• 한국해양공학회지
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• 제35권3호
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• pp.216-228
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• 2021

In the present study, the performance of a floating oscillating water column (OWC) device has been studied in regular waves. The OWC model has the shape of a hollow cylinder. The linear potential theory is assumed, and a matched eigenfunction expansion method(MEEM) is applied for solving the diffraction and radiation problems. The radiation problem involves the radiation of waves by the heaving motion of a floating OWC device and the oscillating pressure in the air chamber. The characteristics of the exciting forces, hydrodynamic forces, flow rate, air pressure in the chamber, and heave motion response are investigated with various system parameters, such as the inner radius, draft of an OWC, and turbine constant. The efficiency of a floating OWC device is estimated in connection with the extracted wave power and capture width. Specifically, the piston-mode resonance in an internal fluid region plays an important role in the performance of a floating OWC device, along with the heave motion resonance. The developed prediction tool will help determine the various design parameters affecting the performance of a floating OWC device in waves.

• 한국정밀공학회지
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• 제14권10호
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• pp.44-49
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• 1997

The damping force is determined by four valves and the components which consist of the shock absorber for vehicle. In this study it is investigated the individual effects of four valves and these components on damping forces using dynamic behaviour analysis program of the shock absorber. In addition, opening of main valves are researched during compression and tension cycle due to up- down reciprocation movement of piston. We are to strictly control the properties and tolerance of componenets having important effects on the damping force. Thus we are intended to produce shock absorber of better quality.

• Tribology and Lubricants
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• 제25권5호
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• pp.305-310
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• 2009

Laser surface texturing (LST) methods are applied recently to generate micro-dimples in machine components having parallel sliding surfaces such as thrust bearings, mechanical face seals and piston rings, etc. And it is experimentally reported by several researchers that the micro-dimpled bearing surfaces can reduce friction force. Until now, however, theoretical results for various dimple parameters are not fully presented. In this paper, a commercial computational fluid dynamics (CFD) code, FLUENT is used to investigate the effect of dimple depth on the lubrication characteristics of parallel thrust bearing. The results show that the pressure, velocity and density distributions within dimples are highly affected by dimple depths and cavitation conditions. Adoption of micro-dimple on the bearing surface can reduce the friction force highly and its levels are affected by dimple depth. The numerical methods and results can be use in design of optimum dimple characteristics to improve thrust bearing performance.

• 동력기계공학회지
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• 제10권3호
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• pp.11-16
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• 2006

The pressure fluctuation in the intake and exhaust pipe of 4 stroke-cycle diesel engine is caused by reciprocating motion of piston for suction of fresh air and exhaust of burned gas. this gas dynamic effect can be utilized for increase the volumetric efficiency. Many empirical studies have been carried out to investigate the effects of intake pulsating flow on the volumetric efficiency. However, when the gas dynamic effects are utilized for the variable speed engine to increase its performance, The speed range in which the maximum volumetric efficiency is limited and there occurs some difficulties in lay-out of intake system because it become too long. During induction process, as waves travel both directions, they are reflected and interacted each other and pressure waves are transmitted through it. Hence, the flow becomes more complex and unsteady flow. These pressure waves act upon intake pulsating flow and affects on the volumetric efficiency. In this paper the effects of pulsating flow of intake and exhaust pipes on volumetric efficiency were examined and evaluated. It was found that volumetric efficiency was affected by pulsating flow of intake and exhaust pipes.

• International Journal of Automotive Technology
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• 제7권5호
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• pp.519-526
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• 2006

For investigating the effect of fuel stratification on flame propagation, initial flame development and propagation were visualized under different axially stratified states in a port injection SI engine. Stratification was controlled by the combination of the port swirl ratio and injection timing. Experiments were performed in an optical single cylinder engine modified from a production engine and images were captured through the quartz window mounted in the piston by an intensified CCD camera. Firstly in this paper, the characteristics under no port-generated swirl condition, i.e. normal conventional case was studied. Under various stratified conditions, flame images were captured at the pre-set crank angles. These were averaged and processed to characterize the flames propagation. The flame stability was estimated by the weighted average of flame area and luminosity. The stability was also evaluated through the standard deviation of flame area and propagation distance, and mean absolute deviation of propagating direction. Results show that stratification state according to injection timing do not affect on the direction of flame propagation. The flame development and the initial flame stability are strongly dependent on the stratified conditions and the initial flame stability is closely related to the engine stability and lean misfire limit.

• 한국전산유체공학회지
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• 제14권1호
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• pp.9-17
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• 2009

In this study we investigated the characteristics of fluid flow and heat transfer within a coolant passage in the cylinder head assembly of an internal combustion engine by using a commercial CFD code, CFX The complex coolant passage of the cylinder head assembly was modelled by suitable choice of a grid system and careful attention was paid in the construction of meshes near the walls where significant cooling occurs. To treat the simultaneous heating and cooling of the combustion walls we invented a methodology allowing a heat source within the solid wall and the convective cooling at the interface between the solid and the fluid. We managed to reproduce the experimental results by adjusting parameters appropriately. We have found that high temperature was concentrated at the surface of the cylinder jacket. It turned out that the effect of oil cooling from the piston head was unexpectedly significant. On the other hand the effect of cooling from the ambient air is almost negligible. The CFD method proposed in this study is believed to be useful in the early stage of the design of the engine-cooling system.

• 한국분무공학회지
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• 제20권4호
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• pp.254-260
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• 2015

The objective of this study is to investigate the effect of multi-hole nozzle on the performance of small CRDI engine. Combustion and exhaust emission characteristics of engine were studied by using CFD simulation with ECFM-3Z combustion model. The conditions of simulation were varied with nozzle geometry, injection timing and injection quantity. In addition, the results were compared in terms of combustion pressure, rate of heat release, $NO_x$ and soot emissions. It was found that combustion pressure was increased when injection timing was advanced. The rate of heat release of 6 hole nozzle was higher than that of 12 hole nozzle since the quantity of fuel impinged at the bottom of piston rim was different under different injection timing conditions. In the case of $NO_x$ emission, 6 hole nozzle generated more $NO_x$ emission than 12 hole nozzle. On the other hand, in the case of soot emission, 12 hole nozzle showed higher value than 6 hole nozzle because injected fuel droplets from multi-hole nozzle were coalesced.

• 태양에너지
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• 제19권4호
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• pp.33-43
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• 1999

The study on ice thermal storage system is to improve total system performance and increase the economical efficiency in actual all-conditioning facilities. To obtain the high charging and discharging efficiencies in ice thermal storage system, the improvement of thermal stratification is essential, therefore the process flow must be piston flow in the cylindrical type. With the relation of the aspect ratio(H/D) in the storage tank, the stratification is formed better as inlet flow rate is smaller. If the inlet and the outlet port are settled at the upside and downside of the storage tank, higher storage rate could be obtainable. In case that the flow directions inside the thermal storage tank are the upward flow in charging and the downward in discharging, thermal stratification is improved because the thermocline thickness is maitained thin and the degree of stratification increases respectively. In the charging process, in case of inlet flow rate the thermal stratification has a tendency to be improved with the lower flow rate and smaller temperature gradient in case of inlet temperature, the large temperature difference between inflowing water and storage water are influenced from the thermal conduction. The effect of the reference temperature difference is seen differently in comparison with the former study for chilled and hot water. In the discharging process, the thermal stratification is improved by the effect of the thermal stratification of the charging process.

• 한국항해항만학회지
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• 제38권5호
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• pp.443-450
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• 2014

The objective of this study is to examine the nonlinear fluid characteristics near and inside a moonpool in various sea conditions. We estimate the flow of the free surface in a moonpool taking into account the viscosity effect and the hydrodynamic forces that affects a moonpool and hull through CFD calculations. The comparison of horizontal forces per wave length shows that the hydrodynamic force is greater for the long wave length than short wave length, and the greatest hydrodynamic force acts on the moonpool when the wave length is equal to the ship's length. The horizontal force decreases as the wave amplitude decreases, and the hydrodynamic force acting on the moonpool in ${\lambda}=LBP$ is 10 times that in ${\lambda}=LBP/3$. The free surface demonstrates the piston mode, in which it oscillates up and down while remaining essentially flat, and the rise of the free surface level increases as the wave length increases. We can assume that the hydrodynamic force acting on the moonpool increases owing to the effect of a strong vortex for ${\lambda}=LBP$ and owing to the rise of the free surface level for ${\lambda}=LBP{\times}2$.

• 한국군사과학기술학회지
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• 제22권2호
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• pp.255-265
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• 2019

In the present study, the effect of opening patterns of a flow control valve on underwater discharge systems using a linear pump was investigated numerically. For that, a improved mathematical model was developed. The improvement is to separate a middle tank from a water cylinder because the cross-section area of the inlet of the middle tank is an important parameter. To validate the improved model, calculation results were compared with a previous study. The results showed that $2^{nd}$ order or more polynomial opening patterns had an advantage over ramp opening patterns. Higher an order of polynomial resulted in wider operating limits. An escape velocity and a maximum acceleration of underwater vehicle were affected by time derivative of the cross-section area of the flow control valve. Besides, as a velocity profile of the vehicle got closer to linearity, the escape velocity got faster and the maximum acceleration got smaller. And velocities of the vehicle and piston had similar variation trend.