• 대한조선학회논문집
• /
• 제48권3호
• /
• pp.260-266
• /
• 2011

For long slender offshore structures, such as cables and pipe lines, their interaction with surrounding fluid flow becomes an important issue for global design of ocean systems. We employ a long circular cylinder as a representative case of slender offshore structure. A flexibly mounted cylinder in cross-flow generates complex vortex shedding and results in oscillation of the structure. In this paper, flow behind a circular cylinder at Re=100 is simulated. The vortex shedding pattern and flow induced motion are examined in the cross flow configuration as well as with various yaw-angled configurations. The "Lock-in" phenomenon is also observed when reduced velocity is approximately 4.0. The MAC Grid system, which is the typical grid system for Cartesian mesh and pressure correction methods, are used for solving the incompressible Navier-Stokes equations. Predictor/Corrector method is applied for obtaining a non-linear response of structure at the flexibly mounted. The existance and motion of the body is represented by the immersed boundary technique.

• 오토저널
• /
• 제14권4호
• /
• pp.39-49
• /
• 1992

Three dimensional numerical calculation carried out to investigate the eccentricity effect of intake valve on the in-cylinder flow fields for the intake stroke and the compression stroke. During the intake stroke, a corner vortex in the vicinity of the valve exit interacted strongly with a toroidal vortex in the case of axisymmetric valve. But a weak interaction between the corner vortex and the toroidal vortex occurred due to the eccentricity of the valve in the narrow region between valve and cylinder wall in the case of offset valve. During the compression stroke, it was found that a solid body rotation was maintained in the radial-circumferential plane in the case of axisymmetric valve. But a weak secondary vortex was formed in the radial-circumferntial plane in the case of offset valve, because of the interaction between swirl flows and inward flows towards cylinder axis. The calculated turbulence intensity presented a similar trend with the experiental results but, in spite of using the modified k-.epsilon. model, it was found that the qualitative difference between the numerical results and experimental results was large in the region where the velocity gradient is substantial.

• International Journal of Naval Architecture and Ocean Engineering
• /
• 제13권1호
• /
• pp.163-177
• /
• 2021

Numerical simulations of the Vortex-Induced Vibration (VIV) about a large-scale flexible pipe subject to shear flow were carried out in this paper. Efficiency verification was performed firstly, validating that the proposed fluid-structure interaction solution strategy is competent in predicting the VIV response. Then, the VIV characteristics related to multi-mode and spanwise hybrid waveform about the flexible pipe attributed to shear flow were investigated. When inflow velocity rises, higher vibration modes are apt to be excited, and the spanwise waveform easily convertes from a standing-wave-dominated status to a hybrid standing-traveling wave status. The multi-mode or even multiple-dominant-mode is prone to occur, that is, the dominant mode is often followed by several apparent subordinate modes with considerable vibration energy. Hence, the shedding frequencies no longer obey Strouhal law, and vibration trajectories become intricate. According to the motion analysis concerning the coupled cross-flow and in-line vibrations, as well as the corresponding wake patterns, a tight coupling interaction exists between the structural deformation and the wake flow behind the flexible pipe. In addition, the evolution of the vortex tube along the pipe span and a strong 3D effect are observed due to the slenderness of the flexible pipe and the variability of the vortex shedding attributed to the shear flow.

• Journal of Mechanical Science and Technology
• /
• 제18권5호
• /
• pp.857-864
• /
• 2004

The change in the NO emission indices (EINO) in a two-dimensional plane has been investigated, which is due to the interaction between a vortex and methane-air flames established at different equivalence ratios, by solving the field equation. After solving the field equation, the spatial distribution of G-values is obtained. The NO emission index is calculated after applying the appropriate relation between the G-values and the NO production rate or the mass fraction of methane obtained from the library of freely propagating flames created from detailed simulations. When a vortex exists in a reacting flow field, in general EINO slightly increases, whereas ElNO is lowered in the vicinity of the vortex regardless of flow direction. A change in vortex size has negligible impact on EINO$\_$T/ but increasing the vortex strength slightly increases EINO$\_$T/ in the domain of this study.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2004년도 추계학술대회
• /
• pp.1377-1382
• /
• 2004

Flow fields in a half ducted propeller fan have been investigated by three-dimensional Reynolds-averaged Navier-Stokes (RANS) simulations and a vortex core identification technique. The simulation at the design operating condition shows that the tip vortex onset point is located at 30 percent tip chord of the suction surface on the blade tip. There is no interaction between the tip vortex and the adjacent blade, so that the tip vortex smoothly convects to the rotor exit. However, the high vorticity in the tip vortex causes the wake and the tip leakage flow to be twined around the tip vortex and to interact with the pressure surface of the adjacent blade. This flow behavior corresponds well with experimental results by Laser Doppler Velocimetry. On the contrary, the simulation at the low-flowrate operating condition shows that the tip vortex onset point is located at the 60 percent tip chord of the suction surface. In contrast to the design operating condition, the tip vortex grows almost tangential direction, and impinges directly on the pressure surface of the adjacent blade.

• International Journal of Naval Architecture and Ocean Engineering
• /
• 제2권2호
• /
• pp.112-118
• /
• 2010

The numerical study about the vortex-induced vibration and vortex shedding in the wake has been presented. Prior to the numerical simulation of flexible riser systems concerning engineering conditions, efficiency validating of the proposed FSI solution method have been performed. The comparison between numerical simulation and published experimental data shows that the CFD method designed for FSI solution could give acceptable result for the VIV prediction of flexible riser/pipe system. As meaningful study on VIV and vortex shedding mode with the focus on flexible riser model systems, two kinds of typical simulation cases have been carried out. One was related to the simulation of vortex visualization in the wake for a riser model subject to forced oscillation, and another was related to the simulation of fluid-structure interaction between the pipes of coupled multi-assembled riser system. The result from forced oscillation simulation shows that the vortex-induced vibration with high response frequency but small instantaneous vibration amplitude contributes to vortex conformation as much as the forced oscillation with large normalized amplitude does, when the frequency of forced oscillation was relatively high. In the multi-assembled riser systems, it has been found that the external current velocity and the distance between two pipes are the critical factors to determine the vibration state and the steady vibration state emerging in quad-pipe system may be destroyed more easily than dual-pipe system.

• 한국안전학회지
• /
• 제25권1호
• /
• pp.9-16
• /
• 2010

Flame structure of diffusion flame interacting with a single vortex was investigated with direct numerical simulation (DNS). A well-known counterflow diffusion flame was used as an initial flat flame and single vortices were made by issuing a high-velocity jet abruptly in fuel- and air-side. The variations in the maximum concentration of major species (CO and $CO_2$) and NOx (NO and $NO_2$) with the stoichiometric scalar dissipation rate were investigated. Unsteady effects in the species concentration variation of the flame interacting with a vortex were identified by comparing with that of steady flame. $NO_2$ formation characteristics of the flame interacting with a vortex were well understood by investigating the $HO_2$ formation. To enhance the prediction performance in the fire simulation, current turbulent combustion modelings are needed to be modified by adopting the unsteady effects in the species concentrations of diffusion flame interacting with a vortex.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2003년도 춘계학술대회
• /
• pp.1690-1696
• /
• 2003

A two-dimensional direct numerical simulation is performed to investigate the flame structure of $CH_4/N_2$-Air counterflow nonpremixed flame interacting with a single vortex. The detailed transport properties and a modified 16-step augmented reduced mechanism based on Miller and Bowman's detailed chemistry are adopted in this calculation. The results show that an initially flat stagnation plane, where an axial velocity is zero, is deformed into a complex-shaped plane, and an initial stagnation point is moved far away from vortex head when the counterflow field is perturbed by the vortex. It is noted that the movement of stagnation point can alter the mechanism of reactants (fuel and oxidizer) fluxes into the flame surface, and then can alter the flame structure.

• 한국연소학회:학술대회논문집
• /
• 한국연소학회 2002년도 제25회 KOSCI SYMPOSIUM 논문집
• /
• pp.49-56
• /
• 2002

A two-dimensional direct numerical simulation is performed to investigate the formation characteristics of a single vortex interacting with $CH_4/N_2$-Air counterflow nonpremixed flame. The numerical method was based on a predictor-corrector scheme for a low Mach number flow. The detailed transport properties and a 16-step augmented reduced mechanism are adopted in this calculation. The budgets of the vorticity transport equation arc examined to reveal the mechanisms leading to the formation, evolution and dissipation of a single vortex interacting with counterflow nonpremixed flame. It is found that the stretching term, which depends on the azimuthal component of vorticity, and radial velocity, mainly generates vortieitv in non-reacting and reacting flows. The viscous and baroclinic torque term destroy the vorticity in non-reacting flow. In addition, the baroclinic torque term due to density and pressure gradient generates vorticity, while viscous and the volumetric expansion terms due to density gradient destroy vorticity in reacting flow.

• 한국전산유체공학회:학술대회논문집
• /
• 한국전산유체공학회 2002년도 춘계 학술대회논문집
• /
• pp.35-40
• /
• 2002

The dynamic behaviors of the single vortex in a reacting and non-reacting methane-air jet flow were investigated numerically. The numerical method was based on a predictor-corrector for low Mach number flow A two-step global reaction mechanism was adopted as a combustion model. After fuel and air were developed entirely in computational domain, the single vortex was generated by an axisymmetric jet that was impulsed to emit a cold fuel. Through comparisons of single vortex in reacting and non-reacting jet flow, it was found that global dynamic behaviors and the mechanisms leading to the formation, transport processes of vortex ring were influenced significantly by heat release from reaction. In addition, the interaction between a single vortex and flame bulge generated by buoyance effect in a reacting jet flow was found.