Georgios Pechlivanoglou

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

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.

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

Vortex generators (VGs) are passive flow control devices 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 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. Reducing the distance between adjacent VGs produced large increases in the static stall angle and maximum lift, but also resulted in a significant increase in drag as well as sharp lift excursions at angles exceeding the static stall angle. The optimal chordwise position of the vortex generators was found to be in the range of x/c = 15%–20%, where a comparatively low parasitic drag and a smooth post-stall lift curve were achieved. 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. Three different rotorblades were designed and several stall and pitch regulated wind turbine models were simulated by means of a Blade Element Momentum (BEM) code (QBlade) developed by Smart Blade GmbH. The performance of the rotorblades with and without VGs was simulated in order to assess their effect on the aerodynamic performance and loads. Finally, previously measured steady state performance curves under high-roughness conditions were used to simulate the detrimental effect of surface roughness on the performance of the aforementioned rotorblades. This allows for an estimate of the potential of the VGs to be employed as retrofit elements for performance recovery of blades with a contaminated surface.© 2012 ASME

Performance Optimization of Wind Turbine Rotors With Active Flow Control

This paper presents a series of investigations performed at the Hermann Fottinger Institute of TU Berlin. The initial scope of the investigations was the identification of Active Flow Control (AFC) solutions with significant implementation potential on wind turbine rotors. Several Active Flow Control solutions were thoroughly investigated based on extensive literature research. The performance of all the investigated solutions was ranked according to objective performance criteria and then the best performing solutions were selected for further numerical and experimental investigation. The selected Active Flow Control solutions were experimentally investigated with steady state wind tunnel measurements as well as steady state CFD simulations. The results of these investigations and the potential of each AFC solution are presented and discussed. The steady state tests were followed by a dynamic wind tunnel test campaign where the performance of one AFC solution (active Gurney flap) on a pitching test wing was investigated. The results of the static and dynamic investigations were very positive and proved the large load reduction potential of AFC on wind turbines.Copyright

Active Aerodynamic Control of Wind Turbine Blades with High Deflection Flexible Flaps

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 eort of the wind turbine industry in the eld of aerodynamics is related to the development of blades which oer better performance, increased reliability and faster control of larger wind turbines. Currently, however, most of the research eort 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 exible ap conguration 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 modied-DU96W180 test airfoil section equipped with the exible ap assembly in ow with Reynolds number Re equal to 1,300,000. The The exible ap was tested in various positive and negative deections in order to extract its complete operational curve. The results showed signicant inuence on both lift and drag as well as strong variations on the pitch behavior of the wing. The paper also discusses the possible benets of the integration of exible ap systems in wind turbine blade structures.

Effects of Airfoil's Polar Data in the Stall Region on the Estimation of Darrieus Wind Turbine Performance

Interest in vertical-axis wind turbines (VAWTs) is experiencing a renaissance after most major research projects came to a standstill in the mid 1990s, in favor of conventional horizontal-axis turbines (HAWTs). Nowadays, the inherent advantages of the VAWT concept, especially in the Darrieus configuration, may outweigh their disadvantages in specific applications, like the urban context or floating platforms. To enable these concepts further, efficient, accurate, and robust aerodynamic prediction tools and design guidelines are needed for VAWTs, for which low-order simulation methods have not reached yet a maturity comparable to that of the blade element momentum theory for HAWTs’ applications. The two computationally efficient methods that are presently capable of capturing the unsteady aerodynamics of Darrieus turbines are the double multiple streamtubes (DMS) theory, based on momentum balances, and the lifting line theory (LLT) coupled to a free vortex wake model. Both methods make use of tabulated lift and drag coefficients to compute the blade forces. Since the incidence angles range experienced by a VAWT blade is much wider than that of a HAWT blade, the accuracy of polars in describing the stall region and the transition toward the “thin plate like” behavior has a large effect on simulation results. This paper will demonstrate the importance of stall and poststall data handling in the performance estimation of Darrieus VAWTs. Using validated CFD simulations as a baseline, comparisons are provided for a blade in VAWT-like motion based on a DMS and a LLT code employing three sets of poststall data obtained from a wind tunnel campaign, XFoil predictions extrapolated with the Viterna–Corrigan model and a combination of them. The polar extrapolation influence on quasi-steady operating conditions is shown and azimuthal variations of thrust and torque are compared for exemplary tip-speed ratios (TSRs). In addition, the major relevance of a proper dynamic stall model into both the simulation methods is highlighted and discussed. [DOI: 10.1115/1.4034326]

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

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.

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

The effects of distributed roughness on wind turbines are extensively investigated in this paper. The sources of roughness are identified and analyzed and their effects on airfoil are estimated from simulations and measured with wind tunnel measurements. In addition to the environmental and manufacturing induced roughness, several forms of roughness-related shape deviations are investigated and their effects on the aerodynamic performance of airfoils is qualitatively predicted through numerical simulations. The actual effects of roughness on wind turbine performance are also presented through power production measurements of wind turbines installed in sandy environments. These measurements are correlated with simulated power predictions, utilizing a steady state BEM code.Copyright

Experimental Investigation of the Aerodynamic Lift Response of an Active Finite Gurney Flap

The passive Gurney flap has proven being a good mean to alter the static aerodynamic lift on an airfoil. The Gurney flap can be applied to either the suction side for lift reduction or to the pressure side for lift enhancement. Measurements on a FX 63 -137 airfoil with an active Gurney flap were conducted in a wind tunnel to investigated the time dependent behavior of two-dimensional as well as finite flaps. The time-dependent pressure response of the deploying flaps were evaluated locally over the airfoils surface and the spanwise time-dependent lift was evaluated. Results have shown, that the lift response behaves differently for a flap deployment than for a flap retraction, where the convergence generally takes longer. Furthermore, the adjacent flap sections of the finite flap have shown to also experience a dynamic lift change. This response is different than the response in front of the flap, which is due to the influence of to the surface pressure expansion around the finite flap as well as the flap tip vortices. The measurements have shown, that the timedependent lift in the adjacent sections do play a relevant part in the overall lift response of the wing.

Configuration and Numerical Investigation of the Adaptive Camber Airfoil as Passive Load Alleviation Mechanism for Wind Turbines

If the number of suitable sites for horizontal axis wind turbines is limited, increasing the rotor diameter is a viable means of increasing the power output of the wind turbine. For a given wind speed the power output theoretically increases with the radius squared. However, the material needed to upscale a classically designed rotor that withstands the also increasing fatigue loads, scales with the radius to the power of three, approximately. Rotor blades contribute to a significant part to the capital costs of a wind turbine. Because their cost scales with the amount of used material, reducing the fatigue loads on wind turbine blades is an efficient way to lower the cost of energy. To achieve the latter, a mechanism is presented that passively alleviates the unsteady aerodynamic loads that act on the rotor. The basic principle of this adaptive camber airfoil will be presented in the following. Subsequently, simulations with a nonlinear lifting line free vortex wake algorithm are performed that estimate the load reduction potential of this passive load control element on the NREL 5MW reference turbine.

Nonlinear Lifting Line Theory Applied to Vertical Axis Wind Turbines: Development of a Practical Design Tool

Recently a new interest in vertical axis wind turbine (VAWT) technology is fueled by research on floating support structures for large scale offshore wind energy application. For the application on floating structures at multi megawatt size, the VAWT concept may offer distinct advantages over the conventional horizontal axis wind turbine (HAWT) design. As an example VAWT turbines are better suited for upscaling and, at multi megawatt size, the problem of periodic fatigue cycles reduces significantly due to a very low rotational speed. Additionally, the possibility to store the transmission and electricity generation system at the bottom, compared to the tower top as in a HAWT, can lead to a considerable reduction of material logistics costs. However, as most VAWT research stalled in the mid 90’s, no established and sophisticated tools to investigate this concept further exist today. Due to the complex interaction between unsteady aerodynamics and movement of the floating structure fully coupled simulation tools, modelling both aero and structural dynamics are needed. A nonlinear Lifting Line Free Vortex Wake code was recently integrated into the open source wind turbine simulation suite QBlade. This paper describes some of the necessary adaptations of the algorithm, which differentiates it from the usual application in HAWT simulations. A focus is set on achieving a high robustness and computational efficiency. A short validation study compares simulation results with those of a U-RANS and a Double Multiple Streamtube (DMS) simulation.