• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 2002년도 춘계학술대회 논문집
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• pp.82-86
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• 2002

The thermal characteristics of high-speed air spindle system with built-in motor are studied. Experiment and finite difference method analysis obtain temperature rise and temperature distribution of housing. For the analysis, air fluid film model is built and temperature rise and distribution in thermal steady state are computed for each rotational speed. Generally, it is said that the heat generation of air bearing is negligible. But the heat generation in air film by heat dissipation can not be negligible especially into high-speed region of the journal. In case that the heat generation of air spindle system is high, natural frequency of the spindle system becomes lower when the thermal state is in steady-state and it means the changes of air bearing stiffness due to the change of bearing clearance. It is shown that the temperature rise of air spindle system causes thermal expansion and induces the variation of bearing clearance. In consequence the stiffness of air bearing becomes smaller.

• Structural Engineering and Mechanics
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• 제46권1호
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• pp.53-73
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• 2013

Prediction of prestressed concrete girder integral abutment bridge (IAB) load effect requires understanding of the inherent uncertainties as it relates to thermal loading, time-dependent effects, bridge material properties and soil properties. In addition, complex inelastic and hysteretic behavior must be considered over an extended, 75-year bridge life. The present study establishes IAB displacement and internal force statistics based on available material property and soil property statistical models and Monte Carlo simulations. Numerical models within the simulation were developed to evaluate the 75-year bridge displacements and internal forces based on 2D numerical models that were calibrated against four field monitored IABs. The considered input uncertainties include both resistance and load variables. Material variables are: (1) concrete elastic modulus; (2) backfill stiffness; and (3) lateral pile soil stiffness. Thermal, time dependent, and soil loading variables are: (1) superstructure temperature fluctuation; (2) superstructure concrete thermal expansion coefficient; (3) superstructure temperature gradient; (4) concrete creep and shrinkage; (5) bridge construction timeline; and (6) backfill pressure on backwall and abutment. IAB displacement and internal force statistics were established for: (1) bridge axial force; (2) bridge bending moment; (3) pile lateral force; (4) pile moment; (5) pile head/abutment displacement; (6) compressive stress at the top fiber at the mid-span of the exterior span; and (7) tensile stress at the bottom fiber at the mid-span of the exterior span. These established IAB displacement and internal force statistics provide a basis for future reliability-based design criteria development.

• Structural Engineering and Mechanics
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• 제44권1호
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• pp.109-125
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• 2012

Post-buckling behavior of Timoshenko beams subjected to uniform temperature rising with temperature dependent physical properties are studied in this paper by using the total Lagrangian Timoshenko beam element approximation. The beam is clamped at both ends. In the case of beams with immovable ends, temperature rise causes compressible forces end therefore buckling and post-buckling phenomena occurs. It is known that post-buckling problems are geometrically nonlinear problems. Also, the material properties (Young's modulus, coefficient of thermal expansion, yield stress) are temperature dependent: That is the coefficients of the governing equations are not constant in this study. This situation suggests the physical nonlinearity of the problem. Hence, the considered problem is both geometrically and physically nonlinear. The considered highly non-linear problem is solved considering full geometric non-linearity by using incremental displacement-based finite element method in conjunction with Newton-Raphson iteration method. The beams considered in numerical examples are made of Austenitic Stainless Steel (316). The convergence studies are made. In this study, the difference between temperature dependent and independent physical properties are investigated in detail in post-buckling case. The relationships between deflections, thermal post-buckling configuration, critical buckling temperature, maximum stresses of the beams and temperature rising are illustrated in detail in post-buckling case.

• 설비공학논문집
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• 제17권12호
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• pp.1139-1144
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• 2005

This study has been performed to investigate the thermal performance of variable conductance heat pipe (VCHP) with meshed wick. The length of condenser portion in a VCHP is varied by the expansion of inert gas with the operation temperature, and the heat transport capacity is thus varied with the operating temperature. In this study, numerical evaluation of the VCHP is made for the thermal performance of VCHP, based on the diffusion model of inert gas. Water is used as a working fluid and nitrogen as a control inert gas in the copper tube. As a result, the thermal performance of VCHP has been compared with that of constant conductance heat pipe (CCHP) according to the variation of operation temperature. Maximum heat transport capacity of VCHP is mainly presented for operation temperature and the variation of operation temperature is also presented for heat transfer rate of VCHP.

• 해양환경안전학회지
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• 제25권3호
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• pp.334-343
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• 2019

The Al-Zour coastal area, located in southern Kuwait, is a region of concentrated industrial water use, seawater intake, and the outfall of existing power plants. The Al-Zour LNG import facility project is ongoing and there are two issues regarding the seawater temperature in this area that must be considered: variations in water temperature under local meteorology and an increase in water temperature due to the expansion of the thermal discharge of expanded power plant. MIKE 3 model was applied to simulate the water temperature from June to July, based on re-analysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the thermal discharge input from adjacent power plants. The annual water temperatures of two candidate locations of the seawater intake for the Al-Zour LNG re-gasification facility were measured in 2017 and compared to the numerical results. It was determined that the daily seawater temperature is mainly affected by thermal plume dispersion oscillating with the phase of the tidal currents. The regional meteorological conditions such as air temperature and tidal currents, also contributed a great deal to the prediction of seawater temperature.

• Composites Research
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• 제25권4호
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• pp.117-125
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• 2012

필름 삽입 사출 시편의 휨은 비대칭적인 잔류응력 분포에 기인한다. 비대칭적 잔류응력과 온도 분포는 삽입된 필름 표면의 수직방향으로 지연되는 열 전달에 의해 발생한다. 사출 공정조건 최적화를 통해 필름 삽입 사출 시편의 휨을 억제할 수 없었기 때문에, 필름 삽입 사출 시편의 휨을 최소화하기 위해서 유리 섬유가 강화된 복합재료를 기판으로 사용하였다. 유리 단섬유의 분포를 마이크로 CT 장비를 이용하여 평가하였다. 복합재료로 구성된 기판을 이용한 필름 삽입 사출 시편의 배향 텐서와 휨을 계산하기 위해서는 적절한 마이크로 역학, 이방적 열팽창계수 및 닫힌 어림법 모델이 선택되어야만 한다. 여섯 종류의 마이크로 역학모델, 세 종류의 열 팽창 계수 모델 및 다섯 종류의 닫힌 어림법 모델을 고려한 후, Mori-Tanaka 모델, Rosen and Hashin 모델 및 third orthotropic 닫힌 어림법 모델을 선택하였다. 수치적으로 계산된 섬유 배향 텐서와 휨에 관한 결과들은 실험결과와 잘 일치하였고, 유리 섬유의 보강효과가 필름 삽입 사출 복합 재료 시편의 휨에 미치는 영향을 파악하였다.

• Composites Research
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• 제33권4호
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• pp.198-204
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• 2020

본 연구에서는 탄소나노튜브 다발을 포함하는 나노복합재료의 열-기계적 특성을 정량적으로 예측하기 위하여 분자동역학 전산모사와 유한요소 기반 균질화 기법을 적용하였다. 응집된 탄소나노튜브의 수가 증가함에 따라 동일한 탄소나노튜브의 체적분율에도 불구하고, 면내 영률 및 면내 전단계수는 감소하였고, 면내 열팽창계수는 증가함을 확인할 수 있었다. 계면의 두께를 조사하기 위하여 밀도의 반경 방향 분포(Radial density distribution)을 조사하였으며, 계면의 두께는 탄소나노튜브의 수와는 거의 무관함을 확인할 수 있었다. 기지와 계면은 등방성 재료로 가정하였으며, 예측한 계면의 열-기계적 특성에 따르면, 응집된 탄소나노튜브의 수가 증가함에 따라 계면의 영률 및 전단계수는 감소하였으며, 열팽창계수는 반대로 증가하였다. 이를 토대로, 탄소나노튜브 다발을 포함하는 PLA 나노복합재료의 열-기계적 특성 예측을 위한 멀티스케일 균질화 모델을 개발하였다.

• 대한기계학회논문집B
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• 제41권4호
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• pp.285-291
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• 2017

본 연구에서는 무인 항공 장비에 장착되는 전자 장비에 상변화 물질을 적용한 방열 설계를 수치적으로 진행하였다. 상변화 물질에 대한 열특성 실험을 통해 용융점($T_m$), 용융시 온도 증분(${\Delta}T_m$) 및 체적 팽창을 확인하였으며, 이를 통해 해석 모델 검증을 진행하였다. 용융시 용융점에서 발생하는 온도 정체 현상을 모사하기 위해 등가 비열법으로 계산한 열 물성치를 상변화 물질의 해석 모델 물성치로 입력하였으며, 실험 결과와의 비교를 통해 해석 모델의 신뢰성을 검증하였다. 검증된 해석 모델을 통해 핀과 함께 상변화 물질이 충진된 장비 하우징의 방열 성능을 향상시키고, 이를 통해 장비의 열적 안정성을 확보하였다. 현재 상변화 물질이 충진된 하우징의 방열 성능 극대화를 위해 핀 최적 설계에 대한 추가적인 연구가 진행 중에 있다.

• Advances in aircraft and spacecraft science
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• 제8권4호
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• pp.345-372
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• 2021

The analysis of nonlinear vibrations, buckling, post-buckling, flutter boundary determination and post-flutter behavior of a homogeneous curved plate assuming cylindrical bending is conducted in this article. Other assumptions include simply-supported boundary conditions, supersonic aerodynamic flow at the top of the plate, constant pressure conditions below the plate, non-viscous flow model (using first- and third-order piston theory), nonlinear structural model with large deformations, and application of mechanical and thermal loads on the curved plate. The analysis is performed with constant environmental indicators (flow density, heat, Reynolds number and Mach number). The material properties (i.e., coefficient of thermal expansion and modulus of elasticity) are temperature-dependent. The equations are derived using the principle of virtual displacement. Furthermore, based on the definitions of virtual work, the potential and kinetic energy of the final relations in the integral form, and the governing nonlinear differential equations are obtained after fractional integration. This problem is solved using two approaches. The frequency analysis and flutter are studied in the first approach by transferring the handle of ordinary differential equations to the state space, calculating the system Jacobin matrix and analyzing the eigenvalue to determine the instability conditions. The second approach discusses the nonlinear frequency analysis and nonlinear flutter using the semi-analytical solution of governing differential equations based on the weighted residual method. The partial differential equations are converted to ordinary differential equations, after which they are solved based on the Runge-Kutta fourth- and fifth-order methods. The comparison between the results of frequency and flutter analysis of curved plate is linearly and nonlinearly performed for the first time. The results show that the plate curvature has a profound impact on the instability boundary of the plate under supersonic aerodynamic loading. The flutter boundary decreases with growing thermal load and increases with growing curvature.

• Coupled systems mechanics
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• 제6권2호
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• pp.207-227
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• 2017

In this paper Timoshenko beam theory is employed to investigate the vibration characteristics of functionally graded carbon nanotube-reinforced composite (FG-CNTRC) Beams with a stiff core in thermal environment. The material characteristic of carbon nanotubes (CNT) are supposed to change in the thickness direction in a functionally graded form. They can also be calculated through a micromechanical model where the CNT efficiency parameter is determined by matching the elastic modulus of CNTRCs calculated from the rule of mixture with those gained from the molecular dynamics simulations. The differential transform method (DTM) which is established upon the Taylor series expansion is one of the effective mathematical techniques employed to the differential governing equations of sandwich beams. Effects of carbon nanotube volume fraction, slenderness ratio, core-to-face sheet thickness ratio, different thermal environment and various boundary conditions on the free vibration characteristics of FG-CNTRC sandwich beams are studied. It is observed that vibration response of FG-CNTRC sandwich beams is prominently influenced by these parameters.