• Journal of the Korean Society for Nondestructive Testing
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• v.29 no.4
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• pp.308-315
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• 2009

Because the attenuation of AE signal in composite materials is relatively higher than that of metallic materials, it is required to develop a damage assessment technique less affected by the attenuation property of composite materials in order to use AE sensing as a damage detection method. In the signal processing procedure, it is profitable to use the leading wave that arrives first because the leading wave is less influenced by the boundary conditions. Using wavelet transform, we investigated the frequency characteristics of impact induced AE signals focused on the leading wave in advance and chose the key factors to discriminate the damaged condition quantitatively. In this research, we established a damage assessment technique using the sharing percentage of the wavelet detail components of AE signal, and conducted a low-velocity impact test on composite laminates to confirm the feasibility of the proposed signal processing method.

• Structural Engineering and Mechanics
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• v.42 no.6
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• pp.831-847
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• 2012

Impact from water-borne debris during tsunami and flood events pose a potential threat to structures. Debris impact forces specified by current codes and standards are based on rigid body dynamics, leading to forces that are dependent on total debris mass. However, shipping containers and other debris are unlikely to be rigid compared to the walls, columns and other structures that they impact. The application of a simple one-dimensional model to obtain impact force magnitude and duration, based on acoustic wave propagation in a flexible projectile, is explored. The focus herein is on in-air impact. Based on small-scale experiments, the applicability of the model to predict actual impact forces is investigated. The tests show that the force and duration are reasonably well represented by the simple model, but they also show how actual impact differs from the ideal model. A more detailed three-dimensional finite element model is also developed to understand more clearly the physical phenomena involved in the experimental tests. The tests and the FE results reveal important characteristics of actual impact, knowledge of which can be used to guide larger scale experiments and detailed modeling. The one-dimensional model is extended to consider water-driven debris as well. When fluid is used to propel the 1-D model, an estimate of the 'added mass' effect is possible. In this extended model the debris impact force depends on the wave propagation in the two media, and the conditions under which the fluid increases the impact force are discussed.

• Wind and Structures
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• v.16 no.2
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• pp.193-211
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• 2013

This paper presents results from a wind tunnel study that examined the drag coefficient and wind flow over an asymmetric wave train immersed in turbulent boundary layer flow. The modeled wavy surface consisted of eight replicas of a statistically-valid hurricane-generated wave, located near the coast in the shoaling wave region. For an aerodynamically rough model surface, the air flow remained attached and a pronounced speed-up region was evident over the wave crest. A wavelength-averaged drag coefficient was determined using the wind profile method, common to both field and laboratory settings. It was found that the drag coefficient was approximately 50% higher than values obtained in deep water hurricane conditions. This study suggests that nearshore wave drag is markedly higher than over deep water waves of similar size, and provides the groundwork for assessing the impact of nearshore wave conditions on storm surge modeling and coastal wind engineering.

• Journal of the Optical Society of Korea
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• v.17 no.1
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• pp.97-102
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• 2013

Frequency tuning characteristics of a THz-wave by varying phase-matching angle and pump wavelength in a noncollinear phase-matching THz-wave parametric oscillator (TPO) are analyzed. A novel scheme to realize the tuning of a THz-wave by moving the cavity mirror forwards and backwards is proposed in a noncollinear phase-matching TPO. The parametric gain coefficients of the THz-wave in a $LiNbO_3$ crystal are explored under different working temperatures. The relationship between the poling period of periodically poled $LiNbO_3$ (PPLN) and the THz-wave frequency under the condition of a quasi-phase-matching configuration is deduced. Such analyses have an impact on the experiments of the TPO.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.11a
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• pp.1463-1466
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• 2007

Visco-elastic damping material for reducing heavy-weight floor impact noise and vibration in reinforced concrete structures was tested according to its thickness in the damping layer. The effect of damping material was compared with 20, 15, 10 and 5mm thickness. The wave propagation characteristics was measured for suggestion of an efficient method to reduce the floor impact noise. The method was proposed using the flexural wave propagation characteristics. The result showed that reduction of the thickness of damping layer made a slight difference; the natural frequency moved to higher frequency and the amplitude increased at low frequencies with 5mm thickness of damping material.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.05a
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• pp.1100-1105
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• 2007

In the gas supply duct, the gas leakage caused by the impact of the construct equipment is serious problem. The identification of the impact position is an important issue and an engineering work. For the basic research of this problem, the principle studies for the acoustic wave propagation in a gas duct are proceeded in this paper. This principal work is based on the identification of the cut-off frequency associated with major modes of the gas duct theoretically and experimentally. The cut-off frequency is confirmed by STFT and cross-correlation function is used to identify the leakage position.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.18 no.8
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• pp.797-804
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• 2008

In the gas supply pipe, the gas leakage caused by the impact of the construct equipment is serious problem. The identification of the impact position is an important issue and an engineering work. For the basic research of this problem, the principle studies for the acoustic wave propagation in a gas pipe are proceeded in this paper. This principal work is based on the identification of the cut-off frequency associated with major modes of the gas pipe theoretically and experimentally The cut-off frequency is confirmed by STFT and cross-correlation function is used to identify the leakage position.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.22 no.1
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• pp.45-52
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• 2009

In this paper, a guideline for designing breakwater in wave loads and in seismic loads is proposed. A simple model structure in breaking wave zone is examined using Morison equation in consideration with the effect of an impact load, for evaluation of the wave loads. As the impact load effect is not significant, pressure distributions according to Goda are applied for evaluation of wave loads on breakwater. Structural behavior of breakwater in wave loads can be obtained using the Goda method, as well. For seismic analysis, Ofunato and Hachinohe models, as well as an artificial seismic acceleration loads model, are adopted. Soil-structure interaction analysis is carried out to find the seismic load effect. It is found that, in certain cases, structural deformation in wave loads is in the same level as deformation that in seismic loads. Thus, it is our recommendation that these two loads are considered at the same level in breakwater design.

• Journal of the Society of Naval Architects of Korea
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• v.44 no.2 s.152
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• pp.148-153
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• 2007

Two typical impact loadings, shock wave and gas bubble pulse, due to UNDEX(UNDerwater EXplosion), should be considered together for the closest response analysis of structure subjected to UNDEX to a reality. Since these two impact loadings have different response time bands, however, their response characteristics of structure are different from each other. It is impossible to consider these effectively under the current computational environment and the mathematical model has not yet been developed. Whereas Hicks model approximates the fluid-structure interaction due to gas bubble pulse as virtual mass effect, treating the flow by the response of gas bubble after shock wave as incompressible ideal fluid contrary to the compressible flow due to shock wave, Geers-Hunter model could make the closest response analysis of structure under UNDEX to a real one as a mathematical model considering the fluid-structure interaction due to shock wave and gas bubble pulse together using acoustic wave theory and DAA(Doubly Asymptotic Approximation). In this study, the application and effectiveness of integrated dynamic response analysis of submerged structure was examined with the analysis of the shock wave and gas bubble pulse together.

• Journal of Navigation and Port Research
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• v.31 no.7
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• pp.579-587
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• 2007

Samchunpo(Sin Hyang) Harbor is located in the bay of Sa Chun, the central south coast of Korean peninsula. The harbor and coastal boundaries have been protecting by natural coastal islands and shoals. Currently, The Sin Hyang harbor needs maintenance and renovation of the sheltered structures against the weather deterioration and typhoon damages. Consequently to support this, the calculation of accurate design wave through the typhoon wave attack is necessary. In this study, calculation of incident wave condition is simulated using steady state spectrum energy wave model(wide area wave model) from 50 years return wave condition. And this simulation results in wide offshore area were used for the input of the extended mild slope wave model at the narrow coastal area. Finally, the calculation of design wave at Sin Hyang harbor entrance was induced by Boussinesq wave model(detail area wave model) simulation. The numerical model system was able to simulate wave transformations from generation scale to shoreline or harbor impact. We hope these results will be helpful to the engineers doing placement, design, orientation, and evaluation of a wide range of potential solutions in this area.