• Journal of computational fluids engineering
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• v.14 no.4
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• pp.41-47
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• 2009

An immersed boundary method(IBM, Kim et al.(2001)) for simulating flows over complex geometries is applied to computation of three-dimensional Floquet stability of a periodic wake. Floquet stability analysis is employed to extract different modes of three-dimensional instability. To verify the present method, a fully-resolved Floquet stability calculation for flow past a circular cylinder is considered. There are two different instability modes with long(mode A) and short (mode B) spanwise wavelengths for the periodic wake of a circular cylinder. The critical Reynolds number and the most unstable spanwise wavelengths of modes A and B are computed using the present method, and compared with other authors' results currently available.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1909-1914
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• 2004

An immersed boundary method for simulation of density-stratified flows is developed and applied to computation of viscous flows over two-dimensional obstacles in a bounded domain under stable density stratification. Density sources/sinks are introduced on the body surface. Two obstacle shapes are used, a vertical barrier and a smooth cosine-shaped hill; weak stratification, defined by $K=ND/{\pi}U{\leq}1$, where U, N, and D are the upstream velocity, buoyancy frequency, and domain height, respectively, is considered. The results are consistent with other authors' calculations, and shed light on computation of density-stratified flows in complex geometries.

• Journal of Mechanical Science and Technology
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• v.20 no.6
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• pp.849-863
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• 2006

IBM (Immersed Boundary Method) with feedback momentum forcing was applied to stationary and moving bodies. The capability of IBM to treat the obstacle surfaces, especially with moving effect has been tested for two dimensional problems. Stationary and oscillating cylinders were simulated by using IBM based on finite volume method with Cartesian coordinates. For oscillating cylinder, lateral and vertical motions are considered, respectively. Present results such as time histories of drag and lift coefficients for both stationary and oscillating cases are in good agreement with previous numerical and experimental results. Also, the instantaneous wake patterns of oscillating cylinder with different oscillating frequency ratios well represented those of previous researches. More feasibility study for IBM has been carried out to two oscillating cylinders. Drag and lift coefficients are presented for two cylinders oscillating sinusoidally with phase difference of $180^{\circ}$.

• Geomechanics and Engineering
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• v.18 no.6
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• pp.651-659
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• 2019

The IB-SEM numerical method combines the spectral/hp element method and the rigid immersed boundary method. This method avoids the problems of low computational efficiency and errors that are caused by the re-division of the grid when the solids move. Based on the Fourier transformation and the 3D immersed boundary method, the 3D IB-SEM system was established. Then, using the open MPI and the Hamilton HPC service, the computational efficiency was increased substantially. The flows around a cylinder and a sphere were simulated by the system. The surface of the cylinder generates vortices with alternating shedding, and these vortices result in a periodic force acting on the surface of the cylinder. When the shedding vortices enter the flow field behind the cylinder, a recirculation zone is formed. Finally, the three-dimensional pore flow was successfully investigated.

• Proceedings of the KSME Conference
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• 2008.11b
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• pp.2477-2482
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• 2008

Effect of computational domain size in simulating of periodic obstacle flow has been investigated for the flow past tube banks. Reynolds number, defined by freestream velocity (U) and cylinder diameter (d), was fixed as 200, and center-to-center distance (P) as 1.5d. In-line square array was considered. Drag coefficient, lift coefficient and Strouhal number were calculated depending on domain size. Circular cylinders were implemented on a Cartesian grid system by using an immersed boundary method. Boundary condition is periodic in both streamwise and lateral directions. Previous studies in literature often use a square domain with a side length of P, which contains only one cylinder. However, this study reveals that size is improper. Especially, RMS values of flow-induced forces are most sensitive to the domain size.

• Journal of computational fluids engineering
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• v.15 no.2
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• pp.21-27
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• 2010

Immersed boundary lattice Boltzmann method (IBLBM) has been applied to simulate a turbulent flow over circular cylinder in a flow field effectively. Although IBLBM is very effective method to simulate the flow over a complex shape of obstacle in the flow field regardless of the constructed grids in the calculation domain, the results, however, become numerically unstable in high reynolds number flow. The most effective suggestion to archive the numerical stability in high Reynolds number flow is applying the multiple relaxation time (MRT) model instead of single relaxation time(SRT) model in the collision term of lattice Boltzmann equation. In the research MRT model for IBLBM was introduced and comparing the numerical results obtained by applying SRT and MRT. The hydraulic characteristic of cylinder in a flow field between two parallel plate at the range of $Re{\leqq}2000$represented and it is also compared the drag and lifting coefficients of the cylinder calculated by IBLBM with SRT and MRT model.

• Journal of computational fluids engineering
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• v.16 no.2
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• pp.24-29
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• 2011

Immersed boundary lattice Boltzmann method has been applied to analyze the characteristics of the self-propelled fish motion swimming robot. The airfoil NACA0012 with caudal fin stroke model was considered to examine the characteristics. The foil in steady forward motion and a combination of steady-state harmonic deformation produces thrust through the formation of a flow downstream from the trailing edge. The harmonic motion of the foil causes unsteady shedding of vorticity from the trailing edge, while forming the vortices at the leading edge as well. The resultant thrust is developed by the pressure difference formed on the upper and lower surface of the airfoil. and the time averaged thrust coefficient increases as Re increase in the region of $Re{\leqq}700$. The suggested numerical method is suitable to develop the fish-motion model to control the swimming robot, however It would need to extend in 3D analysis to examine the higher Re and to determine the more detail mechanism of thrust production.

• Journal of the Society of Naval Architects of Korea
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• v.43 no.3 s.147
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• pp.322-330
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• 2006

Numerical calculations are carried out for flow past a circular cylinder forced oscillating normal to the free-stream flow at a fixed Reynolds number equal to 185. The cylinder oscillation frequency ranged from 0.8 to 1.2 of the natural vortex-shedding frequency, and the oscillation amplitude extended up to 20% of the cylinder diameter. IBM (Immersed Boundary Method) with direct momentum forcing was adopted to handle both of a stationary and an oscillating cylinder Present results such as time histories of drag and lift coefficients for both stationary and oscillating cases are in good agreement with previous numerical and experimental results. The instantaneous wake patterns of oscillating cylinder with different oscillating frequency ratios showed the synchronized wakes pattern in the lock-in region and vortex switching phenomenon at higher frequency ratio than the critical frequency ratio.

• Proceedings of the KSME Conference
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• 2008.11b
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• pp.2489-2494
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• 2008

The two-dimensional circular cylinder freely falling in a channel has been simulated by using Immersed boundary - lattice Boltzmann method in order to analyze the characteristics of motion originated by the interaction between the fluid and the solid. The wide range of the solid/fluid density ratio has been considered to identify the effect of the solid/fluid density ratio on the motion characteristics such as the falling time, the terminal velocity and the trajectory in the vertical and horizontal directions. In addition, the effect of the gap between the cylinder and the wall on the motion of two-dimensional circular cylinder freely falling has been revealed by taking into account a various range of the gap size. The Reynolds number in terms of the terminal velocity is diminished as the cylinder becomes close to the wall at the initial dropping position, since the repulsive force induced between the cylinder and wall constrains the vertical motion. Quantitative information about the flow variables such as the pressure coefficient and vorticity on the cylinders is highlighted.

• 한국전산유체공학회:학술대회논문집
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• 2006.10a
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• pp.66-69
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• 2006

In the present study, we propose a virtual boundary method for simulation of massive inextensible flexible strings immersed in viscous fluid flow. The fluid motion is governed by the Navier-Stokes equations and a momentum forcing is added in order to bring the fluid to move at the same velocity with the immersed surface. A massive inextensible flexible string model is described by another set of equations with an additional momentum forcing which is a result of the fluid viscosity and the pressure difference across the string. The momentum forcing is calculated by a feedback loop. Simulations of several numerical examples are carried out, inlcuding a hanging string which starts moving under gravity without ambient fluid, a string swimming within a uniform flow and a uniform flow over two side-by side strings. The numerical results agree well with the theoretical analysis and previous experimental observations. Preliminary results of a swimming elongated fishlike body will also be presented.