• Proceedings of the KSME Conference
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• 2000.11b
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• pp.397-403
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• 2000

In the present study, flow characteristics of turbulent pulsating flow in a square-sectional $180^{\circ}$ curved duct were experimentally investigated. Experimental studies for air flows were conducted to measure axial velocity and wall shear stress distributions and entrance length in a square-sectional $180^{\circ}$ curved duct by using the LDV with the data acquisition and the processing system. The experiment was conducted in seven sections from the inlet (${\phi}=0^{\circ}$) to the outlet (${\phi}=180^{\circ}$) at $30^{\circ}$ intervals of the duct. The results obtained from the experimentation were summarized as follows ; (1) When the ratio of velocity amplitude ($A_1$) was less than one, there was hardly any velocity change in the section except near the wall and any change in axial velocity distributions along the phase. When the ratio of velocity amplitude ($A_1$) was 0.6, the change rate of velocity was slow. (2) Wall shear stress distributions of turbulent pulsating flow were similar to those of turbulent steady flow. The value of the wall shear stress became minimum in the inner wall aid gradually increased toward the outer wall where it became maximum. (3) The entrance length of turbulent pulsating flow reached near the region of bend angle of $90^{\circ}$, like that of turbulent steady flow. The entrance length was changed by the dimensionless angular frequency (${\omega}^+$).

• Journal of the Korean Society of Radiology
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• v.14 no.6
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• pp.733-740
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• 2020

As the use of radiation for medical purposes increases, the exposure dose of medical workers is also increasing. To reduce this dose, various studies on changing the shielding material have been conducted. Recently, a new method to reduce the dose at the entrance of the radiation treatment room was proposed by using the photoelectric effect that occurs when the radiation is scattered. Because this method is particularly effective for low-energy photons, in this study, a slit-type structure was proposed as a excellent shielding structure against scattered x-ray in a general photography room, and was evaluated the shielding effect by Monte Carlo simulation. As a result of the calculation, this study found that in the case of a structure in which steel plates with a thickness of 2 mm and a width of 5 cm are stacked at 2 mm intervals, a shielding effect was approximately 99.9% or more, excluding the heights of the floor and the patient where scattering occurs directly.

• The KSFM Journal of Fluid Machinery
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• v.11 no.1
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• pp.9-17
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• 2008

A numerical study for three-dimensional laminar flow in the entrance region of helical tubes connected with straight ones is carried out to investigate the effects of Reynolds number, pitch and curvature ratio on the oscillation periods of the flow. The fully elliptic governing equations were solved by means of a finite volume method. The fully developed laminar flow boundary condition was applied at the straight tube inlet. This results cover a curvature ratio range of 1/10${\sim}$1/320, a pitch range of 0.0${\sim}$3.2, and a Reynolds number range of 62.5${\sim}$2000. A comparison is made with previous experimental correlations and numerical data. The developments of velocity, local and average friction factors are discussed. The average friction factors are oscillatory in the entrance region of helical pipes. It has been found that the angle required for the flow to be similarly developed is most affected by the curvature ratio. The pitch and Reynolds number do not have any significant effect on the angle. The characteristic angle ${\phi}_c(={\phi}/sqrt{\delta})$, or the characteristic length to diameter ratio $s_c(=l\sqrt{\delta} cos(atan{\lambda})/d)$, can be useful to represent the development of flow in helical tubes. As the pitch increases and as the curvature ratio and Reynolds number decrease, the amplitude and the number of flow oscillations along the main streamwise direction decrease.

• Journal of the Korean Society of Propulsion Engineers
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• v.22 no.3
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• pp.46-52
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• 2018

A simple Crocco's $n-{\tau}$ time delay model and linear analysis of fluid flow coupled with acoustics are combined to investigate the high frequency combustion instability in the combustion chamber of LOX/hydrocarbon engines. The partial differential equation of the velocity potential is separated into ordinary differential equations, and eigenvalues that correspond to tangential resonance modes in the cylindrical chamber are determined. A general solution is obtained by solving the differential equation in the axial direction, and boundary conditions at the injector face and nozzle entrance are applied in order to calculate the chamber admittance. Frequency analysis of the transfer function is used to evaluate the stability of system. Stability margin is determined from the system gain and phase angle for the desired frequency range of 1T mode. The chamber model with variable baffle length and configurations are also considered in order to enhance the 1T mode stability of the combustion chamber.

• Proceedings of the KSR Conference
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• 2008.06a
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• pp.1794-1801
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• 2008

Fire-driven flow and temperature distribution in a ventilated tunnel was analyzed by Large Eddy Simulation using FDS code. The simulated tunnel is 182m length, 5.4m wide and 2.4m height. A pool fire was located 112m from tunnel entrance and was taken as a heat source of $0.89m^2$. The heat is assumed to be released uniformly throughout the whole simulated time. The fire strength was 2.76MW and the fuel burnt was octane. The parallel computational method was employed to accelerate the computing time and manage the large grid points which is not possible to handle in the one CPU. The total grid points used were $2.4{\times}10^6$ and 7 CPUs were used to calculate the momentum and energy equations. The simulated results were well compared with the experiments.

• Journal of Digital Convergence
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• v.10 no.11
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• pp.613-618
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• 2012

ICT education guidelines revised in 2005 reinforce computer science elements such as algorithm, data structure, and programming covering all schools. And Ministry of Education emphasizes STEAM education. Most important is that "How instruct them". This means necessity of contents. So this paper suggests learning method of Stack and Queue using LEGO MINDSTORMS NXT. The main purpose is that how stack and queue are used, when robot explore realistic maze. Teaching and learning strategies are algorithm, flowchart, and NXT-G programming. Simple maze has path in left or right, but complex maze has three-way intersection. These are developed by authors. Master robot explores maze and push stack, and then return to entrance using stack. Master robot explores maze and transmits path to slave's queue. And then slave robot drives without exploration. Students can naturally learn principles and applications of them. Through these studies, it can improves ability of logical and creative thinking. Furthermore it can apply to ICT and STEAM education.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.72-75
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• 2003

In this research, we focused on the effects of the orifice internal flow such as cavitation and hydraulic flip. The breakup characteristics such as the breakup length and trajectory were measured by changing the orifice diameter (d), the orifice length/orifice diameter (L/d), the injection pressure and the shapes (sharp and round) of orifice entrance to provide a lot of conditions of the orifice internal flow. It is found that cavitation bubbles that occur inside the sharp-edged orifice make the liquid jet ejecting from the orifice turbulent. In the orifices (L/d = 5), the hydraulic flip phenomenon is shown when the injection pressure is high. In case cavitation occurs it breaks up more earlier than that in case of non-cavitation. In case hydraulic flip occurs, since the area of the liquid jet becomes small, the breakup length is also small as that in case of cavitation. But the liquid column trajectories have a similar tendency irrespective of cavitation.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.1
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• pp.244-251
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• 2016

In this study, the relationship between flow characteristics and projection distance, depending on the shape was examined. A numerical investigation technique for fluid analysis of a foam monitor was developed for the prediction, comparison and validation of the actual injection performance. The foam monitor changes the flow pattern of fluid flow according to the shape, The fluid losses were calculated from the numerical investigation affecting the projection distance. The basic form of foam monitor was used as a designed shape in N. The modified model used the length increase model of the flow path, and straight line of the model. The inlet pressure was 6.5bar. The results showed that the length increase model of the flow path and straight line of the model in the nozzle projection distance had improved. The results comparing the error rates projection performance were well matched to the 7.43% obtained from the validity test of the analysis method.

• Proceedings of the Korea Water Resources Association Conference
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• 2019.05a
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• pp.138-138
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• 2019

진해 용원해역은 부산 신항 개발사업의 일환으로 추진된 북 부두조성 사업 및 항만 배후단지 조성사업으로 전면 해역이 매립되어 원활한 해수흐름이 차단되고 송정천 하구 지점의 견마도 북쪽 통로를 이용한 선박이동만이 가능한 좁고 긴 수로형태로 변형된 국내에서 이례적인 폐쇄성 수역으로 변형되었다. 용원수로의 평균 너비는 약 100 m, 길이는 3,600 m정도로 세장비가 아주 큰 수로에 해당되며, 용원수로의 조석 유입유출은 뚜렷한 일차원적 흐름으로 간주할 수 있으며 입구에서 용원수로 내부로 갈수록 조석유입은 점진적으로 감소하여 흐름이 정체되는 양상을 보이고 있다. 이로 인한 지형적, 물리적 특성 때문에 외해와의 해수교환이 제한되어 적은 양의 오염물질이 유입되어도 유입부하가 지속될 경우 수로 바닥에 축적될 가능성이 매우 크며, 이는 수질에 대한 악영향을 끼칠 수 있다. 따라서 폐쇄성 수역인 용원수로를 장기간 모니터링하고 수질에 끼치는 영향을 파악하여 적절한 관리방안을 마련하는 것이 중요하다고 볼 수 있으며, 본 연구에서는 과거 축적된 수환경 자료와 실제로 용원수로 내에서 측정된 자료들을 기반으로 수질변화에 대한 시 공간적인 양상을 분석하였다. 또한 용원수로로 유입되는 토구를 전수조사하고, 강우시와 비강우시 토구 모니터링을 통해 육상에서 기인하는 오염원을 분석하였으며, 용원수로 내에 유입된 오염물질 거동 분석을 위해 추적자 실험을 실시하였다. 특히 용원수로 내 오염도가 높은 내측지점에서 추적자 투입하여 드론 영상을 통해 그 거동을 분석하고, 수로 내로 유입되는 오염원을 조사하여 폐쇄성 정체수역에서의 수질영향을 분석하였다. 이를 바탕으로 용원수로 내 수질영향에 대한 관리방안을 모색하고자 하였다.

• Journal of the Korean Society of Propulsion Engineers
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• v.23 no.5
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• pp.43-52
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• 2019

Cryogenic propellant rockets utilize a natural circulation loop of cryogenic fluid to cool the engine inlet temperature before launch. The geometric information about the circulation system, such as length and diameter of the pipes and the heat input to the system, defines the mass flow rate of the natural circulation loop. We performed experiments to verify the natural circulation mass flow rate and compared the results with the analytical results. The comparison of the mass flow rate between experiments and numerical simulations showed a 12% offset. We also included a prediction of the natural circulation flow rate in the low-gravity section and in the acceleration section in the upper stage of the launch vehicle. The oxygen tank should have 100 kPa(a) of pressure in the acceleration section to maintain a high flow rate for the natural circulation loop. In the low-gravity section, there should be an optimal tank pressure that leads to the maximum natural circulation flow rate.