Active Trailing-edge Flap Research Articles

Effects of a Near Wake model on the performance of a Multi-Megawatt wind turbine with an active flap

The impact of including the near wake model in the aeroelastic models of an SWT-DD-120 wind turbine equipped with an active trailing edge flap system (ATEF) is studied. The ATEF installed on this turbine was tested in full scale in a series of campaigns between 2018 and 2022. Since standard BEM methods cannot accurately capture the induced velocities near the transition areas of the start and end of flap add-ons due to the strong interaction of the local trailed vorticity with the neighboring blade sections, this study aims to investigate if and how the near wake model is relevant in the aerodynamic and aeroelastic characterization of rotor blades with active flaps. It is shown that the Near Wake model, when combined with an active trailing edge flap system, has a marginal impact on the blades’ thrust-related loads, independently from the presence, actuation state, and actuation frequency of the flap. The impact on the torque-related loads is also marginal at the blade root, while it becomes more relevant in the blade area where the flaps are deployed.

Validation of aeroelastic dynamic model of active trailing edge flap system tested on a 4.3 MW wind turbine

Validation of aeroelastic dynamic model of active trailing edge flap system tested on a 4.3 MW wind turbine

Active trailing edge flap system fault detection via machine learning

Abstract. Active trailing edge flap (AFlap) systems have shown promising results in reducing wind turbine (WT) loads. The design of WTs relying on AFlap load reduction requires implementing systems to detect, monitor, and quantify any potential fault or performance degradation of the flap system to avoid jeopardizing the wind turbine's safety and performance. Currently, flap fault detection or monitoring systems are yet to be developed. This paper presents two approaches based on machine learning to diagnose the health state of an AFlap system. Both approaches rely only on the sensors commonly available on commercial WTs, avoiding the need and the cost of additional measurement systems. The first approach combines manual feature engineering with a random forest classifier. The second approach relies on random convolutional kernels to create the feature vectors. The study shows that the first method is reliable in classifying all the investigated combinations of AFlap health states in the case of asymmetrical flap faults not only when the WT operates in normal power production but also before startup. Instead, the second method can identify some of the AFlap health states for both asymmetrical and symmetrical faults when the WT is in normal power production. These results contribute to developing the systems for detecting and monitoring active flap faults, which are paramount for the safe and reliable integration of active flap technology in future wind turbine design.

Optimal Sizing of Trailing Edge Flaps for Helicopter Vibration Reduction: A Response Surface Approach

On-blade active trailing edge flaps are considered for helicopter vibration reduction. A Pareto optimal multi-objective optimization approach is used to obtain the spanwise and chordwise dimensions of single and dual trailing- edge flaps. The objective of this study is to simultaneously achieve minimum hub vibration levels and minimize the flap actuation power requirement. Helicopter aeroelastic analysis is used in conjunction with an optimal controller to minimize hub vibration and flap power levels. It is found that nonlinear polynomial response surfaces based on L9 orthogonal array could adequately describe both the objectives. An intense sampling of the surrogate objective space is used to obtain optimal design points for the mutually conflicting objectives. A Pareto optimal design reduces vibration levels by about 70% and also reduces flap actuation power requirement by about 20%, in comparison to the initial design. The present study leads to an optimal design of trailing edge flaps which expend less power and can be further explored for self-powered control surfaces towards a self-powered smart blade design concept. The optimal designs are robust to uncertainty introduced by manufacturing.

Closed-Loop Control of Transonic Buffet Using Active Shock Control Bump

At transonic flight conditions, the buffet caused by the shockwave/boundary-layer interaction can degrade aircraft performance and even threaten their safety. In this paper, a closed-loop control using an active shock control bump (SCB) has been proposed to suppress the buffet on a supercritical airfoil flying at transonic speeds. A closed-loop control law is designed by using the lift coefficient as the feedback signal and using the bump height as the control variable. The unsteady numerical simulations show that the buffet can be effectively suppressed by an optimal combination of the parameters of the control law, namely the gain and the delay time. Furthermore, the buffet control effectiveness is still acceptably constrained by a prescribed maximum bump height, which is believed to be practically important. In addition to being able to achieve both wave drag reduction and buffet alleviation, the active SCB is less sensitive to the parameters of the control law and has a shorter response time in comparison with the reference active trailing edge flap.

Aeroelastic model validation of an Active Trailing Edge Flap System tested on a 4.3 MW wind turbine

Active Trailing Edge Flap Systems (ATEFS) have shown promising results on reducing Wind Turbine fatigue and ultimate loads and increasing the Annual Energy Production. However, the current lack of field validation creates uncertainty in the fidelity level and accuracy of the engineering models used for the aeroelastic modeling of active flaps, and these systems actual load reduction capabilities are in question. This article describes the validation based on field data of the aeroelastic engineering models of the ATEFS developed for the BEM-based solvers HAWC2 and BHawC for the case of stationary activation of the flap. The validation is based on the field test data from a 4.3 MW Wind Turbine with one blade equipped with an ATEFS and operating in normal power production. With simulation results differing from the measurement of a max of ±3% on mean power, ±0.2 deg on mean pitch angle, ±0.1 rpm on rotor speed, and ±2% on flapwise blade loads, the study showed the HAWC2 and BHAWC aeroelastic ATEFS models provide a reliable and precise estimation of the impact of the ATEFS on the Wind Turbine for the case of stationary activation of the flap.

Repetitive model predictive control for load alleviation on a research wind turbine using trailing edge flaps

SummaryThis paper presents the results of an advanced control strategy that employs active trailing edge flaps to reduce the fatigue loads of an experimental wind turbine. The strategy, called repetitive model predictive control, is a multiple‐input multiple‐output controller that aims at the alleviation of out‐of‐plane blade root bending moments. The strategy incorporates the control commands, output errors, and state deviation from the previous rotation. This way, the time lag in the strain sensor input due to the blade inertia is compensated. Additionally, a strategy to limit the computational costs is presented. The load alleviation performance is evaluated at different yaw cases and compared with different individual flap control strategies. The repetitive model predictive control is able to reduce the fatigue loads by up to 23% compared with the better performing individual flap control strategy. This improvement in load reduction is accompanied by an increase in flap travel of up to 7% compared with the individual flap control strategies.

Modeling and Control of Dynamic Stall Loads on a Smart Airfoil at Low Reynolds Number

This article describes the development and testing of a modified, semi-empirical ONERA dynamic stall model for an airfoil with a trailing edge flap—a “smart airfoil”—pitching at reduced frequencies up to 0.1. The Reynolds number is 105. The model reconstructs the load fluctuations associated with the shedding of multiple dynamic stall vortices (DSVs) in a time-marching solution, which makes it suitable for real-time control of a trailing edge flap (TEF). No other model captures the effect of the DSVs on the aerodynamic loads on smart airfoils. The model was refined and tuned for force measurements on a smart NACA 643-618 airfoil model that was pitching with an inactive TEF and was validated against the measurements when the TEF was activated. A substantial laminar separation bubble can develop on this airfoil, which is challenging for modelers of the unsteady response. A closed-loop controller was designed offline in SIMULINK, and the output of the controller was applied to the TEF in a wind tunnel. The results indicated that the model has a comparable accuracy for predicting loads with the active TEF compared to inactive TEF loads. In the fully separated flow regime, the controller performed worse when dealing with the development of the laminar separation bubble and DSVs.

Active flap control with the trailing edge flap hinge moment as a sensor: using it to estimate local blade inflow conditions and to reduce extreme blade loads and deflections

Abstract. Active trailing edge flaps are a promising technology that can potentially enable further increases in wind turbine sizes without the disproportionate increase in loads, thus reducing the cost of wind energy even further. Extreme loads and critical deflections of the blade are design-driving issues that can effectively be reduced by flaps. In this paper, we consider the flap hinge moment as a local input sensor for a simple flap controller that reduces extreme loads and critical deflections of the DTU 10 MW Reference Wind Turbine blade. We present a model to calculate the unsteady flap hinge moment that can be used in aeroelastic simulations in the time domain. This model is used to develop an observer that estimates the local angle of attack and relative wind velocity of a blade section based on local sensor information including the flap hinge moment of the blade section. For steady wind conditions that include yawed inflow and wind shear, the observer is able to estimate the local inflow conditions with errors in the mean angle of attack below 0.2∘ and mean relative wind speed errors below 0.4 %. For fully turbulent wind conditions, the observer is able to estimate the low-frequency content of the local angle of attack and relative velocity even when it is lacking information on the incoming turbulent wind. We include this observer as part of a simple flap controller to reduce extreme loads and critical deflections of the blade. The flap controller's performance is tested in load simulations of the reference turbine with active flaps according to the IEC 61400-1 power production with extreme turbulence group. We used the lifting line free vortex wake method to calculate the aerodynamic loads. Results show a reduction of the maximum out-of-plane and resulting blade root bending moments of 8 % and 7.6 %, respectively, when compared to a baseline case without flaps. The critical blade tip deflection is reduced by 7.1 %. Furthermore, a sector load analysis considering extreme loading in all load directions shows a reduction of the extreme resulting bending moment in an angular region covering 30∘ around the positive out-of-plane blade root bending moment. Further analysis reveals that a fast reaction time of the flap system proves to be critical for its performance. This is achieved with the use of local sensors as input for the flap controller. A larger reduction potential of the system is identified but not reached mainly because of a combination of challenging controller objectives and the simple controller architecture.

Pressure-based lift estimation and its application to feedforward load control employing trailing-edge flaps

Abstract. This experimental load control study presents results of an active trailing-edge flap feedforward controller for wind turbine applications. The controller input is derived from pressure-based lift estimation methods that rely either on a quasi-steady method, based on a three-hole probe, or on an unsteady method that is based on three selected surface pressure ports. Furthermore, a standard feedback controller, based on force balance measurements, is compared to the feedforward control. A Clark-Y airfoil is employed for the wing that is equipped with a trailing-edge flap of x/c=30% chordwise extension. Inflow disturbances are created by a two-dimensional active grid. The Reynolds number is Re=290 000, and reduced frequencies of k=0.07 up to k=0.32 are analyzed. Within the first part of the paper, the lift estimation methods are compared. The surface-pressure-based method shows generally more accurate results, whereas the three-hole probe estimate overpredicts the lift amplitudes with increasing frequencies. Nonetheless, employing the latter as input to the feedforward controller is more promising as a beneficial phase lead is introduced by this method. A successful load alleviation was achieved up to reduced frequencies of k=0.192.

Aerodynamic Analysis of Active Trailing-Edge Flaps and Passive Surface Roughness on NACA 4412 Airfoil

The focus of this study is to understand the effect of the flow control mechanism on NACA 4412 airfoil. Two configuration of flow control method is used in this investigation. The first configuration is, airfoil with only the trailing-edge flap. This is the active flow control method. The trailing-edge flap can be deflected to various angle accordingly. The second configuration is a combination of, active trailing-edge flap with a passive surface roughness at the leading-edge of the airfoil. The surface roughness is expected to reduce the flow separation at the leading-edge of the airfoil. This is expected to enhance the aerodynamic performance of the airfoil. Both configuration 1 and configuration 2 was simulated and the results compared with the base airfoil without any flow control mechanism. Based on the results, airfoil with configuration 1 performed better compared to the base airfoil. Airfoil with configuration 2, performed well but not better than the airfoil with configuration 1. Upon further analysis, key patterns emphasise airfoil with configuration 2 could perform better if the roughness element parameters further optimised. This airfoil to be used for wind turbine applications.

Investigation of Linear Higher Harmonic Control Algorithm for Rotorcraft Vibration Reduction

Abstract A linear quadratic Gaussian (LQG) controller for active vibratory loads reduction in helicopters is proposed based on a revisited higher harmonic control (HHC) input by active trailing-edge flaps (ATEFs). Conventional individual blade control (IBC) input is redefined using N − 1/rev interblade phase lead, N/rev collective, and N + 1/rev interblade phase lag signals where 1/rev frequency modulation originates from the multiblade coordinate (MBC) transform. A Mach-scaled flap blade is designed and analyzed by the multibody dynamics analysis DYMORE. A linear time-invariant representation is identified from N/rev envelopes of the input and output responses obtained by DYMORE analysis. A matlab/simulink closed-loop control simulation is designed using the identified state-space realization. The N/rev vibratory loads are reduced up to 52% with flap deflections and the linear control results match well with the nonlinear responses obtained from DYMORE. Furthermore, the multivariable closed-loop stability estimated by the loop transfer functions using disk margin analysis reveals sufficient gain and phase margins.

Distributed Aerodynamic Control using Active Trailing-Edge Flaps for Large Wind Turbines

This work presents a numerical framework to investigate distributed aerodynamic control devices for application in large wind turbines. Tool capabilities were extended to facilitate multiple aerodynamic polar tables. The airfoil aerodynamics characteristics were automatically determined, and blade-pitch, generator-torque, and trailing-edge-flap controllers were tuned in-the-loop according to a specific blade design. This automated workflow allows analysis and optimization of trailing-edge flaps, enabling codesign studies. Results targeted reductions of root-flap-bending moment derivatives. The applied trailing-edge-flap control reduced the standard deviation of root-flap-bending moments by more than 6% and benefit related parameters, e.g., reduce blade-tip deflections, by up to 8%. Because of varying thrust distributions along the blade span, different flap designs have nonlinear characteristics in terms of the control objective and show best performance when located at the radial position with maximum thrust. In general, larger flaps provide a greater influence to reduce the target control objective.

Sliding Mode Control of Active Trailing-Edge Flap Based on Adaptive Reaching Law and Minimum Parameter Learning of Neural Networks

Theoretical modeling and the sliding mode control (SMC) of an active trailing-edge flap of a wind turbine blade based on the adaptive reaching law are investigated. The blade is a single-celled thin-walled composite structure using circumferentially asymmetric stiffness (CAS) design, exhibiting displacements of flap-wise/twist coupling. A reduced structural model originated from the variation method is used to model the structure of the blade, the structural damping of which is computed. The trailing-edge flap is a rigid structure that is embedded in and hinged to the blade host structure, and it is driven by two pairs of pneumatic cylinders moving in reverse. Flutter suppression for the large-amplitude vibration of the blade tip is investigated based on an active trailing-edge flap structure and SMC algorithm using the adaptive reaching law. The controlled responses of flap-wise/twist displacements and control inputs (the angles of the trailing-edge flap) are illustrated, with obvious simulation effects demonstrated. An experimental platform for driving the pneumatic cylinders verifies the effectiveness of the control algorithm using the adaptive reaching law and the effectiveness of the selected pneumatic transmission scheme controlled by another adaptive SMC based on the minimum parameter learning of neural networks.

헬리콥터 진동 하중 저감을 위한 능동 뒷전 플랩이 장착된 SNUF 블레이드의 유연보의 설계 및 해석

본 논문에서는 헬리콥터의 전진비행시 발생하는 허브 진동 하중 저감을 위해 설계된 능동 뒷전 플랩이 장착된 SNUF 블레이드의 무베어링 주 로터 적용 설계에 대해 살펴보았다. 이를 위해 EDISON의 박벽 복합재료 회전보 진동해석 프로그램(CORBA77_MEMB)을 이용하여 유연보의 단면 설계가 이루어졌다. 다물체 동역학 해석 프로그램 DYMORE를 이용하여 단면 설계에 따른 블레이드 동특성 및 능동 뒷전 플랩을 이용한 하중 제어의 특성을 예측하였다.

Design improvements and flap deflection evaluations with considering centrifugal load on active trailing edge flap

Purpose The purpose of this paper is to present the design, analysis and experiments of the active trailing-edge flap (Seoul National University Flap, SNUF) for vibration reduction in the helicopter rotor prior to the small-scale blades planned to test in a whirl tower. Design/methodology/approach The predictions of the hinge moment in both steady and unsteady flows were obtained through computational fluid dynamics calculations. When compared with the results originated from analytical formulations, the proposed method showed improved prediction capabilities. To validate the deflection of the flap under the centrifugal load by rotating, static analysis was conducted using both contact and rotating condition of MSC NASTRAN. The corresponding experiment also was performed using the vertical frame for simulating the effect of the centrifugal force. Findings The hinge moment of the flap is predicted through unsteady analysis in the actuation frequency of 3/rev. The material of the guide in the flap mechanism was selected through static analysis under both contact and rotating condition. Finally, reduction of the deflection occurred because of the load in the axial direction of the hinge like the centrifugal load. Practical implications The important aspects, such as design, analysis, and experiments for the active trailing-edge flap were shown. Originality/value This paper showed the relationship of the displacement, block force and voltage of the piezo-actuator, combined with the hinge moment predicted. The methodology and the experiment were presented for simulating the centrifugal force acting on the flap.

Design and Simulation of Active External Trailing-Edge Flaps for Wind Turbine Blades on Load Reduction

AbstractA smart material–based active external trailing-edge flap was designed for wind turbine blades in this study. Its vibration-fatigue load reduction potential was evaluated during normal oper...

Power performance optimization and loads alleviation with active flaps using individual flap control

The present article investigates the potential of Active Trailing Edge Flaps (ATEF) in terms of increase in annual energy production (AEP) as well as reduction of fatigue loads. The basis for this study is the DTU 10 MW Reference Wind Turbine (RWT) simulated using the aeroelastic code HAWC2. In an industrial-oriented manner the baseline rotor is upscaled by 5% and the ATEFs are implemented in the outer 30% of the blades. The flap system is kept simple and robust with a single flap section and control with wind speed, rotor azimuth, root bending moments and angle of attack in flap's mid-section being the sensor inputs. The AEP is increased due to the upscaling but also further due to the flap system while the fatigue loads in components of interest (blade, tower, nacelle and main bearing) are reduced close to the level of the original turbine. The aim of this study is to demonstrate a simple and applicable method that can be a technology enabler for rotor upscaling and lowering cost of energy.

Extreme load alleviation using industrial implementation of active trailing edge flaps in a full design load basis

The application of active trailing edge flaps in an industrial oriented implementation is evaluated in terms of capability of alleviating design extreme loads. A flap system with basic control functionality is implemented and tested in a realistic full Design Load Basis (DLB) for the DTU 10MW Reference Wind Turbine (RWT) model and for an upscaled rotor version in DTU's aeroelastic code HAWC2. The flap system implementation shows considerable potential in reducing extreme loads in components of interest including the blades, main bearing and tower top, with no influence on fatigue loads and power performance. In addition, an individual flap controller for fatigue load reduction in above rated power conditions is also implemented and integrated in the general controller architecture. The system is shown to be a technology enabler for rotor upscaling, by combining extreme and fatigue load reduction.

Design and optimisation of an aerofoil with active continuous trailing-edge flap

ABSTRACTThis paper presents the design and optimisation of an aerofoil with active continuous trailing-edge flap (CTEF) investigated as a potential rotorcraft active control device. Several structural cross-section models are developed: high-fidelity NASA STRucture ANalysis (NASTRAN) and University of Michigan/Variational Asymptotic Beam Section Code (UM/VABS) models and a reduced-order analysis model. The validation of the reduced-order model is established by comparing its predictions of CTEF deformations with those of NASTRAN and UM/VABS analyses, which both show good agreement. The 2D aerodynamic characteristics of the CTEF aerofoil are evaluated using XFOIL and Computational Fluid Dynamics (CFD) analyses: FUN3D and Overset Transonic Unsteady Rotor Navier-Stokes (OVERTURNS). XFOIL, coupled with the reduced-order structure model, is adopted for optimisation study. The accuracy of XFOIL in predicting the aerodynamic pressure of the CTEF aerofoil is verified using CFD simulations, which shows sufficient fidelity. The predicted variations of aerodynamic coefficients with a CTEF angle are compared among the aerodynamic analyses. The optimisation process is developed and applied to two bimorph bender configurations: a Macro-Fibre Composite (MFC) solid bender and an MFC stack bender. The solid bender is used to confirm the functioning of the optimisation procedure and to use its optimal layout as a reference to the stack design, the primary design object. A linear tapered shape is found to be the optimum for a MFC solid bender, which generates an average of 63% more CTEF angles than those of an optimal rectangular bender. An optimised MFC stack bender is shown to resemble the shape of the solid bender. A four-ply bimorph is considered the best choice among the stack layouts because of its large output of CTEF angles and relatively less plies required. The CTEF angle produced by the four-ply optimal layout ranges from 7.6° to 5.3° with speeds from 0 to 200m/s at an angle of attack (AoA) of 6°. The reduction in the CTEF angle with AoA is less steep than that with speed, ranging from 6.5° to 5.8° with AoA from 0 to 8° at speed of 166m/s. An average of 14% increase in CTEF angles is achieved through optimisation for the four-ply bimorph.