• Journal of computational fluids engineering
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• v.21 no.2
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• pp.70-80
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• 2016

The comparison of two commercial codes(FLUENT and STAR-CCM+) and an open-source code(OpenFOAM) are carried out for the aerodynamic analysis of flight vehicles at low speeds. Tailless blended-wing-body UCAV, main wing and propeller of HALE UAV(EAV-3) are chosen as geometries for the investigation. Using the same mesh, incompressible flow simulations are carried out and the results from three different codes are compared. In the linear region, the maximum difference of lift and drag coefficients of UCAV are found to be less than 2% and 5 counts, respectively and shows good agreement with wind tunnel test data. In a stall region, however, the reliability of RANS simulation is found to become poor and the uncertainty according to code also increases. The effect of turbulence models and meshes generated from different tools are also examined. The transition model yields better results in terms of drag which are much closer to the test data. The pitching moment is confirmed to be sensitive to the existence and the location of transition. For the case of EAV-3 wing, the difference of results with ${\kappa}-{\omega}$ SST model is increased when Reynolds number becomes low. The results for the propeller show good agreement within 1% difference of thrust. The reliability and uncertainty of three codes is found to be reasonable for the purpose of engineering use. However, the physical validity and reliability of results seem to be carefully examined when ${\kappa}-{\omega}$ SST model is used for aerodynamic simulation at low speeds or low Reynolds number conditions.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.10
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• pp.955-965
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• 2010

The MH-75 propeller for the MAV propulsion is designed using a free vortex design method which considers design parameters such as the hub-tip ratio, the twist angle distribution, the maximum camber location and the chord length of the propeller blade. Aerodynamic characteristics of the MH-75 propeller are predicted by changing the flight speed using the frequency domain panel method. And, the thrust characteristics of the MH-75 propeller are measured using the balance system of the subsonic wind tunnel for the validation of numerical results. The performance characteristics of the MH-75 propeller satisfied with design requirements. Numerical results of the MH-75, which are predicted by the frequency domain panel method, are more agree with experimental results compare with XFOIL.

• Tribology and Lubricants
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• v.36 no.3
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• pp.153-160
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• 2020

In this paper, we present a hydrostatic bearing design and rotordynamic analysis of a pump-and-drive turbine module for a 250-kW supercritical CO₂ cycle application. The pump-and-drive turbine module consists of the pump and turbine wheel, assembled to a shaft supported by two hydrostatic radial and thrust bearings. The rated speed is 21,000 rpm and the rated power is 143 kW. For the bearing operation, we use high-pressure CO₂ as the lubricant, which is supplied to the bearing through the orifice restrictor. We calculate the bearing stiffness and flow rate for various orifice diameters, and then select the diameter that provides the maximum bearing stiffness. We also conduct a rotordynamic analysis based on the design parameters of the pump-and-drive turbine module. The predicted Campbell diagram shows that there is no critical speed below the rated speed, owing to the high stiffness of the bearings. Furthermore, the predicted damping ratio indicates that there is no unstable mode. We conduct the operating tests for the pump and drive turbine modules within the supercritical CO₂ cycle test loop. The pressurized CO₂, at a temperature of 136℃, is supplied to the turbine and we monitor the shaft vibration during the test. The test results show that there is no critical speed below the rated speed, and the shaft vibration is controlled to below 3 ㎛.

• Tribology and Lubricants
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• v.36 no.4
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• pp.193-198
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• 2020

Fluid film bearings are among the best devices used for overcoming friction and reducing wear. Surface texturing is a new surface treatment technique used for processing grooves and dimples on the lubricated surface, and it helps to minimize friction further and improve the wear resistance. In several studies, parallel surfaces, such as thrust bearings and mechanical face seals, have been investigated, but most sliding bearings have a convergent film shape. This paper presents the third part of a recent study and focuses on the effect of the groove shape on the lubrication performance of inclined slider bearings, following the two previous papers on the effects of the groove position and depth. We adopted the continuity and Navier - Stokes equations to conduct numerical analyses using FLUENT, which is a commercial computational fluid dynamics code. The groove shape adopted in the numerical analysis is rectangular and triangular, and its depth is varied. The results show that the streamlines, pressure distributions, and groove shape significantly influence the lubrication performance of the inclined slider bearing. For both shapes, the load-carrying capacity (LCC) is maximum near the groove depth, where vortices occur. In the shallow grooves, the LCC of the rectangular shape is higher, but in deeper grooves, that of the triangular shape is higher. The deeper the rectangular groove, the higher the decrease in the frictional force. The results of this study can be used as design data for various sliding bearings.

• Geophysics and Geophysical Exploration
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• v.13 no.3
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• pp.198-202
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• 2010

In and around the Korean Peninsula, 18 intraplate earthquake focal mechanisms since 1936 were analyzed to understand the characteristic of focal mechanism and regional stress orientation and tectonics. These earthquakes are largest ones from the last century and may represent the characteristics of earthquake in this region. Focal mechanism of these earthquakes show predominant strike-slip faulting with small amount of thrust components. The average P-axis is almost horizontal ENE-WSW direction. This mechanism pattern and the direction of maximum stress axis is very similar with northeastern part of China and southwestern part of Japan. However they are quite different with the eastern part of East Sea. This indicate that not only the subducting Pacific Plate from east but also the indenting Indian Plate controls focal mechanism in the far east of the Eurasian Plate.

• Tribology and Lubricants
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• v.35 no.6
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• pp.382-388
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• 2019

It is currently well known that surface textures act as lubricant reservoirs, entrap wear debris, and hydrodynamic bearings, which can lead to certain increases in load-carrying capacities. Until recently, the vast majority of research has focused on parallel sliding machine components such as thrust bearings, mechanical face seals, piston rings, etc. However, most sliding bearings have a convergent film shape in the sliding direction and their hydrodynamic pressure is mainly generated by the wedge action. Following the first part of the present study that investigates the effect of groove position on the lubrication performances of inclined slider bearings, this paper focuses on the effects of groove depths and film thicknesses. Using a commercial computational fluid dynamics (CFD) code, FLUENT, the continuity and Navier-Stokes equations are numerically analyzed. The results show that the film thickness and groove depth have a significant influence on the pressure distribution. The maximum pressure occurs at the groove depth where the vortex is found and, as the depth increases, the pressure decreases. There is also a groove depth to maximize the supporting load with the film thickness. The friction force acting on the slider decreases with deeper grooves. Therefore, properly designed groove depths, depending on the operating conditions, can improve the load-carrying capacity of inclined slider bearings as compared to the bearings without a groove.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.7
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• pp.544-551
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• 2014

A program of launch vehicle performance analysis is composed for the education of the conceptual design of launch vehicles and the requirement analysis for the propulsion system design. The program is applied for the mission analysis of space launch vehicles based on KSLV-II with liquid rocket boosters. The 75-ton class liquid rocket engine is assumed for the boosters by referring the mass ratio of KSLV-II second stage. The launch performance analysis is carried out for KSLV-II with 2, 3 and 4 boosters by targeting the circular orbit of 700 km altitude. The trajectory is assumed as two-dimension considering the variation of the flight environment. Payload of advanced KSLV-II could be increased to maximum 3 tons, though it is limited by the thrust performance of the upper stage.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.10
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• pp.891-898
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• 2007

Optimal aerodynamic design for the pitch-controlled horizontal axis wind turbine and its aerodynamic performance for various pitch angles are performed numerically by using the blade element momentum theory. The numerical calculation includes effects such as Prandtl‘s tip loss, airfoil distribution, and wake rotation. Six different airfoils are distributed along the blade span, and the special airfoil i.e. airfoil of 40% thickness ratio is adopted at the hub side to have structural integrity. The nonlinear chord obtained from the optimal design procedure is linearized to decrease the weight and to increase the productivity with very little change of the aerodynamic performance. From the comparisons of the power, thrust, and torque coefficients with corresponding values of different pitch angles, the aerodynamic performance shows delicate changes for just $3^{\circ}$ increase or decrease of the pitch angle. For precisive pitch control, it requires the pitch control algorithm and its drive mechanism below $3^{\circ}$ increment of pitch angle. The maximum torque is generated when the speed ratio is smaller than the designed one.

• Journal of the Korean Society of Propulsion Engineers
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• v.3 no.4
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• pp.38-43
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• 1999

An experimental study was carried out, in order to set up the procedure for evaluation of hot fire test, to investigate the effect of mixture on combustion performance and combustion stability , and to determine the optimum design condition for designing the liquid rocket engine. $HNO_3$/Kerosene uni-element liquid rocket engine(thrust 24 $\iota{b}_f$, chamber pressure 200 psia) using impinging streams doublet injector was designed, and ground hot-fire test was carried out. To prevent or reduce the hard start during ignition period, two step ignition method was used. This was accomplished by maintaining about 25% of the designed operating pressure doting transient period, then chamber pressure was built up to the designed operating pressure. Maximum combustion efficiency was at O/F ratio 3.6, and combustion efficiency is decreased with increasing momentum ratio.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.11a
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• pp.336-339
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• 2011

Hot-fire test facility is one of the most important infrastructure for thruster development and evaluation. During the past three years, Korea Aerospace Research Institute (KARI) and Hanwha Corporation have successfully performed the construction of hot-fire test facility for medium-scale monopellant thruster to the maximum 200N thrust level. In general, thruster hot-firing test should be performed in vacuum conditions to simulate space environment. The hot-fire test facility is divided into three subsystems, vacuum system, propellant supply system and data measurement & control system. The goal of this facility is to extend the capability from small thruster for satellite mission to medium-scale thruster for launch vehicle and lunar mission. In this paper, the progress and overview for thruster hot-fire test facility was introduced and test results were also presented.