• Journal of Ocean Engineering and Technology
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• v.20 no.6 s.73
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• pp.88-92
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• 2006

Large-scale model tests of welded topside joints were carried out to observe the fatigue behavior of API 2W Gr.50 steel produced by POSCO. The fatigue crack behaviors for various loading conditions were measured and investigated around the critical joint sections. The experimental results have been verified with numerical approaches and also compared with the AWS D1.1 and DnV RP-C203 design curves. The large-scale experiment models were fabricated, based on the actual operating east area fixed platform. The dimensions of the models were slightly modified to accommodate the test facilities and capacities. The fatigue test was carried out having ${\Delta}Q$ of T1=705.6kN, T2=749.7kN and T3=793.8kN. The three specimens were statically loaded 20 times, with various loadings of about 50kN intervalsbetween the maximum and minimum loads required in the fatigue tests. This loading removed the residual stress in the specimen before the fatigue tests. The topside joint crack was initiated from the brace heel, where the maximum tensile stress occurred. The API 2W Gr.50 steel satisfied the AWS D1.1 detail category C and DnV RP-C203 detail category F ${\Delta}S-N$ curve.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.12 s.105
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• pp.1378-1388
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• 2005

Spatial control of sound is essential to deliver better sound to the listener's position in space. As it can be experienced in many listening environments. the quality of sound can not be manifested over every Position in a hall. This motivates us to control sound in a region we select. The primary focus of the developed method has to do with the brightness and contrast of acoustic image in space. In particular, the acoustic brightness control seeks a way to increase loudness of sound over a chosen area, and the contrast control aims to enhance loudness difference between two neighboring regions. This enables us to make two different kinds of zone - the zone of quiet and the zone of loud sound - at the same time. The other perspective of this study is on the direction of sound. It is shown that we can control the direction of perceived sound source by focusing acoustic energy in wavenumber domain. To begin with, the proposed approaches are formulated for pure-tone case. Then the control methods are extended to a more general case, where the excitation signal has broadband spectrum. In order to control the broadband signal in time domain, an inverse filter design problem is defined and solved in frequency domain. Numerical and experimental results obtained in various conditions certainly validate that the acoustic brightness, acoustic contrast, direction of wave front can be manipulated for some finite region in space and time.

• Journal of the Korea institute for structural maintenance and inspection
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• v.26 no.1
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• pp.39-48
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• 2022

In earthquake situations, broadcasting and communication services are directly linked to rapid on-site rescue and effective restoration works. Recently, a variety of base isolation devices are widely introduced on building floors to avoid critical seismic damages of telecommunication facilities. However, in buildings with long fundamental periods, those devices may have undesirable amplification of seismic responses due to resonance effect between the building floors and base isolation devices. This study performs the seismic safety evaluation of two types of base isolation devices deployed for telecommunication facilities in mid- and high-rise buildings through numerical and experimental approaches. It is found that mid- and high-rise buildings can have low-frequency dynamic responses at the top floor when being subjected to design basis earthquake loading. Furthermore, bi-directional shake table testing demonstrated that the selected base isolation devices can exhibit unstable dynamic behaviors under such low-frequency excitations of the floor.

• Smart Structures and Systems
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• v.10 no.3
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• pp.229-251
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• 2012

Guided waves have shown a great potential for structural health monitoring (SHM) applications. In contrast to traditional non-destructive testing (NDT) methodologies, a key element of SHM approaches is the high process of automation. The monitoring system should decide autonomously whether the host structure is intact or not. A basic requirement for the realization of such a system is that the sensors are permanently installed on the host structure. Thus, baseline measurements become available that can be used for diagnostic purposes, i.e., damage detection, localization, etc. This paper contributes to guided wave-based inspection in anisotropic materials for SHM purposes. Therefore, computational strategies are described for both, the solution of the complex equations for wave propagation analysis in composite materials based on exact elasticity theory and the popular global matrix method, as well as the underlying equations of two active damage localization algorithms for anisotropic structures. The result of the global matrix method is an angular and frequency dependent wave velocity characteristic that is used subsequently in the localization procedures. Numerical simulations and experimental investigations through time-delay measurements are carried out in order to validate the proposed theoretical model. An exemplary case study including the calculation of dispersion curves and damage localization is conducted on an exemplary unidirectional composite structure where the ultrasonic signals processed in the localization step are simulated with the spectral element method. The proposed study demonstrates the capabilities of the proposed algorithms for accurate damage localization in anisotropic structures.

• Journal of the Korean Geotechnical Society
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• v.28 no.2
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• pp.71-78
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• 2012

Rainfall infiltration test under one dimensional condition is conducted to evaluate the effect of rainfall intensity on seepage velocity and infiltration characteristics for initial unsaturated sediment. Experimental results are compared with those numerical simulations with respect to variations of pore water pressure, degree of saturation and discharge velocity with time, and both results give good agreement. High rainfall intensity tends to increase seepage velocity almost linearly. But it shows rapid increase as rainfall intensity approaches saturated hydraulic conductivity of the sediment. In addition, the upper part of wetting front depth is partially saturated, not fully. Therefore, actual wetting front depth is considered to advance faster than theoretical prediction, which leads to slope instability of unsaturated slope due to surface rainfall.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.8 no.1
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• pp.44-51
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• 1996

The accurate calculation of run-up heights of long waves along the coastline is important in the view of engineering. In this paper the run-up heights of long waves are estimated by using the cnoidal wave theory which also covers both sinusoidal and solitary waves. However, the generation and the calculation of run-up heights of cnoidal waves are difficult both in laboratory and numerical experiments. In this study, the maximum run-up heights of cnoidal waves on steep slopes are computed by using the boundary integral equation model. It has been shown that the run-up heights of cnoidal waves are less than those of solitary waves, while they are larger than those of sinusoidal waves having the same wavelengths and heights. The variation of run-up heights of cnoidal waves is not a monotonic function of the wavelength. However, the run-up heights of cnoidal waves asymptotically approach that of a solitary wave as the wavelength approaches infinity. The calculated run-up heights agreed reasonably with experimental data.

• Ocean Systems Engineering
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• v.5 no.3
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• pp.139-160
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• 2015

The global performance of the 5MW OC4 semisubmersible floating wind turbine in random waves was numerically simulated by using the turbine-floater-mooring fully coupled and time-domain dynamic analysis program FAST-CHARM3D. There have been many papers regarding floating offshore wind turbines but the effects of second-order wave-body interactions on their global performance have rarely been studied. The second-order wave forces are actually small compared to the first-order wave forces, but its effect cannot be ignored when the natural frequencies of a floating system are outside the wave-frequency range. In the case of semi-submersible platform, second-order difference-frequency wave-diffraction forces and moments become important since surge/sway and pitch/roll natural frequencies are lower than those of typical incident waves. The computational effort related to the full second-order diffraction calculation is typically very heavy, so in many cases, the simplified approach called Newman's approximation or first-order-wave-force-only are used. However, it needs to be justified against more complete solutions with full QTF (quadratic transfer function), which is a main subject of the present study. The numerically simulated results for the 5MW OC4 semisubmersible floating wind turbine by FAST-CHARM3D are also extensively compared with the DeepCWind model test results by Technip/NREL/UMaine. The predicted motions and mooring tensions for two white-noise input-wave spectra agree well against the measure values. In this paper, the numerical static-offset and free-decay tests are also conducted to verify the system stiffness, damping, and natural frequencies against the experimental results. They also agree well to verify that the dynamic system modeling is correct to the details. The performance of the simplified approaches instead of using the full QTF are also tested.

• Journal of The Korean Society of Agricultural Engineers
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• v.53 no.1
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• pp.29-36
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• 2011

HPAI (High pathogenic avian influenza) which is a disease legally designated as an epidemic generally shows rapid spread of disease resulting in high mortality rate as well as severe economic damages. Because Korea is contiguous with China and southeast Asia where HPAI have occurred frequently, there is a high risk for HPAI outbreak. A prompt treatment against epidemics is most important for prevention of disease spread. The spread of HPAI should be considered by both direct and indirect contact as well as various spread factors including airborne spread. There are high risk of rapid propagation of HPAI flowing through the air because of collective farms mostly in Korea. Field experiments for the mechanism of disease spread have limitations such as unstable weather condition and difficulties in maintaining experimental conditions. In this study, therefore, computational fluid dynamics which has been actively used for mass transfer modeling were adapted. Korea has complex terrains and many livestock farms are located in the mountain regions. GIS numerical map was used to estimate spreads of virus attached aerosol by means of designing three dimensional complicated geometry including farm location, road network, related facilities. This can be used as back data in order to take preventive measures against HPAI occurrence and spread.

• Journal of Korea Society of Industrial Information Systems
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• v.26 no.1
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• pp.55-65
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• 2021

In this paper, a methodology for reinforcing reverse logistics (RL) activities in a closed-loop supply chain (CLSC) model is proposed. For the methodology, the activities of the recovery center (RC) which can be considered as one of the facilities in the RL are reinforced. By the reinforced activities in the RC, the recovered parts and products after checking and recovering processes of the returned product from customer can be reused in the forward logistics (FL) of the CLSC model. A mathematical formulation is suggested for representing the CLSC model with reinforced RL activities, and implemented using a hybrid genetic algorithm (HGA) approach. In numerical experiment, two different scales of the CLSC model are presented and the performance of the HGA approach is compared with those of some conventional approaches. The experimental results show that the former outperforms the latter in most of performance measures. The robustness of the CLSC model is also proved by regulating various rates of the recovered parts and products in the RC.

• Journal of the Korea institute for structural maintenance and inspection
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• v.6 no.4
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• pp.225-232
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• 2002

Recently the slab span-jointing method has been employed as one of the retrofit technologies to enhance the capacity of existing simple beam bridges in many cases. In general this method makes simple beam bridges behave like multi-span continuous bridges under service loads excluding self weight in company with external prestress force method in the field. In this paper the continuation effect has been studied for the retrofitted bridges by the experimental and numerical approaches. The results show that the deflections and stresses of members are reduced due to the increase of the total stiffness of bridge system and the efficiency of bridge continuation based on the slab span-jointing method is about 40 % when comparing with the case of continuous bridges.