• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.29 no.4
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• pp.167-174
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• 2017

This study attempted to investigate the value of photoelectric sensors in terms of a defrost-control method. Tests were conducted in a calorimeter room under the heating with the defrost-performance test conditions described in KS C 9306. Accordingly, the photoelectric technology is a competitive defrost-control method that can precisely control the operational defrost cycle using the output voltages that are proportional to the frost height. The heating period is gradually reduced because the complex defrost-control method, for which the sensors initiate the defrosting process and the defrosting process is terminated by the time parameter, could not adjust the net defrosting time by itself. Therefore, a complex defrost-control method, for which the photoelectric sensors start the defrosting process and it is terminated by the temperature parameter, is preferred because of the adjustment of the net defrosting time. Regardless of the defrost-control method, the first defrosting cycle is activated earlier than the times that are determined in the second and third cycles and so on, because the first operation cycle can decide the characteristics of the subsequent cycle.

• Nuclear Engineering and Technology
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• v.49 no.6
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• pp.1157-1164
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• 2017

The Monte Carlo criticality simulation of decoupled systems, as for instance in large reactor cores, has been a challenging issue for a long time. In particular, due to limited computer time resources, the number of neutrons simulated per generation is still many order of magnitudes below realistic statistics, even during the start-up phases of reactors. This limited number of neutrons triggers a strong clustering effect of the neutron population that affects Monte Carlo tallies. Below a certain threshold, not only is the variance affected but also the estimation of the eigenvectors. In this paper we will build a time-dependent diffusion equation that takes into account both spatial correlations and population control (fixed number of neutrons along generations). We will show that its solution obeys a traveling wave dynamic, and we will discuss the mechanism that explains this biasing of local tallies whenever leakage boundary conditions are applied to the system.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.24 no.7
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• pp.885-890
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• 2020

In the distance measurement system the time-to-digital conversion circuit used measures the distance using the time interval between the start signal and the stop signal. The time interval is generally converted to digital information using a counter circuit considering the response speed. Therefore, a clock signal with a high frequency is required to improve precision, and a clock signal with a high frequency is also required to measure fine distances. In this paper, a counter circuit was designed to increase the accuracy of distance measurement while using the same frequency. The circuit design was performed using a 0.18㎛ CMOS process technology, and the operation of the designed circuit was confirmed through HSPICE simulation. As a result of the simulation, it is possible to obtain an improvement of four times the precision compared to the case of using a general counter circuit.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2022.11a
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• pp.215-216
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• 2022

Profit, the performance of an apartment development project, is directly affected by the sales ratio, unit sale price, financial costs, land costs and construction costs. However, these factors fluctuate in response to changes in the environment, including various stake holders, and the profits fluctuate as a result. In order to ensure that profits are managed within target levels, these factors must be able to be predicted, controlled and monitored and managed up to the start, sale, and end stages of the project. The purpose of this study is to develop a profit risk management model for apartment development projects. The results of this study will contribute to the establishment of academic basis for the dynamic management of project profits that fluctuate with time and environment. And in practice, it will help project developers manage their business revenue to the proper level. In addition, the risks that occur from time to time can be identified quantitatively and visually, and it is expected that it will be easier to derive consensus points for smooth business progress by reducing conflicts of interest among stakeholders.

• Journal of Computing Science and Engineering
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• v.1 no.1
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• pp.56-73
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• 2007

Demands have been growing in safety-critical application fields for producing networked real-time embedded computing (NREC) systems together with acceptable assurances of tight service time bounds (STBs). Here a service time can be defined as the amount of time that the NREC system could take in accepting a request, executing an appropriate service method, and returning a valid result. Enabling systematic composition of large-scale NREC systems with STB certifications has been recognized as a highly desirable goal by the research community for many years. An appealing approach for pursuing such a goal is to establish a hard-real-time (HRT) component model that contains its own STB as an integral part. The TMO (Time-Triggered Message-Triggered Object) programming scheme is one HRT distributed computing (DC) component model established by the first co-author and his collaborators over the past 15 years. The TMO programming scheme has been intended to be an advanced high-level RT DC programming scheme that enables development of NREC systems and validation of tight STBs of such systems with efforts far smaller than those required when any existing lower-level RT DC programming scheme is used. An additional goal is to enable maximum exploitation of concurrency without damaging any major structuring and execution approaches adopted for meeting the first two goals. A number of previously untried program structuring approaches and execution rules were adopted from the early development stage of the TMO scheme. This paper presents new concrete justifications for those approaches and rules, and also discusses new extensions of the TMO scheme intended to enable further exploitation of concurrency in NREC system design and programming.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.5
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• pp.765-771
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• 2001

Elastic-plastic fracture toughness JIC can be used a s an effective design criterion in elastic-plastic fracture mechanics. Among the JIC test methods approved by ASTM, unloading compliance method was used in this study. In order to examine the relationship between fracture behavior of JIC test and AE signals, the post processing of AE signals has been carried out by Short Time Fourier Transform(STFT), one of the time-frequency analysis methods. The objective of this study is to evaluate the application of characterization of AE signals for unloading compliance method of JIC test. As a result of time-frequency analysis, we could extract the AE from the raw signal and analyze the frequencies in AE signal at the same time. AE signal generated by elastic-plastic fracture of material has some different aspects at elastic and plastic ranges, or the first portion of crack growth by fracture. First of all, increased energy recorded and detected by using AE count method increase rapidly from the start of ductile fracture. The variation of main frequency range with time-frequency analysis method could be confirmed. We could know fracture behavior of interior material by examination AE characteristics generated in real-time when elastic-plastic fracture occurred in material under loading.

• Geomechanics and Engineering
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• v.27 no.5
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• pp.433-445
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• 2021

Artificial ground freezing (AGF) is a commonly used geotechnical support technique that can be applied in any soil type and has low environmental impact. Experimental and numerical investigations have been conducted to optimize AGF for application in diverse scenarios. Precise simulation of groundwater flow is crucial to improving the reliability these investigations' results. Previous experimental research has mostly considered horizontal seepage flow, which does not allow accurate calculation of the groundwater flow velocity due to spatial variation of the piezometric head. This study adopted vertical seepage flow-which can maintain a constant cross-sectional area-to eliminate the limitations of using horizontal seepage flow. The closure time is a measure of the time taken for an impermeable layer to begin to form, this being the time for a frozen soil-ice wall to start forming adjacent to the freeze pipes; this is of great importance to applied AGF. This study reports verification of the reliability of our experimental apparatus and measurement system using only water, because temperature data could be measured while freezing was observed visually. Subsequent experimental AFG tests with saturated sandy soil were also performed. From the experimental results, a method of estimating closure time is proposed using the inflection point in the thermal conductivity difference between pore water and pore ice. It is expected that this estimation method will be highly applicable in the field. A further parametric study assessed factors influencing the closure time using a two-dimensional coupled thermo-hydraulic numerical analysis model that can simulate the AGF of saturated sandy soil considering groundwater flow. It shows that the closure time is affected by factors such as hydraulic gradient, unfrozen permeability, particle thermal conductivity, and freezing temperature. Among these factors, changes in the unfrozen permeability and particle thermal conductivity have less effect on the formation of frozen soil-ice walls when the freezing temperature is sufficiently low.

• Journal of the Korean Association of Oral and Maxillofacial Surgeons
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• v.46 no.5
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• pp.328-334
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• 2020

Objectives: This study aimed to validate the effectiveness of a recently proposed difficulty index for removal of impacted mandibular third molars based on extraction time and suggest a modified difficulty index including the presence of pathologic conditions associated with third molars. Materials and Methods: This retrospective study enrolled 65 male patients younger than 25 years with third molars. Extraction time was calculated from start of the incision to the last suture. The difficulty scores for third molars were based on spatial relationship (1-5 points), depth (1-4 points), and ramus relationship (1-3 points) using cone-beam computed tomography. The difficulty index was defined as follows: I (3-4 points), II (5-7 points), III (8-10 points), and IV (11-12 points). The modified difficulty score was calculated by adding one point to the difficulty score if the third molar was associated with a pathologic condition. Two modified difficulty indices, based on the presence of pathologic conditions, were as follows: the half-level up difficulty index (HDI) and the one-level up difficulty index (ODI) from the recently proposed difficulty index. Results: The correlations between extraction time and difficulty index and or modified difficulty indices were significant (P<0.001). The correlation coefficient between extraction time and difficulty index was 0.584. The correlation coefficients between extraction time and HDI and ODI were 0.728 and 0.764, respectively. Conclusion: Extraction time of impacted third molars exhibited a moderate correlation with difficulty index and was strongly correlated with the modified indices. Considering the clinical implications, the difficulty index of surgical extraction should take into consideration the pathologic conditions associated with third molars.

• Journal of the Korean Data and Information Science Society
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• v.28 no.2
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• pp.371-381
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• 2017

There has never been research on Chaos analysis using real estate auction sale price rate in Korea. In this study, three Chaos analysis methodologies - Hurst exponent, correlation dimension, and maximum Lyapunov exponent - in order to capture the nonlinear deterministic dynamic system characteristics. High level of Hurst exponent and the extremely low maximum Lyapunov exponent provide the tendency and the persistence of the data. The empirical results give two meaningful facts. First, monthly time lags of the correlation dimension are coincident with the time period from the approval auction start day to the sale price fixing day. Second, its weekly time lags correspond to the time period from the last day of request for sale price allocation to the sale price fixing day. Then, this study potentially examines the predictability of the real estate auction price rate time series.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.13 no.10
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• pp.2218-2226
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• 2009

TDMA based MAC protocol supporting wireless mesh network has many advantage rather than 802.11 DCF/EDCA protocol based on packet. But TDMA based MAC protocol require new synchronization method because of mobile point oscillator's difference, and distributed environments. This thesis propose synchronization method for TDMA based MAC protocol. It divides MP(Mobile Points) states into 4 types. If MP is in sync mode, it schedules TDMA local start time in time skew interval using beacon. It proposes compensation algorithms to compensate time skew caused by clock drift. This proposal show that general time error and clock drift rate value reduced and get synchronized result.