A solution‐based stall delay model for horizontal‐axis wind turbines

This work proposes a new solution‐based stall delay model to predict rotational effects on horizontal‐axis wind turbines. In contrast to conventional stall delay models that correct sectional airfoil data prior to the solution to account for three‐dimensional and rotational effects, a novel approach is proposed that corrects sectional airfoil data during a blade element momentum solution algorithm by investigating solution‐dependent parameters such as the spanwise circulation distribution and the local flow velocity acting at a section of blade. An iterative process is employed that successively modifies sectional lift and drag data until the blade circulation distribution is converged. Results obtained with the solution‐based stall delay model show consistent good agreement with measured data along the National Renewable Energy Laboratory Phase VI and Model Experiments in Controlled Conditions rotor blades at low and high wind speeds. Copyright © 2014 John Wiley & Sons, Ltd.     

Estimating the angle of attack from blade pressure measurements on the NREL Phase VI rotor using a free wake vortex model: axial conditions

Blade element momentum (BEM) methods are still the most common methods used for predicting the aerodynamic loads during the aeroelastic design of wind turbine blades. However, their accuracy is limited by the availability of reliable aerofoil data. Owing to the 3D nature of the flow over wind turbine blades, the aerofoil characteristics will vary considerably from the 2D aerofoil characteristics, especially at the inboard sections of the blades. Detailed surface pressure measurements on the blade surfaces may be used to derive more realistic aerofoil data. However, in doing so, knowledge of the angle of attack distributions is required. This study presents a method in which a free wake vortex model is used to derive such distributions for the NREL Phase VI wind turbine under different operating conditions. The derived free wake geometry solutions are plotted together with the corresponding wake circulation distribution. These plots provide better insight into how circulation formed at the blades is eventually diffused into the wake. The free wake model is described and its numerical behaviour is examined. Copyright © 2006 John Wiley &Sons, Ltd.     

A study on rotational augmentation using CFD analysis of flow in the inboard region of the MEXICO rotor blades

This work presents an analysis of data from existing as well as new full‐rotor computational fluid dynamics computations on the MEXICO rotor, with focus on the flow around the inboard parts of the blades. The boundary layer separation characteristics on the airfoil sections in the inboard parts of the rotor are analysed using the pressure and the skin friction data at a range of angles of attack. These data are used to gain insight on the relative behaviour of separated boundary layers in 3D flow compared with 2D flow. It has been found that separation on airfoils in rotating flows is different from that in 2D flows in two respects: (i) there is a chord‐wise postponement (or delay) of the separation point, and (ii) the angle of attack at which separation is initiated is higher in 3D compared with 2D. Comments are made on the mechanism of stall delay, and the main differences between the skin friction and pressure distribution behaviours in 2D and 3D rotating flows are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.     

Measurements on a wind turbine wake: 3D effects and bluff body vortex shedding

The velocity field in the wake of a two‐bladed wind turbine model (diameter 180 mm) has been studied under different conditions using a two‐component hot wire. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the freestream direction (the yaw angle was varied from 0° to 20°). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side, showing the potential of controlling the wake by yawing the turbine. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low‐frequency fluctuation both in the wake and in the flow outside the wake. This fluctuation was found both with and without freestream turbulence and also with a yawed turbine. The frequency expressed as a Strouhal number was shown to be independent of the freestream velocity or turbulence level, but the low frequency was only observed when the tip speed ratio (or equivalently the drag coefficient) was high. The shedding frequency changed also with the yaw angle. This is in agreement with the idea that the turbine sheds structures as a bluff body. The phenomenon, noticeable in all the velocity components, was further investigated using two‐point cross‐correlations of the velocity signals. Copyright © 2005 John Wiley & Sons, Ltd.     

An experimental study on the effects of winglets on the tip vortex interaction in the near wake of a model wind turbine

An experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight‐cut wing tip and a downstream‐facing winglet shape are compared on the same two‐bladed rotor operated at its design tip speed ratio. Phase‐averaged measurements of the velocity vector are synchronized with the rotor position, visualizing the downstream location of tip vortex interaction for the two blade tip configurations. The mean streamwise velocity is found not to be strongly affected by the presence of winglet tip extensions, suggesting an insignificant effect of winglets on the time‐averaged inflow conditions of a possible downstream wind turbine. An analysis of the phase‐averaged vorticity, however, reveals a significantly earlier tip vortex interaction and breakup for the wingletted rotor. In contradistinction, the tip vortices formed behind the reference configuration are assessed to be more stable and start merging into larger turbulent structures significantly further downstream. These results indicate that an optimized winglet design can not only contribute to a higher energy extraction in a rotor's tip region but also can positively affect the wake's mean kinetic energy recovery by stimulating a faster tip vortex interaction.     

A new stall delay algorithm for predicting the aerodynamics loads on wind turbine blades for axial and yawed conditions

Stall delay is a complicated phenomenon that has gained for many years the attention of industry and academics in the fields of helicopter and wind turbine aerodynamics. Since most of the potential flow theories still rely on the use of 2D aerofoil data for simulating loads on a rotating blade, less degree of accuracy is expected because of 3D rotational effects. In this work, a new model for correcting the 2D steady aerodynamic data for 3D effects is presented. The model can reduce the uncertainty in the blade design process and, subsequently, make wind turbines more cost‐effective. This model combines the stall delay model of Corrigan and Schillings, a modified version of an inviscid stall delay model, a new modification factor to account for the effect of the angle of attack changes and a new tip loss factor. Furthermore, the model applies the use of the separation factor of Du and Selig to evaluate the area on the rotor disc where stall delay is most prominent. The new stall delay model was embedded in a free‐wake vortex model to estimate the aerodynamic loads on the National Renewable Energy Laboratory Phase VI rotor blades consisting of the S809 aerofoil sections. The results in this study confirm the validity of the 3D corrections by the proposed new model under both axial and yawed flow conditions. Copyright © 2017 John Wiley & Sons, Ltd.     

Estimating the angle of attack from blade pressure measurements on the National Renewable Energy Laboratory phase VI rotor using a free wake vortex model: yawed conditions

Wind turbine design codes for calculating blade loads are usually based on a blade element momentum (BEM) approach. Since wind turbine rotors often operate in off‐design conditions, such as yawed flow, several engineering methods have been developed to take into account such conditions. An essential feature of a BEM code is the coupling of local blade element loads with an external (induced) velocity field determined with momentum theory through the angle of attack. Local blade loads follow directly from blade pressure measurements as performed in the National Renewable Energy Laboratory (NREL) phase IV campaign, but corresponding angles of attack cannot (on principle) be measured. By developing a free wake vortex method using measured local blade loads, time‐dependent angle of attack and induced velocity distributions are reconstructed. In a previous paper, a method was described for deriving such distributions in conjunction with blade pressure measurements for the NREL phase VI wind turbine in axial (non‐yawed) conditions. In this paper, the same method is applied to investigate yawed conditions on the same turbine. The study considered different operating conditions in yaw in both attached and separated flows over the blades. The derived free wake geometry solutions are used to determine induced velocity distributions at the rotor blade. These are then used to determine the local (azimuth time dependent) angle of attack, as well as the corresponding lift and drag for each blade section. The derived results are helpful to develop better engineering models for wind turbine design codes. Copyright © 2008 John Wiley & Sons, Ltd.     

A numerical study of tilt‐based wake steering using a hybrid free‐wake method

This study investigates the potential of using tilt‐based wake steering to alleviate wake shielding problems experienced by downwind turbines. Numerical simulations of turbine wakes have been conducted using a hybrid free‐wake analysis combining vortex lattice method (VLM) and an innovative free‐wake model called constant circulation contour method (CCCM). Simulation results indicate tilting a horizontal axis wind turbine's shaft upward causes its wake to ascend, carrying energy‐depleted air upward and pumping more energetic replacement air into downstream turbines, thereby having the potential to recover downstream turbine power generation. Wake cross section vorticity and velocity distributions reveal that the wake upward transport is caused by the formation of near‐wake streamwise vorticity components, and furthermore, the wake velocity deficit is weakened because of the skewed wake structure. Beyond the single turbine wake simulation, an inline two‐turbine case is performed as an assessment of the wake steering influence on the two‐turbine system and as an exploratory work of simulating turbine‐wake interactions using the hybrid free‐wake model. Individual and total turbine powers are calculated. A comparison between different tilting angles suggests turbine power enhancement may be achieved by tilting the upstream turbine and steering its wakes away from the downstream turbine.     

Integrating a new flow separation model and the effects of the vortex shedding for improved dynamic stall predictions using the Beddoes–Leishman method

This paper aims at improving dynamic stall predictions on the S809 aerofoil under 2D flow conditions. The method is based on the well‐known Beddoes–Leishman model; however, a new flow separation model and a noise generator are integrated to improve the predictions in the load fluctuations, including those induced by vortex shedding on the aerofoil upper surface. The flow separation model was derived from a unique approach based on the combined use of unsteady aerodynamic loads measurements, the Beddoes–Leishman model and a trial‐and‐error technique. The new flow separation model and random noise generator were integrated in the Beddoes–Leishman model through a new solution algorithm. The numerical predictions of the unsteady lift and drag coefficients were then compared with the Ohio State University measurements for the oscillating S809 aerofoil at several reduced frequencies and angles of attack. The results using the proposed models showed improved correlation with the experimental data. Hysteresis loops for the aerodynamic coefficients are in good agreement with measurements. Copyright © 2016 John Wiley & Sons, Ltd.     

Wake modelling of a VAWT in dynamic stall: impact on the prediction of flow and induction fields

Dynamic stall is a relevant phenomenon in the design and operation of a vertical axis wind turbine (VAWT) as it impacts loading, control and wake dynamics. Although streamtube models and single‐wake vortex models are commonly used for VAWT simulation, they either do not explicitly simulate the distribution of vorticity in the wake (streamtube models) or simplify it into a single‐wake release point (single‐wake vortex models). This can lead to inaccurate predictions of the vorticity distribution and wake dynamics, and therefore of the induction field, rotor loading and wake development, including wake mixing and re‐energizing. In this work, we use a double‐wake panel model developed for the simulation of dynamic stall in a VAWT to analyse (i) what is the flow field in dynamic stall, including the induction field, (ii) what is the error due to assuming a simplified wake, in both vorticity distribution and induction and (iii) how an incorrect simulation of the vorticity distribution can affect the prediction of the dynamics of the near and far wake. The results demonstrate that for mild separation (tip speed ratio λ≥3), single‐wake models can produce acceptable results. However, for lower tip speed ratios (λ < 3), the inaccuracy in the prediction of loads, induction field and vorticity distribution becomes significant because of an inadequate representation of the wake dynamics. These results imply that using lower order models can lead to inaccurate estimations of loads, performance and power control requirements at low tip speed ratios. Copyright © 2014 John Wiley & Sons, Ltd.     

A Tomo‐PIV study of the effects of freestream turbulence on stall delay of the blade of a horizontal‐axis wind turbine

Volumetric velocity fields were measured using tomographic particle image velocimetry on a model of the blade of a 5 kW horizontal‐axis wind turbine to study the effects of freestream turbulence levels (FTLs) at 0.4%, 4% and 13% on stall delay phenomenon at two different global tip speed ratios of 3 and 5 with Reynolds number (Re) ? 5000. Static pressures were measured, and results illustrated that FTL has stronger effect on the surface pressures of the static airfoil. Magnitudes of the absolute velocities within the separated flows above the static airfoil's suction surface increase significantly with higher FTL, while the changes of these velocities above the rotating blade's surface are less obvious. Radial flows from rotating blade's root to tip were also observed with strong spanwise velocity component located in the vicinities of the vortices. At the root and middle sections of the rotating blade, the flows with strong radial velocity component, w, become wider with higher FTL near to the rotating blade's leading edge when the angles of attack (AOAs) are large. At large AOAs, the strength and size of the vortices shed from the rotating blade's leading and trailing edges decrease significantly with higher FTL. However, at small AOAs, the size and coherence of the vortices near the rotating blade's trailing edge increase significantly with higher FTL. Surface streamlines of the rotating blade illustrated that at the rotating blade's root region and at large AOAs, the streamlines tend to lean toward the rotating blade's trailing edge at higher FTL. Copyright © 2014 John Wiley & Sons, Ltd.