Lars Oliver Bernhammer

Aeroelastic Control Using Distributed Floating Flaps Activated by Piezoelectric Tabs

In this paper, a novel aeroservoelastic effector configuration that is actuated by piezoelectric tabs is presented. The effector exploits trailing-edge tabs installed on free-floating flaps (FFFs). These flaps are used to prevent flutter from occurring and to alleviate loads originating from external excitations such as gusts. A vertical tailplane wind-tunnel model with two free-floating rudders and a flutter control mechanism were designed, and the aeroelastic stability and response characteristics have been modeled numerically. The controller uses the tailplane tip acceleration as a sensor and sends control signals to the piezoelectrically actuated tabs. Wind-tunnel experiments were performed to demonstrate the feasibility of the technology. It was demonstrated experimentally that the flutter speed associated with the free rudders could be increased by 80%. The same controller, applied to the external rudder, was used to alleviate the aeroelastic response of the tailplane to the excitation of the other ...

How far is smart rotor research and what steps need to be taken to build a full- scale prototype

During the last decade research on the field of smart rotor has advanced significantly. Fundamental aerodynamics, structural and control concepts have been established and simulators created for distributed flaps on wind turbine blades, which are considered the most promising option. Also a proof of concept has been done under laboratory conditions. However, the results obtained under these conditions can only be partially transfer to the real application as the control authority of smart rotors is limited compared to full pitch control. The steps that need to be taken before smart rotors can be successfully exploited are in the design of reliable systems that can operate under environmental conditions without inspections. Besides that, other potential advantages of distributed control need to be established such as the effect on other components of a wind turbine for example the gear box or the power system. Finally, it is necessary to investigate what benefits can be achieved if blades are designed with distributed control right from the start instead of applying control schemes to already existing turbines.

Active flap control on an aeroelastic wind turbine airfoil in gust conditions using both a CFD and an engineering model

In the past year, smart rotor technology has been studied significantly as solution to the ever growing turbines. Aeroservoelastic tools are used to asses and predict the behavior of rotors using trailing edge devices like flaps. In this paper an unsteady aerodynamic model (Beddoes-Leishman type) and an CFD model (URANS) are used to analyze the aeroservoelastic response of a 2D three degree of freedom rigid body wind turbine airfoil with a deforming trailing edge flap encountering deterministic gusts. Both uncontrolled and controlled simulations are used to asses the differences between the two models for 2D aerservoelastic simulations. Results show an increase in the difference between models for the y component if the deforming trailing edge flap is used as control device. Observed flap deflections are significantly larger in the URANS model in certain cases, while the same controller is used. The pitch angle and moment shows large differences in the uncontrolled case, which become smaller, but remain significant when the controller is applied. Both models show similar reductions in vertical displacement, with a penalty of a significant increase in pitch angle deflections.

Model Validation and Simulated Fatigue Load Alleviation of SNL Smart Rotor Experiment.

In this paper an individual flap controller (IFC) design for smart rotors is presented and compared to the model identification of the Sandia National Labs Smart Rotor experiment. The controller design has been carried out using an in-house aeroservoelastic software - DU_SWAT. The root bending moment response due to a step input of the flap deflection has been linearized. Based on this linearization a Coleman transform combined with a proportionality controller has been used to eliminate the cyclic components in the root bending moment. It was shown that IFC can reduce the 1P mode similar to individual pitch control, while only modest flap deflections of below 5

Gust load alleviation of an unmanned aerial vehicle wing using variable camber

It is vital for an unmanned aerial vehicle to meet contradictory mission requirements originating from different tasks this type of aircraft has to fulfill. The ability to switch between configurations greatly expands the range of possible missions. An unmanned aerial vehicle wing has been developed to demonstrate the capacity to optimize the aerodynamic and structural performances according to the mission stage. The wing is equipped with four macro fiber composite benders that can be controlled individually, and each of these macro fiber composite benders actuates a section of the wing. A numerical study was conducted with XFLR5 to determine the optimal configurations of the flap positions for both range and endurance. A wind tunnel study was performed to verify these results. During the experiment, a maximum attainable increase in lift coefficient of 0.072 could be achieved, while numerically the increase was computed to be 0.079. The wide-frequency bandwidth of the actuators allows using the developed system also for other purposes such as load alleviation. Unmanned aerial vehicles are often light and fly at low airspeeds, which make them very sensitive to gust excitation. For this purpose, the experimental model was equipped with two accelerometers to measure the amplitude of the first two deformation modes. Significant load alleviation capacities with reductions up to 50% in load amplitude could be achieved. This reduction was achieved, even though the wing box contributes largely to the structural damping, as the foam for the construction absorbs a significant proportion of the vibrations.

Energy harvesting for actuators and sensors using free-floating flaps

A novel configuration of an energy harvester for local actuation and sensing devices using limit cycle oscillations has been modeled, designed and tested. A wing section has been designed with two trailing-edge free-floating flaps. A free-floating flap is a flap that can freely rotate around a hinge axis and is driven by trailing edge tabs. In the rotational axis of each flap a generator is mounted that converts the vibrational energy into electricity. It has been demonstrated numerically how a simple electronic system can be used to keep such a system at stable limit cycle oscillations by varying the resistance in the electric circuit. Additionally, it was shown that the stability of the system is coupled to the charge level of the battery, with increasing charge level leading to a less stable system. The system has been manufactured and tested in the Open Jet Wind Tunnel Facility of the Technical University Delft. The numerical results could be validated successfully and voltage generation could be demonstrated at cost of a decrease in lift of 2%.

Experimental investigation of an autonomous flap for load alleviation

This paper presents an experimental aeroservoelastic investigation of a novel load alleviation concept using trailing edge flaps. These flaps are autonomous units, which are self-powered and self-actuated, using trailing edge tabs, thereby demonstrating advantages in comparison with conventional flap systems in terms of wiring and structural integration. The flaps are free-floating and mass underbalanced, such that they may flutter at operation velocities unless suppressed by their own control system. This makes the system very responsive for turbulence and control action. In the wind tunnel campaign presented in this paper, the limit cycle behavior of autonomous, free-floating flaps was investigated. It has been shown that limit cycle oscillation can be reached either through structural limiters or by control actions of the trailing edge tabs. In the latter case, the amplitude of the limit cycle oscillation is adjustable to the required energy output. An energy balance of harvested power and power consumption for actuators and sensing system was made showing that the vibration energy of limit cycle oscillations can be used to keep the amplitude of the limit cycle constant, while the electric batteries that power the load alleviation system are being charged.

Structural Optimization of Multi-Megawatt, Offshore Vertical Axis Wind Turbine Rotors

This paper presents the case study of possible design improvements for 20 MW Vertical Axis Wind Turbine (VAWT) rotors. Structural optimizations of a 3-bladed carbon-fiber H-rotor and Darrieus rotor are performed for different rotor sizes and heights. The results are used to construct rotor mass scaling trends for VAWT rotors. Furthermore, critical failure modes and their driving loads are identified. To mitigate fatigue and buckling in the blade, a non-constant chord distribution is recommended. Furthermore, further research on improving the buckling performance of the H-rotor strut is recommended.

Wind Turbine Structural Model Using Non-Linear Modal Formulations

In this paper a new method to obtain a geometrically non-linear wind turbine structural model based on the full linear finite element model is presented. For this purpose, the wind turbine model is divided into multiple segments, i.e. tower, drive train and blades. For each segment a modal analysis is carried out. Boundary grid points are defined on each segment and loaded by ficticious masses. The modal analysis produces a set of 6 rigid-body modes and elastic modes close to fixed-fixed analysis. For the aeroelastic turbine simulation, the ficticious masses are removed. The elastic modes are used as master modes that describe the deformation, while the rigid-body modes are used as slaves to establish compatibility between the segments. A modal analysis is carried out in the local segment attached reference frame, yielding a local linear solution that is part of a global non-linear analysis. Large rotations and displacements are provided by rigid-body modes in a co-rotational framework.

Experimental and Numerical Investigation of an Autonomous Flap for Load Alleviation

This paper presents the design, a numerical aeroservoelastic investigation, and an experimental proof of concept of an autonomous flap system. Autonomous flaps are load-alleviation devices, intende...