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Published Articles

The Volume 10, No 1, March 2005

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A Coupled FEM-BEM Formulation in Structural Acoustics for Imaging a Material Inclusion

Abhijit Sarkar, Venkata R. Sonti and Rudra Pratap


A rectangular plate, point supported at four corners, is irradiated with a normally incident acoustic wave. The plate has an inclusion embedded in it supposedly at an unknown location. A structural-acoustic coupled formulation using FEM and BEM is developed and the inclusion is imaged (located) from the surface displacement response of the plate. The efficacy of the formulation is demonstrated by varying the location, size, geometry, and material properties. For variations in location, size, and geometry of the inclusion, the method is seen to give satisfactory results. The frequency of the incident wave is limited to 4775 Hz due to the available computing power. This limits the smallest size and clarity of the image for certain cases. For some cases where the material properties are widely different, and where one would expect the results to distinguish the two materials clearly, the image clarity is less than satisfactory. A list of factors is presented for possible future work.

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Shape Optimisation on Constrained Single-Chamber Muffler by Using GA Method and Mathematical Gradient Method

Ying-Chun Chang, Long-Jyi Yeh and Min-Chie Chiu


Shape optimisation to maximise a muffler?s performance is essential when the space volume of the muffler is constrained for the necessity of maintenance and operation in practical design work. This paper presents numerical optimal studies on single chamber mufflers. Additionally, two numerical approaches, the genetic algorithm (GA) method and mathematical gradient searches, are applied to muffler design work. In this paper, the exterior penalty function method (EPFM), interior penalty function method (IPFM), and feasible direction method (FDM) all belong to mathematical optimal searches. The four-pole transfer matrix was used in evaluating the sound transmission loss (STL) in conjunction with the numerical techniques. Furthermore, a numerical case of noise elimination of pure tone noise was studied. Before optimisation, a simple example was tested and compared with the experimental data in order to check the accuracy of the mathematical model. Results reveal that multiple local optimal solutions exist with respect to the GA and gradient methods. The optimal values of STL in the GA and the gradient method are found to be the same. Consequently, we investigated GA parameters and optimal muffler shapes that produced various pure tones. From an economics point of view, GA is indeed a good tool that provides a quick and effective approach in optimising the shape of single-chamber mufflers.

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Experimental Research of Aerodynamic Noise Induced by Condenser of Drying Machine

Nikola Holecek, Brane Sirok, Marko Hocevar, Rudolf Podgornik


In this paper analyse the noise of aerodynamic sources induced by a drying machine condenser. Two condenser versions of identical external geometry that differ in the form of the flow channel in the direction of the secondary flow, were studied. We analysed the integral and local aerodynamic and acoustic characteristics of both condenser types. The measured far-field sound frequency spectra show essential differences between the two cases, discernible in the form and location of specific peaks superimposed on a generic power-law frequency spectrum. Recent theoretical advances in the Lighthill acoustic analogy were used to identify the generic part of the spectrum due to isotropic turbulence. The specific peaks in the spectrum are apparently due to different structural characteristics of the two studied versions of the condenser types. The measured characteristics of the power spectra were used to propose optimisation strategies in the design of the acoustic properties of the drying machine condensers.

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Nonlinear Dynamic Analysis of a Complex Rotor Bearing System

S.P. Harsha and P.K. Kankar


The paper deals with the structural dynamic response of rotor-supported ball bearings. The mathematical model takes into account the sources of non-linearity such as the Hertzian contact force, surface waviness, varying compliance and internal radial clearance. Due to these sources there exists a transition from a non-contact to a contact state between the rolling elements and races. The effects of speed of the balanced rotor in which the ball bearings show nonlinear dynamic behaviour are analysed. The results are presented in the form of Fast Fourier Transformation (FFT), force time diagrams and phase plots. It is implied from the frequency spectra that the peak amplitude of vibrations appears at a varying compliance frequency.

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Demonstrations of Acoustic Re-radiation from A0 and S0 Waves on Submerged Elastic Shells

P.K. Raju, A.C. Ahyi, H. Cao, H. Uberall


Elastic waves propagating on thin shells may be classified similarly to Lamb waves on plates (A0, S0,A1, S1, ?) and Scholte-Stoneley waves (A) in the external fluid loading. The present study deals with evacuated shells of semi-infinite extent with a uniformly-curved front surface, on which acoustic pulses are incident head-on. The incident signals cause the generation of the shell waves at a critical angle of incidence; these may be observed by their re-radiation into the fluid at the same critical angle. Our demonstration of the re-radiated pulses consists of a numerical evaluation of the incident and re-radiated fields and a visualisation of the corresponding pulses in a tank experiment employing the Schlieren method. While the weaker A wave could not be observed here, we were able to demonstrate by both methods the generation of re-radiated pulses of the A0 and the S0 wave, at the same time verifying the value of the critical angle of their generation.

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