• Journal of the Microelectronics and Packaging Society
• /
• v.16 no.1
• /
• pp.13-19
• /
• 2009

A 440MHz wireless and passive surface acoustic wave (SAW) based chemical sensor was developed on a $41^{\circ}YX\;LiNbO_3$ piezoelectric substrate for simultaneous measurement of $CO_2$ gas and relative humidity (RH) using a reflective delay line pattern as the sensor element. The reflective delay line is composed of an interdigital transducer (IDT) and several shorted grating reflectors. A Teflon AF 2400 and a hydrophilic $SiO_2$ layer were used as $CO_2$ and water vapor sensitive films. The coupling of mode (COM) modeling was conducted to determine optimal device parameters prior to fabrication. According to simulation results, the device was fabricated and then wirelessly measured using the network analyzer. The measured reflective coefficient $S_{11}$ in the time domain showed high signal/noise (S/N) ratio, small signal attenuation, and few spurious peaks. In the $CO_2$ and humidity testing, high sensitivity ($2^{\circ}/ppm$ for $CO_2$ detection and $7.45^{\circ}/%$RH for humidity sensing), good linearity and repeatability were observed in the $CO_2$ concentration ranges of $75{\sim}375ppm$ and humidity levels of $20{\sim}80%$RH. Temperature and humidity compensations were also investigated during the sensitivity evaluation process.

• The Journal of Engineering Geology
• /
• v.6 no.3
• /
• pp.137-153
• /
• 1996

The purpose of this study is to investigate the possibilities of identifying and detecting underground cavities using seismic waves recorded by the fixed and mobile stations. During 18 months of field work we recorded chemical explosions near the Bongdarn station. Seismic Stations were installed on the free surface and underground inside the Samba mine. The seismograms at the fixed(lorg-term) seismic station show abrupt change of polarization characteristics which can he associated with the appearance of P-to-S converted phase(PS) at 150 ~ 200 msec after the first P arrival. This result indicates that converted phases are generated very near to the Bongdarn station at a depth of 190m. Shear-wave splitting phenomena have also been observeci The time delay between fast shear(fS) and slow shear(sS) waves ranges between 30 and 60 msec(average is 42 msec). However, exact time delay between the fast and the slow shear waves can not be accurately measured because of the very short time delay and limitation of sampling rate. Chemical explosion experiments were recorded at stations along various paths to contrast the seismic response of areas with and without cavities. The seismograms recorded at the stations installed at cavity areas show an abrupt change of polarization characteristics but not on the other stations. Seismic waves propagating through the cavity are characterized by the attenuation of high frequency waves and predominantly low frequency seismic waves after the S wave arrivals.

• The Journal of Engineering Geology
• /
• v.31 no.4
• /
• pp.603-620
• /
• 2021

A land nodal seismic system was employed to acquire seismic reflection data using stand-alone cable-free receivers in a land-river area. Acquiring reliable data using this technology is very cost effective, as it avoids topographic problems in the deployment and collection of receivers. The land nodal airgun system deployed on the mouth of the Hyungsan River (in Pohang, Gyeongsangbuk Province) used airgun sources in the river and receivers on the riverbank, with subparallel source and receiver lines, approximately 120 m-spaced. Seismic data collected on the riverbank are characterized by a low signal-to-noise (S/N) and inconsistent reflection events. Most of the events are represented by hyperbola in the field records, including direct waves, guided waves, air waves, and Scholte surface waves, in contrast to the straight lines in the data collected conventionally where source and receiver lines are coincident. The processing strategy included enhancing the signal behind the low-frequency large-amplitude noise with a cascaded application of bandpass and f-k filters for the attenuation of air waves. Static time delays caused by the cross-offset distance between sources and receivers are corrected, with a focus on mapping the shallow reflections obscured by guided wave and air wave noise. A new time-distance equation and curve for direct and air waves are suggested for the correction of the static time delay caused by the cross-offset between source and receiver. Investigation of the minimum cross-offset gathers shows well-aligned shallow reflections around 200 ms after time-shift correction. This time-delay static correction based on the direct wave is found essential to improving the data from parallel source and receiver lines. Data acquisition and processing strategies developed in this study for land nodal airgun seismic systems will be readily applicable to seismic data from land-sea areas when high-resolution signal data becomes available in the future for investigation of shallow gas reservoirs, faults, and engineering designs for the development of coastal areas.

• Journal of the Society of Naval Architects of Korea
• /
• v.37 no.1
• /
• pp.67-81
• /
• 2000

An accurate and efficient numerical method for two-dimensional nonlinear radiation problem has been developed. The wave motion due to a moving body is described by the assumption of ideal fluid flow, and the governing Laplace equation can be effectively solved by the higher-order boundary element method with the help of the GMRES (Generalized Minimal RESidual) algorithm. The intersection or corner problem is resolved by utilizing the so-called discontinuous elements. The implicit trapezoidal rule is used in updating solutions at new time steps by considering stability and accuracy. Traveling waves caused by the oscillating body are absorbed downstream by the damping zone technique. It is demonstrated that the present method for time marching and radiation condition works efficiently for nonlinear radiation problem. To avoid the numerical instability enhanced by the local gathering of grid points, the regriding technique is employed so that all the grids on the free surface may be distributed with an equal distance. This makes it possible to reduce time interval and improve numerical stability. Special attention is paid to the local flow around the body during time integration. The nonlinear radiation force is calculated by the "acceleration potential technique". Present results show good agreement with other numerical computations and experiments.

• Journal of Ocean Engineering and Technology
• /
• v.24 no.5
• /
• pp.16-21
• /
• 2010

During the loading/offloading operation of a liquefied natural gas carrier (LNGC) that is moored in a side-by-side configuration with an offshore plant, sloshing that occurs due to the partially filled LNG tank and the interactive effect between the two floating bodies are important factors that affect safety and operability. Therefore, a time-domain software program, called CHARM3D, was developed to consider the interactions between sloshing and the motion of a floating body, as well as the interactions between multiple bodies using the potential-viscous hybrid method. For the simulation of a floating body in the time domain, hydrodynamic coefficients and wave forces were calculated in the frequency domain using the 3D radiation/diffraction panel program based on potential theory. The calculated values were used for the simulation of a floating body in the time domain by convolution integrals. The liquid sloshing in the inner tanks is solved by the 3D-FDM Navier-Stokes solver that includes the consideration of free-surface non-linearity through the SURF scheme. The computed sloshing forces and moments were fed into the time integration of the ship's motion, and the updated motion was, in turn, used as the excitation force for liquid sloshing, which is repeated for the ensuing time steps. For comparison, a sloshing motion coupled analysis program based on linear potential theory in the frequency domain was developed. The computer programs that were developed were applied to the side-by-side offloading operation between the offshore plant and the LNGC. The frequency-domain results reproduced the coupling effects qualitatively, but, in general, the peaks were over-predicted compared to experimental and time-domain results. The interactive effects between the sloshing liquid and the motion of the vessel can be intensified further in the case of multiple floating bodies.

• Journal of Navigation and Port Research
• /
• v.31 no.8
• /
• pp.683-687
• /
• 2007

Hydroelastic deformation of pontoon type floating structure in waves is critical in structural design. Therefore, it is necessary to develop additional technology that make to dissipate the wave energy as the submerged horizontal plate. In this study, we investigate the characteristics of hydrodynamic interaction effect by the submerged plate affecting to the radiation forces on a pontoon type floating structure using numerical analysis. We have developed the numerical method based on the composite grid system that consists of moving and fixed grid to compute the radiation forces due to the heaving motion of pontoon type floating structure and submerged plate. The numerical simulations based on the finite difference method are carried out to solve the fully nonlinear free surface involving the breaking waves and compared with the experimental data to confirm the reliability of the numerical method. Then, we discuss the interaction effects on the hydrodynamic forces that could influence on the hydroelastic response of floating structure.

• Journal of the Society of Naval Architects of Korea
• /
• v.54 no.6
• /
• pp.492-500
• /
• 2017

In the study, numerical simulation on the greenwater impact load of free surface offshore structure in the regular waves using fixed cartesian grid system and Modified Marker-Density (MMD) method were carried out and the results were reviewed. In order to compare numerical simulation and experimental results, the FPSO with the scale ratio of 1/100 model ship with fixed rectangular deck was selected and turbulence characteristic of the flow was considered by applying the Sub-Grid Scale (SGS) in laminar flow. As a result, it is reviewed how the greenwater impact load inflowed from bow in regular headsea wave influence the flow on the deck and the flow characteristic by numerical simulation and the experiment results were compared and reviewed. Based on this study, it would be useful to numerically study the effect of greenwater on offshore structure.

• Journal of Navigation and Port Research
• /
• v.38 no.6
• /
• pp.585-591
• /
• 2014

The primary objective of the current work is to predict speed performance of the medium patrol boat over $F_N=0.5$ employing experimental materials based on the CFD before model tests. In other words, the predicted brake powers according to ship speeds are assessed satisfying the main engine capacity. The subject ships are selected the two different stern hull forms. The flow computation are conducted considering free surface and dynamic trim using a commercial CFD code(STAR-CCM+). The resistances of the bare-hull are obtained from CFD. Wave patterns, pressures and limiting streamlines on the hull and velocity distribution in the propeller plane for the two hull forms are compared using CFD. The effective powers of the object ships are assessed based on CFD. Resistance increase according to the attached appendages and quasi-propulsive efficiency are employed the experimental datas. Speed performance prediction method concerning high speed vessels like a medium patrol boat is developed employing CFD and experimental data.

• Journal of the Korean Society of Fisheries and Ocean Technology
• /
• v.53 no.4
• /
• pp.446-455
• /
• 2017

The improvement of resistance performance for the 4.99 ton class fishing boats was shown. The 4.99 ton fishing boats are the most commonly used one in the Korean coastal region. The evaluation of resistance performance was estimated by the Computational Fluid Dynamics (CFD) analysis. The CFD simulation was performed by the validation for various types of bow shapes on the hull. The optimized hull form from the simulation was selected and showed the best resistance performance. This hull type was tested on the towing tank in the National Institute of Fisheries Science (NIFS). The effective horsepower (EHP) was estimated by the resistance test on the towing tank with the bare hull condition. The drag force on the three service speed conditions was obtained for the resistance analysis to power prediction. The measured drag forces are compared with the results from the CFD simulation with one another. As results of the model tests, it was confirmed that the shape of the bow is an important factor in the resistance performance. The effective horsepower decreased about 30% in comparison with the conventional hull form. Also, the resistance performance improved the reduction of required horsepower, which especially contributed to the energy-saving for the fisheries industry. In the CFD analysis, the resistance performance improved slightly. In this case, the ratio of the residual resistance ($C_R$) in the total resistance ($C_T$) was high. Therefore, the CFD analysis was not enough to satisfy with reflection for the free surface and wave form in the CFD procedure. Both model test and CFD calculation in this study can be applied to the initial design process for the coastal fishing vessel.

• Journal of Korea Water Resources Association
• /
• v.32 no.3
• /
• pp.237-246
• /
• 1999

A finite element model is studied to simulate unsteady free surface flow based on dynamic wave equation and collocation method. The collocation method is used in conjunction with Hermite polynomials, and resulting matrix equations are solved by skyline method. The model is verified by applying to hydraulic jump, nonlinear disturbance propagation and dam-break flow in a horizontal frictionless channel. The computed results are compared with those by Bubnov-Galerkin and Petrov-Galerkin methods. It is also applied to the North Han River to simulate the floodwave propagation. The computed results have good agreements with those of DWOPER model in terms of discharge hydrographs. The suggested model has proven to be one of the promising scheme for simulating the gradually and rapidly varied unsteady flow in open channels.