• 한국소음진동공학회논문집
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• 제27권1호
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• pp.36-42
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• 2017

Helical gear excitation is transmitted to a gearbox through the shafts and bearings and the vibration of the gearbox radiates the noise in the air. Therefore gearbox modeling is essential to evaluate the gear noise. This work deals with vibration and acoustic analysis of a cylindrical shell-type gearbox with/without holes excited by helical gears and focuses on the development of the simple gearbox model. To do so, helical gears and bearing forces are calculated. Gearbox with/without holes is modeled by the aluminum end plates and PMMA cylindrical shell body. The vibration mode and the forced harmonic response were calculated by the commercial FE software and the end plate of the gearbox is more contributed to vibration than the body. Acoustic analysis was also conducted by the commercial acoustic software and a cylindrical shell type gearbox with/without holes has the similar vibro-acoustic characteristics.

• Journal of Advanced Marine Engineering and Technology
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• 제16권5호
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• pp.77-84
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• 1992

In the most of acoustic system with low flow rate and low pressure level, one-dimensional, linear modeling techniques are used very well. At low frequency, the tube is modeled as inertia element and cavity as capacitance element, and to extend the range of frequency normal mode oscillators are represented. Bond graph modelling techniques are proposed to predict TL (Transmission Loss) and time response which is impossible by transfer matrix in muffler system. A simple acoustic filter which consists of several tubes and cawities is analyzed in both time and frequency domain.

• 소음진동
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• 제8권6호
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• pp.1172-1180
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• 1998

음향학적으로 강성인 물체와 연성인 물체로부터의 탄성파의 산란과 음향파의 산란을 수치적으로 해석하고 그 특성들을 비교하였다. 부분파들과. 또한 부분파들의 합인 전체 산란파의 위상 및 크기의 거동을 수치적으로 분석하고 토의하였다. 탄성파의 산란시 발생하는 종파와 횡파 사이의 모드변환 현상이 산란파의 크기와 위상에 주는 영향을 밝혔다.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2005년도 동계학술발표대회 논문집
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• pp.335-340
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• 2005

Nonlinear and forced oscillations of supported viscous droplet were focused in the present study. The droplet has a free contact line with solid plate and inviscid fluid. Natural frequencies of a pendant droplet have been investigated experimentally by imposing the acoustic wave while the frequency is being increased at a fixed amplitude. The evaporation was observed at atmosphere pressure. The droplet was recorded throughout the entire evaporation process and transient variations of the volume was measured. The evaporation process of oscillating droplet with thermofoil has been also observed to investigate analyzing the resonance effect on the thermal characteristics of droplet. It is found that a pendant droplet shows the resonant behaviors at each mode similar to the theoretical analysis. During imposing the acoustic wave, the pendant droplet makes a rotating motion in its longitudinal axis which is a new shape oscillation mode. The evaporation rate of a pendant droplet at resonant frequency is significantly enhanced.

• Smart Structures and Systems
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• 제16권1호
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• pp.213-222
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• 2015

Early detection and precise location of leakage is of great importance for life-cycle maintenance and management of municipal pipeline system. In the past few years, acoustic emission (AE) techniques have demonstrated to be an excellent tool for on-line leakage detection. Regarding the multi-mode and frequency dispersion characteristics of AE signals propagating along a pipeline, the direct cross-correlation technique that assumes the constant AE propagation velocity does not perform well in practice for acoustic leak location. This paper presents an improved cross-correlation method based on wavelet transform, with due consideration of the frequency dispersion characteristics of AE wave and the contribution of different mode. Laboratory experiments conducted to simulate pipeline gas leakage and investigate the frequency spectrum signatures of AE leak signals. By comparing with the other methods for leak location identification, the feasibility and superiority of the proposed method are verified.

• 대한조선학회논문집
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• 제56권1호
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• pp.94-101
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• 2019

Underwater acoustic power should be measured in a free field, but it is not easy to implement. In practice, the measurement could be performed in a reverberant field such as a water-filled steel tank and concrete tank. In this case, the structure and the acoustic field are strongly or weakly coupled according to material properties of the steel and water. So, characteristics of the water tank must be investigated in order to get the accurate underwater acoustic power. In detail, modal frequencies, mode shapes of the structure and frequency response functions of the acoustic field could represent the characteristics of the reverberant water tank. In this paper, the structure-acoustic coupling has been investigated on a reverberant water tank numerically and experimentally. The finite element analysis has been carried out to estimate the structural and acoustical modal parameters under the dry and water-filled conditions, respectively. In order to investigate the structure-acoustic coupling effect, the numerical analysis has been performed according to the structure stiffness change of the water tank. The acoustic frequency response functions were compared with the numerical analysis and acoustic exciting test. From the results, the structural modal frequencies of the water-filled condition have been decreased compared to those of the dry condition in the low frequency range. The acoustic frequency response functions under the coupled boundary conditions showed different patterns from those under the ideal boundary conditions such as the pressure release and rigid boundary condition, respectively.

• International Journal of Aeronautical and Space Sciences
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• 제11권4호
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• pp.319-325
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• 2010

For decades, researchers have rigorously studied the characteristics of flow traveling around blunt objects in order to gain greater understanding of the flow around aircraft, vehicles or vessels. Many different types of flow exist, such as boundary layer flow, flow separation, laminar and turbulent flow, vortex and vortex shedding; such types are especially observed around circular cylinders. Vortex shedding around a circular cylinder exhibits a two-dimensional flow structure possessing a Reynolds number within the range of 47 and 180. As the Reynolds number increases, the Karman vortex changes into a three-dimensional flow structure. In this paper, a numerical analysis was performed examining the flow and aero-acoustic field characteristics around a circular cylinder using an optimized high-order compact scheme, which is a high order scheme. The analysis was conducted with a Reynolds number ranging between 300 and 1,000, which belongs to B-mode flow around a circular cylinder. For a B-mode Reynolds number, a proper spanwise length is analyzed in order to obtain the characteristics of three-dimensional flow. The numerical results of the Strouhal number as well as the lift and drag coefficients according to Reynolds numbers are coincident with the other experimental results. Basic research has been conducted studying the effects an unstable three-dimensional wake flow on an aero-acoustic field.

• 비파괴검사학회지
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• 제27권3호
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• pp.262-267
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• 2007

Detection of gas leak from a tube is a very important issue in the quality control of machines such as the heat exchanger of an air-conditioner, because leakage of operating gas directly reduces the performance of machines. The acoustic emission (AE) method is a common way to detect leak of gas, however its application under the environment of mechanical vibration is restricted since most AE detectors are very sensitive to external vibration noise. In order to overcome this problem, we propose a method based on the mode analysis of the Lamb wave. In this method, the dominant Lamb mode and its frequency are found first, and then a proper band-pass filter is used to retain only this frequency component. In this way, we could improve the SNR (signal-to-noise ratio) of AE signal generated by gas leak from the tube even under vibratory environment.

• 한국음향학회지
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• 제41권1호
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• pp.1-8
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• 2022

본 논문은 마이크로폰 검교정을 위한 음향 챔버형 검교정기의 고주파 검교정 한계가 나타나는 원인을 밝히고 이를 향상시키는 방법을 제시한다. 상용 유한요소 해석 프로그램을 활용하여 고유 주파수 해석과 주파수 영역 해석을 통해 음향 챔버형 검교정기의 검교정 주파수 한계를 분석하였고, 이를 바탕으로 약 2 Hz 부터 6.4 kHz 까지 1 dB 수준 이내로 정교하게 검교정할 수 있는 음향 챔버형 검교정기를 설계 및 제작하고 실험을 통해 그 성능을 검증하였다. 본 연구를 통해 제안된 음향 챔버형 검교정기는 넓은 주파수 범위에서 다수의 마이크로폰을 동시에 검교정 할 수 있는 이점을 가지고 있어 다수의 마이크로폰을 위한 간편한 검교정에 유용하게 사용될 수 있다.

• Journal of Theoretical and Applied Mechanics
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• 제3권1호
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• pp.34-44
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• 2002

Theoretical analysis Is carried out to identify the modal coupling effect between some particular acoustic modes of a vehicle compartment cavity and vibration modes of body panels like side doors, roof or floor. A simplified panel-cavity coupled model is investigated on the coupled resonance frequencies, modes and frequency response characteristics. Through parametric study, It Is possible to explain how the acoustic response of a coupled system will be determined by the vibration and acoustic property of the individual panel and cavity system. Full coupled system shows some interesting features different from those of the semi-coupled system In frequency, mode and acoustic response.