• The Journal of the Acoustical Society of Korea
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• v.39 no.4
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• pp.371-377
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• 2020

Combustion instability occurred in the combustion test of solid rocket motor with large aspect ration Length/Diameter (L/D) and cylinder-slot grain. As a result of spectral analysis of the pressure perturbation, it was confirmed that the central axis longitudinal frequency was dominant, so that the length of the cylinder part was increased to eliminate the coincidence with acoustic node. In addition, acoustic modal analysis and flow analysis were performed to analyze the cause of instability by unsteady flow structure in solid rocket motors. It was confirmed that the combustion instability is reduced by quantitative comparison of the amplitude and frequencies of the pressure inside the combustion chamber using the grain shape before and after the design change. Finally, a combustion test was performed to verify that the combustion instability was resolved as in the flow analysis.

• Journal of the Korean Society of Propulsion Engineers
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• v.25 no.5
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• pp.18-26
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• 2021

In this work, we have developed a 1D network model aimed at predicting eigenvalues for resonance frequency analysis in a lab-scale industrial gas turbine single nozzle combustion system. Modern industrial gas turbines generally adopt combustors with very complex geometry and flow path to meet various design requirements simultaneously. The current study has developed a network model for combustion systems with backflow at the same axial location. The modeling results of resonance frequencies and mode distributions for a given system using the network model were validated from comparisons with prediction results using a 3D Helmholtz solver.

• The Journal of the Acoustical Society of Korea
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• v.42 no.4
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• pp.313-319
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• 2023

In this paper, a time series machine learning model, Long Short Term Memory (LSTM), is applied into the bubble flow noise data and the underwater projectile launch noise data to predict missing values of time-series underwater noise data. The former is mixed with bubble noise, flow noise, and fluid-induced interaction noise measured in a pipe and can be classified into three types. The latter is the noise generated when an underwater projectile is ejected from a launch tube and has a characteristic of instantaenous noise. For such types of noise, a data-driven model can be more useful than an analytical model. We constructed an LSTM model with given data and evaluated the model's performance based on the number of hidden units, the number of input sequences, and the decimation factor of signal. It is shown that the optimal LSTM model works well for new data of the same type.

• The Journal of the Acoustical Society of Korea
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• v.26 no.6
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• pp.293-300
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• 2007

Turbulent boundary layer over an underwater vehicle is formed when it moves underwater and wall pressure fluctuation within the turbulent boundary layer generates flow-induced noise by exciting the elastic hull of the underwater vehicle. One of the methods to reduce this flow noise is to attach a compliant layer on the surface of the vehicle. In order to observe the possibility of noise reduction in the water when the compliant layer treatments are applied on the surface, three types of specimens those are a bare steel plate, a steel plate coated with neoprene and a steel plate with polyurethane coating material are tested at various flow speeds in a low noise cavitation tunnel. This paper presents the results of measurements and analysis of wall pressure fluctuations which is a main source of flow noise, within the turbulent boundary layer on three specimens. Its results could be shown that about 10dB reduction of wall fluctuation pressure at high frequencies was achieved due to the dissipation of turbulent energy by the compliant coating while it makes the turbulent boundary layer thicker and changes the behavior of turbulent flow in the layer.

• Proceedings of the Acoustical Society of Korea Conference
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• spring
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• pp.297-300
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• 2004

Perforated elements are extensively used in mufflers for the intake and exhaust systems of various fluid machines. Perforated elements are usually exposed to grazing flow or cross flow. For analyzing performances of mufflers, the impedance of perforated elements with mean flow is very important. The impedance of perforates under both conditions are measured with different experimental setups. Even if there is no flow, the preceding experimental method for grazing flow shows different values with both theoretical ones and measured under cross flow setup. Using high-order analysis considering phase differences, the experimental method for grazing flow can be modified. The acoustical impedance of perforated impedance contains interaction effects between orifices. After correcting these effects, the measured impedance with grazing flow setup show similar results with both theoretical impedance and measured ones under cross flow setup.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.447-450
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• 2004

선박에서 발생하는 소음 가운데 수중 추진기 소음은 가장 우세한 소음이다. 추진기 소음은 소나에 의해 적을 탐지하는 군용 선박에 경우 생존과 직결된 문제가 되며 쾌적함과 정숙성을 요구하는 여객선의 경우에서도 중요한 문제로 대두되고 있다. 이러한 중요성과 필요성에 따라, 본 연구에서는 수중 추진기의 비공동 및 공동 소음을 수치적으로 해석하였다. 수중 추진기 소음 해석을 위해 유동 해석이 선행되어야 하며 이는 포텐셜을 기반으로 한 패널법을 이용하여 해석한다. 소음 해석은 시간 영역 음향상사법을 이용하였으며, 공동 소음은 홀극 음원으로 모사하여 해석하였다. 또한 향후 수중 추진기의 날개 끝 볼텍스 캐비테이션 해석을 위한 기초연구로서 Eulerian-Lagrangian 접근법을 이용하여 수중익에서의 날개 끝 볼텍스 캐비테이션의 거동과 소음을 예측하였다.

• Proceedings of the Acoustical Society of Korea Conference
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• 1997.06a
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• pp.57-63
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• 1997

실린더 형태의 관은 산업체나 일반가정에서 자주 사용되어진다. 이러한 실린더 형태의 관들은 유체나 기체를 운반하는데 쓰이며, 운반과정에서 소음이 발생한다. 이러한 소음들의 해석은 우선실린더 방정식을유도하여야 한다. 음향학 분야에서 여러 가지 다른 형태의 실린더 운동 방정식이소개되었다. 예로써, Junger and Feit는 실린더의 벽이 매우 얇고 , 무한히 긴 실린더를 가정하에서 실린더운동방정식을 유동하였다. 본 논문에서는 참고문헌[5]에서 유도된 실린더운동방정식고 Junger and Feit 의 실린더운동방정식에서 실린더 좌표 방향으로 향하는 변위의 항을 비교하고, 임의의 실린더 크기를 가정하여 항의 차이를 그래프로 보였다.

• The Journal of the Acoustical Society of Korea
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• v.40 no.4
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• pp.278-289
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• 2021

In this study, the acoustic performance of a dissipative silencer used in the ship with excellent performance compared to its size was predicted and analyzed using a numerical analysis method to reduce the pipe noise. To this end, the performance of the single expansion chamber-shaped silencer was verified using experimental and numerical analysis methods. The acoustic performance of the silencer was expressed using the Transmission Loss (TL), an indicator of its own performance, and the result was derived using the two-load method, which measured by changing the impedance at the end of the pipe. For the numerical analysis method, a general-purpose finite element analysis program was used, and the Delany-Bazley-Miki model with the flow resistivity of the sound absorbing material as an input parameter was applied. Finally, we compared the experimental and simulated results for each of the acoustic performances of the single expansion type and the dissipative silencer to confirm the consistency of the results, and predicted and analyzed the simulation results for four cases according to the properties of the sound absorbing material.

• The Journal of the Acoustical Society of Korea
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• v.40 no.6
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• pp.555-565
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• 2021

The goal of this study is to improve flow and noise performances of existing backward-curved blade centrifugal fan system used for circulating cold air in a refrigerator freezer by optimally designing airfoil shape. The unique characteristics of the system is to drive cold airflow with two volute tongues in combination with duct system in a back side of a refrigerator without scroll housing generally used in a typical centrifugal fan system. First, flow and noise performances of existing fan system were evaluated experimentally. A P-Q curve was obtained using a fan performance tester in the flow experiment, and noise spectrum was measured in an anechoic chamber in the noise experiment. Then, flow characteristics were numerically analyzed by solving the three-dimensional unsteady Navier-Stokes equations and noise analysis was performed by solving the Ffowcs Williams and Hawkins equation with input from the flow simulation results. The validity of numerical results was confirmed by comparing them with the measured ones. Based on the verified numerical method, blade inlet and outlet angles were optimized for maximum flow rate using the two-factor central composite design of the response surface method. Finally, the flow and noise performances of a prototype manufactured with the optimum design were experimentally evaluated, which showed the improvement in flow and noise performance.

• The Journal of the Acoustical Society of Korea
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• v.42 no.1
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• pp.7-15
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• 2023

Recently, as interest in air quality in vehicles increases, the use of fine dust detection sensors for air quality measurement is becoming common. An axial-flow fan is inserted in the fine dust sensor installed in the air conditioning system in the vehicle to prevent dust from sinking directly on the sensor. When the sensor operates, the flow noise caused by the rotation of the axial-flow fan acts as a major noise source of the fine dust sensor. flow noise is recognized as one of the product competitiveness of fine dust sensors. In this study, the noise was gradually reduced at the same flow rate by improving the flow performance of the small axial flow fan. First, a virtual fan performance tester consisting of about 20 million grids was developed to analyze the aerodynamic performance of the target small axial-flow fan. In addition, the flow field was simulated by using compressible Large Eddy Simulation for direct computation of flow noise as well as high-accurate prediction of flow rate. The validity of numerical method are confirmed through the comparison of predicted results with experimental ones. After the effects of pitch angle on flow performance were analyzed using the verified numerical method, the pitch angle was determined to maximize the flow rate. It was found that the flow rate was increased by 8.1 % and noise was reduced by 0.8 dBA when the axial-flow fan with the optimum pitch angle was used.