• Journal of Korea Water Resources Association
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• v.32 no.6
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• pp.607-616
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• 1999

A fractional step finite difference model for the longitudinal dispersion of nonconservative contaminants is developed. It is based on splitting the longitudinal dispersion equation into a set of three equations each to be solved over a one-third time step. The fourth-order Holly-Preissmann scheme, an analytic solution, and the Crank-Nicholson scheme are used to solve the equations for the pure advection, the first-order decay, and the diffusion, respectively. To test the model, it is applied to simulate the longitudinal dispersion of continuous source released into a nonuniform flow field as well as the dispersion of an instantaneous source in a uniform flow field. The results are compared with the exact solution and those computed by an existing model. Compared to the existing model which uses Euler method for the first-order decay equation, the present model yield more accurate results as the decay coefficient increases.

• Korean Journal of Environmental Agriculture
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• v.14 no.1
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• pp.7-14
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• 1995

This research was conducted to apply the QUAL2E model to be adopted to the Nam river under current water quality conditions. The survey area of total 60 Km was divided into five reaches. Each reach was then subdivided into the uniform computational elements of 1.5 Km. Based on the stream characteristics, nine sampling stations consisting of six at main streams and three at tributaries were selected. The field data were obtained from the selected stations twice during October of 1991 and May of 1992, which represented the cold weather and low flow, also the warm weather and low flow conditions, respectively. As the results of sensitivity analysis of the model, the important parameters were the rates of BOD decay, Org-N oxidation, $NH_3-N$ oxidation, Org-P decay. The calibrated and verified results by QUAL2E model were correlation coefficient of $0.45{\sim}0.94$. The results displayed a good agreement between the variables of the field measurements and the model simulations, indicating a potential use of the QUAL2E model for the water quality assessment in the Nam River.

• 제어로봇시스템학회:학술대회논문집
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• 1987.10a
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• pp.729-733
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• 1987

Stability of Co Semigroups perturbed via the steady state Riccati equation (SSRE) is studied. We consider an infinite dimensional system : .chi. over dot = A.chi. + Bu, in, (A), domain of A, where A is the infinitesimal generator of a Co semigroup [T(t), t.geq.0] in H. If the original Co semigroup [T(t), t.geq.0] has a lower bound : vertical bar T(t).chi. vertical bar .geq. k vertical bar .chi. vertical bar, for all .chi. in H. t.geq. 0 and k>0, then the perturbed Co semigroup via the SSRE, where the feedback operator B is compact, cannot be exponentially stable. Physical interpretation of this result is as follows : in real applications, a finite number of actuators are available, therefore the operator B is compact. When the original system is inherently unstable, that is, has an infinite number of unstable modes, the perturbed system via the SSRE cannot be stable with a uniform decay rate.

• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.17 no.2
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• pp.131-139
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• 1988

The thermoacoustic oscillation induced in an air column with variable cross section area is investigated theoretically and experimentally. The onset condition of the oscillation is derived by equating the acoustic power production to the power dissipation. The power production at the heater is predicted by using the efficiency factor obtained by heat transfer analysis for a single wire in a uniform cross flow and considering the interference between heater wires. The power dissipation is estimated by measuring the attenuating coefficient from the pressure decay curve. The theoretical prediction to the onset condition of the oscillation is confirmed experimentally. The effect of the variation of the column cross section area on the onset condition is presented.

• Proceedings of the Korea Water Resources Association Conference
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• 1984.07a
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• pp.7-21
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• 1984

Entrainment and deposition experiments were counducted in fresh water on four groups of sediments: three well-defined sediments of uniform composition and narrow-size distribution (1 to 9 um, 10 to 50 um, and 50 to 90 um), and a fourth group which was a mixture of these three sediments. In the entrainment experiments and at a particular stress, the steady-state suspended sediment concentration of the coarse group was the lowest while the concentrations of the fine and medium groups were higher that that of the coarse group but were similar to each other. Deposition experiments generally showed an exponential decrease of suspended sediment concentration with time with the decay time being a function of particle size and applied stress.

• Proceedings of the Optical Society of Korea Conference
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• 1989.02a
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• pp.20-25
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• 1989

The nonlinear, time dependent photon transport equations of Frantz and Nodvik, which describe the amplification of an optical pulse in an active medium, are modified to a simpler equation which describes only the amplification of energy. with this equation, the output energy of the high power YLF(Nd3+)-Phosphate Glass(Nd3+) Laser System is calculated. When the stored energy density Est is 0.10J/㎤, 0.16J/㎤, 0.228J/㎤, and 0.50J/㎤, and with the assumption of uniform population inversion density, the final output energy of this laser system is 5.38J, 176J, 317J, and 283J, respectively. The gain saturation causes distortion of the output beam. This phenomenon is described in detail at the first three rod amplifier systems in the case of E=0.228J/㎤. The peak current and decay time constant of the flashlamps, which are used to obtain population inversion in the active medium, are investigated. The flashlamp driving circuit which has optimum operational performance should have {{{{ SQRT { LC} }} time about 100$\mu$sec.

• Journal of Mechanical Science and Technology
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• v.17 no.8
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• pp.1148-1155
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• 2003

The finite element method is one of the methods widely applied for predicting vibration in mechanical structures. In this paper, the effect of the mesh size of the finite element model on the accuracy of the numerical solutions of the structural vibration problems is investigated with particular focus on obtaining the optimal mesh size with respect to the solution accuracy and computational cost. The vibration response parameters of the natural frequency, modal density, and driving point mobility are discussed. For accurate driving point mobility calculation, the decay method is employed to experimentally determine the internal damping. A uniform plate simply supported at four corners is examined in detail, in which the response parameters are calculated by constructing finite element models with different mesh sizes. The accuracy of the finite element solutions of these parameters is evaluated by comparing with the analytical results as well as estimations based on the statistical energy analysis, or if not available, by testing the numerical convergence. As the mesh size becomes smaller than one quarter of the wavelength of the highest frequency of interest, the solution accuracy improvement is found to be negligible, while the computational cost rapidly increases. For mechanical structures, the finite element analysis with the mesh size of the order of quarter wavelength, combined with the use of the decay method for obtaining internal damping, is found to provide satisfactory predictions for vibration responses.

• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.179-182
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• 2007

Dynamical friction plays an important role in reducing angular momenta of objects in orbital motions. While astronomical objects usually follow curvilinear orbits, most previous studies focused on the linear-trajectory cases. Here, we present the gravitational wake due to, and dynamical friction on, a perturber moving on a circular orbit in a uniform gaseous medium using a semi-analytic method. The circular orbit causes the density wakes to bend along the orbit into asymmetric configurations, resulting in the drag forces in both opposite (azimuthal) and lateral (radial) directions to the perturber motion, although the latter does not contribute to the orbital decay much. For a subsonic perturber, the bending of a wake is only modest and the resulting drag force in the opposite direction is remarkably similar to the linear-trajectory counterpart. On the other hand, a supersonic perturber is able to overtake its own wake, possibly multiple times, creating a high-density trailing tail. Despite the dramatic changes in the wake morphologies, the azimuthal drag force is in surprisingly good agreement with the formulae of Ostriker for the linear-trajectory cases, provided $V_pt=2R_p,\;where\;V_p\;and\;R_p$ are the velocity and orbital radius of the perturber, respectively.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.4
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• pp.1395-1405
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• 1996

The purpose of this experimental research is to investigate the local heat transfer characteristics in the upward free water jet impinged on a downward flat plate of uniform heat flux. The inner shape of rectangular nozzle used was sine curve type and its contraction ratio of inlet to outlet area was five. Experimental parameters considered were Reynolds number, nozzle exit-flat plate distance, and level of supplementary water. Local Nusselt number was influenced by Reynolds number, Prandtl number, supplementary water level, and distance between the nozzle exit and flat plate. Within the impingement region, the Nusselt number has a maximum value on the nozzle center axis and decreases monotonically outward from center. Outside of the impingement region, on the other hand, the Nusselt number has a secondary peak near the position where the distance from nozzle center reaches four times the nozzle width. However if nozzle exit velocity exceeds 6.2 m/s, the secondary peak appears also in the impingement region. The empirical equation for the stagnation heat transfer is a function of Prandtl, Reynolds, and axial distance from the nozzle exit. The optimum level of supplementary water to augment the heat transfer rate at stagnation point was found to be twice the nozzle width.

• Proceedings of the Korea Water Resources Association Conference
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• 1993.07a
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• pp.175-182
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• 1993

The complex nature of low flow transport and tranformation of nonconservative pollutants in natural streams with pools and riffles has been investigated using a numerical solution of a proposed mathematical model that is based on a set of mass balance equations describing hydrodynamic processes (advection, dispersion, and mass exchange mechanicms in streams and in storage zones) and chemical processes (reaction or decay). In this study, a mathematical model (named "Storage-Transformation Model") has been developed to predict adequately the non-Fickian nature of mixing and transformation mechanisms for decaying substances in natural streams under low flow conditions. Comparisons between the concentration-time curves predicted usingthe proposed model and the measured stream data shows that the Storage-Transformation Model yields better agreements in the goneral shape, peak concentration and time to peak than the 1-D dispersion model. The result of this study also demonstrates the differences between transport in pool-and-riffle streams versus transport in more uniform channels. The proposed model shows significant improvement over the conventional 1-D disperision model in predicting natural mixing and stroage processes in streams through pools and riffles.