• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2008년도 춘계학술대회논문집
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• pp.286-289
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• 2008

Damping materials are widely used to reduce vibration or noise generation of structures. To understand their damping capability and for use in numerical simulations, their viscoelastic properties should be measured in the frequency range of interest. In this study, experimental setup is proposed to measure materials properties of very compliant polymer materials. The polymer materials used in this study are difficult to form into rod shapes, and typical resonance methods are not applicable. In the proposed measurement setup, the damping materials were modeled as a simple viscoelastic support at one end of the beam. Their properties were measured through analysis of their effects on the wave propagation characteristics of the beam structure.

• 한국소음진동공학회논문집
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• 제14권7호
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• pp.577-585
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• 2004

In this paper, reactive shearing structural intensity method is extended to damping patches placement on outer panels of outdoor unit of air-conditioner to reduce its structural borne noise. The structural intensity is calculated from the normal velocities of structures that are measured by using a laser scanning vibrometer, and $textsc{k}$-space (wave-number domain) signal processing is used to obtain the spatial derivatives in formulation of structural intensity. This method is applied to the outdoor unit of air-conditioner on shaker-exciting mode and operating mode. and then damping patches are placed over area of high reactive shearing structural intensity for reducing the radiated noise. Experimental results show the largest reduction of sound pressure level of an outdoor unit by appling small damping patches to optimal position.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 1998년도 가을 학술발표회 논문집
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• pp.137-144
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• 1998

• Ocean Systems Engineering
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• 제3권3호
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• pp.203-217
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• 2013

A PTO (power-take-off) mechanism by using relative heave motions between a floating buoy and its inner mass (magnet or amateur) is suggested. The inner power take-off system is characterized by a mass with linear stiffness and damping. A vertical truncated cylinder is selected as a buoy and a special station-keeping system is proposed to minimize pitch motions while not affecting heave motions. By numerical examples, it is seen that the maximum power can actually be obtained at the optimal spring and damper condition, as predicted by the developed WEC(wave energy converter) theory. Then, based on the developed theory, several design strategies are proposed to further enhance the maximum PTO, which includes the intentional mismatching among heave natural frequency of the buoy, natural frequency of the inner dynamic system, and peak frequency of input wave spectrum. By using the intentional mismatching strategy, the generated power is actually increased and the required damping value is significantly reduced, which is a big advantage in designing the proposed WEC with practical inner LEG (linear electric generator) system.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년도 제31회 추계학술대회논문집
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• pp.11-14
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• 2008

로켓엔진 연소기의 음향 불안정 억제를 위해 고안된 반파장 공명기의 음향학적 최적 설계 조건을 실험적으로 연구하였다. 표준 음향시험 절차에 따라 음향파관에 공명기를 장착하여 음향파 응답 신호를 측정하였다. 측정된 신호를 토대로, 흡음계수를 계산하였고, 이로부터 공명기의 음향감쇠 성능을 파악하였다. 연소실을 모사하는 음향파관의 직경을 변화시켰고, 각 음향파관에 대해 공명기의 최적 면적비를 구하였다. 공명기의 면적비가 최적 값을 초과할 때 과감쇠가 발생하였다. 연소실 직경이 증가함에 따라 최적 면적비가 감소하였고, 공명기 직경이 증가 할수록 더 작은 면적비에서 최적 흡음성능이 나타남을 알았다.

• 한국지반공학회논문집
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• 제15권2호
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• pp.95-104
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• 1999

Set-up현상이 나타나는 지반에 항타된 말뚝의 지지력은 시간경과에 따라 지지력이 증가하게 된다. 파동이론해석법(WEAP해석법)은 말뚝의 항타관입성을 평가하는데 매우 유용한 것으로 평가되고 있다. 그러나 기존의 WEAP해석법은 항타후 변화하는 지반의 강도특성을 고려할 수 없다는 측면에서 재항타시의 신뢰도 있는 지지력을 예측하는데 한계가 있다고 평가된다. WEAP에 의한 항타말뚝의 지지력 평가가 신뢰성 있기 위해서는 set-up효과가 반영되어야만 한다. 그러나 아직까지 이를 고려할 수 있는 만족할 만한 방법이 없다. 본 논문에서는 항타시와 재항타시의 동재하시험 결과들을 통계분석하여 WEAP해석시 set-up효과를 반영할 수 있는 방안을 제시하였다. 또한 현장 시험결과와의 비교를 통해서, 제안된 변수(quake, damping)를 적용한 파동이론해석 결과는 기존의 방법보다 신뢰성이 있음을 알 수 있었다.

• 한국산학기술학회논문지
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• 제18권2호
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• pp.71-76
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• 2017

선박 및 해양구조물 설계에 있어 롤댐핑에 관한 문제는 유체점성과 현상자체의 비선형성으로 인해 공학자들에게 있어 난제로 남아있다. 본 연구에서는 강제동요방법을 이용하여 원형실린더의 점성 롤감쇠에 관한 연구를 수행하였다. 토크 센서를 이용하여 강제동요 시 발생하는 롤모멘트(roll moment)를 강제동요 주기별로 계측하였고 이를 실험식(empirical formula)과 비교 검토 하였다. 점성에 의한 전단력으로부터 계측된 토크의 크기가 상대적으로 작은 값임에도 불구하고 실험식으로부터 얻어진 계산값들과 정성적으로 유사한 결과를 보였고, 일부 주기에서는 정량적으로도 잘 일치하는 결과를 보였다. 또한, PIV 계측기법을 통해 원형실린더 벽면주위의 유동을 면밀히 관찰하였으며, 유체점성으로 인해 원형실린더 벽면근처에서 경계층이 형성되고 자유수면에서 원형실린더의 주기적인 강제회전동요로 인해 미소한 크기의 파가 생성(wave making)됨을 PIV 계측결과의 분석을 통해 확인하였다. 본 연구에서는 점성 롤감쇠로 인한 실험식의 적합성을 모형시험을 통해 확인하였고, PIV 계측기법을 통해 벽면주위의 유체점성으로 인한 조파현상을 입증하였다.

• 대한수학회지
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• 제59권1호
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• pp.205-216
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• 2022

This paper deals with a nonlinear wave equation with boundary damping and source terms of variable exponent nonlinearities. This work is devoted to prove a global nonexistence of solutions for a nonlinear wave equation with nonnegative initial energy as well as negative initial energy.

• 대한조선학회논문집
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• 제55권6호
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• pp.490-496
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• 2018

The consistency of the initially designed waves in the domain is essential for accurate calculation of the added resistance in waves through CFD. In particular, unwanted reflected waves at domain boundaries can cause incorrect numerical solutions due to the superposition with initially designed waves. Euler Overlay Method(EOM) is one of the methods for reducing wave reflections by adding an additional source term to momentum and phase conservation equations, respectively. In this study, we apply the Euler Overlay Method(EOM) to the open-source CFD library, OpenFOAM(R), to simulate the accurate free-surface waves in the domain and the parametric study is performed for efficient implementation of Euler Overlay Method(EOM). Considering that the damping efficiency depends on the selection of the overlay parameter in the added source terms, the size of overlay zone and the wave steepness, the influences of these factors are tested through the wave elevation measured at constant time intervals in the 2D numerical wave tank. Through this process, guidelines for selection of optimal overlay parameter and overlay zone size that can be applied according to the scaling law are finally presented.

• International Journal of Ocean System Engineering
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• 제4권1호
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• pp.1-18
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• 2014

Heave plates have been widely used to enhance viscous damping and thus reduces the heave response of Spar platforms. Single heave plate attached to the keel of the Spar has been reported in literature (Tao and Cai 2004). The effect of double heave plates on hydrodynamic response in random waves has been investigated in this study. The influence of relative spacing $L_d/D_d$ ($D_d$-the diameter of the heave plate) on the hydrodynamic response in random waves has been simulated in wave basin experiments and numerical model. The experimental investigation has been carried out using 1:100 scale model of Spar with double heave plates in random waves for different relative spacing and varying wave period. The influence of relative spacing between the heave plates on the motion responses of Spar are evaluated and presented. Numerical investigation has been carried out to investigate effect of relative spacing on hydrodynamic characteristics such as heave added mass and hydrodynamic responses. The measured results were compared with those obtained from numerical simulation and found to be in good agreement. Experimental and numerical simulation shows that the damping coefficient and added mass does not increase for relative spacing of 0.4 and the effect greater than relative spacing on significant heave response is insignificant.