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유연재 코팅 평판의 난류 변동압력 특성에 관한 실험적 연구
박경훈,이승재,신구균,Park, Kyung-Hoon,Lee, Seung-Jae,Shin, Ku-Kyun 한국음향학회 2007 韓國音響學會誌 Vol.26 No.6
수중운동체가 주행할 때 수중운동체의 표면에는 난류 경계층이 형성되고, 난류 경계층 내 벽면 변동압력은 탄성체인 수중운동체의 표면을 가진시켜 유동 유기 소음을 발생시킨다. 이러한 유체 소음을 감소시키기 위한 하나의 방법으로 수중운동체 표면에 유연재를 부착하여 수중운동체의 유동 유기 방사 소음을 감소시키는 방법이 제안되기도 한다. 본 논문에서는 유연재 부착에 따른 수중에서의 유체소음의 변화 특성을 살펴보기 위하여 유연재가 코팅되지 않은 강판 시편과, 고무 재질의 Neoprene 및 폴리우레탄 재질의 유연재가 부착된 강판시편을 저소음 공동수조에 설치한 후 여러 가지 유속 조건에서 유체소음의 주요 소음원인 난류 경계층 내 벽면 변동압력을 측정하고 그 결과를 비교 분석하였다. 그 결과 유연재를 코팅한 경우에는 유연재로 인하여 난류 경계층의 두께가 두꺼워지는 등 경계층 유동이 변화하지만, 유연재에서의 난류 에너지의 소산으로 인하여 고주파수 대역에서 약 10dB의 벽면 변동압력의 감소 효과를 확인할 수 있었다. 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.
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파수-주파수 분석을 통한 고압 배관 내 수축 확장 노즐의 유동 소음원에 대한 수치적 연구
구가람,이송준,김극수,정철웅,Ku, Garam,Lee, Songjune,Kim, Kuksu,Cheong, Cheolung 한국음향학회 2017 韓國音響學會誌 Vol.36 No.5
일반적으로 감압밸브는 고압 가스에 의한 배관 파손을 방지하기 위해 설치된다. 그러나 감압 밸브를 지나면서 발생하는 급격한 압력 저하는 음향파의 형태로 전파되는 큰 음향 에너지를 발생 시키며, 하류 방향으로 전파되면서 배관의 벽면을 진동시키는 가진원으로 작용하여 배관의 파손을 유발한다. 따라서, 본 연구에서는 단순 수축-확장 배관을 대상으로 LES(Large-Eddy Simulation)기법과 파수-주파수 분석을 통해 유동장 내 비압축성 압력섭동과 압축성 압력 섭동을 분리하고, 밸브 유동에 의한 내부 유동소음을 예측하였다. 수치해석의 수렴성을 향상시키기 위해 먼저 정상상태 Reynolds-Averaged Navier-Stokes 방정식을 해석하여, 고정확도의 비정상 LES해석의 초기 값으로 활용하였으며, 비정상 유동장 결과로부터 파수-주파수 분석을 실시하였다. 파수-주파수 분석을 통해 비압축성 압력섭동과 압축성 압력섭동을 분리하였으며, 이를 통해 배관 내 음향유기진동에 의한 소음원 정보를 정확히 제공할 수 있음을 확인하였다. A pressure relief valve is generally used to prevent piping systems from being broken due to high pressure gas flows. However, the sudden pressure drop caused by the pressure relief valve produces high acoustic energy which propagates in the form of compressible acoustic waves in the pipe and sometimes causes severe vibration of the pipe structure, thereby resulting in its failure. In this study, internal aerodynamic noise due to valve flow is estimated for a simple contraction-expansion pipe by combining the LES (Large-Eddy Simulation) technique with the wavenumber-frequency analysis, which allows the decomposition of fluctuating pressure into incompressible hydrodynamic pressure and compressible acoustic pressure. In order to increase the convergence, the steady Reynolds-Averaged Navier-Stokes equations are numerically solved. And then, for the unsteady flow analysis with high accuracy, the unsteady LES is performed with the steady result as the initial value. The wavenumber-frequency analysis is finally performed using the unsteady flow simulation results. The wavenumber-frequency analysis is shown to separate the compressible pressure fluctuation in the flow field from the incompressible one. This result can provide the accurate information for the source causing so-called acoustic-induced-vibration of a piping system.
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강웅(Woong Kang),김형진(Hyung Jin Kim),성형진(Hyung Jin Sung) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
Turbocharger has been widely used in many passenger cars in application with diesel engines because of high power and fuel efficiency. However, flow-induced noise (whoosh or hissing noise) which is generated within the compressor during its operation at marginal surge line can deteriorate noise characteristics. Hissing noise excitation was associated with the generation of turbulence within the turbocharger compressor and radiated through the transmission path in turbocharger system. In this study, a sharp-edged reactive-type muffler was devised and installed in the transmission path to reduce the hissing noise. Acoustic and fluid dynamic characteristics for the muffler were investigated which is related to the unsteadiness of turbulence and pressure in turbocharger system. A transfer matrix method was used to analyze the transmission loss of the muffler. Simple expansion muffler with extended tube of the reactive type is proposed for the reduction of high frequency component noise. Turbulence computation was carried out by a standard k ? ε model. An optimal design condition of the muffler was obtained by extensive acoustic and fluid dynamic analysis on the engine dynamometer with anechoic chamber. A significant reduction of the hissing noise was achieved at the optimal design of the muffler as compared with the conventional turbocharger system.
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이승배,김휘중,Lee, Seungbae,Kim, Hooi-Joong 대한기계학회 1999 大韓機械學會論文集B Vol.23 No.6
The wall pressure fluctuations of a turbulent boundary layer over a flat plate have been investigated in an anechoic wind tunnel facility. The anechoic wind tunnel consists of acoustically-lined duct, muffler, and splitter-type silencer for noise suppression and vanes for reducing head losses involved. To improve spectra characteristics in high frequency range, a 1/8" pressure-type microphone sensor, which has a pin-holed cap of various diameters, was employed in this experiment. It was shown that the pin-holed microphone sensor with a dimensionless diameter $d^+$ of 7.1 resolved the high frequency pressure fluctuations most effectively among ones with various pin-hole diameters. The measured wall pressure spectra in terms of three types of scaling parameters were in good agreement with other experimental and numerical results. The pressure events of high amplitude were found to contribute to total fluctuating pressure energies in the turbulent boundary layer significantly and supposed to radiate to the far-field effectively.
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