• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.100-100
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• 2022

일반적으로 유체와 구조물간 상호작용의 수리동역학적 모의에서는 벽경계조건을 통하여 유동에 대한 구조물의 영향이 반영된다. 하지만 도심지에서 발생한 홍수를 예측하려는 경우 이러한 방법으로는 밀집한 구조물들 사이에 형성된 좁은 길들로 인하여 세밀한 격자망을 요하여 큰 계산량을 유발하고 빠른 예측 속도를 기대할 수 없게 한다. 최근 이러한 문제를 극복하기 위해 성긴 격자망에서도 구조물의 유체에 대한 영향을 반영할 수 있도록 하는 방법들이 큰 관심을 받고 있다. 그 중에서도 다공성 천수방정식은 벽경계조건 대신 다공도(posority)의 개념을 이용한 모형으로 도시범람모의에 있어 계산량과 정확도를 가장 적절하게 타협한 모형으로 보고되고 있다. 이러한 흐름에 맞추어 본 연구는 다공도 천수방정식을 해석하는 수치 기법을 개발하였고, 여기에 최근 쌍곡선계 방정식의 수치적 연구들에서 소개된 주요 특징들이 반영되도록 설계하였다. 우선, WENO 기법과 Runge-Kutaa 기법을 통하여 공간과 시간에 대한 고차 정확도를 만족시켰다. 이 때, 재구성 변수와 알고리즘를 새롭게 제시하여 정상흐름조건에 대한 플럭스항과 생성·소멸항간 절단오차에 의한 비물리적인 흐름생성을 억제하였다. 또한, 수치모의 중 음수심의 발생으로 인하여 수치모형이 불안정해지는 현상을 막기 위해, 양-보존성 제한자를 구축하였다. 마지막으로 도심지에서 즐비한 인위적인 구조물에 의해 나타나는 지형적인 불연속의 효과를 적절하게 반영할수 있도록 정상파 재구축의 단계를 구축하여 수치 기법에 반영하였다. 이렇게 구성된 수치기법은 리만문제의 해석해에 기반하여 기존의 주요 연구들의 결과와 비교되었고, 그 결과 본 연구의 방법이 정확성, 수렴성, 안전성의 측면에서 가장 우수함을 수치적으로 증명하였다.

• Journal of the Korean Society of Marine Environment & Safety
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• v.30 no.4
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• pp.317-323
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• 2024

According to the section A-VI/3 of the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW), Water-based firefighting training center is mandatory to obtain onboard certificates. This space, being similar to fire situations on ships requires that safety measures be quantified to ensure occupant safety and establish operational standards. For fire safety evaluation, cases were designed based on the presence or absence of smoke control equipment using Pyrosim based on Fire Dynamics Simulation (FDS). Vector analysis was performed to evaluate flow of smoke and heat. Available safe escape time / required safe escpae time (ASET/RSET) analysis was conducted to evaluate safety by comparing the interpreted numerical results through Pathfinder. During safety evaluation of the current operational condition, the appropriateness of the function of each smoke control equipment was numerically and visually indicated. The emergency situation with dust collector stopped was expressed by each evacuation time and safety margin of 111.2 seconds, suggesting that be used as a standard of evacuation time.

• Nuclear Engineering and Technology
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• v.18 no.3
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• pp.200-208
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• 1986

The loss of coolant accident based on a double-ended cold leg break is analyzed with the discharge coefficient (Ca) of 0.4. This analysis covers the whole transient period from the start of depressurization to the complete refilling of the core by using RELAP4/MOD6-EM and RELAP4/ MOD6-HOT CHANNEL for the system thermal-hydraulics and the fuel performance during the blowdown phase respectively, and RELAP4/MOD6-FLOOD and TOODEE2 during the reflood phase. A simple analytical method has been developed to account for the lower plenum filling by approximating steam-water countercurrent flows and superheated wall effects at the downcomer during the refill period. Based on the informations. at the time of EOB (end-of-bypass), the refill duration time and the initial reflooding temperature were estimated and compared with the results from the RELAP4/MOD6, resulting in a good agreement. In addition, some parametric studies on the EOB were performed. The form loss coefficient between upper head and upper downcomer was found to be sensitive to the occurrence of the spurious EOB. Appropriate form loss coefficients should be taken into account to avoid the flow oscillations at the downcomer. The analyses with the six and three volume core nodalizations, respectively, show much similar trends in the system thermal-hydraulic performance, but the former case is recommended to obtain good results.

• Proceedings of the Korean Society for Agricultural Machinery Conference
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• 2017.04a
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• pp.54-54
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• 2017

본 연구는 무인 헬리콥터의 양력을 개선하기 위한 기초 단계로서 V1505A 및 V2008B 기본 두 익형의 400 mm 블레이드 섹션에 해당하는 모델에 대한 풍동실험을 실시하여 양력, 항력 및 동력특성을 분석하고 CFD 시뮬레이션의 결과와 비교하고 검증하였다. 시뮬레이션은 풍동 실험과 유사하게 설정하기 위하여 400 mm 블레이드 섹션의 양 끝을 벽으로 제한하여 3차원 와류현상을 억제하여 모델을 구성하였고, 시뮬레이션의 결과와 비교하여 모델을 검증하였다. 사용된 모델은 로터로부터 $Re=0.32{\times}10^6$ 영역까지는 aminar 모델을 사용하였으며, 그 이후 영역역(>$Re=0.32{\times}10^6$)은 양력 및 저항의 급격한 변화를 올바로 포착할 수 있다는 S-A 모델을 적용하여 확장하였다. 시뮬레이션의 격자는 유동 현상에 있어 박리로 야기된 와류 현상을 관찰하기 위하여 익형 주변에 접하는 부분에 격자를 집중시켰다. 시뮬레이션 방법은 유속은 36~141 m/s 까지 5 수준으로 하였으며, 받음각은 $0{\sim}16^{\circ}$로 7 수준으로 변화 시키면서 공력계수 및 동력을 분석하였다. 양력분석에 있어 익형 V1505A에 비해 익형 V2008B의 특성이 우수하였으나, 익형 V1505A는 실속 이후 양력이 급격히 떨어지지 않고 유지되는 특성을 보였다. 익형 V2008B는 낮은 받음각에서 높은 공력과 낮은 항력을 나타냈다. 동력 분석 결과로 익형 V1505A의 유도동력은 총 동력의 56~72%를 차지하고, 형상동력은 총 동력의 27~43%를 차지하였다. 익형 V2008B는 유도동력은 총 동력의 66~81%를 차지하고, 형상동력은 총 동력의 18~33%를 차지하였다. 익형 V2008B이 익형 V1505A보다 유도동력은 크며, 형상동력은 적게 나와 상대적으로 효율적이라 할 수 있다. 헬리콥터 동력원의 규모는 법률적인 총중량에 의하여 제한되므로 일반적인 농용 소형 무인 헬리콥터 엔진의 사양인 24.5 kW (32PS)를 적용한다면, 익형 V1505A은 받음각 $8{\sim}10^{\circ}$에서 그리고 익형 V2008B은 $7{\sim}9^{\circ}$정도에서 받음각이 제한되며 이때 총 양력은 1200~1300 N 정도로 예상된다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.11
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• pp.767-773
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• 2017

Printed Circuit Heat Exchanger (PCHE) has an advantage for exchanging thermal energy between high-pressure and high-temperature fluids because its core is made by diffusion bonding method of accumulated metal thin-plates which are engraved of flow channel. Moreover, because it is possible that the flow channel can be micro-size hydraulic diameter, the heat transfer area per unit volume can be made larger than traditional heat exchanger. Therefore, PCHE can have higher efficiency of heat transfer. The smaller channel size can make the larger heat transfer area per unit volume. But if high pressure fluid flows inside the channel, the channel wall can be deformed, the structure and shape of flow channel and header have to be designed appropriately. In this study, the design methodology of PCHE channel in high pressure environment based on pressure vessel codes was investigated. And this methodology was validated by computational analysis.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.14 no.2
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• pp.962-969
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• 2013

This study used Computational Fluid Dynamics(CFD) to assess the properties of the air current inflow and the flow velocity distribution in the fume hood. In order to verify the effect of improvement, it was also predicted the characteristics of the flow pattern in case the hood face is structurally improved. The assessment of the capture velocity with the existing fume hood confirmed maximum 23 to 30% difference as compared to the root mean square (RMS). And the hood face showed great difference in flow velocity, with the flow velocity in the upper part is 58 to 68% faster than that in the lower part of the hood. So, as a result of the improvement of the hood designed to maintain a steady exhaust at the hood face (that is, installing a baffle on the inner wall and designing the slot type face), a difference of maximum 7% as compared to RMS appeared while maximum 12% differentiation in flow velocity through sections was predicted, showing mitigation of much of the difference in control velocity as compared to the previous structure.

• Korean Chemical Engineering Research
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• v.49 no.2
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• pp.230-237
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• 2011

In this study, the characteristic of fluid flow in the new type lung assist devices has been established using computational fluid dynamics(CFD). For the modeling, the hollow fiber was ignored, and vertical types and tangential types were used for the model. Which was to analyze the flow characteristics of the fluid flow model when there exists 1 and 2 input/output ports, and when the input/output ports is located at the center of the cylinder and at the tangential direction with the cylinder wall. The modeling results showed that it was possible to eliminate no-flow region(stagnant layer) as shown in the vertical type when an inlet and an outlet were installed on the tangential direction of the cylinder as shown in the tangential type. Also, in the tangential type, vortex-type flow appeared as dominant, and it showed a complicated flow not deviated to one side. When the number of input/output was two, there was no deviated flow, and complicated flows were generated all across the tube. From the test result, it was found that input/output of flow was tangential type and complicated flows with no stagnant layer would be generated when there are two inputs/outputs, respectively.

• Journal of Biomedical Engineering Research
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• v.23 no.6
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• pp.467-475
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• 2002

Hyperthermia using transrectal thermal probes has been used for a noninvasive treatment of prostate diseases. However it is known that heating the rectal wall at excessively high temperature can lead to destruction of the rectal mucous membrane. and it is difficult to maintain an optimum temperature over the entire prostate. Thus, a more accurate understanding of the heat transfer mechanism between prostate and hyperthermia system is needed Numerical analysis was performed to investigate how the cold/warm stimulations on the prostate surface affect the temperature distribution in the prostate model. The general purpose software "FLUENT" was used for obtaining a finite volume solution to the unsteady conduction equation and to calculate the time-varying temperature in the prostate. Effects of the warm/cold stimulations and the stimulation frequency on the temperature distribution were simulated. and we visualized how hyperthermia affected the inside of the prostate. It was found that the effect of hyperthermia by using a typical heating method is limited due to the low thermal conductivity of the prostate. Consecutive repetitions of warm and cold stimulations were considered to provide the thermal irritations inside a prostate. The effects of temperature difference and duration of warm/cold stimulations were investigated, and basic data for the optimum period and effective patterns of stimulations were obtained. A simplified bioheat equation was also solved to describe effects of the blood flow on the blood-tissue heat transfer. The effect of blood flow was not dominant compared to that of warm/cold stimulations. These results might be used as data for design of prostate treating probe, prostatic therapy and thermal stimulation effects on the prostate.

• Applied Chemistry for Engineering
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• v.25 no.4
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• pp.401-408
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• 2014

In membrane bio-reactor (MBR), the aeration control is one of the important independent variables to decrease fouling and to save energy with shear stress change on the membrane surface. The paper was carried out for numerical simulation of 3-dimensional fluid flow phenomena of the cylindrical bioreactor with submerged flat membranes equipped in the center and supplied the air from the bottom by using the COMSOL program. The viscosity and temperature of solution were assumed to be constant, and the specific air demand based on permeate volume ($SAD_p$) defined as scouring air per permeate rates was used as a variable. The calculated CFD velocities were compared with those of the velocity meter measurement and video image analysis, respectively. The results were good agreement each other within 11% error. For fluid flow in the reactor the liquid velocity increased rapidly between the air diffuser and membrane module, but the velocity decreased during flowing of the membrane module. Also, the velocity increased as it was near from the reactor wall to the central axis. The calculated shear stress on the membrane surface showed the highest value at the center part of the module bottom side and increased as aeration rate increased. Especially, the wall shear stress increased dramatically as the aeration rate increased from 0.15 to 0.25 L/min.

• Journal of the Society of Naval Architects of Korea
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• v.33 no.1
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• pp.27-39
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• 1996

The effects of free surface and a strut on the lifting characteristics on a fin attached at the mid-chord of the strut are investigated experimentally in a circulating water channel. Variation of lift force on the fin is investigated with respect to free stream velocity(V), angle of attack of the fin(${\alpha}$) and ratio of the submergence depth of the fin to the chord of the fin(H/C). Attentions are focused on the lifting characteristics of the fin at shallow depths of submergence. Visualization of the free surface deformation along the strut and of the streamline about the fin is made in order to examine the inflow angle to the fin. Lift force on the fin alone i.e. in absence of the strut is also measured to investigate the difference in lifting characteristics of the fin caused by the strut. The results show that lift forces over the fin are largely affected by a free surface in the case of small submergence ratios(HiC<3.0). For HiC>5.0, the effects of the free surface are negligibly small. The inflow angle to the fin is significantly influenced by the strut and flow speed at the shallow depths of submergence. The deformation of the free surface is largely governed by the waves generated by the strut. However, for small submergence depths, the effects of the fin are found also significant.