• Journal of the Society of Naval Architects of Korea
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• v.35 no.4
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• pp.1-10
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• 1998

This paper presents a vorticity-based numerical method for analyzing an incompressible Newtonian viscous flow around an impulsively started cylinder. The Navier-Stockes equations have a natural Helmholtz decomposition. The vorticity transport equation and the pressure equation are derived from this decoupled form. The associated boundary conditions are dynamic for the vorticity and pressure variables representing the coupling relation between them and the force balance on the wall. The various numerical treatments for solving the governing equations are introduced. According to Wu et al.(1994), the boundary conditions are decoupled, keeping the dynamic relation between vorticity and pressure. The vorticity transport equation is formulated by FVM and TVD(Total Variation Diminishing) scheme is used for the convection term. An integral approach similar to the panel method is used to obtain the velocity field for a given vorticity field and the pressure field, instead of the conventional differential approaches. In the numerical process, the structured grid is generated. The results are compared to existing numerical and analytic results for the validity of the present method.

• International Journal of Fluid Machinery and Systems
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• v.9 no.4
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• pp.370-381
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• 2016

As a single-channel pump is used for wastewater treatment, this particular pump type can prevent performance reduction or damage caused by foreign substances. However, the design methods for single-channel pumps are different and more difficult than those for general pumps. In this study, a design optimization method to improve the hydrodynamic performance of a single-channel pump impeller is implemented. Numerical analysis was carried out by solving three-dimensional steady-state incompressible Reynolds-averaged Navier-Stokes equations using the shear stress transport turbulence model. As a state-of-the-art impeller design method, two design variables related to controlling the internal cross-sectional flow area of a single-channel pump impeller were selected for optimization. Efficiency was used as the objective function and was numerically assessed at twelve design points selected by Latin hypercube sampling in the design space. An optimization process based on a radial basis neural network model was conducted systematically, and the performance of the optimum model was finally evaluated through an experimental test. Consequently, the optimum model showed improved performance compared with the base model, and the unstable flow components previously observed in the base model were suppressed remarkably well.

• Journal of the Society of Naval Architects of Korea
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• v.31 no.3
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• pp.80-86
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• 1994

A numerical method for simulations of inviscid incompressible flow fields around a ship advancing on the free surface is developed. A body fitted coordinate system, generated by numerically solving elliptic type partial differential equations is used to conform the ship and free surface configurations. Three dimensional Euler equations transformed to the non-staggered body fitted coordinate system are discretised by finite difference method. Time and spatial derivatives are discretised by forward and centered differencings, respectively, and artificial dissipations are added to discretised convection terms for improvements of numerical stability. At each time steps, free surface elevations are recomputed to satisfy nonlinear free surface conditions. Poisson equations for pressure field are solved iteratively and the velocity field for next time step is extrapolated. To verify the developed numerical method, flow fields around a Wigley model are simulated(Fn=0.250-0.408) and compared with experimental data to show good agreements.

• Journal of the Korean Society of Visualization
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• v.21 no.1
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• pp.80-93
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• 2023

We performed numerical simulations of blood flow in an arterial cerebral artery aneurysm to investigate the hemodynamic behavior after coil embolization. A patient-specific model was created based on CTA data. We also conducted the coil embolization simulation to obtain the coil placement within the aneurysm. Blood was assumed to be an incompressible Newtonian fluid, and both the vessel and coil were considered rigid walls. The pulsatile boundary condition was applied at the inlet, and the outflow boundary conditions were used at the outlets. Our findings demonstrated that the coil embolization significantly reduces the blood volume flowrate entering the aneurysm by effectively blocking the inflow jet, leading to a decrease in both TAWSS and WSS, especially at the systolic peak in the impingement zone. While several high OSI regions disappeared over the aneurysm surface, we observed high OSI regions with a relatively small area where the coil did not completely occlude the aneurysm. Overall, these results quantitatively analyzed the effectiveness of coil embolization by focusing on hemodynamic indicators, potentially preventing aneurysm rupture. The present work could contribute to the development of patient-specific coil embolization.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.20 no.5
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• pp.661-669
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• 2007

Eyer since the Prandtl's experiment in 1934 and X-21 airjet test in 1950 both attempting to reduce drag, it was found that controlling the velocities of surface for extremely fast-moving object in the air through suction or injection was highly effective and active method. To obtain the right amount of suction or injection, however, repetitive trial-and error parameter test has been still used up to now. This study started from an attempt to decide optimal amount of suction and injection of incompressible Navier-Stokes by employing optimization techniques. However, optimization with traditional methods are very limited, especially when Reynolds number gets high and many unexpected variables emerges. In earlier study, we have proposed an algorithm to solve this problem by using step by step method in analysis and introducing SQP method in optimization. In this study, we propose more effective and robust algorithm and techniques in solving flow optimization problem.

• 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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.9
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• pp.597-604
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• 2016

A parallel algorithm of bi-conjugate gradient method was developed based on CUDA for parallel computation of the incompressible Navier-Stokes equations. The governing equations were discretized using splitting P2P1 finite element method. Asymmetric stenotic flow problem was solved to validate the proposed algorithm, and then the parallel performance of the GPU was examined by measuring the elapsed times. Further, the GPU performance for sparse matrix-vector multiplication was also investigated with a matrix of fluid-structure interaction problem. A kernel was generated to simultaneously compute the inner product of each row of sparse matrix and a vector. In addition, the kernel was optimized to improve the performance by using both parallel reduction and memory coalescing. In the kernel construction, the effect of warp on the parallel performance of the present CUDA was also examined. The present GPU computation was more than 7 times faster than the single CPU by double precision.

• Tribology and Lubricants
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• v.31 no.4
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• pp.148-156
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• 2015

In the study, a design process for ensuring optimal clearance in a fuel injection pump(FIP) is suggested. Structure analysis and hydrodynamic lubrication analysis are performed to determine the optimal clearance. The FIP is simulated using Hypermesh, Abaqus 6.12 to evaluate the reduction of clearance when the maximum supply pressure is applied. The reduction in clearance is caused by the difference in the deformations between the barrel and plunger. When the deformation of the plunger is larger than that of the barrel, a reduction in clearance at the head part occurs. On the other hand, the maximum clearance reduction equals the maximum deformation in the stem part, because the deformation of barrel does not occur in this region. The clearance of FIP should be designed to be larger than maximum reduction of clearance in order to avoid contact between the plunger and barrel. In addition, the two-dimensional Reynolds equation is used to evaluate lubrication characteristics with variations of viscosity, clearance and nozzle for a laminar, incompressible, unsteady state flow. The equation is discretized using the finite difference method. The lubrication characteristics of FIP are investigated by comparing film parameter, which is the ratio of the minimum film thickness and surface roughness. The optimal clearance of FIP is to be designed by considering the maximum reduction in clearance, lubrication characteristics, machining limits and tolerance of clearance.

• Nuclear Engineering and Technology
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• v.46 no.5
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• pp.633-640
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• 2014

In this research, the surface roughness affecting the pressure drop in a pipe used as the steam generator of a PWR was studied. Based on the CFD (Computational Fluid Dynamics) technique using a commercial code named ANSYS-FLUENT, a straight pipe was modeled to obtain the Darcy frictional coefficient, changed with a range of various surface roughness ratios as well as Reynolds numbers. The result is validated by the comparison with a Moody chart to set the appropriate size of grids at the wall for the correct consideration of surface roughness. The pressure drop in a full-scale U-shaped pipe is measured with the same code, correlated with the surface roughness ratio. In the next stage, we studied a reduced scale model of a U-shaped heat pipe with experiment and analysis of the investigation into fluid-structure interaction (FSI). The material of the pipe was cut from the real heat pipe of a material named Inconel 690 alloy, now used in steam generators. The accelerations at the fixed stations on the outer surface of the pipe model are measured in the series of time history, and Fourier transformed to the frequency domain. The natural frequency of three leading modes were traced from the FFT data, and compared with the result of a numerical analysis for unsteady, incompressible flow. The corresponding mode shapes and maximum displacement are obtained numerically from the FSI simulation with the coupling of the commercial codes, ANSYS-FLUENT and TRANSIENT_STRUCTURAL. The primary frequencies for the model system consist of three parts: structural vibration, BPF(blade pass frequency) of pump, and fluid-structure interaction.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.16 no.1
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• pp.33-42
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• 2003

This paper deals with the FEM analysis of nonlinear sloshing of incompressible, invicid and irrotational flow in two dimensional rectangular tank. We use laplace equation based on potential theory as governing equation. For large amplitude sloshing motion, kinematic and dynamic free surface conditions derived from Bernoulli equation are applied. This problem is solved by FEM using 9-node elements. For the time integration and accurate velocity calculation, we introduce predictor-corrector time marching scheme and least square method. Also, numerical stability in tracking of free surface is obtained by direct calculation of free surface location to time variation. Numerical results of sloshing induced by harmonic excitations, while comparing with those of linear theory and references, prove the accuracy and stability. After verification of our program, we analyze sloshing response characteristics to the fluid height and the excitation amplitude.