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Interior Noise Reduction of CAB using RADIOSS (implicit)

Kiran Kumar Pedamallu Sr. Analyst CAE

Mahindra Engineering Services, Pune

Manoj Purohit Lead Analyst CAE

Mahindra Engineering Services Pune

Dr. D.W.Pande Dean R & D

College of Engineering Pune

Keywords: vibro-acoustic, Interior Noise

Abstract The structure born noise plays a very important role in the ride and comfort feeling for a person riding a vehicle. NVH studies have been mainly concentrated for the passenger cars and SUV's and less has been done for improvements for commercial vehicles. However, looking at the extended period of driving for the commercial vehicles, improvement in driver’s comfort from NVH point of view is very important considering safety aspects. This paper focuses on the structure borne noise reduction inside the Heavy Commercial Vehicle (HCV) cab using RADIOSS (Implicit) to perform fully coupled vibro-acoustic analysis. Functionalities available in HyperWorks NVH manager were evaluated for pre/post processing of the NVH models. The paper describes the process found useful for identifying the critical frequencies, and contribution of different panels at the critical frequencies. Aspects important to help optimization to reduce interior noise and process of optimization are also discussed.

Introduction

The structure borne noise are limited to those that consist of an enclosed acoustic fluid cavity(air), which is

coupled to a flexible structure and/or a porous sound absorbing material domain, detailed structure-acoustic

analyses need to be performed. The interior noise comfort can also be in conflict with other vehicle

properties such as safety (crashworthiness), so the design process must be conducted in an integrated

fashion that addresses various vehicle properties such as safety, reliability and comfort in the process.

The project focuses on structure borne noise reduction of the commercial vehicle cab in comparison

with the benchmark vehicle available in the market, we are using the available Finite element methods

(Vibro-Acoustic) for the perdition the interior noise, Sound Pressure Level (SPL),Different Panel

contributions are identified for the critical frequencies. Areas of improvement are identified in accordance

with the different Panel contributions to the Response. Optimizations of the critical panels are done to

minimize the response (SPL) at the critical frequencies using OptiStruct and feasible design modifications

are suggested for the areas which are contributing to the vibration.

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Process Methodology

Figure 1: Methodology

FE Model Details: The air cavity is generated by using the OptiStruct NVH module ref Figure 2 .The air cavity is modeled with existing three-dimensional elements: CHEXA, CPENTA, and CTETRA. Fluid elements are defined on the CHEXA, CPENTA, and CTETRA Bulk Data entries. Also, on the referenced PSOLID entry, the character value PFLUID must be specified for FCTN in field 8 and MID, field 3, must reference a MAT10 material entry. The MAT10 entry defines the bulk modulus and the mass density properties of the fluid. If a PSOLlD entry defines fluid elements (FCTN = PFLUID), then lSOP, field 7, is defaulted to 1 (FULL), resulting in a full integration scheme. The interface between the fluid and the structure may be modeled so that the grid points of the fluid are coincident with those of the structure. This is called a matching mesh. If not, then it is called a non matching mesh. In either case and by default, coupling for the stiffness and mass is automatically computed. The fluid-structure interface is determined automatically by the ACMODL card. A non matching interface is defined by entering DIFF in the INTER field and either CP or BW in the METHOD field on the ACMODL Bulk Data entry. The wetted structural elements are determined by comparing grid point locations corresponding to structural elements that are within a tolerance specified by NORMAL on the ACMODL entry.

Figure 2: Acoustic modal

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Material Details: Linear elastic (Isotropic) material properties were used for the CIW and the Air cavity. Simulation Process: The process includes 3 stages of simulation

1. Normal modal analysis 2. Coupled Modal Frequency Response analysis 3. Optimization.

The Eigen valves and Normal modes are extracted up to 200HZ both for the fluid and structure.

The fluid modes are correlated by the Analytical formula available in the literature just by the dimensions of the acoustic cavity as shown below. This is patch test for the proper coupling of the Fluid and structure.

The Fluid modes can be seen in the figure 3.

Coupled Modal Frequency response method is used for the analysis of the fluid- structure system. A constant acceleration (1G) Harmonic excitation across the frequency range of 10Hz to 200Hz is given as the input to the mountings of the Cab. The Harmonic excitation is given as the Enforced displacement to the mounting of the brackets in the respective directions. The Model damping of the system is assumed to be 4% of the critical damping across the frequency range apart from the structural damping for the structure and the air cavity. The sound Pressure level (SPL) at the Drivers ear is measured across the frequency range as shown in the figure 4. The post process of the other parameters like the mode participation factors, panel participation factors are also extracted in the response analysis and prost processed in the NVH utilities to investigate the critical frequencies and the different panel contributing to the critical frequencies as shown in the figure 5. Topography optimization is done on the identified panel which is contributing to the interior noise at the critical frequency. The beads pattern and the size are provided in the topography optimization panel in HyperMesh. The responses are created for Frf displacement at the critical frequency in the dominant direction on the node on the contributing panel at the frequency. The constraint is provided on the displacement magnitude at the critical frequency.

Figure 3: Acoustic mode Shapes

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Figure 4: SPL at driver ear

Figure 5: Modal Participation and Panel Participation

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Figure 6: Optimized Displacement magnitude at first frequency

Results & Discussions The Post processing NVH Utilities are helpful in diagnosing the behavior and identifying the structural panel contributing to the SPL at the Drivers ear for the critical frequency. The sound pressure level can be reduced at the critical frequency with the optimized structure panel. Future Plans A Multi Objective Optimization (MOO) is planned to get optimized structure for Acoustic response without compromise of durability and crash requirements Conclusions

Once the process and the important dynamic parameters of the system are established with testing and simulation. A number of design iterations can be carried out with much lesser time to get the best optimum design for the Cab structure in terms of interior noise.

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Acknowledgements

The authors would like to thank, Chinmay Pawaskar and Deepak Nidgalkar for their guidance in accomplishing the work and to publish the paper.

REFERENCES

[1] Matthew Harrison, (2004) “. Vehicle Refinement: Controlling Noise and Vibration in Road Vehicles”, SAE International and Elsevier

[2] Tyrrell, R J, (2004) “How To Get Started in Acoustics Analysis”, NAFEMS [3] Wu, G., Shi, W., Yang, W., Chen, Z. et al.(2012) "Structure Optimization and Interior Noise Reduction of Commercial Vehicle

Cab," SAE Technical Paper 2012-01-1928. [4] C.R. Fredö, Anders Hedlund , (2005) “NVH optimization of truck cab floor panel embossing pattern” , SAE paper 2005-01-2342