The Path Towards the Future "Smart Wind Turbine" Design.

Proceedings of Renewable Energy 2030 - Expert's Visions, Oldenburg, Germany October 1, 2012
Authors: G. Pechlivanoglou

Wind turbines are being constantly developed with a rapid increase in size and capacity. The continuous upscaling however leads to significant design, manufacturing, transportation and operation challenges. Someof the largest and most heavily loaded components of wind turbines such as the blades are at the limits of their material and design properties. Current large wind turbine blades, bearings and other heavily loaded components suffer from extreme and fatigue loads. Meanwhile wind turbine control systems have reached a limited range of load and power management, therefore new concepts need to be developed. Active flow control (AFC) solutions are consequently becoming very attractive. Such concepts can offer accurate and fastaerodynamic response and therefore achieve a significant load reduction. The latest developments and theresearch efforts of the authors in this field described in this paper as well as the performance of some of theinvestigated solutions. Some of the critical aspects of AFC concepts are also outlined.

Active stall control solutions for power regulation and load alleviation of large wind turbines.

Conference on Modelling Fluid Flow (CMFF’12) The 15 th International Conference on Fluid Flow Technologies Budapest, Hungary September 4, 2012
Authors: G. Pechlivanoglou, C. N. Nayeri, C. O. Paschereit

Large wind turbine blades suffer from the effects of high fluctuating aerodynamic loads, which lead to extreme load and power peaks. The existing blade pitch systems are required to operate reliably for several million cycles through the lifetime of the turbine. At the same time they are expected to be fast enough to adapt the blade positions to the current wind regimes. The pitch rates, however, are also limited by the structural integrity and torsional stiffness of the blades thus creating a very complex system control problem. The current paper proposes the use of active stall control elements on the blades forload alleviation and partial power regulation.Vertical and inclined spoilers as well as inflatable stall ribs are parametrically investigated in the wind tunneland tested on virtual blade simulations.

Vortex Generators for Wind Turbine Blades: A Combined Wind Tunnel and Wind Turbine Parametric Study.

ASME IGTI Turbo Expo Proceedings June 2012
Authors: H. M. Vahl, G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

Vortex generators (VGs) are passive flow control device that are commonly employed to prevent flow separation on wind turbine blades. They mitigate the damaging fatigue loads resulting from stall while increasing lift and consequently lead to rotor torque increase. This work summarizes a comprehensive research project aimed at optimizing the sectional as well as the full rotor-blade aerodynamics using VGs.The effects of chordwise position, spanwise spacing and VG size were studied with force balance measurements of a 2D wing section.  Particle Image Velocimetry measurements were conducted at various chordwise positions to provide insight into the interaction between adjacent streamwise vortices. The experimental aerodynamic performance curves of the optimal VG configuration were used to project their effect on wind turbine blade aerodynamics. 

Performance Optimization of Wind Turbine Rotots with Active Flow Control. Part 2: Active Aeroelastic simulations.

ASME IGTI Turbo Expo Proceedings June 2012
Authors: G. Weinzierl, George Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

This paper presents the continuation of the research efforts of the authors in the direction of the development of ”smart blades” for the wind turbines of the future. Results from previous research work is further in combination with a newly developed simulation code in order to assess the performance of Active Flow Control (AFC) elements implemented on modern wind turbine blade structures. Parametric investigations have been conducted in order to identify the optimal configuration of various AFC elements. These are tested under identical bound-ary conditions to define an overall optimal solution. The results of the research project show that the Active Gurney Flap is the element with a highest probability for the fastest implementation on wind turbine blades for load alleviation purposes. The most promising however overall solution is the Flexible Trailing Edge Flap. With its high control authority and relatively high regulation speed is able to significantly vary the aerodynamic performance of wind turbines.

Performance optimization of Wind Turbine Rotors with Active Flow Control.

Conference Proceedings ASME IGTI Turbo Expo 2011 June 2011
Authors: G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

The paper presents the methodical investigation and evaluation of various AFC solutions by means of extensive literature research and several numerical simulations. The best performing AFC solutions are studied and evaluated in a second step experimentally with constant chord wind tunnel wing sections under steady and unsteady conditions. After these two-dimensional investigations, more realistic configurations are studied in a third phase. This phase includes the integration of the investigated AFC solutions in two different custom wind turbine blade design proposals.

Single and Multi-Element Airfoil Performance Simulation Study and Wind Tunnel Validation.

EUROMECH 2012 - Springer April 2, 2011
Authors:  O. Eisele, G. Pechlivanoglou

The focus of the present investigation is to evaluate the capability of modern state of the art codes to simulate the aerodynamic behavior of typical wind turbine airfoils. Two airfoils were selected. The AH93-W-174 [1] with a thickness of 17.4% as a representative for relatively thin airfoils in the outer region and the relatively thick DU 97-W-300 [8] with a thickness of 30% as a representative for rather thick airfoils in the inner region of wind turbine blades. The third test case was a multi-element configuration composed of the DU-97-W-300 as the main airfoil and a slat based on the NACA 22 airfoil. For all configurations aerodynamic simulations with different levels of complexity were accomplished. First simulations were carried out with the panel code XFOIL and the Euler code MSES. Furthermore steady state RANS computations were conducted by the use of the open source code OpenFOAM.

Experimental Investigation of Dynamic Load Control Strategies using Active Microflaps on Wind Turbine Blades.

EWEA 2011 Scientific Proceedings March 17, 2011
Authors: O. Eisele, G. Pechlivanoglou, Christian Navid Nayeri, Christial Oliver Paschereit

A control system consisting of a force sensor, a controller and the microflap as an actuator was designed and tested in the wind tunnel. For this purpose a test wing with constant cross section, based on the dedicated wind turbine airfoil AH 93-W-174 was equipped with a trailing edge microflap with a flap-chord of 1.6%c.Measurements were accomplished at the large wind tunnel of the Herman Föttinger Institute (HFI) of the TU-Berlin. Wind gusts were simulated by varying the angle of attack of the airfoil model with a maximum angular velocity of 2.2°/s. The microflap could be deflected simultaneously with a deflection speed of approximately 300°/s.

Integration of a WT Blade Design Tool in XFOIL/XFLR5.

Conference Proceedings DEWEK 2010 November 2010
Authors: D. Marten, G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

A blade element momentum code (BEM) has been integrated into the graphical user interface (GUI) XFLR5 of the panel code XFOIL. The resulting software, QBlade, features various correction algorithms, such as the Prandtl tip and hub loss correction, Shens new tip and hub loss correction and Snels correction for 3D crossflow effects. An additional module for the extrapolation of polars to a range of 360° is included in the new software. Furthermore various algorithms, to optimize chord and twist distribution are provided to assist the blade design process. For a validation a simulation has been compared to the results of the software WT_Perf and to experimental data.

Flow Control Using Plasma Actuators at the Root Region of Wind Turbine Blades.

Conference Proceedings DEWEK 2010 November 2010
Authors: O. Eisele, G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

The near root region of a wind turbine blade significantly influences the overall performance of the blade. Because of limitations in the mechanical design the aerodynamic of this blade region strongly suffers. Thick profiles lead to early separation. In the separated region the air is exposed to high centrifugal forces, which are much higher than inertia forces. Thus separation propagates span-wise from the near root region of the blade to the midspan parts. Hence also sections of the blades far away from the root are affected by this cross flow. This paper investigates the feasibility of flow control using dielectric barrier discharge plasma actuators at the near hub region of wind turbine blades. Thereby the root region of the blade was simulated by a circular cylinder model made of a PVC-tube and a thick cambered airfoil model (DU97W300) made of glass fibre reinforced plastic. Both models were equipped with plasma actuators at different positions in order to realize a high frequency unsteady momentum injection to the boundary layer flow. Thus Tollmien-Schlichting as well as Kelvin-Helmholtz instabilities could be triggered dependent on the position of momentum injection. Force mea- surements and flow visualizations using the smoke wire method were accomplished to capture the effect of the momentum injection to the flow around the models. In case of the circular cylinder model stream-wise, radial and counter stream-wise momentum injection for different azimuthal actuator positions was considered. Further more the results were compared with measured data resulting from measurements with the same circular cylinder equipped with a laminar-turbulent transition tripping element. The cylinder measurements were conducted at low Reynolds numbers in the order of 10 5 . Significant drag reduction and lift increase was ob- served for actuator positions towards the natural separation point for all directions of unsteady momentum injection. The visualization results show a significant narrowing of the cylinder wake, when the flow is controlled by the actuator. Force measurements with the DU97W300 airfoil model were accomplished with two different actuator configurations. The first configuration was equipped with one unsteady stream-wise blowing actuator on the suction side at 10% chord length. In a second measurement an ad- ditional actuator was mounted on the pressure side at 20% chord length. The results were compared with the results of baseline measurements. Drag reduction and lift enhancement was observed at angles of attack higher than the angle of stall which was identified during the base- line measurements. Thus the shear layer after the separation point is directly manipulated by the actuator and Kelvin-Helmholtz instabilities are subsequently triggered. The flow visualiza- tion results exhibit a significant delay of separation in case of a controlled flow. Measurements at higher Reynolds numbers however showed dramatic decrease and eventual absence of the plasma actuator control authority to the flow.

Fixed Leading Edge Auxiliary Wing as a Performance Increasing Device for HAWT Blades.

Conference Proceedings DEWEK 2010 November 2010
Authors: G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

Current composite wind turbine blades are produced as two halves with large single piece moulds. In order for the two halves of the blades to connect and form the actual blade, the moulds need to precisely ''meet'' each other and thus form the closed blade structure. Due to the use of this blade manufacturing process, how- ever, it is not possible to create highly twisted blades. Most of the blades therefore have a limited root twist in the range of 8° - 11°. This is far less than the optimal value, therefore the root section of commercial wind turbine blades operates at high angles of attack (often in the post stall regime). The rotation of the wind turbine blade induces three dimensional flow pheno- mena at the blade root region. When the blade root operates at high AoA and stalls, the 3D root flow transports the separated boundaries from the root towards the\n middle region of the blade. Such a phenomenon has adverse effect on the efficiency and performance of the wind turbine blade. Until now this phenomenon was considered as an unavoidable property from the wind turbine blade de- signers. The solutions to encounter this phenomenon were the design of airfoils with desirable stall and post stall characte- ristics and the intensive implementation of vortex generators as a means of stall delay. The proposed solution in this paper is the implementation of a fixed auxiliary leading edge airfoil which can prevent separation at the root section of the blade. Such an implementation would re- duce/eliminate the local flow separation and consequently increase the blade performance. The performance of the blade sections near the root would also benefit from the additional lift produced by the auxiliary airfoil. In order to investigate parametrically the implementation of an auxiliary airfoil to a wind turbine blade root, a precise wind tunnel constant chord test wing was machined based on the DU97W300 airfoil. A full span auxiliary wing was also pre- cisely machined, based on the NACA 22 fixed slat airfoil. Special side plates were fitted to the test wing, which allowed the variation of angle and position of the auxiliary wing. Parametric wind tunnel investigations were performed at the large wind tunnel facilities of the Hermann- Föttinger Institute of the TU-Berlin. The wind tunnel results were corrected for solid blockage and wake blockage and the baseline measurements with the single DU97W300 wing were compared to previous wind tunnel measurements found in the literature. The wind tunnel test results were also compared with steady state CFD inves- tigations using the OpenFOAM CFD toolbox with the simpleFOAM solver. Computations with both Spalart Allmaras and k-omega SST turbulence models were performed in order to investigate the turbulence model sensitivity of the solu- tions. A grid sensitivity analysis was also performed in order to assure grid- insensitive flow simulation results. Based on the wind tunnel and CFD data, a ``baseline'' wind turbine blade design was modified in order to include the root section with the auxiliary wing. Both the baseline and the modified blades were im- ported in a custom Blade Element Momentum Theory (BEM) code where the blade and turbine performance was simu- lated. The BEM simulation showed in a quantitative way the benefits of the auxiliary wing implementation.


The Effect of Distributed Roughness on the Power Performance of Wind Turbines.

ASME Conference Proceedings (ISBN 978-0-7918-3872-3) June 18, 2010
Authors: G. Pechlivanoglou, S. Fuehr, C.N. Nayeri, C.O. Paschereit

It is well known from literature [1,2,3] that distributed roughness can have adverse effects on an airfoils' power performance. In spite of this knowledge nowadays engineering still suffers from setbacks due to roughness, especially in the field of wind energy.
Thereof the current paper tries to deal with specific roughness related aspects of wind turbine aerodynamics to be taken into account. The phenomena created by distributed roughness originating from manufacturing processes or accumulated during operation (roughness from contamination and erosion) are briefly explained. A more detailed analysis on known related investigations presents the actual state of the art in the field of aeronautics and wind energy. Wind tunnel roughness experiments and numerical investigations I on typical wind turbine airfoils as well as aviation airfoils (performed for various Reynolds Number values) are briefly described and thoroughly compared to wind turbine power measurements. The performance data, collected from a wind turbine in the field in combination with photographic material from blade inspections, give evidence regarding the roughness patterns in different span and chord-wise blade locations. Based on these measurements, different cases of theoretical performance models are derived in order to identify the relation between the early boundary layer transition and flow separation due to roughness with the energy production of the wind turbine. The comparison of the field measurements with the existing literature shows significant discrepancies between the estimated and the wind turbine actual performance. These discrepancies affect the characteristic power curve of the wind turbine and consequently its energy yield. General recommendations and trends for airfoil design and simulation are summarised in order to improve the reliability of aerodynamic wind turbine design in respect to the adverse effects of distributed roughness. Finally, some basic improvements for blade manufacturing and design processes are suggested.


Active aerodynamic control of wind turbine blades with high deflection flexible flaps

AIAA Conference Proceedings January 7, 2010
Authors: G. Pechlivanoglou, J. Wagner, C. N. Nayeri, C.O. Paschereit

The implementation of an innovative aerodynamic control technique in wind turbines is
a point under extensive investigation since the conventional wind turbine blade technology
is reaching its limits. Almost all the effort of the wind turbine industry in the field of
aerodynamics is related to the development of blades which offer better performance,
increased reliability and faster control of larger wind turbines. Currently, however, most of
the research effort is focusing on the implementation of aerodynamic elements for dynamic
load alleviation during wind turbine operation rather than rotor stall control or even more
the complete wind turbine power regulation which is the ultimate target of the current
project. The current document presents the test process, methodology and results of
wind tunnel test campaigns on the investigation of the flexible flap configuration as a
possible means of aerodynamic control of wind turbines. The test campaign took place
at the HFI/TU Berlin wind tunnel. Measurements were performed with a model of the
DU96W180 airfoil as well as with the modified-DU96W180 test airfoil section equipped with
the flexible flap assembly in flow with Reynolds number Re equal to 1,300,000. The The
flexible flap was tested in various positive and negative deflections in order to extract its
complete operational curve. The results showed significant influence on both lift and drag
as well as strong variations on the pitch behavior of the wing. The paper also discusses the
possible benefits of the integration of flexible flap systems in wind turbine blade structures.