• Journal of Communications and Networks
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• v.6 no.4
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• pp.376-386
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• 2004

In ad hoc networks, loss-based congestion window progression by the traditional means of duplicate ACKs and timeouts causes high network buffer utilization due to large bursts of data, thereby degrading network bandwidth utilization. Moreover, network-oriented feedbacks to handle route disconnection events may impair packet forwarding capability by adding to MAC layer congestion and also dissipate considerable network resources at reluctant intermediate nodes. Here, we propose a new TCP scheme that does not require the participation of intermediate nodes. It is a purely end-to-end scheme using TCP timestamps to deduce link conditions. It also eliminates spurious reductions of the transmission window in cases of timeouts and fast retransmits. The scheme incorporates a receiver-oriented rate controller (rater), and a congestion window delimiter for the 802.11 MAC protocol. In addition, the transient nature of medium availability due to medium contention during the connection time is addressed by a freezing timer (freezer) at the receiver, which freezes the sender whenever heavy contention is perceived. Finally, the sender-end is modified to comply with the receiver-end enhancements, as an optional deployment. Simulation studies show that our modification of TCP for ad hoc networks offers outstanding performance in terms of goodput, as well as throughput.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.10
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• pp.1377-1384
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• 2003

An analysis has been performed for active thermal control of the KOMPSAT monopropellant rocket engine assembly, i.e., dual thruster module(DTM). The main efforts of this work have been directed at determining proper heater sizes for propellant valves and catalyst beds necessary to maintain their temperatures within specified temperature ranges under KOMPSAT environment and operational conditions. The TAS incorporated with TRASYS thermal radiation analyzer was used to establish a complete heat transfer model which allows to predict the DTM temperature as a function of time. The thermal analysis has been performed in transient mode to verify the appropriate power for catalyst bed heaters necessary to increase catalyst bed temperature to the required value within a specified period of time. Similar analysis has been executed to validate the heater power for the thermostatically controlled primary and redundant heater circuits used to prevent hydrazine freezing, i.e., single fault. Moreover the effect of the radiative property of thermal control coating of heat shield was examined. Thruster firing condition was also simulated for the heat soakback condition. As a consequence, all thermal analysis results for DTM satisfactorily met the thermal requirements for the KOMPSAT DTM under the worst case average voltage, i.e. 25 volt.

• Journal of the Korea Institute of Building Construction
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• v.5 no.4 s.18
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• pp.115-120
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• 2005

In recent years, polymer-modified mortars using polymer dispersions have been widely used as finish and repair materials in the construction industry because of their excellent properties compares to those of ordinary cement mortar. Especially, the adhesion improvement of ordinary cement mortar and concrete has attracted a great deal of attention from researchers, and several unique and simply applicable techniques for the adhesion improvement have been developed. The purpose of this study is to evaluate the abrasion resistance of polymer-modified mortar according various curing methods. The polymer-modified mortar are prepared with various polymer-cement ratios, and are subjected to three curing methods such as dry rure, standard cure and freezing and thawing cure after two curing methods, and then tested for abrasion. From the test results, the polymer-modified mortars with various polymer-cement ratios have some superior abrasion resistance compared with plain mortar. The abrasion resistance of polymer-modified mortars increase with an increase in the polymer-cement ratio, and is better under water cure than any other curing methods. It is concluded that the abrasion resistance of cement mortar is markedly improved by modifying of polymer dispersion.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.17 no.2
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• pp.30-36
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• 2018

Since the freezer compartment and the refrigerating compartment are located side by side in a side-by-side refrigerator, the problems of the door height difference (DHD) and door flatness difference (DFD) have been constantly raised. Deformation of the cabinet of a built-in side-by-side refrigerator under food and thermal loads was analyzed by the finite element software ANSYS. The DHD and DFD, occurring due to the deformation of the cabinet, evaluated. From the results of the analysis of the cabinet, the 3D CAD software CATIA was used to geometrically translate and rotate the freezing and refrigerating compartment doors, in consideration of the displacement of the hinge fastening point. Then, the coordinates of two points on the upper corner of the doors were determined, and the DHD and DFD were obtained. It found that the thermal load, occurring under normal operation conditions, decreases the door height difference, but increases the door flatness difference. Values of the analyzed DHD and DFD appear smaller than the acceptance criteria used by the refrigerator manufacturer.

• The Korean Journal of Food And Nutrition
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• v.33 no.1
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• pp.1-8
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• 2020

The purpose of this study was to prevent excessive softening of seasoned spinach by heating and freezing through the pre-heating treatment in calcium chloride solution. The pre-heating temperature was set at 60℃, which supports the highest activity of pectinesterase around 60℃. Pre-heating spinach in calcium solution does not affect the chromaticity characteristics of spinach leaves and significantly improves its texture compared to conventional blanching only and pre-heating in water. The improvement of the firmness is related to the formation of strongly cross-linkages between carboxyl groups and Ca²⁺ by the action of the pectinesterase in spinach. For reheated spinach, the firmness of calcium chloride treated stems were 37.80~44.44 kg, and the control was 28.73 kg. The firmness of calcium chloride treated leaves was 19.73~40.79 kg, and the control was 9.63 kg. Additionally, the total aerobic bacteria in the fresh samples were 3.25~3.99 log CFU/g and 1.97~2.72 log CFU/g in the reheated samples. And the total coliform was not detected in the reheated samples. Considering the color, texture, microbial characteristics and taste quality, the optimum pre-treatment conditions to prevent excessive softening of spinach were 3% calcium chloride solution and 15minutes of treatment.

• The Journal of Korean Society for School & Community Health Education
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• v.8 no.2
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• pp.79-95
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• 2007

A study was conducted in order to develope HACCP model in school lunch in Korea. Results: 1. Of 22 menus 4(18%) were non heating processes (#1), 2(9%) were food handling by using hands (#2), and 16(73%) were heating processes (#3). Of 279 menus 36(12.9%) belong to process #1, 8(2.9%) to process #2, and 235(84%) to process #3. 2. The critical control points for process #1 were contamination by hands of food handlers, and unsanitary food preparation habits of food handlers. Those for process #2 were improper heating temperature, contaminations by food handlers' hands, and unsanitary food handling habits, and cross contamination by unclean utensils and equipment. 3. Management criteria for the CCPs were conditions of food storage, refrigeration, freezing, food cooking temperature, personal hygiene, washing and sanitization of utensils and equipment. 4. Monitoring criteria for CCPs were observation, temperature checking, inspection of utensils and equipment, and practice of good personal hygiene. 5. Corrective actions were refusal of unsafe products, correction of improper temperature, proper cleaning and sanitization, and proper reheating time and temperature.

• Journal of the Korea Concrete Institute
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• v.17 no.5 s.89
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• pp.857-868
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• 2005

This paper deals with the accumulated damage in concrete structures due to the cyclic freeze-thaw as an environmental load. The cyclic ice body nucleation and growth processes in porous systems are affected by the thermo-physical and mass transport properties, and gradients of temperature and chemical potentials. Furthermore, the diffusivity of deicing chemicals shows significantly higher value under cyclic freeze-thaw conditions. Consequently, the disintegration of concrete structures is aggravated at marine environments, higher altitudes, and northern areas. However, the properties of cyclic freeze-thaw with crack growth and diffusion of chloride ion effects are hard to be identified in tests, and there has been no analytic model for the combined degradations. The main objective is to determine the driving force and evaluate the reduced strength and stiffness by freeze-thaw. For the development of computational model of those coupled deterioration, micro-pore structure characterization, pore pressure based on the thermodynamic equilibrium, time and temperature dependent super-cooling with or without deicing salts, nonlinear-fracture constitutive relation for the evaluation of internal damage, and the effect of entrained air pores (EA) has been modeled numerically. As a result, the amount of ice volume with temperature dependent surface tensions, freezing pressure and resulting deformations, and cycle and temperature dependent pore volume has been calculated and compared with available test results. The developed computational program can be combined with DuCOM, which can calculate the early aged strength, heat of hydration, micro-pore volume, shrinkage, transportation of free water in concrete. Therefore, the developed model can be applied to evaluate those various practical degradation cases as well.

• Journal of Pharmaceutical Investigation
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• v.35 no.2
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• pp.71-74
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• 2005

The aim of this study was to analyze characteristics of the barrier function of excised porcine buccal mucosa to the test compounds, estradiol, propranolol HCI, melatonin, and mannitol with a wide range of partition coefficient values. The permeability of melatonin was measured through frozen, stored, and fresh porcine buccal mucosa to examine the impact of storage conditions on the permeability of porcine buccal mucosa. The results demonstrated that the ex vivo permeability of the porcine buccal mucosa was greater for more lipophilic solutes, which was consistent with a series of molecules transported by passive transepithelial diffusion. The melatonin permeation profiles through frozen, stored, and fresh mucosa illustrated that damage was incurred by the freezing process of the mucosal tissue, leading to loss of the barrier function and thereby an increased permeation coefficient. It can be observed that the influence of compound lipophilicity on the association of the compounds with buccal mucosa was clear. The relationship between permeation coefficient and Log P values for the four compounds investigated demonstrated a proportional relationship, further confirming the importance of the lipophilicity of a compound to permeate the buccal mucosa. These results showed that the ex vivo porcine buccal mucosa model is a suitable tool to screen oral mucosal permeability.

• Computers and Concrete
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• v.19 no.1
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• pp.111-120
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• 2017

This study aimed to design the accelerated life testing (ALT) of concrete, which stimulating the special natural environment maximumly. Its evaluation indexes, such as dynamic elastic modulus, mass and ultrasonic velocity were measured, and the variation of relative mass and relative dynamic elastic modulus of concrete were studied. Meanwhile, the microanalysis method was used. Moreover, an exploratory application of the stochastic approach, the Weibull distribution and the lognormal distribution, were made to assess the durability of concrete structures. The results show that the ALT for simulating natural environment is more close to the service process of concrete structure under actual conditions; The relative dynamic elastic modulus can be used as the dominant durability evaluation parameters, because it is more sensitive to the environmental factors compared with the relative quality evaluation parameters; In the course of the concrete deterioration, the destruction of the salt freezing cycle is the dominant factor, supplemented by other factors; Both of those two stochastic approaches can be used to evaluate the reliability of concrete specimens under the condition of ALT; By comparison, The lognormal distribution method is better to describe the reliability process.

• Journal of Powder Materials
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• v.23 no.3
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• pp.189-194
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• 2016

We report a facile method for preparing KIT-1 mesoporous silicates with two different macroporous structures by dual templating. As a template for macropores, polystyrene (PS) beads are assembled into uniform three dimensional arrays by ice templating, i.e., by growing ice crystals during the freezing process of the particle suspension. Then, the polymeric templates are directly introduced into the precursor-gel solution with cationic surfactants for templating the mesopores, which is followed by hydrothermal crystallization and calcination. Later, by burning out the PS beads and the surfactants, KIT-1 mesoporous silicates with macropores are produced in a powder form. The macroporous structures of the silicates can be controlled by changing the amount of EDTANa4 salt under the same templating conditions using the PS beads and inverse-opal or hollow structures can be obtained. This strategy to prepare mesoporous powders with controllable macrostructures is potentially useful for various applications especially those dealing with bulky molecules such as, catalysis, separation, drug carriers and environmental adsorbents.