

Transmission Loss Result Graph(CFD Mesh Muffler)

Figure.8: TL plot for CFD model

Introduction to HyperMesh

Altair Engineering HyperMesh is a commercial Pre-processing tool for most of the FEA and CFD solver codes. It is used to develop surface, solid and hybrid mesh with 1D, weld, rigid, mass element connections for different types of geometries. Also it is used to develop various solver specific input deck i.e. defining material properties, element properties, loads, boundary conditions and solver settings for performing different CAE analysis such as static linear and Non-linear, modal, thermal, dynamic, buckling, crash, NVH and impact analysis. With the use of batch meshing, in-built templates and macros, our pre-processing time has been reduced in order to quickly analyze the products for design modification and optimization. This will reduce the product development time to minimize the cost and effort.

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Pre-processing methodology using HyperMesh

Figure.9: HyperMesh Pre-processing methodology

Geometry simplification

CAD model of exhaust muffler is imported into HyperMesh. Geometry cleanup activity such as removing of unwanted lines, duplicate surfaces, pinholes and small holes has been done. Midsurface extraction of each components has been done and baffle plate is merged with muffler shell by trim operation. Also all the weld gap between pipe to end plate, baffle plate are merged to have node-to-node connectivity at the time of shell

Import CAD geometry

Geometry cleanup and Mid surface Extraction

Merging of baffle surface and pipe weld surface by trimming operation

Shell meshing of muffler components

Solid meshing of internal pipes with perforations

Organizing of component collector for perforation surface on pipe and baffle plate, pipe outer surface and baffle surface

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

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meshing. Surface to be created at both ends of the inlet pipe, outlet pipe and middle pipe to have enclosed volume.

Figure .10: Muffler Geometry simplification With Internals

Perforated Tube;

Perforated tubes in the muffler as used to escape are deliver the hot exhaust gases through the perforation holes in the tube.The hole diameter and numbr of perforations are calculated as per the muffler volume and requirment. The perforation need have to proper spacing between the each hole for better performance.Here perforations are closed with the creation of surfaces and it should be in separate collector.After geometry simplification,perforation surfaces are to be meshed with shell elements with minimum element length to have more number of nodes on the surface to achieve proper flow of gases and minimize noise level.

Perforated tube

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Figure.11: Perforated Tube and Perforations

Perforated Baffle

Figure.12: Perforated Baffle and Perforations

Baffle plates have many design uses in general in different applications. However, If baffles are created to the pressurized conditions required within our systems design for proper distribution, and flow rate throughout our system. Many baffles are adjustable and some are fixed. Air noise is something that baffles are used to help reduces the backpressure during the hot gas passing through the muffler. Perforated baffles are used to expand the exhaust gases from one room to another room through the use of their perforations. Perforated and non-perforated baffle plates are merged with muffler shell with the use of trim operation.

Shell meshing

Shell meshing of muffler components such as muffler shell, end plates, baffle plates and internal perforated pipes are done with shell elements. Perforation surface are meshed and kept as a separate collector. The muffler assembly meshed model is checked for free edges to achieve enclosed volume. This enclosed volume is necessary to create the solid or volume mesh.

Perforations

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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, the each room should be created as single volume with their respective internals for that the inlet and outlet openings must be closed for creating tetra mesh.

Solid meshing

Initially 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 baffle plates and end plates inside the muffler. Once the volume mesh for each internal pipes are completed, the model is carefully selected for developing the solid mesh for each chamber. Here chamber1 components are considered 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 for developing solid mesh. Likewise chamber3 components are baffle plate2 and plate2 and internal pipes in between baffle plate and end plate are taken for creating solid mesh.

Figure.14: Muffler Solid Mesh from HyperMesh with Wireframe Model

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Development of FE model for 3D Acoustic solver

After creating solid mesh, node sets are created for baffle plates shell mesh without considering perforation surface and internal pipes without perforations.3D Detaching of created node sets for baffle plates and perforated pipes from tetra mesh of each chambers are to be done to have flow of gases in all perforations in baffle plate and internal pipes. Then the model is finally checked for whether the connectivity has been removed 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 for importing it into 3D acoustic simulation solver. The cut section view of the model is verified for correction or proceeds 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)

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Transmission Loss Measurement from Test Lab

NVH test lab setup used to measure the transmission loss for Muffler/Resonators. the equipment which is having 2 quarter inch Microphones and 2 half inch microphones and Up stream, Downstream impedance tube. The transmission loss setup to measure TL by 2 load conditions are i.e.1.Rigid load and 2.Anechoic Termination. 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

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Transmission Loss Result Graph (Test Lab)

Figure.20: TL curve for Experimental method

CFD Muffler TL Vs Test Lab TL Comparison Result

Figure.21: TL comparison curve for CFD model Vs Test Lab

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HyperMesh TL Vs NVH TL Comparison Result

Figure.22: TL comparison curve for HyperMesh model Vs Test Lab

HyperMesh TL Vs CFD Muffler TL Comparison Result

Figure.23: TL comparison plot for HyperMesh model Vs CFD model

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HyperMesh, CFD and NVH Muffler TL Comparison Graph

Figure.24: TL comparison plot for CFD, HyperMesh model Vs Test lab

Results and Discussion

1) In this paper, FE model development of muffler from Altair's 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 Summary

Altair's Pre-processing tool HyperMesh is effectively used to develop FE model of the muffler assembly for calculating transmission loss in the muffler. Also it is used to develop the volume mesh for the developed CFD shell model .HyperMesh is also used for surface repair and trimming operation to have nodal connectivity at the time of meshing.

HyperMesh pre-processing methodology helps in reducing the pre-processing time for muffler assembly with good quality mesh control and element quality parameters such as minimum element length, tria element maximum and minimum angles and tetra collapse are achieved properly to get better correlated results of transmission loss using 3D acoustic simulation solver.

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Challenges

The 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 as through hole and suppose if any internal pipes are passing through half of the chamber has been considered as 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 each perforation on baffle plate and internal pipes has been done by 3D detaching of volume mesh from already created 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 simulation tool, internal components of muffler are not visible if we are viewing in wireframe mode or cut section view.

Future Plans

Altair's HyperMesh pre-processing methodology has to be implemented for different kind of muffler assembly such as glass wool packed muffler ,integrated muffler (in-built catalytic converter),in-built resonator type mufflers to determine transmission loss in the muffler using 3D acoustic simulation tool to achieve correlated results with experimental test setup i.e. Impedance tube.

Conclusion

It has been concluded that Transmission loss of muffler for developed FE model from HyperMesh and CFD tool 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 3D acoustic simulation solver and also method for reducing the meshing time has been explained in this paper.

Acknowledgement

The authors would like to thank Jose bright, Gokulraj, Madhan and Ashwini and Sharda Motor R&D team members for providing constant support through the project.

Reference

1) Steven J.Owen(1996),A Survey of Unstructured Mesh Generation Technology

2) Senesh.K, Balasubramaniam.P, sudhir Kumar.B(1998) HyperMesh Customization to Manage

FEM data For Custom Hybrid Thermal-Deflection Solver

3) CFD software users manual

4) Sandeep Palaksha, Abhijith Balakrishanan(1997) Modelling CFD specific Domain Using HyperMesh

for Display Case used in the Retail Industry

5) Z.Tao and A.F.Seybert " A review at current Techniques for Muffler Transmission loss measurement".