• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2011년도 제36회 춘계학술대회논문집
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• pp.93-96
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• 2011

액체로켓엔진의 최적화를 위해 유전알고리즘을 사용하여 주요 설계변수인 연료와 산화제의 질유량과 연소실 압력을 결정하였다. 대상엔진은 LO2/RP-1을 추진제로 사용하는 개방형 가스발생기 사이클을 대상으로 하였다. 최적설계의 목적함수는 비추력의 최대화이고 펌프-터빈의 에너지 발란스와 요구추력을 제한조건으로 하였다. 연소실의 물성치는 CEA2를 이용하였으며 펌프 및 터빈의 효율, 가스발생기 및 예연소기의 물성치는 문헌 자료를 수학적으로 모델링 하였다. 가스발생기 사이클 계산 결과 참고문헌과 비교하여 비추력에서 약 3~4%, 펌프파워에서 2~6%의 오차를 보였다.

• 대한화학회지
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• 제65권5호
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• pp.370-381
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• 2021

본 연구에서는 한국과 싱가포르의 교육과정 문서를 바탕으로 화학 교육과정을 비교 분석하였다. 양국의 화학 영역 성취기준을 분석한 결과, 우리나라 성취기준이 지식과 기능으로 구성된 것에 비해 싱가포르는 지식, 기능, 가치와 태도로 구성되어 있었다. 싱가포르 성취기준의 지식과 기능의 범위가 우리나라보다 폭넓게 나타났으며, 각 요소가 명시적으로 제시되어 있었다. 화학 학습 내용 측면에서, 양국은 공통적으로 초등 과정에서는 실생활과 관련된 물질을 정성적으로 이해하도록 교육과정을 구성하고, 중학교 과정에서는 물질을 입자적 관점으로 이해하도록 구성하고 있었다. 하지만 주기율표는 싱가포르에서만, 온도와 압력에 따른 기체의 부피 변화는 우리나라에서만 다루는 등 세부 내용 요소에는 차이가 있었다. 지식, 기능, 가치와 태도를 과학교육과정 설계에 통합하여 시행하고 있는 싱가포르의 사례는 국제적인 흐름을 우리나라 교육 현실에 맞게 수용하고 재구성하는데 참고할 수 있을 것이다.

• Physical Therapy Rehabilitation Science
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• 제9권4호
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• pp.302-308
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• 2020

Objective: The endurance and strength of deep neck flexor (DNF) muscles have a major role on the function and stability of the cervical spine. In recent years, there has been a lack of research that have investigated the muscle tone of the superficial neck muscles. The purpose of this study was to examine the relationship of between contraction ratio of the DNF and sternocleidomastoid (SCM) muscles, DNF endurance, and muscle tension of the neck muscles. Design: Cross-sectional study. Methods: Forty-seven subjects (male=20, female=27) participated in this study. The muscle tone of the upper trapezius (UT), SCM, and suboccipital (SO) muscle was measured using a contact soft tissue tone-measuring instrument. For the contraction ratio of the SCM and longus colli muscle, the thickness of the relaxation and maximum contraction state of the SCM and longus colli muscles were assessed using a diagnostic ultrasound measuring instrument and a pressure biofeedback unit. The deep neck flexor endurance test (DNFET) was performed in a cranio-cervical flexion posture. The correlations between the measured variables were investigated. Results: The relationship between the DNFET and SO tone showed a significant negative correlation (p<0.05). The relationship between the DNFET and contraction ratio showed a significant positive correlation (p<0.01). There was no significant correlation between the DNFET and SCM and UT tone. Conclusions: This study confirmed that there is a relationship between DNF endurance, DNF activation, and SO tone. The information on the results of this study may be used as a reference that can be actively applied in the clinical environment.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제13권1호
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• pp.69-85
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• 2019

BeiDou navigation satellite system (BDS) is one of the four main types of global navigation satellite systems. The current system has been widely used by the military and by the aerospace, transportation, and marine fields, among others. However, challenges still remain in the BeiDou system, which requires rapid responses for delay-sensitive devices. A differential positioning algorithm called the data center-based differential positioning (DCDP) method is widely used to avoid the influence of errors. In this method, the positioning information of multiple base stations is uploaded to the data center, and the positioning errors are calculated uniformly by the data center based on the minimum variance or a weighted average algorithm. However, the DCDP method has high delay and overload risk. To solve these problems, this paper introduces edge computing to relieve pressure on the data center. Instead of transmitting the positioning information to the data center, a novel method called edge computing-based differential positioning (ECDP) chooses the nearest reference station to perform edge computing and transmits the difference value to the mobile receiver directly. Simulation results and experiments demonstrate that the performance of the ECDP outperforms that of the DCDP method. The delay of the ECDP method is about 500ms less than that of the DCDP method. Moreover, in the range of allowable burst error, the median of the positioning accuracy of the ECDP method is 0.7923m while that of the DCDP method is 0.8028m.

• 한국추진공학회지
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• 제22권6호
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• pp.84-93
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• 2018

냉각제 분사조건이 변화할 때 혼합가스 사출시스템 (또는 가스-스팀 발사체계) 냉각효율 및 발사체 사출성능의 변화를 확인하기 위해 3차원 수치모사를 수행하였다. 선행연구를 참조하여 구축한 1차원 모델을 통해 3차원 단상(single-phase) 계산모델을 검증한 후, Discrete Phase Model을 활용하여 고온의 기체에 냉각제 분사를 모사한 이상(two-phase) 유동 계산을 수행하였다. 냉각제 유량과 분사구 개수를 각각 변화시키면서 계산을 수행한 결과, 분사구 개수를 증가시켰을 경우 냉각효율이 보다 개선되었다. 또한 분사조건에 따라 냉각제 액적 서로간의 coalescence 발생 빈도 및 공간분포 변화가 발생하여 액적의 직경이 달라졌고, 이는 냉각제 증발률에 영향을 미쳤다. 냉각제의 증발은 breech 내부의 온도를 감소시키는 반면에 압력감소를 억제하여 사출시스템 최적화 설계에 있어 중요한 요소임을 확인하였다.

• Geomechanics and Engineering
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• 제18권4호
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• pp.353-362
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• 2019

Soil-rock mixture (S-RM) is an inhomogeneous geomaterial that is widely encountered in nature. The mechanical and physical properties of S-RM are important factors contributing towards different deformation characteristics and unstable modes of the talus slope. In this paper, the equivalent substitution method was employed for the preparation of S-RM test samples, and large-scale triaxial laboratory tests were conducted to investigate their mechanical parameters by varying the water content and confining pressure. Additionally, a simplified geological model based on the finite element method was established to compare the stability of talus slopes with different strength parameters and in different excavation and support processes. The results showed that the S-RM samples exhibit slight strain softening and strain hardening under low and high water content, respectively. The water content of S-RM also had an effect on decreasing strength parameters, with the decrease in magnitude of the cohesive force and internal friction angle being mainly influenced by the low and high water content, respectively. The stability of talus slope decreased with a decrease in the cohesion force and internal friction angle, thereby creating a new shallow slip surface. Since the excavation of toe of the slope for road construction can easily cause a landslide, anti-slide piles can be used to effectively improve the slope stability, especially for shallow excavations. But the efficacy of anti-slide piles gradually decreases with increasing water content. This paper can act as a reference for the selection of strength parameters of S-RM and provide an analysis of the instability of the talus slope.

• Yonsei Medical Journal
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• 제59권9호
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• pp.1115-1122
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• 2018

Purpose: To investigate biomechanical properties of the cornea using a dynamic Scheimpflug analyzer according to age. Materials and Methods: In this prospective, cross-sectional, observational study, participants underwent ophthalmic investigations including corneal biomechanical properties, keratometric values, intraocular pressure (IOP), and manifest refraction spherical equivalent (MRSE). We determined the relationship of biomechanical parameters and ocular/systemic variables (participant's age, MRSE, IOP, and mean keratometric values) by piecewise regression analysis, association of biomechanical parameters with variables by Spearman's correlation and stepwise multiple regression analyses, and reference intervals (RI) by the bootstrap method. Results: This study included 217 eyes of 118 participants (20-81 years of age). Piecewise regression analysis between Corvis-central corneal thickness (CCT) and participant's age revealed that the optimal cut-off value of age was 45 years. No clear breakpoints were detected between the corneal biomechanical parameters and MRSE, IOP, and mean keratometric values. Corneal velocity, deformation amplitude, radius, maximal concave power, Corvis-CCT, and Corvis-IOP exhibited correlations with IOP, regardless of age (all ages, 20-44 years, and over 44 years). With smaller deformation amplitude and corneal velocity as well as increased CorvisIOP and Corvis-CCT, IOP became significantly increased. We provided the results of determination of confidence interval from RI data using bootstrap method in three separate age groups (all ages, 20-44 years, and over 44 years). Conclusion: We demonstrated multiple corneal biomechanical parameters according to age, and reported that the corneal biomechanical parameters are influenced by IOP.

• 한국음향학회지
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• 제40권1호
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• pp.10-17
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• 2021

심벌 트랜스듀서는 Class V flextensional 트랜스듀서의 소형화된 버전으로서 크기에 비해 낮은 공진 주파수와 고출력 특성을 가진다. 하지만, 높은 품질계수와 낮은 에너지 변환 효율성 때문에 단일보다는 배열형으로 많이 사용된다. 배열형으로 사용하면 구성 트랜스듀서들 간의 상호작용으로 인해 단일 트랜스듀서에 비해 주파수 특성에 많은 변화가 나타난다. 본 연구에서는 이러한 성질을 이용하여 심벌 어레이가 광대역 특성을 가지게 하는 패턴을 설계하였다. 특정 중심 주파수를 가지는 세 개의 심벌 트랜스듀서를 먼저 설계하였고, 설계된 심벌 트랜스듀서들로 3 × 3 평면 배열을 구성할 수 있는 모든 패턴을 찾았다. 그 후, 이들 패턴들의 주파수 특성을 분석하고, 그 결과를 바탕으로 높은 비대역폭을 구현하기 위해 가장 효과적인 배열 패턴을 도출하였다. 도출된 배열 패턴은 기준 모델에 비해 비대역폭이 최대 24.9 % 개선되었다.

• Geomechanics and Engineering
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• 제24권5호
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• pp.479-490
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• 2021

Pre-drainage of groundwater in the roof aquifer by boreholes is the main method for prevention of roof water disaster, and the drop in the water level during the drainage leads to the variation of the local stress in the overlying strata. Based on a multitude of boreholes for groundwater drainage from aquifer above the 1303 mining face of Longyun Coal Mine, theoretical analysis and numerical simulation are used to investigate the local stress variation in the process of borehole drainage. The results show that due to the drop in the water level of the roof aquifer during the drainage, the stress around the borehole gradually evolved. From the center of the borehole to the outside, a stress-relaxed zone, a stress-elevated zone, and a stress-recovered zone are sequentially formed. Along with the expansion of drainage influence, the stress peak in the stress-elevated zone also moves to the outside. When the radius of influence develops to the maximum, the stress peak position no longer moves outward. When the coal mining face advances to the drainage influence range, the abutment pressure in front of the mining face is superimposed with the high local stress around the borehole, which increases the risk of stress concentration. The present study provides a reference for the stress concentration caused by borehole drainage, which can be potentially utilized in the optimal arrangement of drainage boreholes in underground mining.

• 한국해양공학회지
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• 제36권2호
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• pp.83-90
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• 2022

This paper presents the minimum submergence depth of an underwater vehicle that can remove the effect of free surface on the resistance of the underwater vehicle. The total resistance of the underwater vehicle in fully submerged modes comprises only viscous pressure and friction resistances, and no wave resistance should be present, based on the free surface effect. In a model test performed in this study, the resistance is measured in the range of 2 to 10 kn (1.03-5.14 m/s) under depth conditions of 850 mm (2.6D) and 1250 mm (3.8D), respectively, and the residual resistance coefficients are compared. Subsequently, resistance analysis is performed via computational fluid dynamics (CFD) simulation to investigate the free surface effect based on various submergence depths. First, the numerical analysis results in the absence of free surface conditions and the model test results are compared to show the tendency of the resistance coefficients and the reliability of the CFD simulation results. Subsequently, numerical analysis results of submergence depth presented in a reference paper are compared with the model test results. These two sets of results confirm that the resistance increased due to the free surface effect as the high speed and depth approach the free surface. Therefore, to identify a fully submerged depth that is not affected by the free surface effect, case studies for various depths are conducted via numerical analysis, and a correlation for the fully submerged depth based on the Froude number of an underwater vehicle is derived.