• Journal of Aerospace System Engineering
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• v.15 no.4
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• pp.40-46
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• 2021

At launch, satellites are exposed to various types of structural loads, such as quasi-static loads, sinusoidal vibrations, acoustic/random vibrations, and shocks. The launch environment test is aimed at verifying the structural stability of the test object against the launch environment. Various types of launch environments are simulated by simple vibration, acoustic, and shock tests considering possible test conditions in ground. However, the difference between the launch environment and the test environment is one of the causes of excessive testing. To prevent overtesting, a notching technique that adjusts the frequency range and the input load considering the design load is applied. For notching, specific procedures are established considering the satellite development concept, selected launch vehicle, higher system requirements, and test target level. In this study, the notching method, established procedure, and development of a notching toolkit for efficient testing are described.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.22 no.7
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• pp.676-684
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• 2012

Numerical analysis was performed to investigate the heavy-weight impact noise of apartment houses. The FEM is practical method for prediction of low-frequency indoor noise. The results of numerical analysis, the shape of the acoustic modes in room-2 are similar to that of acoustic pressure field at the fundamental frequency of acoustic modes. And the acoustic pressure was amplified at the natural frequency of the acoustic modes and structural modes. The numerical analysis result of sound pressure level at 63 Hz and 125 Hz octave-band center frequency are similar to the test results, but at 250 Hz and 500 Hz have some errors. Considering most of bang-machine force spectrum exists below 100 Hz, the noise at 250 Hz and 500 Hz are not important for heavy-weight impact noise. Thus, the FEM numerical analysis method for heavy-weight impact noise can apply to estimate heavy-weight impact noise for various building systems.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.05a
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• pp.996-1000
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• 2002

A porous tube, comprised of a resin-coated woven fabric has recently been used as an effective component for use in intake systems of internal combustion engines to reduce the intake roaring. For the prediction of the acoustic performance of an engine intake system with a porous woven hose, the acoustic wall impedance of the hose must be known. Because of its peculiar acoustical and structural characteristics, the accurate measurement of the wall impedance ofa porous woven hose is not easy. A new measurement technique is proposed herein, that is valid over the low to mid frequency ranges. The acoustics impedance is inversely estimated from an overdetermined set of measured pressure transmission coefficients for specimens of different lengths and the reflection coefficient of end termination. The method involves only one measurement, and, as a result, it is very simple. The measured TL for samples with arbitrary conditions, arbitrary porous frequency, arbitrary length, and arbitrary mean flow condition, are in reasonably good agreement with values predicted from curve-fitted impedance data.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.14 no.11
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• pp.1083-1090
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• 2004

Two noise reduction systems are proposed in order to overcome the geometric restriction of the reactive muffler such as an expansion chamber. First, membrane is installed as a part of a duct wall and an air cavity is covered outside membrane. Second, membrane is installed inside a duct, which gives no volume change of the duct. Structural-acoustic coupling between membrane and fluid inside the cavity and duct causes rapid impedance mismatching and thereby reflected wave. Theoretical prediction is conducted by using modal expansion approach. The results are compared with the experimental results, which show better noise reduction performance than an expansion chamber.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.05a
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• pp.474-479
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• 2004

Two noise reduction systems are proposed in order to overcome the geometric restriction of the reactive muffler such as an expansion chamber. First, membrane is installed as a part of a duct wall and an air cavity is covered outside membrane. Second, membrane is installed inside a duct, which gives no volume change of the duct. Structural-acoustic coupling between membrane and fluid inside the cavity and duct causes rapid impedance mismatching and thereby reflected wave. Theoretical prediction is conducted by using modal expansion approach. The results are compared with the experimental results, which show better noise reduction performance than an expansion chamber.

• Current Optics and Photonics
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• v.2 no.1
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• pp.59-68
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• 2018

Opto-acoustic systems provide structural and functional information regarding biological tissues. Conventional opto-acoustic systems typically employ continuous or pulsed lasers as transmission sources. Compared to lasers, light emitting diodes (LEDs) are cost-effective and relatively portable excitation sources but are non, coherent. Therefore, in this study, a relatively low cost lens - a type of Ramsden eyepiece - was specially designed to theoretically calculate the illumination and achieve a constant brightness across the pupil of an eye. In order to verify the capability of the developed light-emitting diode-acoustic (LEDA) systems, we carried out experiments on bovine and bigeye tuna eyeball samples, which are of similar size to the human eye, using low frequency (10 MHz) and high frequency (25 MHz) ultrasound transducers. High frequency ultrasound transducers are able to provide higher spatial resolution compared to low frequency ultrasound transducers at the expense of penetration depth. Using the 10 MHz and 25 MHz ultrasound transducers, acceptable echo signals (3.82, 3.94, and 5.84 mV at 10 MHz and 282, 1557, 2356 mV at 25 MHz) from depth greater than 3 cm and 6 cm from the anterior surface of the eye were obtained. We thereby confirmed that the LEDA system using a pulsed LED with the designed Ramsden eyepiece lens, used in conjunction with low and high frequency ultrasound transducers, has the potential to be a cost-effective alternative method, while providing adequate acoustic signals from bovine and bigeye tuna ocular areas.

• Advances in aircraft and spacecraft science
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• v.2 no.4
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• pp.403-425
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• 2015

This work is focused on the definition and the analysis of both complete and incomplete similitudes for the dynamic responses of thin shells. Previous numerical and experimental investigations on both structural and structural-acoustic systems motivated this further analysis, mainly centred on the incomplete (distorted) similitudes. These similitudes and the associated scaling laws are defined by using the classical modal approach (CMA) and by invoking also the Energy Distribution Approach (EDA) in order to take into account both the cinematic and energetic items. The whole procedure is named SAMSARA: Similitude and Asymptotic Models for Structural-Acoustic Research and Applications. A brief summary of the procedure is herein given and the attention is paid to the analytical models of thin stiffened and unstiffened cylindrical shells. By using the well-known smeared model, the stiffened cylinder equations are used as general framework to analyse the possibility to define exact (replicas) or distorted similitudes (avatars). Despite the extreme simplicity of the proposed models, the results are really encouraging. The final aim is to define equivalent models to be used in laboratory measurements.

• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.3
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• pp.237-244
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• 2002

Analyzing acoustic-structural systems such as automobiles and aircraft, the FRF-based substructuring (FBS) method is one of the most powerful tools. In this paper, a general procedure for the parametric sensitivity analysis of vibro-acoustic problems has been presented using the multi-domain FRF-based substructuring formulation. For an acoustic-structural system sub-structured by multiple domains, the substructuring formulation gives the reaction farces on the interface boundaries. The design sensitivity formula is obtained from the direct differentiation of the reaction force expression with respect to the design vector. As a practical application, the proposed design sensitivity formula is applied to an engine mount system of passenger car. An objective of the problem is to identify the most effective engine mounts and bushes in minimizing the interior noise over the concerned rpm range. The comparison of the sensitivity results with those of the finite difference method shows excellent agreement. In addition, stiffness modifications of the mounts and bushes identified through the design sensitivity analysis lead to a successful decrease of the interior noise. This results show usefulness of the present method very well.

• Smart Structures and Systems
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• v.4 no.5
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• pp.565-582
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• 2008

This paper presents a feasibility study of flexible piezoelectric paint for use in wide-band low-profile surface-mount or embeddable ultrasonic sensor for in situ structural health monitoring. Piezoelectric paint is a piezoelectric composite with 0-3 connectivity. Because of its ease of application, piezoelectric paint can be readily fabricated into sensing element with complex pattern. This study examines the characteristics of piezoelectric paint in acoustic emission signal and ultrasonic guided wave sensing. A series of ultrasonic tests including pitch catch and pencil break tests were performed to validate the ultrasonic wave sensing capability of piezoelectric paint. The results of finite element simulation of ultrasonic wave propagation, and acoustic emission generated by a pencil lead break on an aluminum plate are also presented in this paper along with corresponding experimental data. Based on the preliminary experimental results, the piezoelectric paint appears to offer a promising sensing material for use in real-time monitoring of crack initiation and propagation in both metallic and composite structures.

• Nuclear Engineering and Technology
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• v.47 no.2
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• pp.196-203
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• 2015

The aim of this work is to demonstrate a method for exciting and receiving torsional and longitudinal mode guided waves with an electromagnetic acoustic transducer (EMAT) ring array. First of all, a three-dimensional guided wave simulator is developed in order to numerically analyze the propagation of the guided wave. The finite difference time domain method is used for the simulator. Second, two guided wave testing systems using an EMAT ring array are provided: one is for torsional mode (T-mode) guided wave and the other is for longitudinal mode (L-mode). The EMATs used in the both systems are the same in design. A method to generate and receive the T- and L-mode guided waves with the same EMAT is proposed. Finally, experimental and numerical results are compared and discussed. The results of experiments and simulation agree well, showing the potential of the EMAT ring array as a mode controllable guided wave transmitter and receiver.