Showing papers on "Flutter published in 2018"

Enhancement of wind energy harvesting by interaction between vortex-induced vibration and galloping

Abstract: Most wind energy harvesters (WEHs) that have been reported in the literature collect wind energy using only one type of wind-induced vibration, such as vortex-induced vibration (VIV), galloping, and flutter or wake galloping. In this letter, the interaction between VIV and galloping is used to improve the performance of WEHs. For a WEH constructed by attaching a bluff body with a rectangular cross-section to the free end of a piezoelectric cantilever, the measures to realize the interaction are theoretically discussed. Experiments verified the theoretical prediction that the WEHs with the same piezoelectric beam may demonstrate either separate or interactive VIV and galloping, depending on the geometries of the bluff bodies. For the WEHs with the interaction, the wind speed region of the VIV merges with that of the galloping to form a single region with high electrical outputs, which greatly increases the electrical outputs at low wind speeds. The interaction can be realized even when the predicted gallop...

Stability analysis of thin-walled spinning reinforced pipes conveying fluid in thermal environment

Abstract: The divergence and flutter instabilities of the thin-walled spinning pipes reinforced by single-walled carbon nanotubes in thermal environment are investigated. The material properties of carbon nanotube-reinforced composites are assumed to be uniform distribution as well as two types of functionally graded distribution patterns. The thermal effects are also considered and the material properties of carbon nanotube-reinforced composites are assumed to be temperature-dependent. The cantilever pipe conveying fluid is spinning along its longitudinal axis and subjected to an axial force at the free end. Based on the thin-walled Timoshenko beam theory, the governing equations of motion are derived using the extended Hamilton's principle and discretized via the Galerkin method. The resulting thermal-structural-fluid eigenvalue problem is solved and the frequency and the critical fluid velocities are calculated. The effects of carbon nanotubes distributions, volume fraction of carbon nanotubes, compressive axial force, spinning speed, gravity and fluid mass ratio on the critical divergence and flutter velocities of the thin-walled spinning pipe conveying fluid are studied.

Complex modal analysis of transverse free vibrations for axially moving nanobeams based on the nonlocal strain gradient theory

Abstract: We investigate the transverse free vibration behaviour of axially moving nanobeams based on the nonlocal strain gradient theory. Considering the geometrical nonlinearity, which takes the form of von Karman strains, the coupled plane motion equations and related boundary conditions of a new size-dependent beam model of Euler-Bernoulli type are developed using the generalized Hamilton principle. Using the simply supported axially moving nanobeams as an example, the complex modal analysis method is adopted to solve the governing equation; then, the effect of the order of modal truncation on the natural frequencies is discussed. Subsequently, the roles of the nonlocal parameter, material characteristic parameter, axial speed, stiffness and axial support rigidity parameter on the free vibration are comprehensively addressed. The material characteristic parameter induces the stiffness hardening of nanobeams, while the nonlocal parameter induces stiffness softening. In addition, the roles of small-scale parameters on the flutter critical velocity and stability are explained.

Flutter and divergence instability in the Pflüger column: Experimental evidence of the Ziegler destabilization paradox

Abstract: Flutter instability in elastic structures subject to follower load, the most important cases being the famous Beck’s and Pfluger’s columns (two elastic rods in a cantilever configuration, with an additional concentrated mass at the end of the rod in the latter case), have attracted, and still attract, a thorough research interest. In this field, the most important issue is the validation of the model itself of follower force, a nonconservative action which was harshly criticized and never realized in practice for structures with diffused elasticity. An experimental setup to introduce follower tangential forces at the end of an elastic rod was designed, realized, validated, and tested, in which the follower action is produced by exploiting Coulomb friction on an element (a freely-rotating wheel) in sliding contact against a flat surface (realized by a conveyor belt). It is therefore shown that follower forces can be realized in practice and the first experimental evidence is given for both the flutter and divergence instabilities occurring in the Pfluger’s column. In particular, load thresholds for the two instabilities are measured and the detrimental effect of dissipation on the critical load for flutter is experimentally demonstrated, while a slight increase in load is found for the divergence instability. The presented approach to follower forces discloses new horizons for testing self-oscillating structures and for exploring and documenting dynamic instabilities possible when nonconservative loads are applied.

Passive control of transonic flutter with a nonlinear energy sink

Abstract: Among different control approaches, the nonlinear energy sink has been proposed as an effective strategy for passive flutter control toward expanding the flight envelope of aircraft. We validate and introduce the capability for coupling the NES within OpenFOAM to simulate and evaluate its usability in controlling transonic flutter. Particular attention is paid to the extent to which variations in the parameters of the sink impact regions of complete and partial suppression. The results show that in the region of complete suppression the nonlinear energy sink may cause the wing to assume a new equilibrium position, in terms of mean value of the oscillations. In the region of partial suppression, the response is dependent on the initial conditions that may lead to multiple pitch and plunge frequencies or potentially a chaotic response.

Dynamic stability of fluid-conveying thin-walled rotating pipes reinforced with functionally graded carbon nanotubes

Abstract: In this study, vibration and dynamic stability of fluid-conveying thin-walled rotating pipes reinforced with functionally graded carbon nanotubes are studied. The pipe is modeled based on thin-walled Timoshenko beam theory and reinforced by single-walled carbon nanotubes with uniform distribution as well as three types of functionally graded distribution patterns. The governing equations of motion and the associated boundary conditions are derived via Hamilton’s principle. The governing equations of motion are discretized via the Galerkin method, and the eigenfrequency and the stability region of the pipe are found using the eigenvalue analysis. Some numerical examples are presented to study the effects of length–radius ratio, carbon nanotubes distribution, volume fraction of carbon nanotubes, rotational speed and mass ratio on the non-dimensional eigenfrequency and critical flutter velocity of the thin-walled rotating pipe conveying fluid. The results show that the carbon nanotubes distribution has a significant effect on the non-dimensional eigenfrequency and critical flutter velocity. Also, it is found that the rotational speed has a stabilizing effect on the dynamic behavior of the system.

Wind-induced nonlinear behaviors of twin-box girder bridges with various aerodynamic shapes

Abstract: Wind-induced nonlinear oscillations of twin-box girder bridges are very sensitive to the aerodynamic shape of the deck (i.e., slot width ratio (SWR) and wind fairing shape) due to the complicated flow characteristics around the bridge deck. This paper presents a fully integrated finite element (FE) model in time domain, involving a nonlinear aerodynamic force model and a bridge FE model, to allow the investigation of nonlinear oscillation behaviors of long-span twin-box girder bridges with various SWRs and wind fairing shapes. The parameters in integrated FE model were firstly identified by using CFD simulation, and then, the proposed model was validated by conducting wind tunnel testing using sectional models and full-bridge aeroelastic models. It demonstrates that the developed integrated model has the capability of simulating the nonlinear flutter behaviors of twin-box girder bridges with various aerodynamic shapes. Furthermore, the prediction results show that the wind fairing shape has significant impact on the degree of freedom participation in coupled oscillation and failure modes, as well as flutter performance of the bridges. In addition, there is an increase in amplitudes of the limit cycle oscillations with the increase in the SWR of the twin-box girder bridges, and the relationships between the bending-torsional coupled oscillation, failure modes, and SWR of the bridges with anti-symmetric wind fairings are opposite to those with symmetric wind fairings.

Panel flutter suppression with nonlinear energy sinks: Numerical modeling and analysis

Abstract: High-speed flight can cause aircraft skin to exhibit an aeroelastic instability typically called panel flutter. Due to the risk of fatigue failure imposed by this undesired phenomenon, several techniques have been proposed over the years to passively or actively control such aeroelastic vibrations. One relatively new method that has been proven effective for controlling various aeroelastic phenomena is the use of nonlinear energy sinks (NES). Here, for the first time, this technique is employed for suppressing panel flutter and reducing the intensity of limit cycle oscillations (LCOs). The present work consists of a comprehensive study on the numerical modeling of a NES, its coupling to an aeroelastic finite element plate model, and several solutions attained through the resulting system. In order to simulate LCOs, a geometrically nonlinear plate model is employed. The supersonic aerodynamic loads are modeled by piston theory, and the final aeroelastic equations of motion are solved directly in time by an implicit integrator. The energy dissipated by the NES and the energy injected in the panel by the flow are obtained numerically as a function of time. This provides important insights on the mechanism through which a NES can either suppress flutter or mitigate LCOs by partially balancing the aerodynamic work. The performance of the NES is tested in three regimes: At the pre-flutter regime, the NES leads the panel to a faster return to equilibrium after a perturbation; at higher speeds, the NES is still able to suppress flutter, whereas an uncontrolled panel would exhibit LCOs; at even higher speeds, the NES is no longer able to completely suppress the motion but can pump enough energy to reduce the amplitude of the LCOs. In any of these scenarios, the practical outcome would be a longer lifespan for the skin structure. Furthermore, a parametric study is conducted to assess how different NES parameters such as damping coefficient and nonlinear stiffness affect the aeroelastic response. The results reveal that a NES can be used as a lightweight device for passively controlling panel flutter, and that such technique is suitable for optimization-driven design.

Flutter and divergence instability of supported piezoelectric nanotubes conveying fluid

Abstract: In this paper, divergence and flutter instabilities of supported piezoelectric nanotubes containing flowing fluid are investigated. To take the size effects into account, the nonlocal elasticity theory is implemented in conjunction with the Euler-Bernoulli beam theory incorporating surface stress effects. The Knudsen number is applied to investigate the slip boundary conditions between the flow and wall of nanotube. The nonlocal governing equations of nanotube are obtained using Newtonian method, including the influence of piezoelectric voltage, surface effects, Knudsen number and nonlocal parameter. Applying Galerkin approach to transform resulting equations into a set of eigenvalue equations under the simple-simple (S-S) and clamped-clamped (C-C) boundary conditions. The effects of the piezoelectric voltage, surface effects, Knudsen number, nonlocal parameter and boundary conditions on the divergence and flutter boundaries of nanotubes are discussed. It is observed that the fluid-conveying nanotubes with both ends supported lose their stability by divergence first and then by flutter with increase in fluid velocity. Results indicate the importance of using piezoelectric voltage, nonlocal parameter and Knudsen number in decrease of critical flow velocities of system. Moreover, the surface effects have a significant role on the eigenfrequencies and critical fluid velocity.

Aeroelasticity The Continuum Theory

Abstract: Introduction.- Dynamics of Wing Structure.- The Air Flow Model.- The Steady State HStatic L Solution of the Aeroelastic Equation.- Linear Aeroelasticity Theory The Possio Integral Equation.- NonLinear Aeroelasticity Theory in 2 D Aerodynamics Flutter As LCO.- Viscous Flow Theory.-Optimal Control Theory : Flutter Suppression.- Aeroelastic Gust Response.-

Aeroelastic suppression of an airfoil with control surface using nonlinear energy sink

Abstract: Aeroelasticity exists in airfoil with control surface freeplay, which may induce instability in an incompressible flow. In this paper, a nonlinear energy sink (NES) is used to suppress the aeroelasticity of an airfoil with a control surface. The freeplay and cubic nonlinearity in pitch are taken into account. The harmonic balance method is used to analytically determine the limit cycle oscillations (LCOs) amplitudes of the airfoil–NES system. Linear and nonlinear flutter speeds are detected from the airfoil with control surface freeplay. When NES is attached, both the linear flutter speed of airfoil without freeplay and the nonlinear flutter speed of airfoil with a freeplay are increased. Moreover, the LCO amplitude of airfoil is decreased due to NES. Then, the influences of NES parameters on the increase in flutter boundary of airfoil are carefully studied.

AEROM: NASA's Unsteady Aerodynamic and Aeroelastic Reduced-Order Modeling Software.

Abstract: The origins, development, implementation, and application of AEROM, NASA’s patented reduced-order modeling (ROM) software, are presented. Using the NASA FUN3D computational fluid dynamic (CFD) code, full and ROM aeroelastic solutions are computed at several Mach numbers and presented in the form of root locus plots. The use of root locus plots will help reveal the aeroelastic root migrations with increasing dynamic pressure. The method and software have been applied successfully to several configurations including the Lockheed-Martin N+2 supersonic configuration and the Royal Institute of Technology (KTH, Sweden) generic wind-tunnel model, among others. The software has been released to various organizations with applications that include CFD-based aeroelastic analyses and the rapid modeling of high-fidelity dynamic stability derivatives. We present recent results obtained from the application of the method to the AGARD 445.6 wing that reveal several interesting insights.

Aeroelastic analysis of a wind turbine blade using the harmonic balance method

Abstract: Most current wind turbine aeroelastic codes rely on the blade element momentum method with empirical corrections to compute aerodynamic forces on the wind turbine blades. While efficient, this method relies on experimental data and does not allow designers much flexibility for alternative blade designs. Unsteady solutions to the Navier-Stokes equations offer a significant improvement in aerodynamic modeling, but these are currently too computationally expensive to be useful in a design situation. However, steady-state solutions to the Navier-Stokes equations are possible with reasonable computation times. The harmonic balance method provides a way to represent unsteady, periodic flows through coupled a set of steady-state solutions. This method offers the possibility of unsteady flow solutions at a computational cost on the order of a few steady-state solutions. By coupling a harmonic balance driven aerodynamic model with a mode shape-based structural dynamics model, an efficient aeroelastic model for a wind turbine blade driven by the Navier-Stokes equations is developed in this dissertation. For wind turbine flows, turbulence modeling is essential, especially in the transition of the boundary layer from laminar to turbulent. As part of this dissertation, the Spalart-Allmaras turbulence model and the gamma-Re\_theta-t transition model are included in the aerodynamic model. This marks the first time that this transition model, turbulence model, and the harmonic balance method have been coupled to study unsteady wind turbine aerodynamics. Results show that the transition model matches experimental data more closely than a fully turbulent model for the onset of both static and dynamic stall. Flutter is of particular interest as turbines continue to increase in size, and longer and softer blades continue to enter the field. In this dissertation, flutter is investigated for the 1.5 MW WindPACT rotor blade. The aeroelastic model created, which incorporates the harmonic balance method and a fully turbulent aerodynamic model, is the first of its kind for wind turbine flutter analysis. Predictions match those of other aeroelastic models for the 1.5 MW WindPACT blade, and the first flapwise and edgewise modes are shown to dominate flutter for the rotor speeds considered.

Nonlocal instability of cantilever piezoelectric carbon nanotubes by considering surface effects subjected to axial flow

Abstract: In this study, the divergence and flutter instability of a cantilever piezoelectric carbon nanotube (CNT) conveying flowing fluid is investigated by considering surface effects. The size-dependent governing differential equation of a piezoelectric CNT is derived using a Newtonian method based on the Eringen nonlocal elasticity theory and in conjunction with the Euler–Bernoulli beam model. The extended Galerkin method is employed to transform the partial differential equation into a set of ordinary differential equations. The resulting eigenvalue problem is solved numerically to determine the effect of nonlocal parameter, various values of piezoelectric voltage, and surface effects on the divergence and flutter instability of a CNT conveying a fluid. Results show that by increasing the voltage from negative values to positive values, the nondimensional critical velocity of the fluid flow decreases. In addition, it should be noted that the effects of the nonlocal parameter lead to a reduction of the flutter...