• Wind and Structures
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• v.35 no.2
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• pp.109-120
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• 2022

The force coefficients of rotating plates in the acceleration stage will vary with rotation rate from 0 to stable rotation rate w₀, which are important for quasi-steady theory of plate-like windborne debris to simulate the trajectory. In this paper, a wind tunnel experiment is carried out to study the effects of geometry and the Reynolds number on the variations of mean force coefficients of rotating plates. The rotational lift coefficients are sensitive to both geometry effect and Reynolds number effect, while the rotational drag and moment coefficients are only sensitive to geometry effect. In addition, new empirical formulas for the rotational lift coefficient and moment coefficients are proposed. Its accuracy is verified by comparing the predicted results with existing test data. Based on the experimental data of rotating plates, a new rotational force model for quasi-steady theory, which can be applied to a wider scope, is proposed to calculate the trajectory of plate-like windborne debris. The results show that the new model provides a better match with the tested trajectories than previous quasi-steady theories.

• Wind and Structures
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• v.8 no.2
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• pp.107-120
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• 2005

The quasi-steady approaches to simulate the wind induced vibrations of inclined cables, especially on the rain-wind induced vibration, have been tried by many researchers. However, the steady wind force coefficients used in those methods include only the effects of water rivulet, but not the axial flow effects. The problem is the direct application of the conventional techniques to the inclined cable aerodynamics. Therefore, in this study, the method to implement the axial flow effects in the quasi-steady theory is considered and its applicability to the inclined cable aerodynamics is investigated. Then, it becomes clear that the perforated splitter plate in the wake of non-yawed circular cylinder can include the effects of axial flow in the steady wind force coefficients for inclined cables to a certain extent. Using the lateral force coefficients measured in this study, the quasi-steady theory may explain the wind induced instabilities of the inclined cables only in the relatively high reduced wind velocity region. When the Scruton number is less than around 40, the high speed vortex-induced vibration occurs around the onset wind velocity region of the galloping, and then, the quasi-steady approach cannot be applied for estimating the response of wind-induced vibration of inclined cable.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.2 s.245
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• pp.117-125
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• 2006

In the present study the unsteady forces acting on the pitching foils such as a flat plate, NACA0010, NACA0020, NACA65-0910 and BTE have been measured by using a six-axis sensor in a circulating water tunnel at a low Reynolds number region. The unsteady characteristics of the dynamic drag and lift have been compared to the quasi-steady ones which are measured under the stationary condition. The pitching motion is available for keeping the lift higher after the separation occurs. Especially, the characteristics of the dynamic lift are quite different from the quasi-steady one at high pitching frequency regions. As the pitching frequency deceases, the amplitude of the dynamic lift becomes closer to the quasi-steady one. However, the phase remains different between the steady and unsteady conditions even at low pitching frequencies. On the other hand, the dynamic drag is governed strongly by the angle of attack.

• Wind and Structures
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• v.15 no.5
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• pp.385-407
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• 2012

Bridge hanger vibrations have been reported under icy conditions. In this paper, the results from a series of static and dynamic wind tunnel tests on a circular cylinder representing a bridge hanger with simulated thin ice accretions are presented. The experiments focus on ice accretions produced for wind perpendicular to the cylinder at velocities below 30 m/s and for temperatures between $-5^{\circ}C$ and $-1^{\circ}C$. Aerodynamic drag, lift and moment coefficients are obtained from the static tests, whilst mean and fluctuating responses are obtained from the dynamic tests. The influence of varying surface roughness is also examined. The static force coefficients are used to predict parameter regions where aerodynamic instability of the iced bridge hanger might be expected to occur, through use of an adapted theoretical 3-DOF quasi-steady galloping instability model, which accounts for sectional axial rotation. A comparison between the 3-DOF model and the instabilities found through two degree-of-freedom (2-DOF) dynamic tests is presented. It is shown that, although there is good agreement between the instabilities found through use of the quasi-steady theory and the dynamic tests, discrepancies exist-indicating the possible inability of quasi-steady theory to fully predict these vibrational instabilities.

• Journal of computational fluids engineering
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• v.4 no.2
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• pp.31-38
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• 1999

Numerical analysis is conducted on the deformation of free surface of magnetic fluid. Steady magnetic fields are induced by a circular current loop. Governing equations of magnetic fields are solved by using the concept of vector potential. The free surface of magnetic fluid is formed by the balance of surface force, gravity, pressure difference, magnetic normal pressure and magnetic body force. The deformations of free surface of magnetic fluid are qualitatively clarified. And, the patterns of steady non-uniform magnetic fields induced by a circular current loop are quantitatively presented. The shape of free surface attained by the polar fluid approach is rougher and higher than that attained by the quasi-steady approach.

• Wind and Structures
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• v.37 no.1
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• pp.57-78
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• 2023

In this study, a generalized three-degree-of-freedom (3-DoF) analytical model is formulated to predict linear aerodynamic instabilities of a prism under quasi-steady (QS) conditions. The prism is assumed to possess a generic cross-section exposed to turbulent wind flow. The 3-DoFs encompass two orthogonal horizontal directions and rotation about the prism body axis. Inertial coupling is considered to account for the non-coincidence of the mass center and the rotation center. The aerodynamic force coefficients-drag, lift, and moment-depend on the Reynolds number based on relative flow velocity, angle of attack, and the angle between the wind and the cable. Aerodynamic forces are linearized with respect to the static equilibrium configuration and mean wind velocity. Routh-Hurwitz and Liénard and Chipart criteria are used in the eigenvalue problem, yielding an analytical solution for instabilities in galloping and static divergence types. Additionally, the minimum structural damping and stiffness required to prevent these instabilities are numerically determined. The proposed 3-DoF instability model is subsequently applied to a conductor with ice accretion and a full-scale dry inclined cable. In comparison to existing models, the developed model demonstrates superior prediction accuracy for unstable regions compared with results in wind tunnel tests.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.1298-1302
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• 2000

Electrorheological(HER) and magnetorheologica(MR) fluids have a unique ability to increase the dynamic yield stress of the fluid substantially when electric or magnetic field is applied. ER and MR fluid-based dampers are typically analyzed using Bingham-plastic shear flow analysis under Quasi-steady fully developed flow conditions. An alternative perspective, supported by measurements reported in the literature, is to allow for post-yield shear thinning and shear thickening. To model these, the constant post-yield plastic viscosity in Bingham model can be replaced with a power-law model dependent on shear strain rate that is known as the Herschel-Bulkley fluid model. The objective of this paper is to predict the damping forces analytically in a typical ER bypass damper for variable electric field, or yield stress using Herschel-Bulkley analysis.

• International Journal of Aeronautical and Space Sciences
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• v.14 no.1
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• pp.46-57
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• 2013

Flutter stability and buffeting response have been the topics of most concern in the design state of long-span suspension bridges. Among approaches towards the aerodynamic stability, the aerodynamic-based control method which uses control surfaces to generate forces counteracting the unstable excitations has shown to be promising. This study focused on the mechanically controlled system using flaps; two flaps were attached on both sides of a bridge deck and were driven by the motions of the bridge deck. When the flaps moved, the overall cross section of the bridge deck containing these flaps was continuously changing. As a consequence, the aerodynamic forces also changed. The efficiency of the control was studied through the numerical simulation and experimental investigations. The values of quasi-steady forces, together with the experimental aerodynamic force coefficients, were proposed in the simulation. The results showed that the passive flap control can, with appropriate motion of the flaps, solve the aerodynamic instability. The efficiency of the flap control on the full span of a simple suspension bridge was also carried out. The mode-by-mode technique was applied for the investigation. The results revealed that the efficiency of the flap control relates to the mode number, the installed location of the flap, and the flap length.

• Coupled systems mechanics
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• v.7 no.4
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• pp.407-420
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• 2018

In solid-liquid two phase flow, the knowledge of how descending solid particles affected by the presence of downstream wall is important. This work studies at what interstitial distance the velocity of a vertically descending sphere is affected by a downstream wall as a consequence of wall-modified hydrodynamic forces through a validated dynamic model. This interstitial distance-the hydrodynamic coupling distance ${\delta}_c-is$ found to decay monotonically with the approach Stokes number St which compares the particle inertia to viscous drag characterized by the quasi-steady Stokes' drag. The scaling relation ${\delta}_c-St-1$ decays monotonically as literature below the value of St equal to 10. However, the faster diminishing rate is found above the threshold value from St=10-40. Furthermore, an empirical relation of ${\delta}_c-St$ shows dependence on the drop height which clearly indicates the non-negligible effect of unsteady hydrodynamic force components, namely the added mass force and the history force. Finally, we attempt a fitting relation which embedded the particle acceleration effect in the dependence of fitting constants on the diameter-scaled drop height.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.11a
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• pp.784-787
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• 2003

Electrorheological(ER) and magnetorheological(MR) fluid-based dampers are typically analyzed using Bingham-plastic shear model under quasi-steady fully developed flow conditions. A Herschel-Bulkley constitutive shear flow relationship is that the linear shear stress vs. strain rate behavior of Bingham model is replaced by a shear stress that is assumed to be proportional to a power law of shear rate. This power is called the flow behavior index. Depending on the value of the flow behavior index number, varying degrees of post-yield shear thickening or thinning behavior can be analyzed. But it is not practical to analyze the damping force in a flow mode damper using Herschel-Bulkley model because it is needed to solve a polynomial equation. A useful guide is suggested to analyze the damping force in a damper using the Herschel-Bulkley model.