Elastic Blade Research Articles

INCREASING THE EFFICIENCY OF THE SEED SHAFT TURBER OF THE LINTER MACHINE

It is noted that the main factor in reducing the quality of the lint is its contamination caused by the increased content of crushed seeds. The latter is explained by the mechanical impact on the seed roller from the rigid elements of the working bodies of the linter machine, such as saws, grates and agitator. The magnitude of this impact largely depends on the density of the seed roller itself. In addition, its high density is the cause of additional energy costs for its rotation. To reduce the dynamic loads on the seeds in the roller, an original design of agitator with elastic inclined blades separated by elastic bushings is proposed. The proposed agitator provides intensive mixing of seeds in the seed roller both in the axial direction and in the direction of its rotation. It is shown that agitator with elastic blades allows to reduce energy costs by 20 % and reduce seed damage by 0.3 %

An efficient numerical solver for highly compliant slender structures in waves: Application to marine vegetation

This paper presents a fully explicit coupled wave–vegetation interaction model capable of efficiently solving the coupled wave dynamics and flexible vegetation motion with large deflections. The flow model is formulated using the continuity equation and linearized momentum equations of an incompressible fluid, with additional terms within the canopy region accounting for the presence of vegetation. This linearized flow solver is unconditionally stable and second-order accurate. The flow model is validated and verified against experimental measurements and analytical solutions for waves over a rigid canopy, demonstrating its capability to accurately capture the wave dissipation and flow velocity profiles, even with a relatively coarse grid. A truss-spring model is proposed to capture vegetation motion with substantial deflections, and is proven to be mathematically consistent with the governing equation for the flexible vegetation motion. It allows for explicit time integration with large time steps when dealing with highly compliant vegetation. The truss-spring model is validated and verified by experimental and numerical results for large-amplitude motions of a single elastic blade subjected to waves and sinusoidal oscillatory flows. The coupled model, combining the linearized flow solver and the truss-spring model, is applied to investigate wave propagating over a heterogeneous, suspended, and flexible canopy, showing high efficiency and good agreement with the experiments concerning wave attenuation and the hydrodynamic loads on the vegetation.

Effect of Induction and Blade Elasticity Modelling on Wind Turbine Rotor Performance Predictions

This study investigates the impact of blade induction modelling on the accuracy of wind turbine rotor aeroelastic predictions. It extends the capabilities of AEOLIAN (AErOeLastic sImulAtioN), a Fluid Structure Interaction (FSI) solver based on Blade Element Momentum Theory (BEMT) coupled with a Lumped Mass approach to represent the blade structure. Herein, AEOLIAN’s analytical wake induction engineering model is replaced with the outcomes of a physically-consistent three-dimensional Free-Vortex Wake (FVW) formulation initially employed in AeroROTOR. This versatile aeroelastic simulation tool is implemented within the framework of MATLAB Simulink/Simscape-Multibody©, a modular environment suitable for industry analysts, researchers, and academic users focusing on wind turbine aero-servo-elastic applications. Furthermore, it serves to lay the groundwork for the development of advanced control laws for multi-megawatt rotors, fostering innovation in the design and optimization of the next-generation wind turbines. The presented analyses focus on predicting the aeroelastic behavior of the bottom-fixed NREL 5MW rotor in uniform axial flow over the operating range, complemented by more detailed investigations at the rated condition undergoing inflow with/without wind misalignment (yaw). The study on key performance parameters is conducted by comparing with the higher-fidelity data from available Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) coupled with CFD.

Design strategy for resonance avoidance to improve the performance of tension leg platform-type floating offshore wind turbines

The objective of this study is to determine the effect of elasticity of the tower and blades of a large tension leg platform (TLP)-type FOWT on the eigenmodes of the entire system and suggest reference directions to be implemented in the conceptual design stage. Typically, TLP-type FOWTs are connected by a mooring system with strong tension and exhibit an extremely short natural period of motion compared with a semi-submersible platform. Furthermore, the wind turbine tower mounted on the floating platform is flexible in the fore-aft and side-to-side directions, forming a vibrating system in itself; therefore, it is coupled with the motion of the floating platform. In particular, the tower elasticity is known to influence the roll and pitch motions of the platform. In this study, the motion response characteristics of the platform were investigated according to the changes in the tower design parameter (bending stiffness). The influence of blade elasticity was also confirmed, verifying that the physical characteristics of both the tower and blade significantly affect the motion of the TLP. Additionally, the degree of influence based on environmental loads was identified to suggest a design direction for FOWTs while avoiding resonance.

Rotor Power Savings and Pitch-Link Load Reductions with Spanwise-Varying Active Camber Morphing

The effect of spanwise-varying active-camber morphing on helicopter rotor power and pitch-link loads was investigated computationally for advance ratios ranging from hovering to high-speed forward flight (μ=0.35) and was compared to results obtained with spanwise uniform camber actuation. Different actuation strategies were applied, such as constant (0P) deflection and superimposed harmonic actuation based on a 0P deflection combined with 1P (one per revolution) and 2P harmonics. A comprehensive rotor aeromechanics model of an isolated Bo-105 rotor, including elastic blade modeling and free vortex wake aerodynamics, was used for this analysis. Two different active-camber section sizes were investigated, one ranging from 0.22R to 0.9R and one from 0.5R to 0.9R. The greatest benefits from active-camber actuation were found in hover and high-speed flight. It was possible to reduce both the rotor power consumption and the peak-to-peak pitch-link loads. A relevant contribution of the spanwise variability to these power gains was obtained in hovering flight. Toward higher flight speeds, the effect of spanwise variable camber morphing became less important in terms of rotor power savings. However, it allowed a notable reduction of the actuation amplitude far outboard on the rotor blade.

The theory of impact interaction of a flexible cleaning blade with the head of a carrot root crop during its cleaning from the remains of tops on the root

Purpose. Determination of the kinematic and force parameters of the interaction of an elastic blade with the head of carrot roots at the root based on a theoretical study of its movement during cleaning from the remains of tops using a two-shaft cleaner with a horizontal axis of rotation. Methods. In the course of the research, the methods of building computational and mathematical models of agricultural machines based on the composition and solution of systems of equations, as well as numerical modeling of the application of programs compiled on a PC, were used. Results. The theory of interaction of the flexible blade of the two-shaft cleaner, which is installed on the drive shaft with a horizontal axis of rotation, with the root head of the carrot on the root has been developed. For the theoretical study of this process, a calculated mathematical model of the process of interaction of a flexible cleaning blade with the head of a carrot root was built and a mathematical model was developed that allows determining the speed of the blade after impact and the magnitude of the impact impulse. Conclutions. As a result of the calculations, it was found that at a low speed of movement of the unit , the influence of the angular speed of the blade has a significant effect on the value after the impact speed of the blade. Thus, at a speed of = 0.5 m·s–1, after the impact velocity is equal to 0.71 m·s–1 at = 10 rad·s–1 and 2.55 m·s–1 at = 50 rad·s–1. And at the speed of movement of the cleaner = 3.0 m·s-1, after the impact velocity increases by only 1.3–30.2 % for changes in the angular speed of the blade in the range from 10 rad·s-1 to 50 rad·s-1. The conducted research can be used in the future to develop a theoretical apparatus that describes the process of the post-impact movement of the shovel along the head of the carrot root crop and the combing of the burdock root. Keywords: carrot, gorse, cleaning heads, flexible blade, equivalent circuit, mathematical model.

OF2: coupling OpenFAST and OpenFOAM for high-fidelity aero-hydro-servo-elastic FOWT simulations

Abstract. The numerical study of floating offshore wind turbines (FOWTs) requires accurate integrated simulations which consider the aerodynamic, hydrodynamic, servo and elastic responses of these systems. In addition, the floating system dynamics couplings need to be included to calculate the excitation over the ensemble accurately. In this paper, a new tool has been developed for coupling NREL's aero-servo-elastic tool OpenFAST with the computational fluid dynamics (CFD) toolbox OpenFOAM. OpenFAST is used to model the rotor aerodynamics along with the flexible response of the different components of the wind turbine and the controller at each time step considering the dynamic response of the platform. OpenFOAM is used to simulate the hydrodynamics and the platform's response considering the loads from the wind turbine. The whole simulation environment is called OF2 (OpenFAST and OpenFOAM). The OC4 DeepCWind semi-submersible FOWT together with NREL's 5 MW wind turbine has been simulated using OF2 under two load cases. The purpose of coupling these tools to simulate FOWT is to obtain high-fidelity results for design purposes, thereby reducing the computational time compared with the use of CFD simulations both for the rotor aerodynamics, which usually consider rigid blades, and for the platform's hydrodynamics. The OF2 approach also allows us to include the aero-servo-elastic couplings that exist on the wind turbine along with the hydrodynamic system resolved by CFD. High-complexity situations of floating offshore wind turbines, like storms, yaw drifts, weather vanes or mooring line breaks, which imply high displacements and rotations of the floating platform or relevant non-linear effects, can be resolved using OF2, overcoming the limitation of many state-of-the-art potential hydrodynamic codes that assume small displacements of the platform. In addition, all the necessary information for the FOWT calculation and design processes can be obtained simultaneously, such as the pressure distribution at the platform components and the loads at the tower base, fairleads tension, etc. Moreover, the effect of turbulent winds and/or elastic blades could be taken into account to resolve load cases from the design and certification standards.

RESEARCH OF PRODUCTION TECHNOLOGY AND PROPERTIES OF THE BLADE IS MADE BY THE MINE WELDING METHOD

Problem statement. One of the main requirements for the blade is to achieve significant indicators of corrosion resistance and elasticity, the ability to withstand lateral and contact loads. Thanks to the combination of high-carbon and stainless steels with the help of pit welding, the effect of zone hardening is achieved in the subsequent heat treatment. Zone hardening technology is a combination of different hardness indicators on one knife blade, which leads to a significant increase in the elasticity and durability of the blade, reduces the risk of hardening cracks. Mine welding allows you to combine into a composite steels that have a high content of carbon (С = 0,9…1 %, С = 0,4…0,45 %) and chromium (Cr = 13…14 %), which with an ordinary electric arc method is practically impossible to do. High carbon equivalent and chromium doping complicates existing welding processes. With the appropriate modes of thermoplastic deformation, we get in the zone of forge welding a monolithic connection of two steels at the atomic level, which in itself is already a composite (combination of two materials with different properties to obtain a third starting material). Materials and methods. Etching the blade in an acid solution. To obtain blanks for blacksmithing (pit welding), we took steel bar ШХ15Ш (ТУ141594 DSTU4738:007) Ø – 20 mm, length L − 80 mm for the cutting part of the blade, steel bar 40Х13 DSTU 7838:007, for covers Ø − 20 mm, L − 150 mm. The total weight of the package is 0,42 kg. The results of the experiment. When conducting four experimental attempts to determine the tempering temperature-time regimes for a composite knife blade (ШХ15Ш in 40Х13 facings), the following parameters were obtained: 1. heating temperature 260…270 °С; 2. exposure for 1,8…2 hours; 3. cooling − air. The hardness of the blade is 58−59 HRC, the cutting part is 52−54 HRC of the coating. With such indicators, the maximum indicators of wear resistance of the cutting edge and high indicators of blade elasticity were obtained. For steel n690: 1. heating temperature for tempering 200…220 °С; 2. exposure for 1 hour; 3. cooling − air. Conclusions. Knife blades are proposed to be made by manual forging, without the use of pneumatic and hydraulic hammers, to ensure the purity of the experiment. The work includes calculations of heat treatment regimes, analysis of the microstructures of the obtained monosteel and composite, economic feasibility, advantages and disadvantages of one method of obtaining a blade blade over another.

Including Blade Elasticity into Frequency-Domain Propeller Whirl Flutter Analysis

Whirl flutter as an aeroelastic instability can be a limiting factor in the design and certification of turboprop aircraft configurations, especially for the engine suspension. Whirl flutter prediction for these configurations is currently done in the frequency domain using rigid propeller aerodynamic derivatives. Blade flexibility is neglected in this process, although it is known to have an impact on whirl flutter predictions. This paper uses frequency-domain transfer matrices for the propeller hub loads identified from a time-domain multibody simulation model of an isolated turboprop propeller and included into a frequency-domain flutter analysis to study the impact of blade elasticity on propeller whirl flutter. Results demonstrate a significantly stabilizing effect of blade elasticity on propeller whirl flutter due to a reduction of the destabilizing pitch-yaw cross-coupling moment. The method demonstrated in this paper is applicable to arbitrary time-domain propeller models and compatible with standard frequency-domain flutter processes, allowing for increased fidelity in the flutter prediction process.

Development and validation of a rotating blade-casing rubbing model by considering the blade deformation and abradable coating

Aiming at the phenomenon that blade bending deformation and coating wear are rarely considered in existing rubbing force models, a blade-casing rubbing model based on the Archard wear model was established in this paper. First, the bending deformation of the blade was deduced according to the torque balance relationship, the equivalent deformation of the coated casing was analysed using the equivalent stiffness method, and the clearance between the blade and casing was calculated with the consideration of coating wear based on the Archard model. Second, the influences of the casing stiffness, blade size, and Young's modulus of the coating on the rubbing force were analysed and compared with the existing rubbing force models. Subsequently, a rubbing force characterisation test bench was built considering the AlSi-polyester coating. A strain gauge was used to analyse the blade deformation, the rubbing force was compensated based on the acceleration sensor, and a grating displacement sensor was used to measure the penetration depth, which made the relative error between the experimental and simulation results was less than 15%. Finally, the rubbing force model was applied to the rotor–blade–casing system, and the influence of coating and coating wear on the dynamic characteristics of the rotor system is analyzed. The results show that the model proposed in this paper is more suitable for the rub-impact faults of the elastic blade with an abradable coating on the casing surface. The rubbing force model considering the coating wear is beneficial to accurately predict the rubbing force.

Computational Fluid‐Structure Interaction towards Simulating Large Wind Turbines with openFOAM and deal.II Coupled via preCICE

AbstractWind energy is an essential part of the Green Deal. The trend to increase the size of wind turbines, especially offshore, introduces additional dynamic effects at the long and flexible blades. Embedded in the CRC 1463, DFG, we are working on the fluid‐structure interaction to avoid dynamic stall and investigate flutter effects and blade breathing of ultra‐slim blades [1,2]. This requires an accurate numerical setup that reliably captures the fluid‐structure interactions due to the highly turbulent flow and large deformations of the blades. In preliminary work, the Unsteady Reynolds‐averaged Navier‐Stokes method (URANS) in openFOAM [3] was used to simulate the flow around rotating helicopter blades with a changing angle of attack. [4] successfully predicted the distinct dynamic stall hysteresis with moderate computational effort and captured extreme values (load peaks) within the experimental uncertainties. This aerodynamic solver is to be coupled with a structural solution, for which deal.II [5] provides the linear elastic blade model. The fluid and the structure solvers are coupled via the software preCICE [6] and solved with a staggered approach. Numerical results are presented for a simplified 2D cross‐section of a rectangular solid of carbon‐fiber‐reinforced polymers and a steady inflow velocity. Key challenges for the coupling of the solvers are discussed and the future work is outlined.

Aeroelastic Characteristics of Rotor Blades of Last Stage of a Powerful Steam Turbine

Blades of powerful steam turbines are subjected to significant unsteady loads, which, in some cases, can lead to the appearance of self-excited oscillations or auto-oscillations. These fluctuations are extremely dangerous and negatively affect the life time of the blading. When developing new or upgrading existing turbine stages, it is necessary to carry out research on the aeroelastic behavior of the rotor blades. As a result of the modernization of a low-pressure cylinder of a 1000 MW steam turbine, the length of the rotor blades of the last stage increased to 1650 mm. In this regard, a numerical analysis of the aeroelastic characteristics of the last-stage rotor blades in the nominal operation mode was carried out. The analysis used the method of solving the coupled problem of unsteady aerodynamics and elastic blade vibrations, which allows the prediction of the amplitude-frequency spectrum of unsteady loads and blade vibrations in a viscous gas flow. The paper presents the results of numerical analysis of aeroelastic characteristics of the last stage rotor blades both for the mode of controlled harmonic oscillations with a given amplitude and inter-blade phase shift, and for the mode of coupled oscillations of the blades under influence of unsteady aerodynamic forces. The results of the simulation of coupled oscillations of blades for the first five natural forms are presented in the form of the time distribution of displacement of the blade peripheral cross-section, as well as the time distribution of forces and moments acting on the peripheral cross-section. The corresponding amplitude-frequency spectra of displacements and loads in the peripheral section are also given. The results of the calculations showed a positive damping of oscillations, the absence of flutter and auto-oscillations for the first five natural forms of oscillations of the blades in the nominal operation mode of the steam turbine

On the influence of elasticity on propeller performance: a parametric study

The aerodynamic performance of propellers strongly depends on their geometry and, consequently, on aeroelastic deformations. Knowledge of the extent of the impact is crucial for overall aircraft performance. An integrated simulation environment for steady aeroelastic propeller simulations is presented. The simulation environment is applied to determine the impact of elastic deformations on the aerodynamic propeller performance. The aerodynamic module includes a blade element momentum approach to calculate aerodynamic loads. The structural module is based on finite beam elements, according to Timoshenko theory, including moderate deflections. Several fixed-pitch propellers with thin-walled cross sections made of both isotropic and non-isotropic materials are investigated. The essential parameters are varied: diameter, disc loading, sweep, material, rotational, and flight velocity. The relative change of thrust between rigid and elastic blades quantifies the impact of propeller elasticity. Swept propellers of large diameters or low disc loadings can decrease the thrust significantly. High flight velocities and low material stiffness amplify this tendency. Performance calculations without consideration of propeller elasticity can lead to decreased efficiency. To avoid cost- and time-intense redesigns, propeller elasticity should be considered for swept planforms and low disc loadings.

Aeroelastic Behavior of Long Blades of Last Stage of a 220 MW Steam Turbine

The phenomenon of flutter in steam turbines is quite rare, but when it occurs, the risk of damaging the blades and putting the whole unit out of service increases significantly. Currently, there is a tendency to increase the length of the blades of the last stages of powerful steam turbines, as a result of which the amplitude-frequency characteristics of the blade oscillations change, which can lead to the occurrence of uncontrolled self-excited oscillations. The paper presents the results of a numerical analysis of the aeroelastic characteristics of blade row of the last stage rotor of a 200 MW steam turbine, in which, during modernization, the length of the blades increased to 1200 mm, which made it necessary to check the resistance to flutter in operating modes. The analysis used the method of solving the coupled problem of unsteady aerodynamics and elastic blade vibrations, which allows to predict the amplitude-frequency spectrum of unsteady loads and blade vibrations in a viscous gas flow. The results of the calculations showed the absence of flutter and self-oscillations on the first five natural forms of blade oscillations in the nominal mode of operation of the steam turbine.

Performance Advantages and Resonance Analysis of a Variable Speed Rotor Using Geometrically Exact Beam Formulations

The efficiency and operating envelope of rotorcraft are constrained by the speed of the rotor. Most helicopters operate with a constant rotor speed. Varying the speed of the rotor based on the operating condition could significantly improve the rotor's performance. In this study, a hingeless rotor model with elastic blades is built-in DYMORE to study various aspects of variable speed rotor technology. The rotor blades are modeled as one-dimensional beams using state-of-the-art beam theory known as the geometrically exact beam theory. An unsteady aerodynamics model with dynamic stall and finite-state dynamic inflow is used to obtain the aerodynamic loads acting on the rotor. The power savings that can be achieved at various advance ratios by varying the speed of the rotor is evaluated. Maximum power savings of 41% was achieved at a nominal advance ratio of 0.2. However, changing the rotor speed leads to vibration issues when a rotor blade passes through a resonance point. A methodology to identify the important resonance points for a given flight condition and rotor speed transition is also provided. The forces acting on the rotor blade during resonance crossings at different advance ratios are evaluated. It is found that the amplitude increase during resonance crossing is strongly dependent on the amplitude of the cyclic pitch angles during resonance.

Investigation of blade-mast fluid-structure interaction of a tidal turbine

In this article, we investigate the movement and vibrations of a blade due to the presence of the mast. When the blade passes in front of the mast, a sudden pressure spike induces vibrations in the blade. To study the influence of stiffness, two different structures were studied. We present our numerical schemes concerning the resolution of the flow, the behavior of the structure and the coupling of the two systems. Then, we validate two methods against an experiment (Bahaj et al., 2007). In a third section, we present cases of fluid-structure interaction. Several structures are setup by modifying the stiffness of the material. Their steady open-water (without a mast) behaviors are compared. And finally, two dynamic fluid-structure computations are performed to compare the behavior of an elastic blade passing next to a mast. For all the cases, we use K–FSI developed by K-Epsilon to solve the fluid-structure interaction (FSI).

Primary and combined multi-frequency parametric resonances of a rotating thin-walled composite beam under harmonic base excitation

A clamped rotating thin-walled composite elastic blade subjected to periodic base excitation is considered in this paper. The mathematical model of the structure captures the complex coupled bending–shear–torsional deformations of the beam resulting from the layout of the composite material. The partial differential equations of motion are formulated based on the Hamilton’s principle and next discretized according to the Galerkin’s method. The coefficients of the final state equations are found to be time dependent and have two distinct frequencies, namely the base excitation and the angular velocity of the system. The derived and initially coupled set of governing equations of motion is uncoupled and the method of multiple time scales is used to determine the transition lines separating stability regions of the system on Ince-Strut diagram. Numerical results are presented to illustrate the influence of the composite reinforcing fibres orientations as well as rotating speed and base excitation frequency on the dynamic stability of the structure.

Control Allocation Reconfiguration for Actuator Failure on a Coaxial-Pusher Helicopter

An elastic blade flight dynamics model for a coaxial helicopter platform based on the Sikorsky X2 Technology™ Demonstrator is presented and validated with steady trim and frequency response flight-test data. A full authority explicit model following control architecture along with pseudoinverse control allocation is implemented for the model in hover and cruise at 180 kt using CONDUIT® in order to stabilize the vehicle and meet a set of stability, handling qualities, and performance requirements. Different fault scenarios are considered including failure of rotor swashplate actuators and tail surface actuators in hover and forward flight, which are compensated for by recalculating the pseudoinverse control mixing accordingly. The approach is shown to maintain aircraft stability through the fault transient and into a new steady trim state for the vehicle. Though the implemented controller is successful in maintaining the aircraft state, different fault cases lead to violations in rotor tip clearance limits, which will require additional effort to account for in flight.

A Numerical Study of a Rotor Induced Flow Based on a Finite-State Dynamic Wake Model.

A Helicopter rotor undergoes unsteady aerodynamic loads ruled by the aeroelastic coupling between the elastic blades and the dynamic wake induced by rotary wings. Modeling the dynamic interaction between the structural and aerodynamic fields is a key point to understand aeroelastic phenomena associated with rotor stability, flow induced vibration and noise generation, among others. In this study, we address the Generalized Dynamic Wake Model, which describes the inflow velocity field at the rotor disk as a superposition of a finite number of induced flow states. It is a mature model that has been validated based on experimental data and numerically investigated from an eigenvalue problem formulation, whose eigenvalues and eigenvectors provide a deeper insight on the dynamic wake behavior. The paper extends the results presented in the literature to date in order to support physical interpretation of inflow states drawn from the finite-state wake model for flight conditions varying from hover to edgewise flight. The discussion of the wake model mathematical formulation is also oriented towards practical engineering applications to fill a gap in the literature.

Wind Turbine Trailing Edge Noise: Mitigation of Normal Amplitude Modulation by Individual Blade Pitch Control

This work deals with the elimination or mitigation of amplitude modulation (AM) at any single listener position in the near proximity of a state-of-the-art three-bladed horizontal axis wind turbine by applying individual azimuthal pitch control to the blades. The investigation is restricted to normal AM (NAM) in the close vicinity of the turbine. The effects of the listener position, shear rate of the inflow, and flexibility of the blades are studied. The methodology is model-based: Among others, the fundamental sub-models are the Brooks, Pope and Marcolini airfoil self-noise model, Howe's directivity model for trailing edge sound sources, Rozenberg's convective amplification model for sound sources in motion and an atmospheric damping model. The aeroelastic behaviour of the complete wind turbine is modelled via a multibody approach as encoded in the Horizontal Axis Wind turbine simulation Code 2nd generation (HAWC2) from the Danish Technical University (DTU). The complete model is integrated into an optimisation scheme that seeks an azimuthal pitch control of each blade for minimal modulation depth of the overall sound pressure level at an arbitrarily chosen listener position. The sound prediction model is validated by experimental data from 160 commercial 3.2 MW turbines and the University of Siegen small-scale research wind turbine. As a result, individual blade pitch control as obtained by the optimisation scheme eliminates NAM at any targeted listener position, at the expense of an increase of NAM at other ground locations. Cancelling NAM for a lateral listener position requires the largest amplitude and gradients in the azimuthal pitch control. In case of elastic blades, the required amplitude of the optimised azimuthal pitch control is substantially larger as compared to rigid blades. It is also shown that azimuthal pitch control causes small fluctuations of the axial thrust and aerodynamic power output of the turbine.