• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1998.03a
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• pp.214-220
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• 1998

The determination of sound pressure radiated from periodic plate structures is fundamental in the estimation of noise levels in aircraft fuselages and ship hull structures. As a robust approach to this problem, here a very general and comprehensive analytical model for prediction the sound radiated by a vibration plate stiffened by periodically spaced orthogonal symmetric beams subjected to a sinusoidally time varying point load is developed. The plate is assumed to be infinite in extent, and the beams are considered to exert both line force and moment reactions on it. Structural damping is included in both plate and beam materials. From this theoretical model, the sound pressure levels on axis in a semi-infinite fluid(water) bounded by the plate with the variation in the loactions of an external using three numerical tools such as the Gauss-Jordan method, the LU decomposition method and the IMSL numerical package.

• The Journal of the Acoustical Society of Korea
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• v.20 no.1E
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• pp.25-30
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• 2001

The unstable signature that is defined as frequency change with respect to the time or frequency modulation, is caused by the external loading variation in specific machinery component and Doppler shift etc. In this study, we analyze the generation mechanism of the unstable signature and apply the Extended Kalman filter (EKF) algorithm for its detection. The performance of Extended Kalman Filter is examined for numerical and measured signals and the results show its validity for unstable signature detection.

• Journal of the Korean Society of Visualization
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• v.20 no.3
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• pp.128-135
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• 2022

Cavitation occurs inevitably in marine propellers rotating at high speed in the water, which is a major cause of underwater radiated noise. Cavitation-induced noise from propellers rotating at a specific frequency not only reduces the sonar detection capability, but also exposes the ship's location, and it causes very fatal consequences for the survivability of the navy vessels. Therefore cavity inception speed (CIS) is one of the important factors determining the special performance of the ship. In this study, we present a method using computer vision that can detect and quantitatively estimate tip vortex cavitation on a propeller rotating at high speed. Based on the model test results performed in a large cavitation tunnel, the effectiveness of this method was verified.

• Journal of the Institute of Electronics Engineers of Korea SP
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• v.37 no.6
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• pp.76-82
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• 2000

To reduce the radiated noise of ships, the noises which are generated from onboard machinery, propulsion system and transfer characteristics of structure must be identified. While the ship is operating, however, we can not directly measure each signal of inputs and characteristics of transfer passage, because measured signals are superimposed by multi source and multi transfer passage. In this paper, the signal processing method for separating noise sources and transfer functions from the measured response signal by the cepstrum technique is proposed. The proposed method is verified by application of simulated signal and impact test and shows usefulness by application of real ship test.

• The Journal of the Acoustical Society of Korea
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• v.43 no.1
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• pp.19-28
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• 2024

This paper proposes a method to improve the performance of ship identification through lofargram analysis of ship noise by applying the Hough Transform to a Convolutional Neural Network (CNN) model. When processing the signals received by a passive sonar, the time-frequency domain representation known as lofargram is generated. The machinery noise radiated by ships appears as tonal signals on the lofargram, and the class of the ship can be specified by analyzing it. However, analyzing lofargram is a specialized and time-consuming task performed by well-trained analysts. Additionally, the analysis for target identification is very challenging because the lofargram also displays various background noises due to the characteristics of the underwater environment. To address this issue, the Hough Transform is applied to the lofargram to add lines, thereby emphasizing the tonal signals. As a result of identification using CNN models on both the original lofargrams and the lofargrams with Hough transform, it is shown that the application of the Hough transform improves lofargram identification performance, as indicated by increased accuracy and macro F1 scores for three different CNN models.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.1185-1191
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• 2000

In general, the strength of hull structures can be estimated from stress evaluation considering static and hydro-dynamic load due to sea-wave. However, war ships such as submarine, have frequently experienced the underwater explosion and local structures of ship as well as hull girder can be damaged by the dynamic response excited from underwater non-contact explosion. When explosion happens at underwater, shock wave is radiated In early short time, then gas bubbles are generated, and expansion and contraction are repeated as they float to the surface. The shock wave causes the damage of equipment and its supporting structures, on the other hand, the hull girder strength can be lost by resonance between bubble pulsation and lowest ship natural vibration period. In this paper, the hydro-Impulse force due to bubble was calculated. Based on these results the hull girder strength of submarine was estimated from transient response analysis by using NASTRAN. Also, shock analysis for some equipment supporting structures was carried out by using DDAM. In order to evaluate the strength of these local structures due to shock wave.

• Journal of the Society of Naval Architects of Korea
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• v.38 no.1
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• pp.108-115
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• 2001

The noise level of the navigation bridge as well as topside of the superstructure is dominated by the exhaust gas noise of the high-powered main engine and generator engine of special purpose vessels. In the case of the noise radiated from the top of the funnel. the exhaust pipe can be fitted with a silencer to reduce the propagated noise level. This paper is prepared based on an experimental performance test results of the silencers for generator-engine exhaust gas noise with consideration of air velocity. Two silencers were examined to check the performance of noise reduction in cases of air velocity 0m/s and 32m/s. In the sound reduction test, 400mm and 600mm diameter pipe ducts equipped with an axial fan were used as exhaust gas pipe system in the actual ship. The test procedure and results are presented in detail.

• Journal of the Korea Institute of Military Science and Technology
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• v.20 no.4
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• pp.505-513
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• 2017

Cavitation is a phenomenon caused by vapour cavities that is produced in rapid pressure changes. When the cavitation happened, the sound pressure level of a underwater radiated noise is increased rapidly. As a result, it can increase the probability of the identification or classification of a our warship's acoustic signature by an enemy ship. However, there is a problem that it is hard to precisely detect the occurrence of a cavitation noise. Therefore, this paper presents recent improvements in terms of the cavitation noise measurement by using continuous wavelet transform and DEMON(Detection of Envelope Modulation on Noise) signal processing. Then, we present that the suggested scheme is more suitable for detecting the cavitation than existing algorithms.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.7 no.1
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• pp.41-50
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• 1998

The detemination of sound pressure radiated from peoriodic plate structures is fundamental in the estimation of noise levels in aircraft fuselages and ship hull structures. As a robust approach to this problem, here a very general and comprehensive analytical model for predicting the sound radiated by a vibrating plate stiffened by periodically spaced orthogonal symmetric beams subjected to a sinusoidally time varying point load is developed. The plate is assumed to be infinite in extent, and the beams are considered to exert both line force and moment reactions on it. Structural damping is included in both plate and beam materials. A space harmonic series representation of the spatial variables is used in conjunction with the Fourier transform to find the sound pressure in terms of harmonic coefficients. From this theoretical model. the sound pressure levels on axis in a semi-infinite fluid (water) bounded by the plate with the variation in the locations of an external time harmonic point force on the plate can be calculated efficiently using three numerical tools such as the Gauss-Jordan method, the LU decomposition method and the IMSL numerical package.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.10a
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• pp.732-739
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• 2005

The determination of sound pressure radiated from periodic plate structures is fundamental in the estimation of noise level in aircraft fuselages or ship hull structures. As a robust approach to this problem, here a very general and comprehensive analytical model is developed for predicting the sound radiated by a vibrating plate stiffened by periodically spaced orthogonal symmetrical beams subjected to a sinusoidally time varying point load. In this these, we experiment with the numerical analysis using the space harmonic series and the SYSNOISE for measuring the vibration mode and character of response caused by sound radiation with adding the harmonic point force in the thin isotropic plate supported by the rectangular lattice reinforcement. We used the reinforcements, beams of open type section like the style of 'ㄷ' letter; the space of the beams were chosen to be 0.2m, 0.3m, 0.4m. We studied the behavior of sound pressure levels, analysis of vibration mode between support points, connection between frequency function and sound pressure levels, and connection between position function and sound pressure levels.