• Wind and Structures
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• v.6 no.2
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• pp.141-150
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• 2003

This paper presents galloping analysis of multiple-degree-of-freedom (MDOF) structural roofs with multiple orientations. Instead of using drag and lift coefficients and/or their combined coefficient in traditional galloping analysis for slender structures, this study uses wind pressure coefficients for wind force representation on each and every different orientation roof, facilitating the galloping analysis of multiple-orientation roof structures. In the study, influences of nonlinear aerodynamic forces are considered. An energy-based equivalent technique, together with the modal analysis, is used to solve the nonlinear MDOF vibration equations. The critical wind speed for galloping of roof structures is derived, which is then applied to galloping analysis of roofs of a stadium and a high-rise building in China. With the aid of various experimental results obtained in pertinent research, this study also shows that consideration of nonlinear aerodynamic forces in galloping analysis generally increases the critical wind speed, thus enhancing aerodynamic stability of structures.

• Journal of Biomedical Engineering Research
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• v.13 no.3
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• pp.235-244
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• 1992

In this paper, fluttering behavior of mechanical monoleaflet tilting disc heart valve prostheses during the opening phase was analyzed taking into consideration the impact between the occluder and the guiding strut at the fully open position. The motion of the valve occluder was modeled as a rotating system, and equations were derived by employing the moment equilibrium principle. Forces due to lift, drag, gravity and buoyancy were considered as external forces acting on the occluder. The 4th order Runge-Kutta method was used to solve the governing equations. The results iimonstrated that the occludes reaches steady equilibrium position only after damped vibration. Fluttering frequency varies as a function of time after opening and is in the range of 8-84 Hz. Valve opening appears to be affected by the orientation of the valve relative to gravitational force. The opening velocities are in the range of 0.65-1.42m/sec and the dynamic loads by impact of the occludes and the strut are in the range of 90-190 N.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2857-2861
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• 2007

Flow and aerodynamic characteristics were analyzed numerically for a commercial passenger airplane, Boeing 747-400, flying in the cruising condition. The model geometry with 100:1 in scale was obtained by the photo scanning measurement with the maximum error of 1.4% comparing with the real airplane dimension. The three-dimensional inviscid steady compressible governing equations were solved by the finite volume method in the unstructured grid system. The convective terms were treated by the Crank-Nicholson and first-order upwind schemes. In the computational results, the strong wing-tip vortices were clearly observed and the pressure contours on the airplane surface were suggested. The lift and drag forces in the wing with engines increase by 1.49% and 3.9%, respectively compared with the case without engines. The aerodynamic forces were estimated quantitatively for each element which consists of the airplane.

• Journal of Advanced Marine Engineering and Technology
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• v.25 no.4
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• pp.857-868
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• 2001

The aerodynamic forces and wake structures of the non-rotating downstream cylinder which is located behind the spinning upstream cylinder in tandem and staggered arrangement have been investigated by experimental method at Re= $1.32{\times}10^4$. The measurements of wake flow and pressure distributions of downstream cylinder are carried out in various spin parameters by combination of both longitudinal spacing rations L/d=1.5, 3.0, 4.5 and transverse spacing ratios T/d =0.0, -0.5, 0.5. For the present experiment, it has been found that the spin parameter of spinning upstream cylinder affect more easily the downstream cylinder in tandem arrangement than that in staggered arrangement.

• Proceedings of the KIEE Conference
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• 2001.04a
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• pp.90-92
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• 2001

The lift and drag forces on various magnet moving with a velocity is developed above a closed loop coil. These forces are affect by resistance and inductance of circuit, velocity of magnet. To obtain strong magnetic flux density, the magnet is replaced by Halbach array. This paper presents analysis of force between linear motion Halbach array and a closed loop having the constant resistance and inductance as a various speed.

• Journal of Biomedical Engineering Research
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• v.12 no.3
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• pp.149-156
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• 1991

In this paper, fluttering behavior of mechanical bileaflet heart valve prosthesis was analyzed taking into consideration of the impact between valve plate and stopper Vibration system of the valve was modeled as a rotating system, and equations are induced by moment equilibrium equations. Lift force, drag force, gravity and buoyancy were considered as external forces acting on the valve plate/ The 4th order Runge-Kutta method was used to solve the equations. Valve plate does not come to the static equilibrium position at a stretch, but come to that position after under damping vibration. Damping ratio increases as the cardiac optput increases, and the mean damping ratio is in the range of 0.16~40.25. Fluttering frequency does not have any specific value, but varies as a function of time. It is in the range of 10~40Hz. Valve opening appears to be affected by the orientation of the of the valve relative to gravitational forces.

• Journal of Mechanical Science and Technology
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• v.15 no.4
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• pp.500-509
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• 2001

Offshore subsea pipelines must be stable against external loadings, which are mostly due to waves and currents. To determine the stability of a subsea pipeline on the seabed, the Morrison equation has been applied with prediction of inertia and drag forces. When the pipeline is placed in a trench, the force acting on it is reduced considerably. Therefore, to consider the stability of a pipeline in a trench, one must employ reduction factors. To investigate the stability of various trenches, we numerically simulated flows over various trenches and compared them with experimental data from PIV (Particle Image Velocimetry) measurements. The present results were produced ar Reynolds numbers ranging from 6$\times$10$^3$to 3$\times$10(sub)5 based on the diameter of the cylinder. Quasi-periodic flow patterns computed by large-eddy simulation were compared with experimental data in terms of mean flow characteristics fro typical trench configurations (W/H=1 and H/D=3, 4). The stability for various trench conditions was addressed in terms of mean amplitudes of oscillating lift and drag, and the reduction factor for each case was suggested for pipeline design.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.38 no.5
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• pp.627-634
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• 2018

In the study, computational fluid dynamics analysis was performed to estimate wind force coefficients for a box-type concrete girder bridge under the influence of wind. The drag, lift and pitching moment coefficients were obtained for the bridge section without noise barrier and compared with those of the bridge section with noise barriers of various heights. The shear stress transport $k-{\omega}$ turbulence model was employed to estimate the wind force coefficients, and the contribution of the friction drag force to the total drag force was investigated. It was found from the study that the drag force coefficients increased as the height of noise barrier increased when a wind blew horizontally, and that the contribution of the friction drag force was highest for the bridge section without noise barrier. It is concluded that the impact of the height of noise barriers should be considered in the design of bridges, and the friction force played an important role in evaluating wind forces on bridges.

• Wind and Structures
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• v.35 no.1
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• pp.55-66
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• 2022

Tall buildings are often subjected to steady and unsteady forces due to external wind flows. Measurement and mitigation of these forces becomes critical to structural design in engineering applications. Over the last few decades, many approaches such as modification of the external geometry of structures have been investigated to mitigate wind-induced load. One such proven geometric modification involved the rounding of sharp corners. In this work, we systematically analyze the impact of rounded corner radii on the reducing the flow-induced loading on a square cylinder. We perform 3-Dimensional (3D) simulations for high Reynolds number flows (Re=1 × 105) which are more likely to be encountered in practical applications. An Improved Delayed Detached Eddy Simulation (IDDES) method capable of capturing flow accurately at large Reynolds numbers is employed in this study. The IDDES formulation uses a k-ω Shear Stress Transport (SST) model for near-wall modelling that prevents mesh-induced separation of the boundary layer. The effects of these corner modifications are analyzed in terms of the resulting variations in the mean and fluctuating components of the aerodynamic forces compared to a square cylinder with no geometric changes. Plots of the angular distribution of the mean and fluctuating coefficient of pressure along the square cylinder's surface illustrate the effects of corner modifications on the different parts of the cylinder. The windward corner's separation angle was observed to decrease with an increase in radius, resulting in a narrower and longer recirculation region. Furthermore, with an increase in radius, a reduction in the fluctuating lift, mean drag, and fluctuating drag coefficients has been observed.

• Wind and Structures
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• v.30 no.5
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• pp.465-474
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• 2020

Vortex-induced vibrations of a yawed flexible cylinder near a plane boundary are numerically investigated at a Reynolds number Re_n= 500 based on normal component of freestream velocity. Free to oscillate in the in-line and cross-flow directions, the cylinder with an aspect ratio of 25 is pinned-pinned at both ends at a fixed wall-cylinder gap ratio G/D = 0.8, where D is the cylinder diameter. The cylinder yaw angle (α) is varied from 0° to 60° with an increment of 15°. The main focus is given on the influence of α on structural vibrations, flow patterns, hydrodynamic forces, and IP (Independence Principle) validity. The vortex shedding pattern, contingent on α, is parallel at α=0°, negatively-yawed at α ≤ 15° and positively-yawed at α ≥ 30°. In the negatively- and positively-yawed vortex shedding patterns, the inclination direction of the spanwise vortex rows is in the opposite and same directions of α, respectively. Both in-line and cross-flow vibration amplitudes are symmetric to the midspan, regardless of α. The RMS lift coefficient C_L,rms exhibits asymmetry along the span when α ≠ 0°, maximum C_L,rms occurring on the lower and upper halves of the cylinder for negatively- and positively-yawed vortex shedding patterns, respectively. The IP is well followed in predicting the vibration amplitudes and drag forces for α ≤ 45° while invalid in predicting lift forces for α ≥ 30°. The vortex-shedding frequency and the vibration frequency are well predicted for α = 0° - 60° examined.