• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2005년도 추계학술대회논문집
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• pp.812-817
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• 2005

The objective of the present work is to develop a time-domain numerical method of broadband noise in a cascade of airfoils. This paper focuses on dipole broadband noise sources, resulting from the interaction of turbulent inflows with the flat-plate airfoil cascade. The turbulence response of a two-dimensional cascade is studied by solving both of the linearised and full nonlinear Euler equations employing accurate higher order spatial differencing, time stepping techniques and non-reflecting inflow/outflow boundary condition. The time-domain result using the linearised Euler equations shows good agreement with the analytical solution using the modified LINSUB code. Through the comparison of the nonlinear time-domain result using the full nonlinear Euler equations with the linear, it is found that the acoustic mode amplitude of the nonlinear response is less than that of the linear response due to the energy cascade from low frequency components to the high frequency ones. Considering the merits of the time-domain methods over the typical time-linearised frequency-domain analysis, the current method is expected to be promising tools for analyzing the effects of the airfoil shapes, non-uniform background flow, linear-nonliear regimes on the broadband noise due to gust-cascade interaction.

• Advances in aircraft and spacecraft science
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• 제3권3호
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• pp.331-349
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• 2016

Stiff and light fibre reinforced composites as used in air- and space-craft applications tend to high sound emission. Therefore, the damping properties are essential for the entire structural and acoustic engineering. Viscous damping is an established and reasonably linear model of the dissipation behaviour. Commonly, it is assumed to be isotropic and constant over all modes. For anisotropic materials it depends on the fibre orientation as well as the elastic and thermal material properties. To portray the orthogonal anisotropic behaviour, a model for unidirectional fibre reinforced plastics (frp) has been developed based on the classical laminate theory by ADAMS and BACON starting in 1973. Their approach includes three damping coefficients - for longitudinal damping in fibre direction, damping transversal to the fibres and shear based dissipation. The damping of a laminate is then accumulated layer wise including the anisotropic stiffness. So far, the model has been applied mainly to thermoset matrix materials. In this study, an experimental parameter estimation for different thermoplastic frp with angle ply and cross ply layups was carried out by measuring free vibrations of cantilever beams. The results show potential and limits of the ADAMS/BACON damping criterion. In addition, a possibility of modelling the anisotropic damping is shown. The implementation in standard FEA software is used to study the influence of boundary conditions on the damping properties and numerically estimate the radiated sound power of thin-walled frp parts.

• Geomechanics and Engineering
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• 제18권5호
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• pp.481-492
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• 2019

Due to the underground openings, the tangentially concentrated stress of the tunnel remains larger at excavation boundary and decreases toward the interior of the surrounding rock with a certain gradient. In order to study the effect of different gradient stress on rockburst, the true-triaxial gradient and hydraulic-pneumatic combined test apparatus were carried out to simulate the rockburst processes. Under the different gradient stress conditions, the rock-like specimen (gypsum) was tested independently through three principal stress directions loading--fast unloading of single surface--top gradient and hydraulic-pneumatic combined loading, which systematically analyzed the macro-mesoscopic damage phenomena, force characteristics and acoustic emission (AE) signals of the specimen during rockburst. The experimental results indicated that the rockburst test under the gradient and hydraulic-pneumatic combined loading conditions could perfectly reflect the rockburst processes and their stress characteristics; Relatively high stress loading could cause specimen failure, but could not determine its mode. The rockburst under the action of gradient stress suggested that the failure mode of specimen mainly depended on the stress gradient. When the stress gradient was lower, progressive and static spalling failure occured and the rockburst grades were relatively slight. On the other hand, shear fractures occurred in rockbursts accounted for increasingly large proportion as the stress gradient increased and the rockburst occurred more intensely and suddenly, the progressive failure process became unconspicuous, and the rockburst grades were moderate or even stronger.

• Geomechanics and Engineering
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• 제18권6호
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• pp.605-614
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• 2019

The present paper seeks to investigate propagation and reflection of waves at free surfaces of homogeneous, anisotropic and rotating micropolar fibre-reinforced medium with voids. It has been observed that, in particular when P-wave is incident on the free surface, there exist four coupled reflected plane waves traveling in the medium; quasi-longitudinal displacement (qLD) wave, quasi-transverse displacement (qTD) wave, quasi-transverse microrotational wave and a wave due to voids. Normal mode Analysis usually called harmonic solution method is adopted in concomitant with Snell's laws and appropriate boundary conditions in determination of solution to the micropolar fibre reinforced modelled problem. Amplitude ratios which correspond to reflected waves in vertical and horizontal components are presented analytically. Also, the Reflection Coefficients are presented using numerical simulated results in graphical form for a particular chosen material by the help of Mathematica software. We observed that the micropolar fibre-reinforced, voids and rotational parameters have various degrees of effects to the modulation, propagation and reflection of waves in the medium. The study would have impact to micropolar fibre-reinforecd rotational-acoustic machination fields and future works about behavior of seismic waves.

• 한국음향학회지
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• 제27권4호
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• pp.183-190
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• 2008

본 논문에서는 미소 비대칭 원통쉘의 맥놀이 음 방사 특성을 해석적으로 검토한다. 미소 비대칭 원통쉘은 한국종의 특징인 맥놀이 현상을 설명할 수 있는 유용한 모델이다. 축대칭 구조물에 부가된 미소 비대칭성은 매우 근접한 주파수 쌍을 만들며, 이 주파수 쌍의 간섭으로 맥놀이 현상이 생긴다. 본 연구에서는 맥놀이 음 방사의 해석적인 모델을 이용하여 방사 음장을 결정하고, 방사 음의 맥놀이 지도를 이용하여 방사 방향 및 거리에 따른 맥놀이 음의 변화특성을 검토하였다. 이론 모델에 의한 해석결과는 경계 요소법에 의한 결과와 비교하여 신뢰도를 검증하였다.

• Structural Engineering and Mechanics
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• 제89권3호
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• pp.265-281
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• 2024

The sound radiation responses of multi-layer composite plates subjected to harmonic mechanical excitation in hygrothermal environment is numerically investigated. A homogenized micromechanical finite element (FE) based on the higher-order mid-plane kinematics replicating quadratic function as well as the through the thickness stretching effect together with the indirect boundary element (IBE) scheme has been first time employed. The isoparametric Lagrangian element (ten degrees of freedom per node) is used for discretization to attain the hygro-thermo-elastic natural frequencies and the modes of the plate via Hamilton's principle. The effective material properties under combined hygrothermal loading are considered via a micromechanical model. An IBE method is then implemented to attain structure-surrounding coupling and the Helmholtz wave equation is solved to compute the sound radiation responses. The effectiveness of the model is tested by converging it with the similar analytical/numerical results as well as the experimentally acquired data. The present scheme is further hold out for solving diverse numerical illustrations. The results revealed the relevance of the current higher-order FE-IBE micromechanical model in realistic estimation of hygro-thermo-acoustic responses. The geometrical parameters, volume fraction of fiber, layup, and support conditions alongside the hygrothermal load is found to have significant influence on the vibroacoustic characteristics.

• Structural Engineering and Mechanics
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• 제90권4호
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• pp.357-370
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• 2024

Due to the fact that the mechanism of the effects of temperature and initial geometric imperfection on low-velocity impact problem of axially moving plates is not yet clear, the present paper is to fill the gap. In the present paper, the nonlinear dynamic behavior of axially moving imperfect graphene platelet reinforced metal foams (GPLRMF) plates subjected to lowvelocity impact in thermal environment is analyzed. The equivalent physical parameters of GPLRMF plates are estimated based on the Halpin-Tsai equation and the mixing rule. Combining Kirchhoff plate theory and the modified nonlinear Hertz contact theory, the nonlinear governing equations of GPLRMF plates are derived. Under the condition of simply supported boundary, the nonlinear control equation is discretized with the help of Gallekin method. The correctness of the proposed model is verified by comparison with the existing results. Finally, the time history curves of contact force and transverse center displacement are obtained by using the fourth order Runge-Kutta method. Through detailed parameter research, the effects of graphene platelet (GPL) distribution mode, foam distribution mode, GPL weight fraction, foam coefficient, axial moving speed, prestressing force, temperature changes, damping coefficient, initial geometric defect, radius and initial velocity of the impactor on the nonlinear impact problem are explored. The results indicate that temperature changes and initial geometric imperfections have significant impacts.

• 비파괴검사학회지
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• 제16권4호
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• pp.215-224
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• 1997

Al-Li 8090 합금을 재료로 인장과 파괴 시험 중의 음향방출 거동을 조사하여, 미세조직과 방향성에 따른 변형 기구를 규명해 보고자 하였다. 이것을 위해 시편은 미세조직(${\delta}'$상이 지배적인 조직과 $S'+{\delta}'$조직)과 방향성(L방향과 ST방향)에 따라 제조하였으며, 인장과 파괴 시험을 수행하면서 음향방출 거동을 관찰하였고, 이를 투과 및 주사 전자 현미경의 결과와 비교하여 전위의 미세기구에 관하여 고찰해 보았다. ${\delta}'$ 조직에서는 L방향과 ST방향의 시편 모두에서 낮은 RMS의 연속형 방출 신호가 나타났고, $S'+{\delta}'$조직아서는 돌발형 방출 신호들만이 관찰되었다. 여러 연구 결과를 통해 ${\delta}'$조직에서의 연속형 형태의 음향방출은 주로 정합인 ${\delta}'$상의 전단에 기인한 것인 반면, $S'+{\delta}'$조직에서의 돌발형 형태의 음향방출은 부정합 ${\delta}'$상의 전단 또는 microcracking때문인 것으로 추론할 수 있었다. 시편의 방향성에 관해서는, ST방향의 시편들의 경우 LT 방향의 시편들보다 많은 돌발형 신호들을 보였다. ST방향 시편들에서 나타나는 이러한 돌발형 신호들은 주로 균열 전파 방향에 평행하게 놓여져 있는 입계를 따라, 급격하게 진행되는 균열 전파에 의해서 나타나는 것으로 보인다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2010년도 춘계학술대회 논문집
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• pp.49-49
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• 2010

The most commonly used numerical modelling techniques for acoustics and vibration are based on element based techniques, such as the nite element and boundary element method. Due to the huge computational eorts involved, the use of these deterministic techniques is practically restricted to low-frequency applications. For high-frequency modelling, probabilistic techniques such as SEA are well established. However, there is still a wide mid-frequency range, for which no adequate and mature prediction techniques are available. In this frequency range, the computational eorts of conventional element based techniques become prohibitively large, while the basic assumptions of the probabilistic techniques are not yet valid. In recent years, a vast amount of research has been initiated in a quest for an adequate solution for the current midfrequency problem. One family of research methods focuses on novel deterministic approaches with an enhanced convergence rate and computational eciency compared to the conventional element based methods in order to shift the practical frequency limitation towards the mid-frequency range. Amongst those techniques, a wave based prediction technique using an indirect Tretz approach is being developed at the K.U.Leuven - Noise and Vibration Research group. This paper starts with an outline of the major features of the mid-frequency modelling challenge and provides a short overview of the current research activities in response to this challenge. Next, the basic concepts of the wave based technique and its hybrid coupling with nite element schemes are described. Various validations on two- and threedimensional acoustic, elastic, poro-elastic and vibro-acoustic examples are given to illustrate the potential of the method and its benecial performance as compared to conventional element based methods. A closing part shares some views on the open issues and future research directions.

• 지구물리와물리탐사
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• 제17권4호
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• pp.216-230
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• 2014

동해 울릉분지 북서 해역에서 취득한 3차원 탄성파 탐사자료의 해석에 의하면 연구해역 내 천부퇴적층은 광역반 사면에 의해 분리되는 5개의 탄성파 단위로 구분되며, 다수의 단층을 수반하는 배사구조가 발달한다. 가스하이드레이트 안정영역 내에는 높은 RMS 진폭을 보이는 해저모방반사면과 낮은 RMS 진폭을 보이는 음향공백대가 특징적으로 분포한다. 탄성파 속성 분석에 의하면 가스하이드레이트와 자유가스가 동시에 부존한 경우 그 경계를 따라 순간진폭이 높게 나타나며 하부에서 순간주파수가 급격히 감소한다. 반면 가스하이드레이트만 존재하는 경우에는 하부에서 순간진폭이 낮고 순간주파수가 감소하지 않는다. 해저모방반사면의 주파수 성분 분해 결과 특정 고주파수 영역에서 나타나는 강한 진폭은 하부 자유가스에 의한 튜닝효과를 지시한다. 가스하이드레이트, 자유가스 및 퇴적층의 경사를 고려한 네 가지 가스하이드레이트 부존모델을 제시하고 그 특징을 기술하였다.