Quantifying the effect of vortex generator installation on wind power production: An academia-industry case study.

Renewable Energy, Volume 113, 2017, Pages 1589-1597
Authors: Hoon Hwangbo, Yu Ding, Oliver Eisele, Guido Weinzierl, Ulrich Lang,
Georgios Pechlivanoglou

This paper presents an academia-industry joint study concerning effective methods to estimate and quantify the effect of vortex generator installation on wind power production. This problem has pre- sented a challenge to the wind industry, because (a) vortex generator installation may lead to a moderate 1e5% extra power production, but this level of improvement is difficult to be accurately detected; and (b) it is equally difficult to validate the estimated effect of vortex generator installation because a controlled experiment is practically impossible to conduct to provide a credible baseline. An academic institute and a wind technology company team up to tackle this challenge. The two teams develop their own version of quantification methods, which are profoundly different. The academic method uses 10-min data and makes use of both power and environmental data, whereas the company method uses high-frequency data via primarily a direct power comparison approach that relies less on the environmental data. When applying the respective methods to two inland wind farms, each of which presents four pairs of turbines, the quantification results from the two methods are surprisingly consistent. We believe the consistent outcome presents a strong case of cross validation, testifying to the respective method's capability and credibility.

Parametric Investigations of Gurney Flaps for the Use on Wind Turbine Blades.

Proceedings of ASME Turbo Expo 2017,June 26-30, 2017, Charlotte, NC USA
Authors: Joerg Alber, Georgios Pechlivanoglou, Christian Oliver Paschereit, Jochen Twele, Guido Weinzierl

This paper presents a modeling strategy of the aerodynamic Gurney flap effect on two-dimensional airfoils and, subsequently, on the rotor blade performance of horizontal axis wind turbines. The first part consists of the parametric investigation of 23 airfoil polar data-sets, derived from different, but comparable, wind tunnel experiments. They are evaluated and processed in terms of the lift and drag increase caused by Gurney flaps in comparison to each Baseline configuration. Consequently, a modeling strategy is developed, transforming Baseline- into Gurney flap polar data for varying flap-heights. The results of the emerging Gurney Flap Polar Calculator are successfully validated against the experimental lift and drag curves. In the second part, polar data-sets are generated for a wide range of Gurney flap-heights based on the blade design of the NREL 5 MW Reference Turbine, which are imported into the rotor simulation software Qblade. Thereupon, blade optimization strategies are examined regarding the two main Gurney flap applications on rotor blades: the retrofit and the design solution. The optimized retrofit solution on existing blades indicates power performance improvements, albeit at the expense of increasing structural loads. The optimized design solution on to-be-constructed blades, on the other hand, suggests chord-length reductions, while keeping the performance characteristics on a similar or even enhanced level. It is concluded that, in general, aerodynamic improvements are achieved by relatively small Gurney flap-heights, which are applied at specific blade positions. Guido Weinzierl SMART BLADE ® GmbH Berlin, Germany g.weinzierl@smart-blade.com

Backflow flaps as stall control elements on wind turbines.

Proceedings of International Symposium on Transport Phenomena and Dynamics of Rotating Machinery (ISROMAC) 2016
Authors: George Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

Wind turbine blades suffer from static and dynamic stall effects during their normal operation. At the same time strong secondary flows (cross-flows) exist at the inner and the outermost regions of the blade. Most of the modern wind turbine blades are equipped with flow control devices such as vortex generators (VGs) in an effort to avoid stall and stabilize their aerodynamic performance. The current invention and research work proposes a solution to the unsteady stall issues and at the same time offers a performance boosting potential. The solution comprises a passively actuated backflow flap positioned in span-wise or chord-wise orientation depending on the local flow characteristics. Wind tunnel tests in combination with extensive field preliminary work as a part of the VG development efforts of the authors form the ground work for a new and innovative aerodynamic solution for wind turbines.

Wind Turbine Waste Heat Recovery - A Short-Term Heat Loss Forecasting Approach.

Challenges 2015, 6(2), 188-201
Authors: George Xydis, George Pechlivanoglou, Navid Christian Nayeri

The transition from the era of massive renewable energy deployment to the era of cheaper energy needed has made scientists and developers more careful with respect to energy planning compared with a few years ago. The focus is—and will be—placed on retrofitting and on extracting the maximum amount of locally generated energy. The question is not only how much energy can be generated, but also what kind of energy and how it can be utilized efficiently. The waste heat coming from wind farms (WFs) when in operation—which until now was wasted—was thoroughly studied. A short-term forecasting methodology that can provide the operator with a better view of the expected heat losses is presented. The majority of mechanical (due to friction) and electro-thermal (i.e., generator) losses takes place at the nacelle while a smaller part of this thermal source is located near the foundation of the wind turbine (WT) where the power electronics and the transformers are usually located. That thermal load can be easily collected via a working fluid and then be transported to the nearest local community or nearby agricultural or small scale industrial units using the necessary piping.

Utility Scale Wind Turbine Yaw From a Flow Visualization View.

Proceedings of ASME Turbo Expo 2015, June 15 - 19, 2015, Montréal, Canada
Authors: Stefan Vey, Henning M. Lang, Christian N. Nayeri, Christian O. Paschereit, Georgios Pechlivanoglou, Guido Weinzierl

Results from a quantitative tuft flow visualization of a utility scale wind turbine undergoing a yaw movement are presented. Based on the turbine’s SCADA data suitable pre- and post-yaw timeframes were defined and the surface flowfields were anal- ysed. A distinct asymmetry between the surface flow patterns of the 90 ◦ and 270 ◦ azimutal blade positions was observed in the pre-yaw timeframe. After the turbine yawed back into the wind the symmetry was restored. Yaw-misalignment is a source of dy- namic loads which limit a turbine’s life time. The characteriza- tion of the involved flow structures and their dynamics is an es- sential step towards possible future load alleviation techniques. The quantitative tuft flow visualization technique is a measure- ment tool that can be used to assess the surface flow field.

Wake Analysis of a Finite Width Gurney Flap.

Proceedings ASME Turbo Expo 2015, Montreal, Quebec, Canada, June 15–19, 2015
Authors: D. Holst, A. B. Bach, C. N. Nayeri, C. O. Paschereit, G. Pechlivanoglou

The results of stereo Particle-Image-Velocimetry measurements are presented in this paper to gain further insight into the wake of a finite width Gurney flap. It is attached to an FX 63-137 airfoil which is known for a very good performance at low Reynolds numbers and is therefore used for small wind turbines and is most appropriate for tests in the low speed wind tunnel presented in this study. The Gurney flaps are a promising concept for load control on wind turbines but can have adverse side effects, e.g. shedding of additional vortices. The investigation focuses on frequencies and velocity distributions in the wake as well as on the structure of the induced tip vortices. Phase averaged velocity fields are derived of a Proper-Orthogonal-Decomposition based on the stereo PIV measurements. Additional hot-wire measurements were conducted to analyze the fluctuations downstream of the finite width Gurney flaps. Experiments indicate a general tip vortex structure that is independent from flap length but altered by the periodic shedding downstream of the flap.

The influence of Gurney flaps on a small wind turbine is investigated by simulating a small 40 kW turbine in Q-Blade. They can serve as power control without the need of an active pitch system and the starting performance is additionally improved. The application of Gurney flaps imply tonal frequencies in the wake of the blade. Simulation results are used to estimate the resulting frequencies. However, the solution of Gurney flaps is a good candidate for large scale wind turbine implementation as well. A FAST simulation of the NREL 5MW turbine is used to generate realistic time series of the lift. The estimations of control capabilities predict a reduction in the standard deviation of the lift of up to 65%. Therefore finite width Gurney flaps are promising to extend the lifetime of future wind turbines.

Finite micro-tab system for load control on a wind turbine.

Science of making torque from wind, 2014, Copenhagen, Denmark
Authors: A. Bach, M. Lennie, G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

Finite micro-tabs have been investigated experimentally to evaluate the potential for load control on wind turbines. Two dimensional full span, as well as multiple finite tabs of various aspect ratios have been studied on an AH93W174 airfoil at different chord wise positions. A force balance was used to measure the aerodynamic loads. Furthermore, the wake vortex system consisting of the Karman vortex street as well as the tab tip vortices was analyzed with a 12-hole probe and hot wire anemometry. Finally, conventional oil paint as well as a quantitative digital flow analysis technique called SMARTviz were used to visualize the flow around the finite tab configurations.

Results have shown that the devices are an effective solution to alleviate the airfoils overall load. The influence of the tab height, tab position as well as the finite tab aspect ratio on the lift and lift to drag ratio have been evaluated. It could be shown, that the lift difference can either be varied by changing the tab height as well as by altering the aspect ratio of the finite tabs. The drag of a two-dimensional flap is directly associated with the vortex street, while in the case of the finite tab, the solidity ratio of the tabs has the strongest effect on the drag. Therefore, the application of a finite tab system showed to improve the lift to drag ratio.

 

Extracting quantitative data from tuft flow visualizations on utility scale wind turbines.

Science of making torque from wind, 2014, Copenhagen, Denmark
Authors: S. Vey, H. Lang, G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

First results of a novel measurement technique that allows to extract quantitative data from tuft flow visualizations on real-world wind turbine blades are presented. The instantaneous flow structure is analyzed by tracking individual flow indicators in each of the snapshot images. The obtained per-tuft statistics are correlated with logged turbine data to provide an insight into the surface flow structure under the influence of wind speed. A histogram filter is used to identify two flow states: a separated flow state that occurs at higher wind speeds and a maximal attached flow state that mainly occurs in the lower wind speed range.

Comparative Study of CFD Solver Models for Modeling of Flow Over Wind Turbine Airfoils.

Proceedings of ASME Turbo Expo 2014, Düsseldorf, Germany, June 16–20, 2014
Authors: William T. Kirk, V. R. Capece, G. Pechlivanoglou, C. N. Nayeri, C. O. Paschereit

 

This paper presents an evaluation of XFOIL and a commercially available CFD solver to predict the two-dimensional lift and drag coefficients of wind turbine airfoil sections for attached and separated flows. The computational solutions are correlated with the experimental data for the DU 96-W-180 airfoil that has been generated from wind tunnel testing performed at TU Delft and TU Berlin. CFD solutions are presented for turbulent calculations using the Shear-Stress Transport (SST) [1] and Spalart-Allmaras [2] turbulence models and transition calculations using the SST γ-θ model. Transition points from the CFD simulations are compared to the results obtained using XFOIL [3].

The paper culminates in a quantitative analysis identifying the deviations of the XFOIL and CFD solutions from the experimental data. This analysis uses the experimental polars generated by TU Delft and TU Berlin as a baseline for the comparison with the end goal of determining the best computational source of design polars for use in industry.

Development and Validation of a Modal Analysis Code for Wind Turbine Blades.

Proceedings of ASME IGTI Turbo Expo 2014 ASME/IGTI, 2014, Düsseldorf, Germany
Authors: M. Lennie, D. Marten, G. Pechlivanoglou, V. Capece, C.N. Nayeri, C.O. Paschereit

QBlade is an open source wind turbine design and simulation tool developed at the Berlin Institute of Technology. To enable a coupling with the aeroelastic simulation tool FAST from NREL an aditional module, called QFEM, was created and integrated with QBlade. This module performs a modal analysis on rotor blades designed in QBlade using isotropic tapered Eu-ler Beam elements. The newly developed module now provides structural properties to the National Renewable Energy Labo-ratorys aeroelasticity simulation tool FAST. The 2D structural properties of the beam elements are created using integration methods. A number of test cases show that the 2D integration methods and beam element code work with adaquete accuracy. The integration of the modal analysis code greatly facilitates the structural design and analysis of rotor blades and will be made available to the public under an open source license.

Utilization of Modern Large Scale HAWT Blade Design Techniques for the Development of Small HWAT Blades.

Proceedings of ASME IGTI Turbo Expo 2014 ASME/IGTI, 2014, Düsseldorf, Germany
Authors: G. Pechlivanoglou, G. Weinzierl, I.T. Masmanidis, C.N. Nayeri, T.P. Philippidis, C.O. Paschereit

Lately the need for modern medium capacity wind turbines for self-consumption within urban and rural areas has risen again. The great majority however of the large wind turbine manufacturers are not interested in this small profit market. SMART BLADE GmbH in cooperation with the CORE Team of the University of Patras have decided to develop modern, sophisticated rotor blades with state of the art tools from the multi-MW wind turbine industry in order to boost the development of the upcoming modern small capacity wind turbines. The current paper presents the design of a modern 4m wind turbine blade with optimal aerodynamic design aimed at a variable speed hybrid stall operation.

Development of a Fluidic Actuator for Adaptive Flow Control on a Thick Wind Turbine Airfoil.

Proceedings of ASME IGTI Turbo Expo 2014 ASME/IGTI, 2014, Düsseldorf, Germany Authors: S. Niether, B. Bobusch, D. Marten, G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

Wind turbines are exposed to unsteady incident flow conditions such as gusts or tower interference. These cause a change in the blades’ local angle of attack, which often leads to flow separation at the inner rotor sections [1]. Recirculation areas and dynamic stall may occur, which lead to an uneven load distribution along the blade. In this work a fluidic actuator is developed that reduces flow separation. The functional principle is adapted from a fluidic amplifier. High pressure air fed by an external supply flows into the interaction region of the actuator. Two control ports, oriented perpendicular to the inlet, allow for a steering of the actuation flow. One of the control ports is connected to the suction side, the other to the pressure side of the airfoil. Depending on the pressure difference that varies with the angle of attack, the actuation air is directed into one of four outlet channels. These guide the air to different chordwise exit locations on the airfoil’s suction side. The appropriate actuation location adjusts automatically according to the pressure difference between the control ports and therefore incidence. Suction side flow separation is delayed as the boundary layer is enriched with kinetic energy. Experiments were conducted on a DU97-W-300 airfoil [2] at Re = 2.2 · 105. Compared to the baseline, changes in lift with angle of attack were reduced by an order of magnitude. An AeroDyn simulation of a full wind turbine rotor was performed that compares the baseline to a rotor design with adaptive flow control.

Comparative Study of CFD Solver Models for Modeling of Flow over Wind Turbine Airfoils.

Proceedings of ASME IGTI Turbo Expo 2014 ASME/IGTI, 2014, Düsseldorf, Germany
Authors: W. Kirk, V.R. Capece, G. Pechlivanoglou, C.N. Nayeri, C.O. Paschereit

This paper presents an evaluation of XFOIL and a commercially available CFD solver to predict the two-dimensional lift and drag coefficients of wind turbine airfoil sections for attached and separated flows. The computational solutions are correlated with the experimental data for the DU 96-W-180 airfoil that has been generated from wind tunnel testing performed at TU Delft and TU Berlin. CFD solutions are presented for turbulent calculations using the Shear-Stress Transport (SST) [1] and Spalart-Allmaras [2] turbulence models and transition calculations using the SST γ-θ model. Transition points from the CFD simulations are compared to the results obtained using XFOIL [3].

The paper culminates in a quantitative analysis identifying the deviations of the XFOIL and CFD solutions from the experimental data. This analysis uses the experimental polars generated by TU Delft and TU Berlin as a baseline for the comparison with the end goal of determining the best computational source of design polars for use in industry.

Vortex Generators for Wind Turbine Blades: Wind Tunnel Tests, Field Simulations and Structural Analysis.

Proceedings of the Conference on Wind Energy Science and Technology, RUZGEM 2013 October 3, 2013
Authors: G. Pechlivanoglou, S.Vey, O.Eisele, T.P. Philippidis,  Christian Navid Nayeri, C.O. Paschereit

This paper presents wind tunnel and field investigations on the aerodynamic effects of Vortex Generators(VGs) on wind turbine performance. Field results validate the aerodynamic simulations with respect to the expected energy production benefits of VGs on wind turbines.Additionally structural analysis simulations were performed in order to assess the effect of additional loading due to VGs on the blade structure. The results of the simulations show that the additional loading due to VGs has very low impact on the blade structure, significantly lower than its design load levels.

Experimental & numerical investigation of inflow turbulence on the performance of wind turbine airfoils.

Proceedings of ASME IGTI Turbo Expo 2013, San Antonio, Texas, USA
Authors: G. Pechlivanoglou, C.N. Naye-ri, C.O. Paschereit

Wind turbine blade design is currently based on the combination of a plurality of airfoil sections along the rotorblade span. The two-dimensional airfoil characteristics are usually measured with wind tunnel experiments or computed by means of numerical simulation codes. The general airfoil input for the calculation of the rotorblade power characteristics as well as the subsequent aerodynamic and aeroelastic loads are based on these two-dimensional airfoil characteristics. In this paper, the effects of inflow turbulence and wind tunnel test measurement deviations are investigated and discussed, to allow considerations of such effects in the rotorblade design process. The results of CFD simulations with various turbulence models are utilized in combination with wind tunnel measurements in order to assess the impact of such discrepancies. It seems that turbulence, airfoil surface roughness and early transition effects are able to contribute significantly to the uncertainty and scattering of measurements. Various wind tunnel facilities generate different performance characteristic curves, while grid-generated turbulence is generally not included in the wind tunnel measurements during airfoil characterization. Furthermore the correlation of grid-generated wind tunnel turbulence with the atmospheric turbulence time and length scales is not easily achieved. All the aforementioned uncertainties can increase the performance scattering of current wind turbine blade designs as well as the generated aeroelastic loads. A brief assessment of the effect of such uncertainties on wind turbine performance is given at the last part of this work by means of BEM simulations on a wind turbine blade.

Development and Application of a Simulation Tool for Vertical and Horizontal Axis Wind Turbines of Wind Turbine Airfoils.

Proceedings of ASME Turbo Expo 2013: Turbine Technical Conference and Exposition June 3, 2013
Authors:  J. Wendler, D. Marten, G. Pechlivanoglou, C. N. Nayeri, C. O. Paschereit

A double-multiple-streamtube vertical axis wind turbine simulation and design module has been integrated within the open-source wind turbine simulator QBlade. QBlade also contains the XFOIL airfoil analysis functionality, which makes the software a single tool that comprises all functionality needed for the design and simulation of vertical or horizontal axis wind turbines. The functionality includes two dimensional airfoil design and analysis, lift and drag polar extrapolation,rotor blade design and wind turbine performance simulation.The QBlade software also inherits a generator module, pitch and rotational speed controllers, geometry export functionality and the simulation of rotor characteristics maps. Besides that,QBlade serves as a tool to compare different blade designs and their performance and to thoroughly investigate the distribution of all relevant variables along the rotor in an included post processor. The benefits of this code will be illustrated with two different case studies. The first case dealswith the effect of stall delaying vortex generators on a vertical axis wind turbinerotor. The second case outlines the impact of helical blades and blade number on the time varying loads of a vertical axis wind turbine.

Experimental & Numerical Investigation of Inflow Turbulence on the Performacne of Wind Turbine Airfoils.

Proceedings of ASME IGTI Turbo Expo 2013 ASME/IGTI, 2013, San Antonio, Texas, USA June 3, 2013
Authors: O. Eisele, G. Pechlivanoglou, C. N. Nayeri, C. O. Paschereit

Wind turbine blade design is currently based on the combination of a plurality of airfoil sections along the rotorblade span. The two-dimensional airfoil characteristics are usually measured with wind tunnel experiments or computed by means of numerical simulation codes. The general airfoil input for the calculation of the rotorblade power characteristics as well as the subsequent aerodynamic and aeroelastic loads are based on these two-dimensional airfoil characteristics. In this paper, the effects of inflow turbulence and wind tunnel test measurement deviations are investigated and discussed, to allow considerations of such effects in the rotorblade design process.

QBlade: An Open Source Tool for Desing and Simulation of Horizontal and Vertical Axis Wind Turbines.

International Journal of Emerging Technology and Advanced Engineering February 2013
Authors:  J. Wendler, D. Marten, G. Pechlivanoglou, C. N. Nayeri, C. O. Paschereit

The software QBlade is developed as an open source framework for the simulation and design of wind turbines. QBlade utilizes the Blade Element Momentum (BEM) method for the simulation of horizontal axis-and a Double Multiple Stream tube (DMS) algorithm for the simulation of vertical axis wind turbine performance. For the design of custom airfoils and the computation of airfoil lift- and drag coefficient polars the viscous-inviscid coupledpanel method code XFOIL is integrated within the graphical user interface ofQBlade. Additionally a module for the extrapolation of airfoil polars, beyond the stall point, for a 360° rangeofangles of attack is integrated. The resulting functionality allows the use of QBlade as a comprehensive toolfor wind turbine design. QBlade is constantly being maintained, validated and advanced with newfunctionality. This paper describes the software and its modules, at the current state, in theory andapplication.

Development of a Flexible Trailing Edge Flap and System Integration Concept for Wind Turbine Blades.

Conference Proceedings DEWEK 2012 November 7, 2012
Authors: J. Fischer, G. Weinzierl,  G. Pechlivanoglou, J. Wagner

This paper describes the design of a flexible trailing edge flap and its system integration for an exemplary 250kW prototype test turbine with a forward look to a multi megawatt approach. Necessary requirements forthe envisioned system are derived by aerodynamic simulations as well as general system studies. The development of the flap system is documented from the points of structural design of the flap and the choice of suitable actuators in connection with the system integration. A wind tunnel test model equipped with the flap prototype was built for aerodynamic and mechanical investigations in the wind tunnel facilities of HFI/ISTA at the Berlin Institute of Technology. Steady state wind tunnel tests were conducted.