Geometrik doğrusalsızlık ve sıkıştırılabilirlik içeren kompozit kanat ve rüzgar türbin kanatlarının aeroelastik analizi.

Aeroelastic behaviour of composite wings and wind turbine blades in the incompressible and compressible flow regimes is investigated utilizing a geometrically nonlinear Thin Wall Beam (TWB) theory incorporating non uniform geometric features such as sweep, taper, pretwist, warping inhibition and transverse shear strain. The structural equations of motion are obtained in the most general form based on the kinematic relations governing thin walled beams, including the nonlinear strain displacement relations, and utilizing the principles of analytical dynamics. Unsteady aerodynamic loads in the incompressible and compressible flow regime are expressed using indicial functions in the time-domain. The aeroelastic system of equations is augmented by the differential equations governing the aerodynamics lag states to come up with the final coupled fluid-structure equations of motion. Time response of the nonlinear aeroelastic system is obtained via the Runge-Kutta direct integration algorithm. The effect of the compressibility on the flutter characteristics of aeroelastically tailored bend-twist coupled (BTC) composite blades designed for the MW sized wind turbine is investigated. Flutter analyses are performed for the baseline blade and the BTC blades designed for the MW sized wind turbine. Beam model of the blade is developed by making analogy with the structural model of the prewisted rotating TWB and utilizing the Variational Asymptotic Beam Section (VABS) method for the calculation of sectional properties of the blades designed. To investigate the effect of compressibility on the flutter characteristics of the wind turbine blades, aeroelastic analyses are performed in frequency and time domain utilizing both incompressible and compressible unsteady aerodynamics via indicial function approach.Ph.D. - Doctoral Progra

Flutter study of flapwise bend-twist coupled composite wind turbine blades

Bending-twisting coupling induced in big composite wind turbine blades is one of the passive control mechanisms which is exploited to mitigate loads incurred due to deformation of the blades. In the present study, flutter characteristics of bend-twist coupled blades, designed for load alleviation in wind turbine systems, are investigated by time-domain analysis. For this purpose, a baseline full GFRP blade, a bend-twist coupled full GFRP blade, and a hybrid GFRP and CFRP bend-twist coupled blade is designed for load reduction purpose for a 5 MW wind turbine model that is set up in the wind turbine multi -body dynamic code PHATAS. For the study of flutter characteristics of the blades, an over-speed analysis of the wind turbine system is performed without using any blade control and applying slowly increasing wind velocity. A detailed procedure of obtaining the flutter wind and rotational speeds from the time responses of the rotational speed of the rotor, flapwise and torsional deformation of the blade tip, and angle of attack and lift coefficient of the tip section of the blade is explained. Results show that flutter wind and rotational speeds of bend-twist coupled blades are lower than the flutter wind and rotational speeds of the baseline blade mainly due to the kinematic coupling between the bending and torsional deformation in bend-twist coupled blades

Structural dynamics analysis and passive control of wind turbine vibrations with tuned mass damper (TMD) technique

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.This paper investigates the use of a passive control device, tuned mass damper (TMD) for the control of vibrations of a simplified wind turbine. In the wind turbine model, tower and blades are modeled with continuous beam structure. Concentrated mass is considered as the nacelle. TMD system is placed at top of tower and attached to the nacelle. The coupled governing equation of motions and associated boundary conditions for the tower and the blades are obtained based on Euler-Bernoulli beam theory. In the present study, centrifugal force of blades due to rotation, lateral acceleration of the nacelle, self-weight and moment of inertia of nacelle and tower are considered in the analysis and aerodynamics loads are excluded in the analysis. Applying the Galerkin’s method with two admissible functions, the solution of the governing equations is obtained. In first part of the article, the coupled natural frequencies and mode shapes of wind turbine without TMD are calculated for various angular velocities of the blade. In the second part, it is shown how the TMD uniformly damps out the vibration of the selected coupled tower and blades mode

Classical Aeroelastic Stability Analysis of Large Composite Wind Turbine Blades

To achieve higher energy production bigger wind turbine systems with very long blades are increasingly used in the wind turbine industry. As the length of the wind turbine blades is increased, blades become more flexible in bending and torsion. Increased bending and torsional flexibility of long wind turbine blades may cause torsional divergence and flapwise bending-torsion flutter at high speeds. Therefore, it is important that aeroelastic stability characteristics of the blades be investigated to ensure that wind turbine system is free of any aeroelastic instability. In this study, classical aeroelastic stability approach is applied to a simplified composite blade model. For the purpose of the study, the composite wind turbine blade is modeled as an elastic cantilevered rotating thin-walled composite box beam with the developed Circumferentially Asymmetric Stiffness (CAS) structural model. Circumferentially asymmetric stiffness structural model takes into account a group of non-classical effects such as the transverse shear, the material anisotropy and warping restraint. The aerodynamic strip method based on indicial function in unsteady incompressible flow is used to simulate incompressible unsteady aerodynamic effects. Hamilton’s principle and the extended Galerkin’s method are used to obtain the coupled linear governing system of dynamic equations. Preliminary results show that fiber angle of the CAS structural model affects the aeroelastic instability speed significantly and fiber angle also controls the aeroelastic instability mode

Aeroelastic Stability Evaluation of Bend Twist Coupled Composite Wind Turbine Blades Designed for Load Alleviation in Wind Turbine Systems

Wind turbine blades for turbines with large rotor diameter tend to be very flexible in order to remain weight and cost effective. Bending-twisting coupling induced in big composite wind turbine blades is one of the passive control mechanisms which is exploited to alleviate loads incurred due to the flexing of the blades. In the present study, aeroelastic stability characteristics of bend-twist coupled blades, designed for load alleviation in wind turbine systems, is investigated to check whether the bending twisting coupling significantly affects the aeroelastic stability characteristics of the blades or not. For this purpose, different full GFRP and hybrid GFRP and CFRP bend-twist coupled blades are designed for load reduction purpose for a 5 MW wind turbine model that is set up in a wind turbine multi-body dynamic code that uses non-linear beam blade definition and allows bend-twist coupling. For the study of aeroelastic stability characteristics of the blades, overspeed analysis of the wind turbine system is performed without using any blade control and applying slowly increasing wind velocity. Time responses of the torsional and the flapwise bending deformation of the blades obtained in the overspeed analysis are processed to predict the flapwise bending-torsion flutter wind and rotational speeds and the flutter frequencies. Flutter analysis results show that hybrid GFRP and CFRP bend-twist coupled blades have lower flutter speeds compared to the baseline and the bend-twist coupled GFRP blade

Classical flutter analysis of composite wind turbine blades including compressibility

For wind turbine blades with the increased slenderness ratio, flutter instability may occur at lower wind and rotational speeds. For long blades, at the flutter condition, relative velocities at blade sections away from the hub center are usually in the subsonic compressible range. In this study, for the first time for composite wind turbine blades, a frequency domain classical flutter analysis methodology has been presented including the compressibility effect only for the outboard blade sections, which are in the compressible flow regime exceeding Mach 0.3. Flutter analyses have been performed for the baseline blade designed for the 5-MW wind turbine of NREL. Beam-blade model has been generated by making analogy with the structural model of the prewisted rotating thin-walled beam (TWB) and variational asymptotic beam section (VABS) method has been utilized for the calculation of the sectional properties of the blade. To investigate the compressibility effect on the flutter characteristics of the blade, frequency and time domain aeroelastic analyses have been conducted by utilizing unsteady aerodynamics via incompressible and compressible indicial functions. This study shows that with use of compressible indicial functions, the effect of compressibility can be taken into account effectively in the frequency domain aeroelastic stability analysis of long blades whose outboard sections are inevitably in the compressible flow regime at the onset of flutter

Free Vibrations of Composite Plates Stiffened by Two Adhesively Bonded Plate Strips

In this study, the free flexural (or bending) vibration response of composite base-plate or panel systems stiffened by two adhesively bonded plate strips are theoretically analyzed in detail and numerically solved in terms of the mode shapes with their natural frequencies. Additionally, some important parametric studies are also included in the present study. The aforementioned bonded and stiffened system is composed of an orthotropic Mindlin base plate or panel stiffened or reinforced by the dissimilar, orthotropic two-bonded stiffened-plate strips. The two relatively thin, in-between adhesive layers are assumed as the linearly elastic continua with dissimilar material properties. The plate elements of the system are analyzed in terms of the Mindlin plate theory, which takes into account the transverse (or bending) and the rotatory moments of inertia as well as the normal and the transverse shear deformations. The dynamic equations of each plate element of the system are combined together with the stress resultant displacement expressions and, where appropriate, with the adhesive-layer equations. After some manipulations and combinations, the aforementioned dynamic equations are finally reduced to a new set of the governing system of the first-order ordinary differential equations in the state vector forms. These equations are numerically integrated by means of the modified transfer matrix method (with interpolation polynomials). In the numerical results, the mode shapes with their natural frequencies, up to the sixth mode, are graphically presented, for various sets of the boundary conditions. The significant effects of some of the important parameters, such as the aspect ratio, the stiffener length (or width) ratio, and the bending stiffness ratio on the natural frequencies, are studied and presented up to the sixth mode for several sets of the support conditions. The serious influence of the adhesive-layer material characteristics on the mode shapes and on the natural frequencies in terms of the hard (or relatively still) and the soft (or relatively flexible) adhesive layers are also investigated and are shown for the various sets of the support conditions. Furthermore, some very important conclusions related to the analysis and the design of such bonded and stiffened systems are presented

Performance Study of Wind Turbines with Bend-Twist Coupled Blades at Underrated Wind Speeds

Use of bend-twist coupled blades is one of the ways to alleviate fatigue loads in wind turbine systems. Load reduction is achieved by placing off-axis layers in the spar caps of composite wind turbine blades. Off-axis layers provide twisting of the blade in the feathering direction thereby decreasing the aerodynamic loads due to the reduced effective angle of attack. Reduction of fatigue loads in the wind turbine system is generally measured by the damage equivalent load. In the present study, performance of bend-twist coupled blades designed for a 5 MW wind turbine is investigated for the wind speeds ranging from the cut-in speed to the overrated wind speeds. The initial analysis of the wind turbine is done at the overrated speed of 15 m/s and it is shown that reduction in damage equivalent loads is achieved at almost no loss in the rated power compared to the reference wind turbine with the baseline blade. However, it is also demonstrated that at the underrated speeds; although reduction in damage equivalent loads can still be achieved with the bend-twist coupled blade, power loss occurs compared to the reference turbine. This study aims to make a performance study of wind turbine systems with bend-twist coupled blades in terms of load reduction achieved and power production and to propose modifications to simultaneously reduce the generator power losses and damage equivalent loads. As a preliminary design modification, it is shown that by reducing the pre-twist angle of the bend-twist coupled sections of the blades, it has been possible to eliminate the power loss disadvantage of wind turbines at the underrated wind speeds, while still achieving reduction in damage equivalent loads

Reduced order nonlinear aeroelasticity of swept composite wings using compressible indicial unsteady aerodynamics

Nonlinear dynamic aeroelasticity of composite wings in compressible flows is investigated. To provide a reasonable model for the problem, the composite wing is modeled as a thin walled beam (TWB) with circumferentially asymmetric stiffness layup configuration. The structural model considers nonlinear strain displacement relations and a number of non-classical effects, such as transverse shear and warping inhibition. Geometrically nonlinear terms of up to third order are retained in the formulation. Unsteady aerodynamic loads are calculated according to a compressible model, described by indicial function approximations in the time domain. The aeroelastic system of equations is augmented by the differential equations governing the aerodynamics lag states to derive the final explicit form of the coupled fluid-structure equations of motion. The final nonlinear governing aeroelastic system of equations is solved using the eigenvectors of the linear structural equations of motion to approximate the spatial variation of the corresponding degrees of freedom in the Ritz solution method. Direct time integrations of the nonlinear equations of motion representing the full aeroelastic system are conducted using the well-known Runge-Kutta method. A comprehensive insight is provided over the effect of parameters such as the lamination fiber angle and the sweep angle on the stability margins and the limit cycle oscillation behavior of the system. Integration of the interpolation method employed for the evaluation of compressible indicial functions at any Mach number in the subsonic compressible range to the derivation process of the third order nonlinear aeroelastic system of equations based on TWB theory is done for the first time. Results show that flutter speeds obtained by the incompressible unsteady aerodynamics are not conservative and as the backward sweep angle of the wing is increased, post-flutter aeroelastic response of the wing becomes more well-behaved