Flexible Blades Research Articles

Performance improvement of horizontal axis wind turbines by aerodynamic shape optimization including aeroealstic deformation

Aerodynamic shape optimization of horizontal axis wind turbines was performed to minimize the cost of energy. The optimum blade was obtained by modifying the blade section contours at selected blade radial stations. For the optimization, the design variables were defined as the coordinates of the blade section contours perpendicular to the chord. An aerodynamic performance database was constructed for those blade configurations defined by random combinations of design variables selected within a given range by Latin-Hypercube sampling. An artificial neural network was applied to derive the approximate functions between the design variables and the aerodynamic performance of the database. By utilizing the approximate functions, a genetic algorithm was adopted to search for an optimized blade. To construct the database, a CFD flow solver based on unstructured meshes was utilized. To consider the effects of aeroelastic deformation of the structurally flexible blades, a coupled CFD-CSD method was also adopted. The applications were made for the NREL phase VI and NREL 5 MW reference wind turbine rotor blades. After optimization, the cost of energy was reduced by 0.82% for the NREL phase VI rotor blade and one percent for the NREL 5 MW reference wind turbine blade, respectively.

Design and Experimental Study of Dragonfly-Inspired Flexible Blade to Improve Safety of Drones

A flexible blade that is easy to fabricate and repair is presented: It has a low impact load when colliding with obstacles, and can generate enough thrust for drone flight. The flexible blade has a camber structure that mimics the feature of dragonflies to increase wing stiffness. It makes the blade's deformation decline when it receives a lift force and drag force. A commercially available drone that conformed to standards was used to design, fabricate, and test the flexible blade. The flexible blade consists of a pivot column made by three-dimensional printer and a polyethylene film. The design variables were defined as values with thrust and efficiency that were determined to be similar to those of the original blade through experiments. Experiments for safety verification were conducted by comparing the impact and sharpness of the original and the flexible blades. Experimental results show that the flexible blade's impact force is approximately a tenth of the original blade's impact force, and only the original blade damaged a 100-μm film obstacle. In general terms, the suggested flexible blade is safer and more economical than a plastic blade, in addition to having performance close to that of the original blade during flight.

Erratum to: Experimental Investigation on a Single NACA Airfoil’s Nonlinear Aeroelasticity in Wake Induced Vibrations

In order to meet the economic and environmental requirements, turbomachine blades and aircraft wings are becoming more light and flexible, and bearing more mechanical and aerodynamic loads. However aerodynamic excitation would bring more variables into the structural vibration, and becoming an aeroelasticity problem. Unlike mechanical resonance vibration, the structure would interact with the aerodynamic excitation, and the aerodynamic excitation frequency would lock into structural natural frequency even the frequency margin is more than 10%. This phenomenon extends the high amplitude response range and should be noticed in safety design in order to deal with the margin in specific resonance conditions. In this paper, the aerodynamic excitation induced forced response is investigated with experimental setup including upstream cylinder and a downstream single NACA airfoil in wind tunnel. The upstream cylinder generates the vortices imposed on the NACA airfoil, brings periodic excitation on the flexible blade. Flow velocity is measured with hot wire anemometer (HWA) at upstream and downstream of the blade synchronously. Numerical simulation is conducted based experimental condition and verified by the measurements. Proper Orthogonal Decomposition (POD) is applied to obtain the major flow structure at one typical flow condition. The structural properties of the airfoil including natural frequency and damping are evaluated through finite element analysis and hammer test. Based on the fluid and structure properties, coupled test and analysis can be conducted. The vibration characteristics of NACA airfoil at 1st and 2nd order modes are explored by altering the freestream velocity and cylinder diameter. The forced vibration of 1st order mode has the lock-in phenomenon, and the maximum amplitude point is not at the resonance point. But 2nd order mode shows typical resonance behavior.

Energy harvesting using an aerodynamic blade element at resonant frequency with air excitation

In this research, we propose an energy harvesting structure with a flexible blade element vibrating at its first mode to maximize the power output of the piezoelectric material. For this purpose, a piezoelectric patch was attached on the blade element used in a small-scale wind turbine, and air load was applied with a suitable angle of attack in the stall zone. The aerodynamic load created by air excitation vibrates the blade element in its first natural frequency and maximizes the voltage output of the piezoelectric patch. The variation of power outputs with respect to electrical resistance, air speed, and extra mass is experimentally investigated for various cases. An analytical model is constituted using a single-mode blade element with piezoelectric patch dynamics, and the power outputs of the obtained model are compared with experimental results.

Vibration control of horizontal axis offshore wind turbine blade using SMA stiffener

Modern multi-megawatt wind turbines have large flexible blades that often exhibit significant vibration when exposed to turbulent wind. The present study proposes longitudinal stiffening of blades to address this issue using tendon made of shape memory alloy. A reduced order model of the combined system is developed that incorporates blade stiffening. The nonlinear material behavior of the stiffener is modeled by combining the principle of thermodynamics with the constitutive model as proposed in the literature. Thus, super-elastic effects are utilized in active mode (i.e. using Jule heating with the help of current flow) to apply actuation force that opposes blade deformation. Three-dimensional wind field passing through the rotor plane is simulated using TurbSim package freely available from National Renewable Energy Laboratory, USA. Aerodynamic loads are computed using modified Blade Element Momentum theory and wave loads are simulated using Morison’s equation where wave time histories are simulated from the JONSWAP spectrum. Using these loads as input, the response of a benchmark wind turbine is simulated to show the performance of the proposed control strategy. Numerical results presented in this paper clearly demonstrate the efficiency and advantage of the SMA based stiffener to improve the vibration characteristics of the large turbine blades.

Active vibration control of a blade element with uncertainty modeling in PZT actuator force

The aim of this research is to attenuate the vibrations of a blade structure with an attached piezoelectric actuator using robust multi-objective control. The force obtained from a piezoelectric patch loading has uncertainties due to the complicated shape (airfoil) of the blade element. A parameter-dependent model of the force equation is developed to understand the possible variation range of the actuation force. The modal analysis of the blade is performed to find vibration mode frequencies, and an aerodynamic load is generated experimentally to create steady-state vibration on the blade. A state-space model is obtained by considering certain vibration modes and the parameter-dependent part of the force in the input vector is taken outside of the plant model. The robust stability filter is modified with parameter dependency to have a cluster of the filter. Two different multi-objective controllers are designed with different design objectives. The designed controllers are implemented in experiments and performances of the controllers are compared using frequency and time domain responses. It is shown that the flexible blade vibrations are successfully suppressed with the proposed mixed norm robust controllers under the effect of steady-state aerodynamic disturbance with different air speeds. It is observed in experimental results that the performances of the controller are better than the controller.

Full-field optical deformation measurement and operational modal analysis of a flexible rotor blade

Abstract This study describes the deformation measurement and operational modal analysis (OMA) of an extremely flexible rotor blade in hover. Blade deformation of a 0.45 m-diameter two-bladed rotor was measured at two different root pitch angles and two rotational speeds up to 1500 RPM by a time-resolved digital image correlation (DIC) technique. The DIC technique successfully measured the time history of three dimensional displacements over the entire rotor blade at approximately 900 measurement locations for the flap, lead-lag, and torsional degrees of freedom. Measured blade deformation data were then processed with the Complexity Pursuit (CP) algorithm, to determine the dynamic characteristics of the rotor blade without input excitation information. The identified modal frequencies were within 5% of those obtained by another OMA technique called NExT-ERA as well as a numerical model. There was good agreement between the mode shapes estimated by the CP algorithm and the numerical model; however there was some discrepancy with the mode shapes identified by NExT-ERA. Although modal damping values were extracted by the CP algorithm, they were less than 3 % for all test cases, which is too low to account for aerodynamic damping. Overall, the modal identification process with the CP algorithm was found to be easier to implement and more accurate than the conventional NExT-ERA approach.

Intracardiac Turbines Suitable for Catheter-Based Implantation-An Approach to Power Battery and Leadless Cardiac Pacemakers?

Cardiac pacemakers are powered by batteries, which become exhausted after a few years. This is a problem in particular for leadless pacemakers as they are difficult to explant. Thus, autonomous devices powered by energy harvesters are desired. We developed an energy harvester for endocardial implantation. The device contains a microgenerator to convert a flexible turbine runner's rotation into electrical energy. The turbine runner is driven by the intracardiac blood flow; a magnetic coupling allows hermetical sealing. The energy harvester has a volume of 0.34 cm3 and a weight of 1.3g. Computational simulations were performed to assess the hemodynamic impact of the implant. The device was studied on a mock circulation and an in vivo trial was performed in a domestic pig. In this article, we show that an energy harvester with a 2-bladed 14-mm-diameter turbine runner delivers 10.2 ± 4.8 μW under realistic conditions (heart rate 80/min, stroke volume 75 ml) on the bench. An increased output power (>80 μW) and power density (237.1 μW/cm3) can be achieved by higher stroke volumes, increased heart rates, or larger turbine runners. The device was successfully implanted in vivo. The device is the first flow-based energy harvester suitable for catheter-based implantation and provides enough energy to power a leadless pacemaker. The high power density, the small volume, and the flexible turbine runner blades facilitate the integration of the energy harvester in a pacemaker. This would allow overcoming the need for batteries in leadless pacemakers.

An efficient computational model for fluid-structure interaction in application to large overall motion of wind turbine with flexible blades

Abstract This paper proposes a very efficient computational model for fluid–structure interaction problems corresponding to steady-state flow representing a large overall motion of wind turbines with flexible blades. The model of turbine blades is based upon the 3D solids finite elements with drilling rotations. The proposed 3D solids model is fully able to describe the flexibility blades large overall motion and easily capture both bending and torsional motion thanks to using an enhanced strain field. The model efficiency is further reinforced by using the 3D panel method that fits naturally with proposed 3D solid finite elements for blades. The proposed panel method is the corresponding modification of the potential fluid flow by introducing a vorticity layer at the fluid–structure interface and Bernoulli’s conservation momentum equation in order to provide quantification for the blade thrust. The fluid–structure interaction is enforced through an efficient iterative procedure providing on one hand a very fast computation of aerodynamic loads, which is sufficiently accurate for computing the overall thrust on the blade, and on the other hand a sufficiently accurate representation of the stress states suitable for fatigue studies. The proposed computational model performance is illustrated with several numerical simulations, including the practical case of a full-scale NREL 5MW wind turbine.

Using Flexible Blades to Improve the Performance of Novel Small-Scale Counter-Rotating Self-Adaptable Wave Energy Converter for Unmanned Marine Equipment

Unmanned marine equipment has been increasingly developed for open seas. The lack of efficient and reliable power supply is currently one of the bottlenecks restricting the practical application of these devices. In order to provide a viable power supply method for unmanned marine equipment, such as sonic buoys and sea robots, we originally propose a novel small-scale flexible blade wave energy converter (WEC) based on self-adaptable counter-rotating operation mechanism. The flexible blade WEC is designed on the basis of the rigid blade WEC with the caging device. This paper identifies the key factors affecting WEC performance through theoretical analysis. According to the numerical simulation analysis, the output mechanical power of the double-layer absorber is 12.8 W, and the hydraulic efficiency is 36.3%. The results of the verification experiment show that the peak power of WEC is 5.8 W and the average power is 3.2 W. The WEC with 65Mn flexible blade under most experimental conditions has the best performance when the blade thickness is 0.10 mm. The study shows that the new generation WEC can effectively overcome the excessive fluctuation of the output power of the previous generation WEC. The output power curve of the novel WEC is relatively smooth, which is conducive to its smooth operation and subsequent utilization and storage of electrical energy.

Aeroelastic stability analysis of a wind turbine blade section with trailing edge flap using a flexible unsteady blade elements momentum theory

This study aims to develop an unsteady aerodynamic model and blade element momentum theory to analyze the aeroelasticity of a rotating airfoil with trailing edge flap of a wind turbine blade. The Hamiltonian principle serves to define the equations of motion. Theodorsen and Loewy aerodynamic models alone and together with blade element momentum (BEM) theory are applied to form aeroelastic equation of motion. Theodorsen or Loewy aerodynamics together with BEM theory present a new flexible unsteady blade element momentum aerodynamic model. Verification of developed aeroelastic equation motion is conducted on a fixed and a rotating airfoil successfully. Flutter boundary results imply the superiority of proposed aerodynamic model. Besides, we investigated theoretical effects of pitch and flap stiffness changes on flutter. The derived results demonstrate that pitch stiffness is the dominant mode on flutter boundary of the rotating blade section with a control surface.

Experimental Investigation on a Single NACA Airfoil’s Nonlinear Aeroelasticity in Wake Induced Vibrations

In order to meet the economic and environmental requirements, turbomachine blades and aircraft wings are becoming more light and flexible, and bearing more mechanical and aerodynamic loads. However aerodynamic excitation would bring more variables into the structural vibration, and becoming an aeroelasticity problem. Unlike mechanical resonance vibration, the structure would interact with the aerodynamic excitation, and the aerodynamic excitation frequency would lock into structural natural frequency even the frequency margin is more than 10%. This phenomenon extends the high amplitude response range and should be noticed in safety design in order to deal with the margin in specific resonance conditions. In this paper, the aerodynamic excitation induced forced response is investigated with experimental setup including upstream cylinder and a downstream single NACA airfoil in wind tunnel. The upstream cylinder generates the vortices imposed on the NACA airfoil, brings periodic excitation on the flexible blade. Flow velocity is measured with hot wire anemometer (HWA) at upstream and downstream of the blade synchronously. Numerical simulation is conducted based experimental condition and verified by the measurements. Proper Orthogonal Decomposition (POD) is applied to obtain the major flow structure at one typical flow condition. The structural properties of the airfoil including natural frequency and damping are evaluated through finite element analysis and hammer test. Based on the fluid and structure properties, coupled test and analysis can be conducted. The vibration characteristics of NACA airfoil at 1st and 2nd order modes are explored by altering the freestream velocity and cylinder diameter. The forced vibration of 1st order mode has the lock-in phenomenon, and the maximum amplitude point is not at the resonance point. But 2nd order mode shows typical resonance behavior.

Effect of Platform Motion on Aerodynamic Performance and Aeroelastic Behavior of Floating Offshore Wind Turbine Blades

In the present study, a numerical framework for predicting the aerodynamic performance and the aeroelastic behavior of floating offshore wind turbine rotor blades involving platform motion was developed. For this purpose, the aerodynamic and structural analyses were conducted simultaneously in a tightly coupled manner by exchanging the information about the aerodynamic loads and the elastic blade deformations at every time step. The elastic behavior of the turbine rotor blades was described by adopting a structural model based on the Euler-Bernoulli beam. The aerodynamic loads by the rotor blades were evaluated by adopting a blade element momentum theory. The numerical simulations were conducted when the platform of the wind turbine independently moves in each of the six degrees-of-freedom directions consisting of heave, sway, surge, roll, pitch, and yaw. It was observed that flexible blades exhibit complicated vibratory behaviors when they are excited by the aerodynamic, inertia, and gravitational forces simultaneously. It was found that the load variation caused by the platform surge or pitch motion has a significant influence on the flapwise and torsional deformations of the rotor blades. The torsional deformation mainly occurs in the nose-down direction, and results in a reduction of the aerodynamic loads. It was also found that the flapwise root bending moment is mainly influenced by the platform surge and pitch motions. On the other hand, the edgewise bending moment is mostly dictated by the gravitational force, but is not affected much by the platform motion.

Fluid-structure coupled investigations of the NREL 5 MW wind turbine for two downwind configurations

This work investigates the performance of a wind turbine in a downwind configuration using two different tower concepts. Being subjected to the tower shadow, wind turbine blades on downwind machines can experience higher loads compared to upwind turbines. Using alternative tower types like truss towers could influence those negative effects and therefore affect the performance of the downwind machines. For this purpose, an in-house framework for fluid-structure coupled CFD computations is used and numerical simulations are conducted on the NREL 5 MW wind turbine for two different tower concepts, namely a tubular and a truss-type tower geometry. To quantify the effect of the tower shadows on the rotor performance, first the isolated tower geometries are investigated. In the second step, the complete wind turbine with flexible blades is simulated. The obtained numerical results indicate, that the truss tower causes a more severe tower shadow than its tubular counterpart, which also recovers slower. Following that, the wind turbine blades experience more pronounced reductions in global and local forces for the truss tower configuration. Furthermore, both downwind turbine types show significant torsional blade vibrations caused by the crossing of the tower shadow. In terms of blade deflections, only small deviations can be observed.

Mathematical modelling for flexible blade coater with magnetohydrodynamic and slip effects in blade coating process

A new mathematical model for a flexible blade coater is proposed and analysed for slip and magnetohydrodynamic (MHD) effects in blade coating process. The slip is considered at the blade surface and magnetic field is imposed normal to the flow. To obtain the velocity profile, pressure, pressure gradient, volumetric flow rate and maximum pressure both exact and numerical solutions are utilized. In order to obtain the numerical solution shooting technique is applied. The interesting physical quantities like load and deflection are calculated and presented in graphical and tabulated form. The influence of the Hartman number the slip parameter and normalized coating thickness parameter on the flow and deflection are discussed graphically. In the presence of magnetic field and slip the fluid velocity and hence blade deflection can be controlled.

Qualitative yaw stability analysis of free-yawing downwind turbines

Abstract. This article qualitatively shows the yaw stability of a free-yawing downwind turbine and the ability of the turbine to align passively with the wind direction using a model with 2 degrees of freedom. An existing model of a Suzlon S111 upwind 2.1 MW turbine is converted into a downwind configuration with a 5∘ tilt and a 3.5∘ downwind cone angle. The analysis shows that the static tilt angle causes a wind-speed-dependent yaw misalignment of up to −19∘ due to the projection of the torque onto the yaw bearing and the skewed aerodynamic forces caused by wind speed projection. With increased cone angles, the yaw stiffness can be increased for better yaw alignment and the stabilization of the free-yaw motion. The shaft length influences the yaw alignment only for high wind speeds and cannot significantly contribute to the damping of the free-yaw mode within the investigated range. Asymmetric flapwise blade flexibility is seen to significantly decrease the damping of the free-yaw mode, leading to instability at wind speeds higher than 19 m s−1. It is shown that this additional degree of freedom is needed to predict the qualitative yaw behaviour of a free-yawing downwind wind turbine.

Characteristics of the fluid–structure interaction within Darrieus water turbines with highly flexible blades

Vertical-axis water turbines (VAWT), the target of the present study, provide a higher area-based power density when used in arrays, and are a promising alternative to horizontal-axis hydro-kinetic turbines (HAWT). Nevertheless, they operate under highly dynamic conditions near or even beyond dynamic stall at their best-efficiency-point. The abrupt loss of lift and strong increase of drag associated with hydrofoil stall can produce cyclic loads and possible damage of turbomachines due to material fatigue.The effect of flexible structures in a highly dynamic flow regime including separation and stall is here studied systematically in an experimental setup which permits observations of all regimes ranging from quasi-static state up to the occurrence of deep dynamic stall and beyond. The process is studied using a surrogate model consisting of an oscillating NACA0018 hydrofoil in a closed water channel, following a motion law comparable to the real angle of incidence of a Darrieus turbine blade along its rotation. The investigated parameters are the oscillation frequency and tip speed ratio, for one rigid as well as for three flexible hydrofoils of different stiffnesses. The coupling process is therefore investigated for multiple machine designs and working points. Lift and drag measurements have been carried out in a systematic manner.Results show that at tip speed ratios for which highly dynamic flow regimes occur, flexible blades provide not only higher thrust, but also reduced normal forces and reduced peak-to-peak cyclic normal force variations. This reduction of stress loads would translate into significantly increased turbine lifetime. This supports the need for further investigations in order to identify optimal blade flexibility and check further turbine designs.

Analytical Modeling of Process Damping in Machining

The machining process induced damping caused by the indentation of the cutting edge into the wavy cut surface greatly affects the process stability in low-speed machining of thermally resistant alloys and hardened steel, which have high-frequency vibration marks packed with short wavelengths. This paper presents an analytical model to predict the process damping forces and chatter stability in low-speed machining operations. The indentation boundaries are evaluated using the cutting edge geometry and the undulated surface waveform. Contact pressure due to the interference of the rounded and straight sections of the rigid cutting edge with the elastic-plastic work material is analytically estimated at discrete positions along the wavy surface. The overall contact pressure is obtained as a function of the cutting edge geometry, vibration frequency and amplitude, and the material properties of the workpiece. The resulting specific indentation force is evaluated by integrating the overall pressure along the contact length. Then, the process damping force is linearized by an equivalent specific viscous damping, which is used in the frequency domain chatter stability analysis. The newly proposed analytical process damping model is experimentally validated by predicting the chatter stability in orthogonal turning, end milling, and five-axis milling of flexible blades. It is shown that the proposed model can replace currently used empirical models, which require extensive experimental calibration approach or computationally prohibitive finite elements based numerical simulation methods.

A strongly-coupled model for flexible rotors

A fluid–structure model describing the equilibrium state of a flexible blade rotor with its own wake is derived for various external axial flow conditions. The model is based on three building blocks. The two-dimensional lifting-line theory is first used to compute the local aerodynamic loads and the blade circulation profile. The blade deformation is then obtained by solving the nonlinear equations for bending and twisting angles deduced from a one-dimensional beam model. Finally, the wake is obtained using a Joukowski model. In this wake model, the wake of each blade is modeled by two small-core-size counter-rotating vortices emitted from the rotor axis and blade tip. The velocity field induced by these vortices is computed using the Biot–Savart law. We show that, in the rotor frame, we can obtain a stationary vortex structure for almost any vertical flight regimes. This wake solution can then be used to compute the induced velocity in the rotor plane and apply the two-dimensional lifting-line theory again. By iterating a few times this loop, we converge toward a nonlinear solution of the problem for which the aerodynamics loads, blade deformation and wake structure are compatible.As illustration, this newly-developed model is applied to two rotors. We analyze the effects of the external wind conditions, geometry and material properties of the blades on the blade deformation and wake characteristics. We show that we can describe slow descending regimes for which the classical momentum theory does not apply.

Blade dynamics in combined waves and current

Submerged aquatic vegetation (SAV), such as seagrass, is flexible and reconfigures (bends) in response to waves and current. The blade motion and reconfiguration modify the hydrodynamic drag. The modified drag can be described by an effective blade length, le, which is defined as the length of a rigid blade that results in the same drag as a flexible blade of length l. In many natural settings SAV is exposed to combinations of waves and current. This study derived and used laboratory measurements to validate new predictions of effective blade length for combined waves and current based on a Cauchy number, which describes the ratio of hydrodynamic drag to the restoring force due to rigidity of blade. Force measurements on and digital images of blades exposed to waves with a 2-s period and with a range of wave velocity (Uw) and current speed (Uc) were used to estimate the effective blade length. The measurements were also used to validate a numerical simulation of blade motion. Once validated, the simulation was used to expand the investigated parameter space to a wider range of wave conditions, and in particular longer wave periods. ForUc<14Uw, the blade motion and hydrodynamic drag were wave-dominated. For Uc>2Uw, the blade motion and hydrodynamic drag were current-dominated.