Search

Aerodynamic admittance functions (AAFs) of twin-box decks are particularly complex due to the presence of a central gap, so require the performance of dedicated studies to determine them. Two main approaches are available for this purpose, which consist of studying the aerodynamic forces produced by the section when impinged by sinusoidal gusts or continuous turbulence, respectively. However, owing to the difficulty in the generation of large-scale turbulence in traditional wind tunnel tests, the two aforementioned approaches often provide remarkably different results. To address this problem, this study aims to systematically investigate the admittance function of twin-box decks by comparing three approaches: active wind tunnel tests, and large eddy simulations (LESs) using both sinusoidal gusts and continuous synthetic turbulence. In particular, wind tunnel tests equipped with an active gust generator are conducted to generate a series of single-frequency vertical sinusoidal gusts. The effects of gust amplitudes and gap widths on the AAFs are experimentally investigated. In addition, LESs are performed aiming to provide valuable insights regarding the discrepancy between AAFs obtained in the two inflow conditions. Results obtained adopting all the aforementioned methodologies are compared, showing good agreement among the three approaches and values sometimes higher than the Sears function. It is also shown that buffeting forces are mainly produced by the upstream deck, so explaining the good agreement obtained between sinusoidal gust and continuous turbulent inflows. [ABSTRACT FROM AUTHOR]

When the critical wind speed of vortex-induced resonance is close to that of quasi-steady galloping, a type of coupled wind-induced vibration that is different from divergent galloping can easily occur in a rectangular bar. It is a type of "unsteady galloping" phenomenon wherein the response amplitude increases linearly with the increase in the wind speed, while a limit cycle oscillation is observed at each wind speed, whose mechanism is still in research. Mass and damping are the key parameters that affect the coupling degree and amplitude response estimation. For a set of rectangular section member models with a width-to-height ratio of 1.2, by adjusting the equivalent stiffness, equivalent mass, and damping ratio of the model system and performing comparative tests on the wind-induced vibration response of the same mass with different damping ratios, it is possible to achieve the same damping ratio with different masses and the same Scruton number with different masses and damping combinations under the same Reynolds number. The results show that the influence of the mass and damping parameters on the "unsteady galloping" amplitude response is independent, and the weight is the same in the coupling state. The Scruton number "locked interval" (12.4–30.6) can be found in the "unsteady galloping" amplitude response, and the linear slope of the dimensionless wind speed amplitude response curve does not change with the Scruton number in the "locked interval." In addition, a "transition interval" (26.8–30.6) coexists with the "locked interval" wherein the coupling state of the wind-induced vibration is converted into the uncoupled state. The empirical formula for estimating the "unsteady galloping" response amplitude is modified and can be used to predict the amplitude within the design wind speed range of similar engineering members. [ABSTRACT FROM AUTHOR]

Classical flutter of a wind turbine blade is a concerned issue to hinder the wind utilization to a large extent. Recent predictions showed a decreasing flutter margin (the ratio of flutter speed to rated rotor speed) with the increase in wind turbine size. To address this issue, a new blade configuration called the double-pitched blade is proposed and analytically investigated for its potential to enhance the flutter suppressing capability of modern large-size wind turbine blades. This new blade comprise an inner part and a tip part, where the tip part can rotate (or pitch) independently with respect to the inner part through a tip actuator commanded by a feedback control law. The aerodynamic loads of blade tip due to the actively controlled rotation of the tip part provide a torque on the inner part, which provides damping for the torsional mode of the wind turbine blade. The effectiveness of this new double-pitched blade for suppressing flutter is verified through a simulation study conducted on a 907-DOF aero-servo-elastic wind turbine model. Parametric studies are performed on two main design parameters, i.e. the length of the tip part and the associated chordwise location of tip shaft with respected to the blade cross section, and flutter control performance are obtained by numerical optimization process. Simulation results show the optimal length of tip part is around 3.3 % of blade length, and the associated chordwise location of tip shaft is around 45 % of chord length, the flutter amplitude of the conventional blade can be mitigated to around 4 % using this double-pitched blade. • Anew double-pitched bladefor suppressing wind turbineflutter is proposed. • An aero-servo-elastic wind turbine model is developedto validate this design. • Parametric study on structural design parameters of this double-pitched blade is performed. • Flutter amplitude of blade can be significantly reduced. [ABSTRACT FROM AUTHOR]

Split type box girder is a recommendable structural style for long-span cable-supported bridges due to its excellent aeroelastic stability. A major issue of this kind of girder is its susceptibility to vortex-induced vibration (VIV), which has not been adequately investigated. This paper presents an experimental study on VIV of a triple-box girder with three different web modifications (namely curved web and linear-type webs with different slopes). Both small- and large-scale sectional models were employed for wind tunnel tests considering the Reynolds number effect. Results indicated that the triple-box girder showed poor VIV performances in both construction and completed phases. Generally, linear-type web was superior to curved web in terms of VIV performance. Increasing the structural damping and turbulence intensity of incoming flow were helpful for VIV mitigation. Geometric profiles and arrangement of appurtenant facilities on the girder have significant effects on the VIV performance. Aerodynamic optimal schemes were proposed for VIV mitigation both in construction and completed phases. Obvious discrepancies in VIV responses were observed between the small- and large-scale tests for several scenarios. Possible factors being responsible for the discrepancies were Reynolds number, turbulence intensity, nonlinearity of model structural damping, blockage ratio and geometric accuracy of the sectional models. [ABSTRACT FROM AUTHOR]

Classical flutter of wind turbine blades is defined as an aeroelastic instability where a torsional blade mode couples to a flapwise bending mode to result in a mutual rapid growth of the amplitude of the flapwise and torsional motions beyond certain rotational speed of the rotor. Till date, flutter instability of wind turbine rotor has not been considered to be a serious problem. However, with the advent of larger turbines fitted with relatively softer blades, classical flutter may become a more important design consideration. As an economically viable alternative to increasing the torsional rigidity (which is more expensive), the use of a type of viscous damper mounted inside the wind turbine blades is proposed to promote the flutter critical rotational speed of the rotor. The flutter critical rotational speed is determined by using a time-domain aeroelastic simulation technique based on the quasi-steady aerodynamic forces and also considering turbulence intensity of incoming wind. The optimal damper location and the optimal damping constant values are investigated for the DTU 10 MW wind turbine. It is shown that at an optimal tuning of the damper, the flutter critical rotational speed is only marginally dependent on the turbulence intensity of the incoming wind field, on the mean wind velocity and on the structural damping of the flutter torsional eigenmode. The novel tuned damper device proposed in this paper may increase the flutter critical rotational speed with more than 100%. [ABSTRACT FROM AUTHOR]

• Wave absorption of cylindrical flap is better than the rectangular ones. • Flap motions can be simply adopted to evaluate the wave absorption. • Order of the effects: load condition, hinge height, damping, thickness, mass. • Damping is negatively related to the wave absorption performance. • Proposed equations can be regarded as references in the OWSC designs. An oscillating wave surge converter extracts energy from sea waves and converts it into electricity. Its main components are a flap, hinge, base, and power take-off system. In this paper, an oscillating wave surge converter comprising a cylindrical flap, made up of several cylinders, is numerically investigated under regular waves using the smoothed particle hydrodynamics method. A comparison between cylindrical and box-type rectangular flaps is performed firstly, in which the former extracts more energy and is more suitable in bearing water pressures than the latter. Subsequently, the effects of five parameters (load conditions, total flap mass, hinge height from the seabed, damping of the power take-off system, and the flap thickness) are systematically examined. To investigate the effects of motion resonance, the natural periods of the structures are also studied. Meanwhile, correlation, partial correlation, and linear regression analysis are conducted to obtain a detailed understanding of the effects of the parameters. The details about how to apply the conclusions to full scale model or even real sea states are discussed also. Numerical results show that the heavier and thinner the flap, or the higher the hinge, the better the wave-absorption performance will be. As the damping increases, the extracted energy increases initially and then decreases. Wave conditions contribute the most to wave absorption and are the key parameter in the designs of the converters, followed by hinge height, damping, thickness, and mass of the flap. Motion-resonance also benefits the wave-absorptions. The proposed equations in the linear regression analysis all show high statistical significance level, which can be regarded as reference when designing this kind of converters, accelerating the design process. [ABSTRACT FROM AUTHOR]

In this paper, an investigation on the nonlinear dynamics of parked large wind turbine blades considering harmonic inertial excitation is presented. The investigation is based on the continuum mathematical model which is derived using the extended Hamilton principle. Employing the Euler–Bernoulli beam model, the equations of motion are derived by considering the flap-wise and edge-wise deformations as well as the cubic nonlinearity. The proposed model is validated through comparisons with both published results and numerical simulations. The nonlinear dynamics of the NREL 5-MW wind turbine blade is investigated numerically using a bifurcation toolbox MATCONT. The steady state primary resonances of the first three modes are compared and analyzed with varying excitation frequency and excitation direction. Results show that for parked blades the aerodynamic damping increases with the increase of inflow wind speed, and the response under low wind speed becomes more unfavorable than high wind speed. The blade response can suddenly jump up or down when the excitation direction varies slightly because of the saddle node bifurcation. The 3:1 internal resonance between the 1st and 3rd modes is also observed in this study, suggesting the possible interaction of the two modes during large amplitude vibration. Quasi-periodic motions can also be induced because of the Hopf bifurcation in the case of negative aerodynamic damping. • Nonlinear dynamics of parked wind turbine blade is studied based on proposed model. • The model is derived using Hamilton principal keeping up to cubic stiffness terms. • Harmonic inertial excitation and aerodynamic damping are adopted for the analysis. • Various factors, say excitation amplitude, direction and damping, are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The flutter instability is the most hazardous wind-induced vibration for the long-span suspension bridge. In this study, the first-built long-span triple-tower suspension bridge in China, the Taizhou Yangtze River Bridge, is taken as an example. The three-dimensional flutter stability analysis of the bridge is conducted based on ANSYS, and the parametric analysis on the structural flutter stability is performed. The parameters studied in this paper include the sag-to-span ratio, the stiffness of the main girder, the dead load of the main girder, the rigid central buckle, the longitudinal stiffness of the middle tower and the cable system. The result shows that the torsional stiffness of the main girder, the sag-to-span ratio of cables, the central buckle and the cable system has significant influence on the flutter stability of the bridge. Among the sensitive parameters found in this paper, the influence of the cable system is prominent. However, the influences of the stiffness of the main girder, the longitudinal stiffness of the middle tower and the dead load of the deck are limited. The results can provide the reference information for wind resistance design of long-span triple-tower suspension bridges in the future. [Copyright &y& Elsevier]