• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.48 no.9
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• pp.725-730
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• 2020

This study was conducted to reduce the computation time required for the computational acoustic analysis of the supersonic rocket jet plume. In order to reduce the computation time, computational acoustic analysis was performed assuming that the supersonic jet plume is a two-dimensional axis-symmetric problem. The results of computational acoustic analysis showed similar results to the acoustic load measurement results. Through this study, it was confirmed that the acoustic load prediction of the supersonic rocket jet plume can be predicted using a two-dimensional axis-symmetric computational analysis.

• Journal of the Korean Ceramic Society
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• v.40 no.4
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• pp.393-397
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• 2003

To investigate the effect of AIN c-axis orientation on the resonance performance of film bulk acoustic wave resonators, solidly mounted resonators with crybtallographically different AIN piezoelectric films were prepared by changing only the bottom electrode surface conditions. As increasing the degree of c-axis texturing, the effective electromechanical coupling coefficient ($\kappa$$\_$eff/)$^2$ in resonators increased gradually. The least 4 degree of full width at half maximum in an AIN(002) rocking curve, which corresponds to $\kappa$$^2$$\_$eff/ of above 5%, was measured to be necessary for band pass filter applications in wireless communication system. The longitudinal acoustic wave velocity of AIN films varied with the degree of c-axis texturing. The velocity of highly c-axis textured AIN film was extracted to be about 10200 n/s by mathematical analysis using Matlab.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.16 no.8
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• pp.716-722
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• 2003

ZnO thin films were deposited on Al and Pt electrodes by an RF reactive sputtering system for the fabrication of FBAR (film bulk acoustic wave resonator), and the effect of thermal treatment temperature on their c-axis preferred orientation was investigated. SEM experiments show that columnar structure of ZnO thin films were grown with c-axis normal to electrode material, and XRD experiments show that both ZnO films were grown with (002) plane preferred orientation, but larger diffraction peak was observed with Pt electrode. The peak intensity increased with higher thermal treatment temperature, but c-axis preferred orientation was diminished. The surface roughness of Al thin film was higher than that of Pt, and these affect the surface roughness of ZnO film deposited on the electrode. Though the preferred orientation with respect to Pt(111) plane was improved with higher thermal treatment temperature, this could not improve the c-axis orientation of ZnO film.

• The Journal of the Acoustical Society of Korea
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• v.36 no.3
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• pp.165-171
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• 2017

The effect of the concentric solid tube inserted inside the vibrator on the sound field distribution was analyzed for the sound waves focused on the center axis in the fluid - filled cylindrical piezoelectric transducer. The sound waves radiated from the inside of the cylindrical piezoelectric vibrator are transmitted through the fluid medium and are reflected or transmitted on the wall surface of the solid tube, and are focused on the central axis. At this time, the sound field distribution centered on the acoustic tube varies depending on the acoustic impedance and the thickness of the solid tube. In order to theoretically analyze this, the transfer matrix for each medium is derived, and the sound pressure level at the center axis is theoretically analyzed. For the acrylic tube with various thicknesses, the changing trend in the sound pressure level measured on the central axis agrees well with the result of the theoretical analysis, and it confirmed that the sound pressure formed at the center changes very sensitively with the thickness of the solid tube.

• 한국신재생에너지학회:학술대회논문집
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• 2009.06a
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• pp.504-507
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• 2009

The purpose of this work is to predict the low frequency aero-acoustic noise generated from the horizontal axis wind turbine, NREL Phase VI using large eddy simulation and Ffowcs-Williams and Hawkings model provided in the commercial code, FLUENT. Calculated aerodynamic performances such as shaft torque and power are compared with experimentally measured value. Performance results show a good agreement with experimental data within about 0.8%. If the distance by two times is changed from 32D to 64D toward the downstream region, sound pressure level is reduced by about 6.4dB.

• Journal of Sensor Science and Technology
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• v.4 no.2
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• pp.45-50
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• 1995

The excellent c-axis oriented zinc oxide thin films were prepared by the RF magnetron sputtering method on glass substrates. Optimum fabrication conditions of the ZnO films were such that RF power, substrate temperature, and gas pressure of mixture Ar(50%):$O_{2}$(50%) were 150 W, $200^{\circ}C$, and 5 mTorr, respectively. In these conditions, the deposition rate was $310\;{\AA}/min$, and the resistivity of the film was $1{\times}10^6\;{\Omega}{\cdot}cm$. The ZnO film also showed high c-axis orientation and crystalinity according to XRD pattern and SEM photograph. A fabricated interdigital transducer generated 1st mode surface acoustic wave at 46.6 MHz and 2nd mode surface acoustic wave at 52.5 MHz. At the 1st mode, the phase velocity of surface acoustic wave and the electromechanical coupling coefficient were 2795 m/sec and 0.031 %, respectivly. At the 2nd mode, they were 3149 m/sec and 0.019 %. respectivly.

• The Journal of the Acoustical Society of Korea
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• v.33 no.6
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• pp.351-357
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• 2014

In this paper, flow noise characteristics of the hybrid vertical-axis wind turbine is investigated. Hybrid vertical-axis wind turbines consisting of two types of vertical-axis wind turbines, Savonius and Darrieus, are devised to maximize merits of one turbine and thus minimize demerits of the other turbine. In order to predict flow noise radiating from hybrid vertical-axis wind turbines, hybrid computatioinal aero acoustic techniques are used. First, unsteady flow fields around the turbine are predicted using computational fluid dynamics method. Then, the flow noise radiations from the turbines are predicted by applying acoustic analogy to the predicted flow fields. Based on numerical results, noise characteristics of a hybrid vertical-axis wind turbine is investigated and is compared with those of Savonius and Darrieus wind turbines.

• Journal of KSNVE
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• v.3 no.2
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• pp.169-178
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• 1993

Internal & External duct acoustic fields are calculated by using a finite element method. The geometry is assumed as an axisymmetric duct. External acoustic field; outside the duct, and combined internal & external acoustic fields are solved. For both cases a far field's nonreflecting boundary condition is enforced by using a wave envelope element, which is a kind of finite element. First, a pulsating sphere and an oscillating sphere problem are calculated to verify the external problems, and the results are compared with exact solutions. When the wave envelope element is applied at the far boundary, the calculated finite element solutions show good agreements with the exact solutions. Secondly, the combined internal & external duct acoustic fields are calculated and visualized when monopole sources are distributed inside the duct. It is observed that the directivity of sound intensity outside the duct is beaming toward the axis for high frequency sources.

• The Journal of the Acoustical Society of Korea
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• v.39 no.5
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• pp.400-405
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• 2020

To analyze the flow distribution formed by multiple acoustic sources, the distribution of acoustic streaming speed caused by an ultrasonic transducer composed of two identical piezoelectric vibrators was examined for various angles between the sound sources. In order to measure the distribution of the speed along the acoustic axis of the transducer, a simple measurement method using a droplet indicator having density similar to that of water is suggested. The simulation results calculated by a numerical method and experimental results showed a similar tendency, and the change of flow speed distribution with the intersection angle between acoustic beams radiated from two acoustic sources was analyzed.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.56 no.5
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• pp.905-909
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• 2007

Crystalline and micro-structural characteristics of ZnO thin films which were deposited on p-Si(100) with cooling naturally down of pre-heated substrate during RF magnetron sputter deposition, were investigated by XRD and SEM in this paper. The film which was prepared on the substrate which was pre-heated to $400^{\circ}C$ before deposition and then cooled naturally down during deposition, showed the most outstanding c-axis preferred orientation. The ZnO thin film having the best crystalline result were applied to SMR type FBAR device and resonance properties of the device were investigated by network analyzer. It showed that resonance frequency was 2.05 GHz, return loss was -30.64 dB, quality factor was 3169 and electromechanical coupling factor was 0.4 %. This deposition method would be very useful for application of surface acoustic wave filter or film bulk acoustic wave resonator.