• Journal of the Korean Society for Precision Engineering
• /
• v.22 no.3 s.168
• /
• pp.154-161
• /
• 2005

In this study, the leaflet motion of a mechanical heart valve and the characteristics of two-dimensional transient blood flow in an elastic blood vessel have been numerically investigated by using fluid-structure interaction method. Here, blood has been assumed as a Newtonian, incompressible fluid. Pressure profiles have been used as boundary conditions at the ventricle and the aorta. As a result, closing motion of the leaflet is faster than opening one. While opening angles of leaflet grow up, vortex is detected at the sinus and backward of the leaflets. When the leaflet is fully closed, vortex is detected at the ventricle and at that moment maximum displacement of the elastic blood vessel is observed in the vicinity of the sinus region. Maximum displacement is caused in association with the blood flow that is oriented toward the elastic blood vessel.

• Journal of Ocean Engineering and Technology
• /
• v.2 no.1
• /
• pp.46-58
• /
• 1988

The experimental investigations on the motion characteristics of a marine riser both in air and water were performed. The static deflections and natural frequencies of the riser in air including the effect of static offset, were obtained from the experiment. These results were compared with those of theoretical prediction by using a simple asymptotic formula. In order to investigate the nonlinear motion characteristics of the riser subject to nonlinear viscous drag and large displacement, the forced oscillation tests both in air and water were performed. In the forced oscillation tests in air, it was found that the transverse motion due to geometrical nonlinearity grows when the amplitude of in-line oscillation exceeds a certain critical value, say, order of 1-2 diameters. The planar motions of the riser in water due to vortex shedding and the geometrical nonlinearity were described. Some of these results were also compared with those of theoretical analysis, which uses a numerical perturbation technique based on the derived linear asymptotic solutions, and found to be generally in good agreement.

• Wind and Structures
• /
• v.1 no.1
• /
• pp.59-66
• /
• 1998

The vibrations of bodies subjected to fluid flow can cause modifications in the flow conditions, giving rise to interaction forces that depend primarily on displacements and velocities of the body in question. In this paper the linearized equations of motion for bodies of arbitrary prismatic or cylindrical cross-section in two-dimensional cross-flow are presented, considering the three degrees of freedom of the body cross-section. By restraining the rotational motion, equations applicable to circular tubes, pipes or cables are obtained. These equations can be used to determine stability limits for such structural systems when subjected to non uniform cross-flow, or to evaluate, under the quasi static assumption, their response to vortex or turbulent excitation. As a simple illustration, the stability of a pipe subjected to a bidimensional flow in the direction normal to the pipe axis is examined. It is shown that the approach is extremely powerful, allowing the evaluation of fluid-structure interaction in unidimensional structural systems, such as straight or curved pipes, cables, etc, by means of either a combined experimental-numerical scheme or through purely numerical methods.

• Journal of the Korean Society of Marine Environment & Safety
• /
• v.28 no.7
• /
• pp.1201-1208
• /
• 2022

In general, to maximize the supply and efficiency of floating offshore wind power generation energy, the motion caused by wave attenuation of the substructure must be reduced. According to previous studies, the motion response was reduced due to the vortex viscosity generated by the damping plate installed in the lower structure among the waves. In this study, a 5 MW semi-submersible OC5 platform and two platforms with attenuation plates were designed, and free decay experiments and numerical calculations were performed to confirm the effect of reducing motion due to vortex viscosity. As a result of the model test, when the heave free decay tests were conducted at drop heights of 30 mm, 40 mm, and 50 mm, compared with the OC5 platform, the platform with two types of damping plates attached had relatively improved motion damping performance. In the model test and numerical calculation results, the damping plate models, KSNU Plate 1 and KSNU Plate 2, were 1.1 times and 1.3 times lower than OC5, respectively, and the KSNU Plate 2 platform showed about two times better damping performance than OC5. This study shows that the area of the damping plate and the vortex viscosity are closely related to the damping rate of the heave motion.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.1
• /
• pp.139-149
• /
• 1994

A numerical simulation of unsteady axisymmetric turbulent flow was performed for a reciprocating engine including port/valve assembly. The governing equations based on a nonorthogonal coordinate formulation with Cartesian velocity components were used and discretised by the finite volume method with non-staggered variable arrangements. The modified $\kappa-\xi$. turbulence model which included the effect of compressibility was used. The results of twodimensional transient calculation for the axisymmetric configuration were compared with the experimental data. Although slightly low rms velocity was predicted compared to the experimental data, predicted velocity distributions at the valve exit and in-cylinder region showed good agreements with the experimental data. The flow at the valve exit was separated at the same valve lift position with the experimental data. Two vortices incylinder region were generated during the initial intake process. The clockwise main vortex became strong and moved upward to the top wall. The counter-clockwise second vortex became weak and stick to the upper left corner of the cylinder. After middle intake process, new vortex adjacent to upper cylinder wall appeared by the piston motion and therefore, the in-cylinder flow was formed into three vortices. The cylinder pressure just before bottom dead center of piston was higher than inlet pressure and then the reverse flow occured at the valve exit. The in-cylinder flow characteristics were strongly dependent on piston motion, but insensitive to valve motion.

• Proceedings of the KSME Conference
• /
• 2001.06d
• /
• pp.864-870
• /
• 2001

Flame propagation along vortex tube was experimentally investigated. The vortex tube was generated by the ejection of propane from a nozzle through a single stroke motion of a speaker and the ignition was induced from a single pulse laser. Non-reactive flow fields were visualized using shadow technique. From these images, vortex ring size and translational velocity were measured in order to determine the ignition time and position. Flame structure and flame speed were measured using high speed CCD camera. Flame speed was accelerated during the initial stage of flame kernel growth, and reached near constant value during steady propagation period. Near the completion of propagation, flame speed was decelerated and then extinguished. Flame speed along the non-premixed vortex tube was found to be linearly proportional to circulation, which was similar to that of the flame propagation along premixed vortex ring. Ignition position minimally affects the propagation characteristics. These imply that flame is propagating along the maximum speed locus expected to be along stoichiometric contour and also support the existence of tribrachial flames.

• Journal of the Korean Society of Visualization
• /
• v.9 no.3
• /
• pp.59-64
• /
• 2011

In this study the change of free surface vortex is expressed through the time volume fraction using multiphase unsteady condition in sump, because in previous studies of the pump sump did not represent the behavior of the free surface vortex exactly due to the reason it was calculated using single phase and steady condition. The reliability of the computational analysis is verified through comparing experimental results with that of present numerical analysis. Homogeneous free surface model is used to apply interactions of air and water. The results show that the free surface vortex can be identified on the isotropic surface at air volume fraction 1%~5%. The vortices make an air column from the free surface to the sump intake and are created and destroyed repeatedly. The behavior of free surface vortex at numerical analysis is quite similar to experimental test. The result of vortex motion according to time, works on a cycle.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.25 no.3
• /
• pp.381-388
• /
• 2001

Presented are heat data which describe the effect of interaction between bulk flow pulsations and a vortex embedded in a turbulent boundary layer. The pulsation frequencies are 3 Hz, 15 Hz and 30 Hz. A half delta wing with the same height as the boundary layer thickness is used to generate the vortex flow. The convection heat transfer coefficients on a constant heat-flux surface are measured by embedded 77 T-type thermocouples. Spanwise profiles of convection heat transfer coefficients show that upwash region of vortex flow is influenced by bulk flow pulsations. The local heat transfer coefficient increases approximately by 7 percent. The increase in the local change of convection heat transfer coefficient is attributed to the spanwise oscillatory motion of vortex flow especially at the low Strouhal number and to the periodic change of vortex size.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.25 no.3
• /
• pp.397-404
• /
• 2001

The behavior of unsteady wake vortices for the two-dimensional flat plate is simulated by a discrete vortex method. The flat plates and their wakes are represented by vortex sheets. The vortex sheets are replaced with discrete vortices. The freely deforming wake sheets are computed as a part of solution and the ground effect is included by a image method. In order to predict wake shapes accurately and to model closely coupled aerodynamic interference, a vortex core model and a vortex core addition scheme are used. The simulated wake shapes convecting behind the plates in unsteady motion are compared to a flow visualization result and other numerical results. The present results agree well with them. The present method is also applied to the aerodynamic analysis of flat plates in tandem configuration in ground effect.

• Journal of the Korean Society of Visualization
• /
• v.18 no.1
• /
• pp.67-73
• /
• 2020

Flexible structures have been adopted in heat transfer systems as vortex generators. The flexible vortex generators immersed in a flow show a self-sustained oscillatory motion, which enhances fluid mixing and heat transfer. In the present study, the vortex generators in a two-dimensional channel flow are numerically investigated, and they are symmetrically mounted on the upper and lower walls with an inclination angle. The momentum interaction and heat transfer between the flexible vortex generators and the surrounding fluid are considered by using an immersed boundary method. The inclination angle is one of the important factors in determining the flapping kinematics of the flexible vortex generators. The flapping amplitude increases as the inclination angle increases, thereby enhancing fluid mixing. The heat transfer is enhanced up to 80% comparing to the baseline channel flow.