• 한국음향학회지
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• 제18권6호
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• pp.31-37
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• 1999

본 논문은 근거리 음장 측정 자료로부터 원거리 음장 예측을 위한 기술에 대한 것이다. 음원의 음장 분포 특성은 원거리에서 측정된 자료의 해석으로 이루어지는 것이 일반적 방법이나, 음향수조 또는 무향실과 같은 제한된 공간에서는 근거리 영역에서 측정이 이루어지는 경우가 발생한다. 따라서 근거리 영역에서의 측정으로부터 원거리 음장이 예측되어야 한다. 이 경우 음원을 둘러싼 근거리 음장의 측정점수는 원거리 음장 예측치의 정확도와 자료 처리의 계산량과 상관된다. 기존 연구 결과는 최적측정점수는 음원의 kL에 비례하고 음원의 기하학적 형태 또는 지향특성에 따라 kL의 의존성이 다르게 나타난다고 되어 있으나 정확한 기준이 없다. 따라서 본 논문에서는 최적측정지점수에 대한 기준을 유도하기 위해 Helmholtz 적분식과 Green 함수를 근간으로 한 원거리 음장 예측 기술인 경계배치법(Boundary Collocation Method)을 분석하여 최적측정점수는 kL이 증가함에 따라 0.54kL로 수렴한다는 결과를 얻었다. 기존의 연구 결과 보다 최적측정점수를 1/2 정도로 줄였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.1122-1127
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• 2004

an isolated droplet combustion exposed to pressure perturbations in stagnant gaseous environment is numerically conducted. Governing equations are solved for flow parameters at gas and liquid phases separately and thermodynamic parameters at the interfacial boundary are matched for problem closure. For high-pressure effects, vapor-liquid interfacial thermodynamics is rigorously treated. A series of parametric calculations in terms of mean pressure level and wave frequencies are carried out employing a n-pentane droplet in stagnant gaseous air. Results show that the operating pressure and driving frequency have an important role in determining the amplitude and phase lag of a combustion response. Mass evaporation rate responding to pressure waves is amplified with increase in pressure due to substantial reduction in latent heat of vaporization. Phase difference between pressure and evaporation rate decreases due to the reduced thermal inertia at high pressure. In addition to this, augmentation of perturbation frequency also enhances amplification of vaporization rate because the time period for the pressure oscillation is much smaller than the liquid thermal inertia time. The phase of evaporation rate shifts backward due to the elevated thermal inertia at high acoustic frequency.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2007년도 추계학술대회논문집
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• pp.1476-1481
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• 2007

Numerical investigation on radiation characteristics of discrete frequency noise from asymmetry aero-intakes was carried out. The near-field predictions were obtained by solving the linearized Euler equations with computational aeroacoustic techniques consisting of high order finite difference scheme, non-reflecting boundary conditions, oversetgrid techniques. For the prediction of far-field directivity pattern, the Kirchhoff integral method was applied. By comparing the directivities of noise radiating from the scarf and the scoop aero-intakes with that from an axisymmetric aero-intake, it is shown that noise reduction at downward peak radiation angle can be achieved. The scattering of the radiating acoustic wave by background mean flow shifts the peak lobe radiation angle toward ground and increases the amplitude of the acoustic pressure compared with the cases without mean flow effect.

• 비파괴검사학회지
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• 제28권3호
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• pp.292-301
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• 2008

A linearized model for hysteretic acoustic nonlinearity of imperfectly joined interface is proposed and analyzed by using Coulomb damping to investigate the characteristics of the reflection and transmission coefficients for harmonic waves at the contact interface. Closed crack is modeled as non welded interface that has nonlinear discontinuity condition in displacement across its boundary. Based on the hysteretic contact stiffness of the contact interface, the reflected and transmitted waves are determined by deriving the tractions on both sides of the interface in terms of the discontinuous displacements across the interface. It is found that the amplitudes of the reflected and transmitted waves are dependent on the frequency and the hysteretic stiffness. As the frequency of the incident wave increases, the higher reflection and lower transmission are obtained. It also shows that the hysteresis of the interface increases the reflection coefficient, but reduces the transmission coefficient. A fatigue crack is also made in aluminum specimen to demonstrate these characteristics of the reflection and transmission of contact interfaces.

• 한국소음진동공학회논문집
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• 제25권3호
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• pp.191-198
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• 2015

Noise propagated through the ventilation openings of enclosures is actively controlled using an FIR filter. The enclosures considered in this paper are used for isolating noise due to machinery. This method is of limited use because of the ventilation openings through which most of noise is propagated. Feedforward control strategy is incorporated to minimize the acoustic power propagated through the openings. For the real-time implementation, although it is numerical study, the controller is implemented using an FIR filter. The acoustic transfer functions of the pressure on the openings of the enclosure to the primary source and to the secondary source are numerically calculated using the boundary element method. The performance analysis of the active control is conducted with the time-domain simulation using Matlab Simulink.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2004년도 제28회 KOSCO SYMPOSIUM 논문집
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• pp.42-48
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• 2004

Lean laminar premixed propane and methane flames which were anchored by a hydrogen-pilot flame in a tube were investigated experimentally. The flame shapes were observed by varying mean velocity from 10cm/s to 140cm/s and equivalence ratio from 0.45 to 0.8. In this study, behaviors of flames are divided into five regions such as tail-out, flash-back, flickering, stable and vibrating flames with respect to the mean velocity and the equivalence ratio. Although the flames are unstable in both the flickering and the vibrating region, they have different characteristics such as the frequency, sound generation and creation process of flame curvature. The flickering region exists near the flammability limit and the flame flickers in a frequency of about 10Hz. When flame front is bended, the propane flame front is straightened and the methane flame front is bended more by thermo-diffusive instability. In the vibrating region, the flame vibrates emitting audible sound in a frequency of about 100Hz. In the boundary of vibrating region, the vibration of flame changes between two modes such as single frequency vibration and dual frequency vibration. Increase and decrease of vibration in each mode are determined by thermo-acoustic instability.

• Journal of Mechanical Science and Technology
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• 제15권12호
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• pp.1882-1891
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• 2001

The present paper investigated the effect of ultrasonic vibrations on the melting process of a phase-change material (PCM). Furthermore, the present study considered constant heat flux boundary conditions unlike many of the previous researches adopted constant wall temperature conditions. Therefore, in the present study, modified dimensionless parameters such as Ste* and Ra* were used. Also, general relationships between melting with ultrasonic vibrations and melting without ultrasonic vibrations were established during the melting of PCM. Experimental observations show that the effect of ultrasonic vibrations on heat transfer is very important throughout the melting process. The results of the present study reveal that ultrasonic vibrations accompany the effects like agitation, acoustic streaming, cavitation, and oscillating fluid motion. Such effects are a prime mechanism in the overall melting process when ultrasonic vibrations are applied. They enhance the melting process as much as 2.5 tildes, compared with the result of natural melting. Also, energy can be saved by applying ultrasonic vibrations to the natural melting. In addition, various time-wise dimensionless numbers provide conclusive evidence of the important role of ultrasonic vibrations on the melting phenomena.

• 한국소음진동공학회논문집
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• 제19권11호
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• pp.1133-1143
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• 2009

좁은 슬롯을 포함한 디스크 브레이크 로터의 소음 방사특성을 전산해석과 이론적 계산을 합성한 방법으로 검토하였다. 첫 단계로 로터와 기본 치수가 동일하고 동일한 슬롯을 보유한 후판 환형 디스크의 소음방사 특성을 유한요소해석을 구한 모달 진동 데이터를 바탕으로 기존의 해법을 이용하여 계산하고 수치해석결과로 검증하였다. 다음으로 이 결과를 유한요소해석으로 구한 샘플 로터의 고유진동 특성에 적용, 고유진동으로 인한 소음방사를 계산한 다음 그 결과를 경계요소법을 이용하여 검증하였다. 마지막으로 이 결과를 바탕으로 로터에 고정된 조화가진 및 로터 주위를 회전하는 조화력에 의해 방사되는 소음 및 진동 특성을 검토하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.810-815
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• 2003

Flow-induced noise propagated from flow over a sphere is numerically investigated for laminar flow at Re = 300 and 425, and for turbulent flow at Re = 3700 and $10^4$, where the Reynolds number is based on the freestream velocity and the sphere diameter. The numerical method used for obtaining the flow over a sphere is based on an immersed boundary method in a cylindrical coordinate system. The Curle’s solutions of the Lighthill’s acoustic analogy with and without the far-field and compact-source approximation are used in order to investigate the noise field from flow over a sphere. Since the drag and lift forces change irregularly in time at Re = 425, 3700 and $10^{4}$, the noise propagates in a complicated manner. At Re = 300, 425 and $10^{4}$, the noise from dipole sources is much larger than that from quadrupole sources. On the other hand, at Re = 3700, the quadrupole source becomes dominant. The temporal variation of the flow-induced noise around a sphere is obtained at some observation points, which shows that the peak frequency corresponds to the Strouhal number associated with the wake instability.

• 소음진동
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• 제10권5호
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• pp.843-848
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• 2000

BEA(Boundary Element Analysis) based on Kirchhoff-Helmholtz integral equation is widely used in the prediction of sound radiation problems of vibrating structures. Accurate estimation of sound pressure distribution by BEA can be [possible if and only if dynamic behavior of the relating structure was described correctly. Another plausible method of sound radiation phenomena could be the NAH(Nearfield Acoustic Holography) method. NAH also based on the identical governing equation with BEA could be one of the best acoustic imaging schemes but it has disadvantages of the complexity of measurement and of the need of large amount of measuring points. In this paper, modal expansion method is presented for taking accurate dynamic data of the structures efficiently. This method makes use of vibration principle an arbitrary dynamic behavior of the structure is described by the summation of that structures mode shapes which can be calculated by FEA easily and accurately. Sound pressure field from a vibration flat plate is calculated using the combination of vibration signal on that flat plate from experiment, and of the natural mode shapes form FEA. When sound pressure field from vibration signal is calculated the importance of the phase information was emphasized.