• Membrane and Water Treatment
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• v.1 no.1
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• pp.13-37
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• 2010

The performance of an electrodialyzer for concentrating seawater is predicted by means of a computer simulation, which includes the following five steps; Step 1 mass transport; Step 2 current density distribution; Step 3 cell voltage; Step 4 NaCl concentration in a concentrated solution and energy consumption; Step 5 limiting current density. The program is developed on the basis of the following assumption; (1) Solution leakage and electric current leakage in an electrodialyzer are negligible. (2) Direct current electric resistance of a membrane includes the electric resistance of a boundary layer formed on the desalting surface of the membrane due to concentration polarization. (3) Frequency distribution of solution velocity ratio in desalting cells is equated by the normal distribution. (4) Current density i at x distant from the inlets of desalting cells is approximated by the quadratic equation. (5) Voltage difference between the electrodes at the entrance of desalting cells is equal to the value at the exits. (6) Limiting current density of an electrodialyzer is defined as average current density applied to an electrodialyzer when current density reaches the limit of an ion exchange membrane at the outlet of a desalting cell in which linear velocity and electrolyte concentration are the least. (7) Concentrated solutions are extracted from concentrating cells to the outside of the process. The validity of the computer simulation model is demonstrated by comparing the computed results with the performance of electrodialyzers operating in salt-manufacturing plants. The model makes it possible to discuss optimum specifications and operating conditions of a practical-scale electrodialyzer.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.28 no.3
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• pp.124-131
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• 2016

In this study, a physical experiment was performed to investigate the change in the stability coefficient, defined by Hudson equation, for the tetrapod of different specific densities. The experiment was carried out once (with no repetition) for a rubble mound breakwater with 1:1.5 slope. In this experiment, the stability coefficient for the high-density tetrapod was greater than that for the normal-density tetrapod. This indicates variability of the stability coefficient according to change in the density of tetrapod. Further experiments and detailed analysis are required to investigate the effect of the density on the stability coefficient of tetrapod.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.5
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• pp.1150-1160
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• 1993

The behavior of polystyrene in the strip cavity during the packing and cooling stages for an injection molding process is examined numerically. The mathematical model is based on the unified post-filling model and finite element/finite difference methods are used to solve simultaneously the continuity, momentum and energy equations coupled to an equation of state. Simulated results show that the density of the molded parts is lower in the core than at the skin, and that the hotter the melt or the higher the packing pressure, the higher the density in the core. The density variation during the packing stage comes up to 50% compared with the total density variation. Also, the density variation after gate sealing and the effect of cooling rate on the equation of state are negligible.

• Imaging Science in Dentistry
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• v.41 no.2
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• pp.59-62
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• 2011

Purpose : This study was performed to evaluate the changes of jaw bone density around the dental implant after placement using computed tomography scan (CT-Scan). Materials and Methods : This retrospective study consisted of 30 patients who had lost 1 posterior tooth in maxilla or mandible and installed dental implant. The patients took CT-Scan before and after implant placement. Hounsfield Unit (HU) was measured around the implants and evaluated the difference of HU before and after implant installation. Results : The mean HU of jaw bone was 542.436 HU and 764.9 HU before and after implant placement, respectively (p<0.05). The means HUs for male were 632.3 HU and 932.2 HU and those for female 478.2 HU and 645.5 HU before and after implant placement, respectively (p<0.05). Also, the jaw bone with lower density needed longer period for implant procedure and the increased change of HU of jaw bone was less in the cases which needed longer period for osseointegration. Conclusion : CT-Scan could be used to assess the change of bone density around dental implants. Bone density around dental implant was increased after placement. The increased rate of bone density could be determined by the quality of jaw bone before implant placement.

• Textile Coloration and Finishing
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• v.17 no.5 s.84
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• pp.45-52
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• 2005

The optimum fabric design plan is recently needed with the relevant fabric shrinkage in dyeing and finishing processes for the various sensible fabrics for garment. For responding this need, the difference of fabric design plan such as fabric density, yarn count and finishing shrinkage has to be surveyed with weaving looms such as water jet, air-jet and rapier looms, and also has to be analysed with weave pattern such as plain, twill and satin according to the fabric manufacturers. Therefore, in this study, 420 polyester weave pattern sheets are collected from 5 fabric manufacturers, and the weave density coefficients of these fabrics are calculated using yarn count, fabric density and 1 repeat weave pattern. These weave density coefficients are analysed according to the fabric manufacturers, weaving looms and various weave patterns. And also the weave density coefficients of 420 polyester fabrics are analysed with finishing shrinkage according to the fabric manufacturers and various weave patterns such as plain, twill and satin fabrics.

• Ocean and Polar Research
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• v.34 no.1
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• pp.85-91
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• 2012

Physical properties such as sound speed contrast (h) and density contrast (g) of the interested target are key parameters to understand acoustic characteristics by using theoretical scattering models. The density and sound speed of moon jellyfish (common jellyfish, Aurelia aurita s.l.) were measured. Sound speed contrast (h) was measured from travel time difference (time-of-flight method) of an acoustic signal in a water tank for APOP studies (Acoustic Properties Of zooplankton). Density contrast (g) was measured by the displacement volume and wet weight (dual-density method). The sound speed remained almost constant as the moon jellyfish increased in bell length. The mean values${\pm}$standard deviation of h and g were $1.0005{\pm}0.0012$ and $0.9808{\pm}0.0195$), respectively. These results will provide important input for use in theoretical scattering models for estimating the acoustic target strength of jellyfish.

• Solar Energy
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• v.8 no.2
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• pp.12-18
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• 1988

The effect of surface recombination current density on the saturation current density in Si solar cell has been studied. Theoretical model for surface recombination current was set up from emitter transparent model of M.A. Shibib, and saturation current of Si solar cell made by ion implantation method was also measured by digital electrometer. The theoretical surface recombination current density which is the same as saturation surface recombination current density in Shibib model was $10^{-11}[A/cm^2]$ and the measured value was ranged from $8{\times}10^{-10}$ to $2{\times}10^{-9}[A/cm^2]$. Comparing with the ideal p-n junction of Shockley, transparent emitter model shows improved result by $10^2$ order of saturation current density. But there still exists $10^2$ order of difference of saturation current density between theoretical and actual values, which are assumed to be caused by 1) leakage current through solar cell edge, 2) recombination of carriers in the depletion layer, 3) the series resistance effect and 4) the tunneling of carriers between states in the band gap.

• Journal of Korean Society of Water and Wastewater
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• v.30 no.5
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• pp.553-559
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• 2016

There has been increasing global interest in the environmental pollution problems produced by fossil fuel consumption and greenhouse gas emissions. In order to tackle these issues, new renewable energy such as solar, wind, bio gas, fuel cell and pressure retarded osmosis(PRO) have been developed extensively. Among these energy sources, PRO is one of the salinity gradient power generation methods. In PRO, energy is obtained by the osmotic pressure generated from the concentration difference between high and low concentration solutions separated by a semipermeable membrane. The development for high power density PRO membranes is imperative with the purpose of commercialization. This study investigates development of thin film composite PRO membrane and spiral wound module for high power density. Also, the influence of membrane backing layer on power density was identified, and the characteristic factors of PRO membranes was determined. Different backing layers were used to improve power density. As expected, the PRO membrane with more porous backing layer showed higher power density.

• Journal of muscle and joint health
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• v.18 no.1
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• pp.73-82
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• 2011

Purpose: The purpose of this study was to investigate the effects of a 12-week Tai Chi on the bone mineral density and bone metabolic markers in postmenopausal women Methods: Data were collected from March to July, 2009. Fifty postmenopausal women were recruited for the study. Twenty two women were allocated to experimental group, and 28 to control group. The experimental group underwent Tai Chi exercise twice a week for twelve weeks. The control group was only notified with results of bone mineral density and bone metabolic markers. Bone mineral density was measured by using of DTX-200 (Osteometer MediTech, Hawthorne, CA, USA) at distal radius site and bone metabolic markers were measured by radioimmunoassay method. Collected data were analyzed by t-test, $X^2$-test, and Mann-Whitney test. Results: After 12 weeks of treatment, the Tai Chi group showed a significant difference in bone mineral density compared to control group but no significant effect on osteocalcin and deoxypyridinoline level. Conclusion: Our results suggest that 12 weeks of Tai chi may delay bone loss in postmenopausal women.

• Proceedings of the Korean Society for Emotion and Sensibility Conference
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• 2001.11a
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• pp.169-172
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• 2001

There are many people who suffer from simulation sickness when immersing in virtual reality. In this study, we analyzed two photoplethysmogram(PPG) parameters - a second derivative parameter and power spectral density ratios - in order to relate PPG parameters with simulation sickness. 36 young, healthy subjects were participated in the experiment, and each subject was equipped with a PPG electrode during his or her immersion. Simulation sickness section was defined as a 7 - second section which starts from the point where a subject reported simulation sickness, and normal section as a same-length section where no physical stimuli was presented to him or her. We compared the PPG parameters of the simulation sickness sections with the normal sections, - d/a ratio is believed to have lower value during vasodilation and higher value during vasoconstriction, however, we could not find much difference in the parameter between normal and simulation sickness sections. We also compared 1 to 10Hz power spectral density ratios in normal sections with in simulation sickness section, and found that 6 density ratios among them have different value. Therefore, the density ratios might be utilized as parameters to detect simulation sickness of subjects.