Cyclic Pitch Control Research Articles

Cyclic pitch control for aerodynamic load reductions of floating offshore wind turbines under pitch motions

Floating offshore wind turbines suffer severe aerodynamics under platform motions. Novel control methods are urgently needed to improve power and load performances. A mechanical cyclic pitch control strategy is introduced in this paper, and the load reduction effect on the NREL 5-MW wind turbine is investigated under dominant pitch motions. The simulation is performed by the standalone Aerodyn_Driver code. Results show that the relative airflow under pitch motions can be equated to a linear sheared flow and is only related to wind velocity and pitch angular velocity. Cyclic pitch control hardly affects power and thrust but significantly changes aerodynamic moments. The resultant moment and moment axis azimuth are defined, and they are controlled respectively by pitch amplitude and phase. The optimal pitch parameter equations to minimize pitching and yawing moments are derived from fitting and derivation, and the standard deviations of aerodynamic moments are reduced by over 70 % under the sinusoidal pitch motion and steady inflow below rated operation. This paper provides a new idea for the individual pitch control structure, and the mechanism study for aerodynamic control is important and instructive to fill the technological gap in mature pitch control strategies for floating offshore wind turbines.

Reducing aerodynamic vibration of rigid rotors with retreating side active control avoidance

This paper proposes a new approach to eliminate aerodynamic lift oscillation, called the Dominant Sector Individual Blade Control (DS-IBC) method for rigid rotor helicopters. An Advancing Blade Concept (ABC) rotor model for aerodynamic analysis based on the free-wake method is applied. DS-IBC avoids applying active control on the rotor’s retreating side by employing and restricting active control inputs to a sector area of the rotor disc. Outside this sector, only primary collective and cyclic pitch control are used. Each blade takes turns entering the sector, creating a “relay” active control form to ensure continuous control inputs. The method also includes outer-trim and inner-trim iteration modules. Results show that DS-IBC can eliminate aerodynamic lift oscillation using much smaller control inputs than the sine-trim method. By focusing active control on the rotor’s advancing side, DS-IBC improves the effective lift-to-drag ratio and reduces the implementation difficulty of active rotor control for aerodynamic oscillation elimination, especially at a large lift-offset.

A directly coil-actuated cyclic pitch control based rotor: Design, modeling, and analysis

This work introduces a directly coil-actuated cyclic pitch control method for the swashplate-less aircraft. This method drives the rotor to rotate periodically around the pitch axis through a magnetic field. The sensitive parameters that affect pitch motion are analyzed and optimized by simulation. Next, the aerodynamic model of the rotor is established by idealizing the blade as a centrally hinged rigid body with a spring restraint, and then the kinematic model of the rotor system is obtained. A state feedback attitude control method is proposed based on the kinematic model. The feasibility of the directly coil-actuated cyclic pitch control method is verified by comparing the results of mechanical experiments and numerical calculations. Moreover, the attitude response of the rotor system is evaluated through attitude experiments.

Study Method of Pitch-Angle Control on Load and the Performance of a Floating Offshore Wind Turbine by Experiments

Offshore wind energy is a renewable energy source that is developing fast. It is considered to be the most promising energy source in the next decade. Besides, the expanding trend for this technology requires the consideration of diversified seabeds. In deep seabeds, floating offshore wind technology (FOWT) is needed. For this latter technology, such as for conventional WT, we need to consider aspects related to performance, aerodynamic force, and forces during operation. In this paper, a two-bladed downwind wind turbine model is utilized to conduct experiments. The collective pitch and cyclic pitch angle are adjusted using swashplated equipment. The fluid forces and moments acting on the rotor surface are measured by a six-component balancing system. By changing the pitch angle of the wind turbine blades, attempts are made to manage the fluid forces generated on the rotor surface. Under varied uniform wind velocities of 7, 8, 9, and 10 m/s, the effect of collective pitch control and cyclic pitch control on the power coefficient and thrust coefficient of FOWT is then discussed. Furthermore, at a wind speed of 10 m/s, both the power coefficient and loads are investigated as the pitch angle and yaw angle change. Experimental results indicate that the combined moment magnitude can be controlled by changing the pitch-angle amplitude. The power coefficient is adjusted by the cyclic pitch-angle controller when the pitch-angle phase changes. In addition, the thrust coefficient fluctuated when the pitch angle changed in the oblique inflow wind condition.

Performance based systematic design methodology for development and flight testing of fuel engine powered quadrotor Unmanned Aerial System for industrial applications

This paper presents a systematic study of the design optimization, development, vibration analysis and flight testing of a single power plant variable pitch quadrotor Unmanned Aerial System (UAS) for industrial applications. The use of variable pitch rotors enables the use of a two-stroke internal combustion engine for powering the four rotors through a mechanical transmission. The proposed methodology is described by carrying out the design of a quadrotor UAS with 2–3 kg payload capacity and have endurance in excess of one hour. The study is conducted in a systematic manner. First, the design of rotors is optimized for minimum power consumption during hover as well as forward flight condition using physics based simulation developed and validated for this purpose. From the rotor optimization, an untwisted rotor blade with taper ratio of 0.6 and aspect ratio of 10 appeared to be an optimal choice for the current design. Next, the powerplant selection is carried out based on the optimized rotor designed earlier. Next, two different approaches for mechanical power transmission design is considered: torque tube-bevel gear and belt–pulley system based designs. The transmission system designed using belt and pulley was nearly 14% lighter and showed greater tolerance to manufacturing imperfections. Next, two prototypes are designed and its static and dynamic analysis is carried out to ensure its airworthiness. A Proportional Integral Derivative (PID) based attitude controller is developed using quarternions for attitude stabilization and trajectory tracking to test the flight in simulation and then subsequently on the actual vehicle by implementing it on PixHawk autopilot board. Finally, system integration is carried out to build two prototypes of engine powered quadrotor UAS. Then, vibration data is recorded using the sensors onboard to conduct the vibration analysis and design an efficient vibration isolator. Performance analysis is also, carried out to estimate the speed for best endurance (11 m/s) and best range (27 m/s) for the variable pitch quadrotor designed. The estimated best endurance achieved is 2.2 h and estimated best range is more than 150 km. This design revisits and solves the challenge of developing an agile helicopter without cyclic pitch control which was first explored nearly a century ago.

Wind tunnel and numerical study of a floating offshore wind turbine based on the cyclic pitch control

Offshore wind energy is one of the most important sources of renewable energy to make sure supplying enough electricity for the world in the future. The floating offshore wind turbine (FOWT) operates under many influences such as both the aerodynamic force, the hydrodynamic force and coupled motion between the two. Therefore, in this paper about the effect of collective pitch control and cyclic pitch control on power and loads of FOWT are discussed under the uniform wind velocity of 10 m/s with wind tunnel and numerical investigation. FAST tool (Fatigue, Aerodynamics, Structures, and Turbulence) is used to simulate FOWT under the same wind conditions as wind tunnel experiments. A two-bladed downwind turbine model is used for this experiment to estimate the fluctuations of the power coefficient, thrust coefficient and moments. The pitch moment and the yaw moment are measured at the top of the tower and the fluctuation of loads is compared. The phase angle of the pitch angle is controlled by three actuators through a swash plate equipment. From the simulated results, the effect of the pitch angle change on the power coefficient is higher than the yaw angle change. For the yawed wind condition, the load of the wind turbine at the phase angle of ξ = 60° was equal to the optimum operating point. Thence, the load of the wind turbine was decreased when the cyclic pitch control method was applied.

Improved hydrodynamic performance of a collective and cyclic pitch propeller by numerical evaluation of the blade design

The collective and cyclic pitch propeller (CCPP) is a novel concept designed to propel and manoeuvre autonomous underwater vehicles (AUVs). Collective pitch control governs the generated thrust, while through cyclic pitch control a manoeuvring force/side-force is generated. Recent work concluded that new solutions are needed to achieve large side-forces without compromising the observed phase shift. In this paper, increasing the blade surface area is rationalised as the most effective pathway to achieve improved CCPP performance. A three-dimensional numerical RANS-based periodic numerical model was used to evaluate three alternative blade designs. Clear performance improvements were realised by the new blade design, manifested in the ability to generate larger side-forces, both absolute and relative based on the actual surface area, at much smaller phase shifts. A performance improvement relation was established, stating that optimal improvement is realised by increasing both the blade's surface area and aspect ratio. At higher aspect ratios, the side-force is produced in a more efficient manner, both from a general force behaviour and a side-force force generation perspective. Finally, an important note was made regarding the future introduction of blade twist in order to address an observed effect on the ‘neutral’ pitch angle.

Study on cyclic pitch control of HAWT by using instantaneous inflow observation

The power output of a wind turbine depends on both wind speed and turbine control. There is a fluctuation in the natural wind speed, which changes the turbine power output. In this paper blade pitch control based on detailed monitoring of the inflow wind, including vertical velocity distributions, has been experimentally evaluated in a field test wind turbine. The inflow wind speeds were observed by 11 sets of ultrasonic anemometers. In this first test, the blade pitch is changed to keep optimum angle of attack based on the local inflow wind speed in the rotor plane. The blade pitch is independently controlled as a reference radial position of 80% of rotor radius. As a result of the pitch controls, the wind turbine power output was increased and the power output fluctuation was decreased for the cyclic pitch control based on the local wind speed. With the cyclic pitch control, the fluctuation in rotor thrust and average rotor thrust were also reduced compere to collective pitch control.

Dynamics research on actively controlled swashplateless rotor

This study presents the design, dynamic model, and dynamics research of a new cyclic pitch control strategy in motor-driven rotorcraft. In this strategy, the control response and flapping feature of conventional rotor systems can be obtained by imposing (1) a lag-pitch coupling on rotor blade and (2) an additional sinusoidal rotational speed of rotor shaft without any actuators or swashplate. This study establishes a refined nonlinear dynamic model including the effects of pitch motion, and figures out the fundamental dynamic characteristics of this novel configuration. Analyses of its mechanical stability, dynamic responses and stability in hover are also conducted. The model in this research, compared with the previous linear model, improves the predicting accuracies of dynamic responses remarkably. A preliminary understanding of this novel rotor's dynamic mechanisms is gained by this study.

Numerical Investigations of Dynamic Stall on a Rotor with Cyclic Pitch Control

Numerical investigations of three-dimensional dynamic stall on a two-bladed Mach-scaled rotor (R = 0.65m, Ma75 = 0.21, Re75 = 3.5 × 105) with 1/rev cyclic pitch control are presented and compared to experimental surface pressure and PIV data. In addition to URANS simulations using the finite-volume flow solvers FLOWer and TAU, a delayed detached-eddy simulation (DDES) with Menter SST as underlying RANS model is carried out with FLOWer. Facing dynamic stall and flow separation, the DDES reproduces high-frequency load fluctuations, cycle-to-cycle variations and small-scale vortical structures as seen in the experiment, which is not the case with URANS. However, common hybrid RANS-LES issues – grid-induced separation and the grey area problem – play a role in this DDES and influence loads severely. FLOWer SST simulations yield load peaks of the same magnitude as individual, non-phase-averaged measurements. With TAU SST the dynamic stall event is delayed and weakened compared to FLOWer SST and experimental results. FLOWer and TAU results using the SA turbulence model are fairly comparable but in bad agreement with the experiment at the outboard station at r/R = 0.77, where they exhibit no dynamic stall at all.

Hydrodynamic Performance Analysis of a Collective and Cyclic Pitch Propeller Under Bollard Pull Condition Through Numerical Evaluation of Two-Dimensional Pitching Hydrofoils

Propulsion and maneuvering of autonomous underwater vehicles require a combination of effective and efficient operation at both high and low speeds. The collective and cyclic pitch propeller (CCPP) is a novel system designed to provide the required operational flexibility through control of the propeller's blade pitch. Collective pitch control governs the forward generated thrust, whereas cyclic pitch control governs the generated maneuvering force(s)/side-force(s). In this article, a numerical analysis into the CCPP's hydrodynamic performance at bollard pull is set-up, reducing the complex three-dimensional flow problem to a two-dimensional problem. Through a force break-down model, the CCPP's hydrodynamic performance is related and matched to the operation of a pitching hydrofoil. Analysis of the two-dimensional numerical results can thereby provide insights into the performance of the three-dimensional CCPP. First, the performance of the pitching hydrofoils is investigated as such, relating the generated lift, drag, and moment to the occurrence of dynamic stall. Next, the methodology's applicability and limitations are discussed by comparing the numerical results with recent experiment CCPP work to allow the model to be used for a numerical evaluation of the CCPP's performance. Under the evaluated conditions, testing a range of collective and cyclic pitch angles under bollard pull, the side-force generation by the CCPP is shown to be highly dependent on the generated drag force at higher collective pitch angles. At low pitch angles, the side-force generation is controlled by the lift produced over the pitching blades, and efficient but not highly effective. As the collective pitch is increased, the generated drag affects both the effectiveness of the side-force and the side-force efficiency, defined by the large resulting side-force orientation. At larger collective pitch angles, the lift forces are overtaken by the drag generation, resulting in effective but inefficient side-force generation. Autonomous underwater vehicles (AUVs) have become a widely used and researched tool for underwater exploration and reconnaissance (Alam et al. 2014). AUVs distinguish themselves from other unmanned underwater vehicles in their ability to complete a pre-determined mission autonomously over large distances and long time periods, i.e., without the need for regular human interaction. The diversity in industry applications for AUVs has resulted in a wide range of AUV shapes and designs (Button et al. 2009). Applications include different areas such as underwater pipe-line inspection in the oil and gas industry, sample collection for marine biology research, and military surveillance missions (Chyba 2009). One key requirement of any AUV design, as a result of their specific mission profile and inherent functionality, is the combination of efficient long-endurance travelling capabilities with effective maneuverability at low speeds (Wernli 2000). Traditional maneuvering systems using control surfaces lose their efficiency at low speeds and lowspeed maneuvering aids such as side-or podded-thrusters reduce the long-endurance travelling efficiency. A novel propulsion and maneuvering system, aimed at providing both efficient long-endurance propulsion and effective maneuvering at all speeds, is the collective and cyclic pitch propeller (CCPP).

Experiment and Simulation Effects of Cyclic Pitch Control on Performance of Horizontal Axis Wind Turbine

Offshore wind is generally stronger and more consistent than wind on land. A large part of the offshore wind resource is however located in deep water, where floating wind turbines can harvest more energy. This paper describes a systematic experiment and a simulation analysis (FAST code) about the cyclic pitch control of blades. This work was performed to investigate performance fluctuation of a floating wind turbine utilizing cyclic pitch control. The experiment was carried out in an open wind tunnel with mainstream wind velocity of 10 m/s with the front inflow wind and the oblique inflow wind conditions. A model wind turbine is two-bladed downwind wind turbine with diameter of 1.6 m. Moment and force acts on the model wind turbine were measured by a six-component balance. Fluctuation of power coefficient and thrust coefficient was investigated in the cyclic pitch control. The model wind turbine and the experimental conditions were simulated by FAST code. The comparison of the experimental data and the simulation results of FAST code show that the power coefficient and thrust coefficient are in good agreement. Keywords: Floating Offshore Wind Turbine, Aerodynamic Forces, Cyclic Pitch Control, FAST Code, Wind Tunnel ExperimentArticle History: Received February 11st 2017; Received in revised form April 29th 2017; Accepted June 2nd 2017; Available onlineHow to Cite This Article: Sang, L.Q., Maeda, T., Kamada, Y. and Li, Q. (2017) Experiment and simulation effect of cyclic pitch control on performance of horizontal axis wind turbine to International Journal of Renewable Energy Development, 6(2), 119-125.https://doi.org/10.14710/ijred.6.2.119-125

Experimental investigation of the cyclic pitch control on a horizontal axis wind turbine in diagonal inflow wind condition

Offshore wind power has huge energy potential. For deep water areas, FOWTs (Floating Offshore Wind Turbines) are sensitive to large oscillations due to the aerodynamic force of wind and the hydrodynamic force of wave. In the natural environment, the wind is usually unstable and characterized by the diagonal inflow wind. In this study, the potential of alleviating blade load variations induced by the cyclic pitch control evaluated under the condition of the diagonal inflow wind. An experiment in an open wind tunnel was performed to investigate the effect of the diagonal inflow wind on the aerodynamic forces of FOWTs. A two-bladed downwind FOWT with a diameter of 1.6 m used for this experiment. The forces and moments applied to the entire wind turbine were measured by a six-component balance. A swash plate used to adjust the pitch angle in the cyclic pitch control. From the experimental results, the average value of the aerodynamic load acting on the rotor face in the diagonal inflow wind condition at phase angle of ξ = 60° equals the moment coefficient in the optimum operating condition at the front inflow wind. The load change on yaw system can be controlled by cyclic pitch control method.

Numerical and Experimental Study of Wake Redirection Techniques in a Boundary Layer Wind Tunnel

The aim of the present paper is to validate a wind farm LES framework in the context of two distinct wake redirection techniques: yaw misalignment and individual cyclic pitch control. A test campaign was conducted using scaled wind turbine models in a boundary layer wind tunnel, where both particle image velocimetry and hot-wire thermo anemometers were used to obtain high quality measurements of the downstream flow. A LiDAR system was also employed to determine the non-uniformity of the inflow velocity field. A high-fidelity large-eddy simulation lifting-line model was used to simulate the aerodynamic behavior of the system, including the geometry of the wind turbine nacelle and tower. A tuning-free Lagrangian scale-dependent dynamic approach was adopted to improve the sub-grid scale modeling. Comparisons with experimental measurements are used to systematically validate the simulations. The LES results are in good agreement with the PIV and hot-wire data in terms of time-averaged wake profiles, turbulence intensity and Reynolds shear stresses. Discrepancies are also highlighted, to guide future improvements.

Wake redirection: comparison of analytical, numerical and experimental models

This paper focuses on wake redirection techniques for wind farm control. Two control strategies are investigated: yaw misalignment and cyclic pitch control. First, analytical formulas are derived for both techniques, with the goal of providing a simple physical interpretation of the behavior of the two methods. Next, more realistic results are obtained by numerical simulations performed with CFD and by experiments conducted with scaled wind turbine models operating in a boundary layer wind tunnel. Comparing the analytical, numerical and experimental models allows for a cross-validation of the results and a better understanding of the two wake redirection techniques. Results indicate that yaw misalignment is more effective than cyclic pitch control in displacing the wake laterally, although the latter may have positive effects on wake recovery.

Time-Dependent Aeroelastic Adjoint-Based Aerodynamic Shape Optimization of Helicopter Rotors in Forward Flight

A formulation for sensitivity analysis of fully coupled time-dependent aeroelastic rotors in forward flight with trim constraints is given in this paper. The formulation includes the effect of blade shape parameters as well as blade cyclic pitch control parameters to enable analysis and optimization of rotors in a trimmed condition. Both forward sensitivity and adjoint sensitivity formulations are derived that correspond to analogs of the nonlinear aeroelastic analysis problem. Both sensitivity analysis formulations make use of the same iterative disciplinary solution techniques used for analysis, and they make use of an analogous coupling strategy. The fully coupled aeroelastic analysis formulation is first verified to effectively perform performance predictions of a four-bladed rotor in forward flight started impulsively from rest. Upon successful verification of the fully coupled adjoint formulation, it is used to perform trim and aerodynamic shape design optimization for helicopter rotors in forward-flight conditions.

Fundamental study on aerodynamic force of floating offshore wind turbine with cyclic pitch mechanism

Wind turbines mounted on floating platforms are subjected to completely different and soft foundation properties, rather than onshore wind turbines. Due to the flexibility of their mooring systems, floating offshore wind turbines are susceptible to large oscillations such as aerodynamic force of the wind and hydrodynamic force of the wave, which may compromise their performance and structural stability. This paper focuses on the evaluation of aerodynamic forces depending on suppressing undesired turbine's motion by a rotor thrust control which is controlled by pitch changes with wind tunnel experiments. In this research, the aerodynamic forces of wind turbine are tested at two kinds of pitch control system: steady pitch control and cyclic pitch control. The rotational speed of rotor is controlled by a variable speed generator, which can be measured by the power coefficient. Moment and force acts on model wind turbine are examined by a six-component balance. From cyclic pitch testing, the direction and magnitude of moment can be arbitrarily controlled by cyclic pitch control. Moreover, the fluctuations of thrust coefficient can be controlled by collective pitch control. The results of this analysis will help resolve the fundamental design of suppressing undesired turbine's motion by cyclic pitch control.

Relevance of aerodynamic modelling for load reduction control strategies of two-bladed wind turbines

A new load reduction concept is being developed for the two-bladed prototype of the Skywind 3.5MW wind turbine. Due to transport and installation advantages both offshore and in complex terrain two-bladed turbine designs are potentially more cost-effective than comparable three-bladed configurations. A disadvantage of two-bladed wind turbines is the increased fatigue loading, which is a result of asymmetrically distributed rotor forces. The innovative load reduction concept of the Skywind prototype consists of a combination of cyclic pitch control and tumbling rotor kinematics to mitigate periodic structural loading. Aerodynamic design tools must be able to model correctly the advanced dynamics of the rotor. In this paper the impact of the aerodynamic modelling approach is investigated for critical operational modes of a two-bladed wind turbine. Using a lifting line free wake vortex code (FVM) the physical limitations of the classical blade element momentum theory (BEM) can be evaluated. During regular operation vertical shear and yawed inflow are the main contributors to periodic blade load asymmetry. It is shown that the near wake interaction of the blades under such conditions is not fully captured by the correction models of BEM approach. The differing prediction of local induction causes a high fatigue load uncertainty especially for two-bladed turbines. The implementation of both cyclic pitch control and a tumbling rotor can mitigate the fatigue loading by increasing the aerodynamic and structural damping. The influence of the time and space variant vorticity distribution in the near wake is evaluated in detail for different cyclic pitch control functions and tumble dynamics respectively. It is demonstrated that dynamic inflow as well as wake blade interaction have a significant impact on the calculated blade forces and need to be accounted for by the aerodynamic modelling approach.Aeroelastic simulations are carried out using the high fidelity multi body simulation software SIMPACK. The aerodynamic loads are calculated using ECN's AeroModule and NREL's BEM code Aerodynl3.

Cyclic pitch control for the reduction of ultimate loads on wind turbines

In this paper we study the use of individual blade pitch control as a way to reduce ultimate loads. This load alleviation strategy exploits the fact that cyclic pitching of the blades induces in general a reduction of the average loading of a wind turbine, at least for some components as the main bearing, the yaw bearing, or the tower.When ultimate loads are generated during shutdowns, the effect of the use of cyclic pitch results in reduced peak loads. In fact, as the machine starts from a less stressed condition, the response to an extreme gust or other event will result in reduced loading on its components. This form of load mitigation can be seen as a preventative load mitigation strategy: the effect on load reduction is obtained without the need to detect and react to an extreme event, but by simply unloading the machine so that, in case an extreme event happens, the result will be less severe.The effect of peak load mitigation by preventative cyclic pitch is investigated with reference to a multi-MW wind turbine, by using high-fidelity aeroelastic simulations in a variety of operating conditions.

Investigation of rotor control system loads

This paper concentrates on the aeroelasticity analysis of rotor blade and rotor control systems. A new multi-body dynamics model is established to predict both rotor pitch link loads and swashplate servo loads. Two helicopter rotors of UH-60A and SA349/2, both operating in two critical flight conditions, high-speed flight and high-thrust flight, are studied. The analysis shows good agreements with the flight test data and the calculation results using CAMRAD II. The mechanisms of rotor control loads are then analyzed in details based on the present predictions and the flight test data. In high-speed conditions, the pitch link loads are dominated by the integral of blade pitching moments, which are generated by cyclic pitch control. In high-thrust conditions, the positive pitching loads in the advancing side are caused by high collective pitch angle, and dynamic stall in the retreating side excites high-frequency responses. The swashplate servo loads are predominated by the rotor pitch link loads, and the inertia of the swashplate has significant effects on high-frequency harmonics of the servo loads.