• Journal of the Korean Society of Propulsion Engineers
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• v.20 no.1
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• pp.82-89
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• 2016

The unique phenomenon that jet flow kinetic energy is converted to thermal energy through thermoacoustic resonance can be applied for the multiple ignition of liquid rocket engines. The present article includes the basic principle and theory behind the phenomenon as well as major outstanding, previous research works. The thermoacoustic phenomenon is affected by underexpanded jet flow characteristics from a nozzle, geometries of a nozzle and a resonance tube, and chemical composition of jet flow. The paper concludes with discussion what should be considered as crucial issues for the future research on the development of a multiple ignition system of liquid rocket engines.

• The Journal of the Acoustical Society of Korea
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• v.37 no.6
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• pp.396-403
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• 2018

In this paper, a high-performance and low-noise centrifugal fan is developed for cooling automotive seats which provide a driver with pleasant driving environment. First, the flow characteristics of the existing fan unit was analyzed using a fan performance tester and CFD (Computational Fluid Dynamics) simulations. The analysis of the predicted flow field indicated vortex flow near the tip of fan hub and stagnation flow on the top of fan hub. Two design points are devised to reduce the vortex flow and the stagnation flow observed in the existing fan unit. First, the cut-off clearance which is the minimum distance between the fan blade and the fan housing is increased to reduce the vortex strength and, as a result, to reduce the overall sound pressure level. Second, the hub shape is more modified to eliminate the stagnation flow. The validity of proposed design is confirmed through the numerical analysis. Finally, a prototype is manufactured with a basis on the numerical analysis result and its improved flow and noise performances are confirmed through the P-Q curves measured by using the Fan Tester and the SPL (Sound Pressure Level) levels measured in the anechoic chamber.

• The Journal of the Acoustical Society of Korea
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• v.41 no.6
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• pp.698-704
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• 2022

In this paper, the radiated aerodynamic noise generated from sound sources of a nozzle inflow is quantitatively investigated and compared with experimental results of externally radiated noise. A high-resolution unsteady compressible Large Eddy Simulation (LES) technique is used to accurately predict the internal and external flow of three types of nozzle shape. Through using the vortex sound source for sound sources, the geometry of nozzle neck is identified as most significant aerodynamic noise sources. For validation of quantitative analysis, the vortex sound source intensity of internal nozzle flow is compared with results of external radiated noise of calculation and experiment.

• 한국전산유체공학회:학술대회논문집
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• 2008.03a
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• pp.50-56
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• 2008

본 논문에서는 실린더 주위유동의 B-mode에 해당하는 레이놀즈 수 300${\sim}$1000범위에서 고차 고해상도기법(OHOC Scheme)을 이용하여 원형 실린더 주위의 유동장 및 음향장 특성에 대해서 연구하였다. B-mode 레이놀즈 수 범위에서 스팬방향 길이에 따른 3차원 원형실린더의 스트롤 수, 양 항력계수의 상관관계에 대해 수치계산 및 실험 결과와 비교 분석한 결과 매우 잘 일치하는 것을 보였다. 그리고 이 결과를 토대로 하여 B-mode 불안정성 영역에서 보다 정확한 2차 와류 모사를 위한 적절한 스팬방향을 찾고, 3차원 후류유동의 불안정성이 음향장의 변화에 미치는 영향을 정성적으로 고찰하여, 3차원 원형실린더의 공력소음 대한 기초적인 연구를 수행하였다.

• 한국전산유체공학회:학술대회논문집
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• 2008.10a
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• pp.50-56
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• 2008

본 논문에서는 실린더 주위유동의 B-mode에 해당하는 레이놀즈 수 $300{\sim}1000$범위에서 고차 고해상도 기법(OHOC Scheme)을 이용하여 원형 실린더 주위의 유동장 및 음향장 특성에 대해서 연구하였다. B-mode 레이놀즈 수 범위에서 스팬방향 길이에 따른 3차원 원형실린더의 스트롤 수, 양 항력계수의 상관관계에 대해 수치계산 및 실험 결과와 비교 분석한 결과 매우 잘 일치하는 것을 보였다. 그리고 이 결과를 토대로 하여 B-mode 불안정성 영역에서 보다 정확한 2차 와류 모사를 위한 적절한 스팬방향을 찾고, 3차원 후류유동의 불안정성이 음향장의 변화에 미치는 영향을 정성적으로 고찰하여, 3차원 원형실린더의 공력소음 대한 기초적인 연구를 수행하였다.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2006.05a
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• pp.1372-1375
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• 2006

The simplified impedance model which can consider a combined flow condition was suggested. Although the strength and position of the shear layer cannot be obtained by a linear sum of two separate contributions when both flows occur together, it was simply assumed that the impedance under the combined flow follows from summing the separate flow impedance. To validate the simplified impedance model, acoustic properties of a concentric resonator was predicted and measured. The predicted transmission loss using the simplified model shows reasonable agreements with measurements. One can find that the simplified impedance model obtained by the superposition of the separate flow impedances can be adjusted to predict the acoustic properties of a concentric resonator.

• The Journal of the Acoustical Society of Korea
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• v.39 no.4
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• pp.364-370
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• 2020

Rocket noise generated from the exhaust plume produces the enormous acoustic loading, which adversely affects the integrity of the electronic components and payload (satellite) at liftoff. The prediction of rocket noise consists of two steps: the supersonic jet exhaust is simulated by a method of the Computational Fluid Dynamics (CFD), and an acoustic transport method, such as the Helmholtz-Kirchhoff integral, is applied to predict the noise field. One of the difficulties in the CFD step is to remove the boundary reflection artifacts from the finite computation boundary. In general, artificial damping, known as a sponge layer, is added nearby the boundary to attenuate these reflected waves but this layer demands a large computational area and an optimization procedure of related parameters. In this paper, a cost-efficient way to separate the reflected waves based on the two microphone method is firstly introduced and applied to the computation result of a laboratory-scale supersonic jet noise without sponge layers.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.14 no.5
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• pp.432-438
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• 2004

Acoustic levitation induced by ultrasonic flexural vibration at 28.4 KHz with a vibration amplitude of 10 micrometers is presented. Levitation of multiple objects along the length of the beam in a gap of 8.3 mm which is the half of acoustic wavelength is experimentally demonstrated. Analytical analysis predicts that levitated objects for the gap of half-the wavelength converges to the center of the gap, which is experimentally verified. It is observed that levitated objects with well-balanced mass distribution are set into rotation due to acoustic streaming. For cylinder-shaped Styrofoam with a diameter of 1.8 mm and a length of 3 mm, measured rotational velocity is 2400 revolution per minute. Applications of standing wave field levitation (SWFL) include manipulation of biological cells and blood constituents in biotechnology, and fine powder in material engineering.

• The Journal of the Acoustical Society of Korea
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• v.14 no.2
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• pp.5-15
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• 1995

An application of the cross correlation technique by adopting ultrasonic waves for water pipe flow measuring purpose is studied. It is a non-intrusive flow metering method by determining the time of the flight of the flow turbulent noise and its non-obstructing mechanism enables to reduce process energy loss due to the flowmeter obstruction. A digital signal processor for the purpose of the real time Fourier transform was employed for the fast time calculation of the flow velocity. The overall accuracy was found as about $1\%$ for flow velocities from 0.25 m/s up to 16 m/s and for the pipe inside diameters from 50mm to 248mm. The cross correlation technique can be used for the tap water utility including most common liquid flows.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.135-140
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• 2006

In the experimental study of $N_2$ purge cold flow test of impinging(FOOF) injector for determining of instability region, the whistling sound which has a specific frequency is generated. The frequency of whistling is proportional to the gas flow velocity in part of the oxidizer orifice and due to the coupling of the vibrating gas column and the natural frequency of pipe-orifice shape, the discontinuous jumping phenomena arises. The whistling phenomena have no effect on the combustion instability. Compared the damping factor of 1T1L mode with the hot fire test, the instability region of $N_2$ purge cold flow test is very much like that. It means that flow instability by impinging or mixing of jet is the main reason of combustion instability of impinging injector(FOOF) in the hot firing test.