• 한국안전학회지
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• 제30권6호
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• pp.18-25
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• 2015

Computational study of a gravity current prior to the backdraft was conducted using fire dynamic simulator (FDS). Various initial conditions of mixture compositions and compartment temperature as well as four opening geometries (Horizontal, Door, Vertical, and Full opening) were considered to figure out their effects on the gravity current. The density difference ratio (${\beta}$) between inside and outside of compartment, the gravity current time ($t_{grav}$) and velocity ($v_{grav}$), and non-dimensional velocity ($v^*$) were introduced to quantify the flow characteristics of the gravity current. Overall fluid structure of the gravity current at the fixed opening geometry showed similar development process for different ${\beta}$ conditions. However, $t_{grav}$ for entering air to reach the opposed wall to the opening geometry increased with ${\beta}$. Door, Vertical, and Horizontal openings where openings are attached on the ground showed similar development process of the gravity current except for Horizontal opening, which located on the middle of the opening wall. The magnitude of $v_{grav}$ at fixed ${\beta}$ was, from largest to smallest, Full > Vertical > Door > Horizontal, but it depended on both the size and location of the opening. On the other hand, $v^*$ was found to be independent to ${\beta}$, and only depended on the geometry of the opening.

• Journal of Mechanical Science and Technology
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• 제17권4호
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• pp.617-625
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• 2003

Effects of Injector nozzle geometry and operating pressure conditions such as opening pressure, ambient pressure. and injection pressure on the transient fuel spray behavior have been examined by experiments. In order to clarify the effect of internal flow inside nozzle on the external spray, flow details Inside model nozzle and real nozzle were alto investigated both experimentally and numerically. for the effect of injection pressures, droplet sizes and velocities were obtained at maximum line pressure of 21 MPa and 105 MPa. Droplet sizes produced from the round inlet nozzle were larger than those from the sharp inlet nozzle and the spray angle of the round inlet nozzle was narrower than that from the sharp inlet nozzle. With the increase of opening pressure, spray tip penetration and spray angle were increased at both lower ambient pressure and higher ambient pressure. The velocity and size profiles maintained similarity despite of the substantial change in injection pressure, however, the increased injection pressure produced a higher percentage of droplet that are likely to breakup.

• 한국화재소방학회논문지
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• 제27권3호
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• pp.20-29
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• 2013

실규모 환기부족 구획화재에서 수직 개구부의 형상 및 위치변화에 따른 열 및 화학적 화재특성 변화를 수치적으로 검토하기 위하여, 이론적 최대 공기 유입량을 결정하는 환기인자($A\sqrt{h}$)와 heptane pool 화재의 질량 감소율이 모든 조건에 대하여 동일하게 설정되었다. 주요 결과로서, 출입문 형상의 변화는 구획 내부의 열 및 화학적특성 변화에 큰 영향을 미친다. 창문 위치의 변화는 화재지속시간 및 재순환 유동구조를 포함한 추가적인 화재특성의 복잡한 변화를 초래하였다. 이들 결과는 개구부 유동 및 연료/공기의 혼합현상을 포함한 다차원 유동 및 화재특성을 통해 상세히 분석되었다.

• Journal of Magnetics
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• 제20권2호
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• pp.176-185
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• 2015

We present the design optimization of the magnetic pole and slot design options that minimize the cogging torque of permanent-magnet (PM) brushless generators for small wind turbine generators. Most small wind-turbines use direct-driven PM generators which have the characteristics of low speed and high efficiency. Small wind-turbines are usually self-starting and require very simple controls. The cogging torque is an inherent characteristic of PM generators, and is mainly caused by the generator's geometry. The inherent the cogging torque can cause problems during turbine start-up and cut-in in order to start softly and to run a power generator even when there is little wind power during turbine start-up. Thus, to improve the operation of small turbines, it is important to minimize the cogging torque. To determine the effects of the cogging torque reductions, we adjust the slot opening width, slot skewing, mounting method of magnets, magnet shape, and the opening and combinations of different numbers of slots per pole. Of these different methods, we combine the methods and optimized the design variables for the most significant design options affecting the cogging torque. Finally, we apply to the target design model and compare FEA simulation and measured results to validate the design optimization.

• 한국화재소방학회논문지
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• 제33권1호
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• pp.30-38
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• 2019

메탄 연료성분이 가득 찬 축소 구획실을 대상으로 개구부에서 유입된 공기와 내부 연료의 혼합특성과 백드래프트 발생특성을 규명하기 위해 대와동모사를 수행하였다. 통상의 문 형태(Door)와 가로 형태의 문이 벽면 상단($Slot_U$), 중단($Slot_M$) 및 하단($Slot_L$)에 있는 구획실의 4가지 개구부 조건들에 대해서 검토를 수행하였다. 점화원이 없을 경우 구획실 내부로 유입되는 산소의 양과 외부로 유출되는 연료의 양은 Door > $Slot_U$ ~ $Slot_M$ > $Slot_L$의 순서로 크지만 $Slot_U$의 경우가 구획실 내부에서 연료와 산소가 전체적으로 가장 잘 혼합되었고 $Slot_L$의 경우에는 연료와 산소가 층을 이루어 혼합이 가장 잘 이루어지지 않는 것으로 나타났다. 구획실 내 산소량과 연료량으로 정의되는 총괄당량비는 구획실에서 발생하는 백드래프트의 강도와 잘 연관되지 않음을 확인하였다. 백드래프트 발생 시의 구획실 내부의 최고 압력은 혼합이 가장 잘 이루어진 $Slot_U$가 가장 높게 나타났으며 $Slot_L$의 경우에는 압력상승이 낮아 백드래프트가 발생하지 않았다. 백드래프트 발생 시 Door와 $Slot_M$ 조건에서의 최고 압력값은 $Slot_U$ 다음 순서로 나타났으며, 각 조건들의 최고압력은 백드래프트 발생순간까지의 총 열발생량과 잘 연관되어 설명될 수 있었다.

• Steel and Composite Structures
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• 제28권5호
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• pp.617-628
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• 2018

Steel shear wall possesses priority over many of the current lateral load-bearing systems due to reasons like higher elastic stiffness, desirable ductility and energy absorption, convenience in construction and implementation technology, and economic criteria. Besides these advantages, this system causes increase in the dimensions of other structural elements due to its high stiffness as one of its intrinsic characteristics. One of the methods for stiffness reduction is perforating the wall panel and creating openings in the wall that can also be used as windows or ducts in buildings service period. The aim of the present study is probing the appropriate geometric shape and location of opening to fulfil economic criterion plus technical and seismic design criteria. In the present research, a number of possible while reasonable opening shapes and locations are defined in various sizes for some steel shear wall specimens. The specimens are modelled in ABAQUS finite elements software and analyzed using nonlinear pushover analysis. Finally, the analyses' results are reported as force-displacement diagrams and the strength, the initial stiffness and the energy absorption are calculated for all specimens and compared together. The obtained results show that both shape and location of the openings affect the seismic parameters of the shear wall. The specimens in which the openings are further from the center and closer to the columns possess higher stiffness and strength while the specimens in which the openings are closer to the center show more considerable changes in their seismic parameters in response to increase in opening area.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 1999년도 제29회 춘계학술대회
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• pp.197-202
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• 1999

A mechanical face seal is a tribe-element intended to control the leakage of working fluid at the interface of a rotating shaft and its housing. The leakage of working fluid decreases as the seal surfaces get closer each other. But a very small seal clearance results in a drastic reduction of seal life because of high wear and heat generation. Therefore, in the design of mechanical face seals the compromise between low leakage and acceptable life is important and presents a difficult design problem. And the gap geometry of seal clearance affects seal performance very much and becomes an important design variable. In this study the Reynolds equation for the sealing dam of mechanical face seals is numerically analyzed using the Galerkin Finite Element Method, which can be readily applied to various seal geometries. The film pressures of the sealing dam are analyzed, including the effects of the seal face coning and tilt. Then, opening forces, restoring moments, leakages, and dynamic coefficients are calculated.

• 한국자동차공학회논문집
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• 제18권5호
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• pp.136-144
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• 2010

Exhaust gas recirculation (EGR) is an emission control technology allowing significant NOx emission reduction from light-and heavy duty diesel engines. The future EGR type, dual loop EGR, combining features of high pressure loop EGR and low pressure loop EGR, was developed and optimized by using a commercial engine simulation program, GT-POWER. Some variables were selected to control dual loop EGR system such as VGT (Variable Geometry Turbocharger)performance, especially turbo speed, flap valve opening diameter at the exhaust tail pipe, and EGR valve opening diameter. Applying the dual loop EGR system in the light-duty diesel engine might cause some problems, such as decrease of engine performance and increase of brake specific fuel consumption (BSFC). So proper EGR rate (or mass flow) control would be needed because there are trade-offs of two types of the EGR (HPL and LPL) features. In this study, a diesel engine under dual loop EGR system was optimized by using design of experiment (DoE). Some dominant variables were determined which had effects on torque, BSFC, NOx, and EGR rate. As a result, optimization was performed to compensate the torque and BSFC by controlling start of injection (SOI), injection mass and EGR valves, etc.

• 한국산학기술학회논문지
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• 제17권9호
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• pp.688-693
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• 2016

본 논문에서는 현재 고고도 이탈 및 저고도 개산강하(HALO, High Altitude Low Opening)용으로 사용하고 있는 군용 낙하산의 훈련 시뮬레이터 개발을 위해 필요한 낙하산 모델링 및 시뮬레이션 결과를 정리하였다. 대상인 군용 낙하산은 파라포일(Parafoil) 형태의 사각 낙하산으로 원형 낙하산과는 달리 강하자가 조종을 통해 원하는 위치로 유도할 수 있는 기동성이 뛰어나 공수부대원들의 적진 침투시에 주로 이용된다. 실재 낙하산의 형상자료를 이용하여 파라포일과 낙하물의 질점 모델을 기반으로 9-자유도 비선형 운동방정식을 유도하고, 각각의 관성모멘트와 공력 미계수를 산출하여 MATLAB/Simulink 기반의 비선형 시뮬레이션을 수행하여 그 결과를 나타내었다. 특히 낙하산과 같은 공기부양(LTA, Lighter-Than-Air) 비행체는 일반적인 항공기 비선형 운동과 달리 부가질량(Added Mass) 및 부가 관성모멘트(Added Moment of Inertia)의 효과가 크기 때문에 이에 대한 경험수식을 바탕으로 동역학 모델링에 포함하여 고려하였다. 수행된 낙하산 운동 모델링의 검증을 위해 비대칭 조종입력을 통한 나선형 강하 비행조건을 시뮬레이션하여 대상 군용 낙하산에서 제시된 실재 성능값과 시뮬레이션 결과치를 비교하여 유도된 운동모델이 타당함을 검증하고 그 결과를 나타내었다.

• 대한기계학회논문집A
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• 제27권6호
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• pp.890-897
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• 2003

This paper compares engineering estimation schemes of C＊ and creep COD for circumferential and axial through-wall cracked pipes at elevated temperatures with detailed 3-D elastic-creep finite element results. Engineering estimation schemes included the GE/EPRI method, the reference stress method where reference stress is defined based on the plastic limit load and the enhanced reference stress method where the reference stress is defined based on the optimized reference load. Systematic investigations are made not only on the effect of creep-deformation behaviour on C＊ and creep COD, but also on effects of the crack location, the pipe geometry, the crack length and the loading mode. Comparison of the FE results with engineering estimations provides that for idealized power law creep, estimated C＊ and COD rate results from the GE/EPRI method agree best with FE results. For general creep-deformation laws where either primary or tertiary creep is important and thus the GE/EPRI method is hard to apply, on the other hand, the enhanced reference stress method provides more accurate and robust estimations for C* and COD rate than the reference stress method.