• Journal of the Korean Society for Precision Engineering
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• v.21 no.10
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• pp.127-136
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• 2004

This paper is the first of two companion papers concerning the analysis and design of a pneumatic vibration isolation system. The design optimization of the pneumatic vibration isolation system is required for the reduction of cost, endeavor and time, and it needs modeling and calculation algorithm. The nonlinear models are devised from the fluid mechanical expression for components of the system and the calculation algorithm is derived from the mathematical relationship between the models. It is shown that the orifice makes the nonlinear property of the transmissibility curve that the resonant frequency changes by the amplitude of excited vibration. Linearization of the nonlinear models is tried to reduce elapsed time and truncation error accumulation and to enable the transmissibility calculation of the system with multi damping chambers. The equivalent mechanical models generated by linearization clarify the function of each component of the system and lead to the linearized transfer function that can give forth to the transmissibility exactly close to that of nonlinear models. The modified successive under-relaxation method is developed to calculate the linearized transfer function.

• Proceedings of the SAREK Conference
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• 2009.06a
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• pp.1256-1261
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• 2009

Seawater amounts to 70% of the earth and represents a quite unlimited resource for the production of fresh water by desalination processes and for the extraction of dissolved salts present in it. Recently, the falling film evaporation has increased in interest as an efficient method for seawater desalination system. In the desalination system, the flow characteristics of the falling film is very important issue to make highly efficient system. So, this study is taken to investigate numerically the falling film thickness on the inlet Renold Number ranges are 400 to 700. Numerical simulations are performed using FLUENT6.3.26, a commercial CFD code.

• Nuclear Engineering and Technology
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• v.44 no.8
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• pp.871-888
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• 2012

This paper aims at presenting the state of the art, the recent progress, and the perspective for the future, in the modelling of two-phase flow in the horizontal legs of a PWR. All phenomena relevant for safety analysis are listed first. The selection of the modelling approach for system codes is then discussed, including the number of fluids or fields, the space and time resolution, and the use of flow regime maps. The classical two-fluid six-equation one-pressure model as it is implemented in the CATHARE code is then presented and its properties are described. It is shown that the axial effects of gravity forces may be correctly taken into account even in the case of change of the cross section area or of the pipe orientation. It is also shown that it can predict both fluvial and torrential flow with a possible hydraulic jump. Since phase stratification plays a dominant role, the Kelvin-Helmholtz instability and the stability of bubbly flow regime are discussed. A transition criterion based on a stability analysis of shallow water waves may be used to predict the Kelvin-Helmholtz instability. Recent experimental data obtained in the METERO test facility are analysed to model the transition from a bubbly to stratified flow regime. Finally, perspectives for further improvement of the modelling are drawn including dynamic modelling of turbulence and interfacial area and multi-field models.

• New & Renewable Energy
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• v.8 no.2
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• pp.6-13
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• 2012

Due to global warming, the need to secure an alternative resource has become more important nationally. Having very strong current on the west coast with up to 10 m tidal range, there are many suitable site for the application of TCP (Tidal Current Power) in Korea. On the south west regions between many islands that create strong current in the narrow channels. The rotor is one of the essential components which can convert tidal current energy into rotational energy to generate electricity. The design optimization of rotor is very important to maximize the power production. The performance of rotor can be determined by various parameters including number of blades, shape, sectional size, diameters and etc. This paper introduces the multi-layer vertical axis tidal current power system which can be applied to offshore jetties and piers effectively. Various cases of VAT turbine were designed. Specifically, the number of blades and turbine shape are changed in several cases. Also, performance analysis was carried out by CFD.

• Journal of the Korean Society for Precision Engineering
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• v.19 no.4
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• pp.202-208
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• 2002

An electro-hydraulic manipulator using hydraulic actuators has many nonlinear elements, and its parameter fluctuations are greater than those of an electrically driven manipulator. So it is relatively difficult to realize not only stable contact work but also accurate force control for the autonomous assembly tasks using hydraulic manipulators. In this report, we applied a compliance control which is based on the position control by a disturbance observer for our manipulator system. And a reference trajectory modification method is proposed in order to achieve accurate force control even though the stiffness and position of environment change. Experimental results show that highly robust force tracking by a 6-link electro-hydraulic manipulator could be achieved under various environment conditions.

• International Journal of Ocean System Engineering
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• v.1 no.4
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• pp.222-229
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• 2011

In this paper, we present the concept and main mission of the Crabster, an underwater walking robot. The main focus is on the modeling of drag and lift forces on the legs of the robot, which comprise the main difference in dynamic characteristics between on-land and underwater robots. Drag and lift forces acting on the underwater link are described as a function of the relative velocity of the link with respect to the fluid using the strip theory. Using the translational velocity of the link as the rotational velocity of the joint, we describe the drag force as a function of joint variables. Generalized drag torque is successfully derived from the drag force as a function of generalized variables and its first derivative, even though the arm has a roll joint and twist angles between the joints. To verify the proposed model, we conducted drag torque simulations using a simple Selective Compliant Articulated Robot Arm.

• Journal of the Korean Society for Geothermal and Hydrothermal Energy
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• v.14 no.2
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• pp.8-14
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• 2018

A boiling heat transfer is used in various industry such as power generation systems, heat exchangers, air-conditioning and refrigerations. In the boiling heat transfer system, the critical heat flux (CHF) is the important factor, and it indicated safety of the system. It has kept up studies on the CHF enhancement. Recently, it is reported the CHF enhancement, when working fluid used the nanofluid with excellent thermal properties. Therefore, in this study, we investigated the influence of nano particles coated surface for heat transfer enhancement in pure water, oxidized multi-wall carbon nanotube nanofluid (OMWCNT), and oxidized graphene nanofluid (OGraphene). Nanoparticles were coated for 120 sec on the surface, and we measured the CHF at the flow velocities of 0.5, 1.0, and 1.5 m/sec, respectively. As the results, both of the OMWCNT and OGraphene nanofluids increased up to about 34.0 and 40.0%.

• Journal of computational fluids engineering
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• v.5 no.1
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• pp.1-7
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• 2000

In general, FDM(finite difference method) and FVM(finite volume method) are used for analyzing the fluid flow numerically. However it is difficult to apply them to problems involving complex geometries, multi-connected domains, and complex boundary conditions. On the contrary, FEM(finite element method) with coordinates transformation for the unstructured grid is effective for the complex geometries. Most of previous studies have used commercial codes such as KIVA or STAR-CD for the flow analyses in the engine cylinder, and these codes are mostly based on the FVM. In the present study, using the FEM for three-dimensional, unsteady, and incompressible Navier-Stokes equation, the velocity and pressure fields in the engine cylinder have been numerically analyzed. As a numerical algorithm, 4-step time-splitting method is used and ALE(arbitrary Lagrangian Eulerian) method is adopted for moving grids. In the Piston-Cylinder, the calculated results show good agreement in comparison with those by the FVM and the experimental results by the LDA.

• Journal of computational fluids engineering
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• v.20 no.4
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• pp.1-6
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• 2015

Centrifugal pumps consume considerable amount of energy in various industrial applications. Therefore, improvement of the efficiency of these machines has become a major challenge. Cavitation is a phenomenon which decreases the pump efficiency and even causes structural demage. Hence, the goal of this paper is to investigate the cavitation problem in the single-stage and double-stage centrifugal pumps. The Volume of Fraction (VOF) method has been used for the numerical simulations together with Rayliegh-Plesset model for the gas-liquid two-phase flow inside the pump. In order to capture the turbulent phenomena, the standard k-${\varepsilon}$ turbulence model has been adopted, and the simulations have been done as unsteady cases. In addition, the motion of the rotating parts has been simulated using Multi Reference Frame(MRF) method. The results are presented and compared in terms of hydraulic head and NPSH for both the single-stage and double-stage pumps. The H-Q curves show the effects of cavitation on decreasing the pumps performances.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.252-255
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• 2008

In this study, the characteristics of crossing a river of Ground Armored Vehicle (GAV) were evaluated by numerical method and real size tests. 3-D hybrid mesh systems were constructed by 3-D models of the GAV, and a commercial software, FLUENT, was used in numerical analysis. In order to deal with multi-phase problem (air and water), Volume Of Fluid (VOF) method was used, and Moving and Deforming Mesh (MDM) was adapted for unsteady motion of GAV. There were two steps in this research. Firstly, stability of the GAV which cruised a river was evaluated by changing several shapes of water-proof-front-wing of the GAV in steady state, and compared results (free surface shape and drag value in 10km/h) with those of real size tests. Secondly, results of unsteady analysis considering weight and moment of inertia of the GAV were presented. There were showed a maximum velocity with a designed water jet and dynamic stability including pitch, roll, and yaw moment. Based on these results, the optimal shape of water-proof-front-wing of the GAV was determined for a proto-type of the GAV.