Christophe Leclerc

A Viscous Three-Dimensional Differential/Actuator-Disk Method for the Aerodynamic Analysis of Wind Farms

Computational Fluid Dynamics (CFD) is a promising tool for the analysis and optimization of wind turbine positioning inside wind parks (also known as wind farms) in order to maximize power production. In this paper, 3-D, time-averaged, steady-state, incompressible Navier-Stokes equations, in which wind turbines are represented by surficial forces, are solved using a Control-Volume Finite Element Method (CVFEM). The fundamentals of developing a practical 3-D method are discussed in this paper with an emphasis on some of the challenges that arose during their implementation. For isolated turbines, results have indicated that the proposed 3-D method attains the same level of accuracy, in terms of performance predictions, as the previously developed 2-D axisymmetric method and the well-known momentum-strip theory. Furthermore, the capability of the proposed method to predict wind turbine wake characteristics is also illustrated. Satisfactory agreement with experimental measurements has been achieved. The analysis of a two-row periodic wind farm in neutral atmospheric boundary layers demonstrate the existence of positive interference effects (venturi effects) as well as the dominant influence of mutual interference on the performance of dense wind turbine clusters.

Appropriate Dynamic-Stall Models for Performance Predictions of VAWTs with NLF Blades

This paper illustrates the relative merits of using Natural Laminar Flow (NLF) airfoils in the design of Vertical Axis Wind Turbines (VAWT). This is achieved by the application of the double-multiple-streamtube model of Paraschivoiu to the performance predictions of VAWTs equipped with conventional and NLF blades. Furthermore, in order to clearly illustrate the potential benefit of reducing the drag, the individual contributions of lift and drag to power are presented. The dynamic-stall phenomena are modelled using the method of Gormont as modified by several researchers. Among the various implementations of this dynamic-stall model available in the literature, the most appropriate and general for NLF applications has been identified through detailed comparisons between predicted performances and experimental data. This selection process is presented in the paper. It has been demonstrated that the use ofNLF airfoils in VAWT applications can lead to significant improvements with respect to conventional design only in a very low wind speed range, the extent of which is negligible with respect to the VAWT operational wind speeds.

TOWARD BLADE-TIP VORTEX SIMULATION WITH AN ACTUATOR-LIFTING SURFACE MODEL

This paper presents a new method based on the imposition of velocity discontinuities to model °ow perturbation due to existence of vortical structures. The proposed method uses actuator disk and lift- ing line concepts in order to provide a framework of analysis that respects conservation laws for mo- mentum, energy and vorticity, which is not the case of classical actuator disk methods used in the wind industry. The °oweld is described by the Euler equations. In the proposed mathematical model, the attitude toward °ow determination is entirely linked to the vorticity structure of the °ow, which is modeled by velocity discontinuities. Results are produced for three basic problems of interest : 2D uniform vorticity distribution, actuator disk with uniform loading andnite wing with prescribed dis- tribution of circulation. These cornerstone prob- lems have shown that numerical method gives fairly precise values regarding °ow streamlines, lift and induced velocities predictions for all problems in- vestigated. Induced drag prediction for thenite wing problem could not be measured since proper grid-independent solution to this problem could not be attained.

Wind Turbine Performance Predictions Using a Differential Actuator-Lifting Disk Model

This paper presents a method based on the imposition of velocity discontinuities to model flow perturbation due to the existence of vortical structures. The proposed method uses actuator-disk and lifting line concepts in order to provide a framework of analysis that respects conservation laws for momentum, energy, and vorticity, which is not always the case for engineering methods used in the wind industry. The flow field is described by the Euler equations. In the proposed mathematical model, the attitude toward flow determination is entirely linked to the vorticity structure of the flow, which is modeled by velocity discontinuities. The numerical method has been applied to four wind turbines: NREL phases 11, IV, and VI rotors, as well as to the Tjaereborg rotor, and has shown satisfactory predictions compared to measurements up to peak power. Comparisons have also been undertaken with the results of a previous method, developed by the same authors, where the velocity field is not allowed to be discontinuous and the actuator disk is analyzed as a source of external forces only. In the stall regime of the turbine, the relative differences in power output between the two methods have been evaluated at 5% on the average.

Prediction of aerodynamic performances and loads of HAWTs operating in unsteady conditions

A numerical method for the prediction of unsteady wind turbine aerodynamics is presented. The actuatordisk concept is used to model the wind turbine, and its effects on the flow are prescribed by the blade-element theory. The flow around the actuator disk is governed by the Navier-Stokes equations. The normal component of the forces, per unit area, associated with the presence of the rotor are balanced by a pressure discontinuity in the fluid. Dynamic stall effects on the aerodynamic characteristics of the blade are evaluated with the Gormont model. A control-volume finite-element method is used to solve the proposed formulation in its three-dimensional form. The fully implicit scheme is used for time discretization. Comparisons between computed and measured blade loading for the 2 MW Tjaereborg and the NREL Combined Experiment wind turbines undergoing sharp blade-pitch changes, operating in unsteady wind conditions, and operating in strongly yawed flows show the ability of the proposed method to predict instantaneous loading of wind turbines during hansient situations.

Abnormally High Power Output of Wind Turbine in Cold Weather: A Preliminary Study

According to popular belief, air temperature effects on wind turbine power output are produced solely by air density variations, and power is proportional to air density. However, some cases have been reported, all involving stall-controlled wind turbines, in which unexpected high power output was observed at very low temperatures.

A survey of the temperature influence on power output for 3 wind turbines operating in cold climates

The present study is motivated by several observations of unexpected, recurring, high levels of power for stallregulated wind turbines operating under very low temperatures. As power levels recorded largely exceed design levels of the rotor, operation in such conditions can cause dramatic damage to the mechanical or electrical components of the turbine. This study aims to understand the origin of such phenomenon by analyzing experimental data gathered from three stall-controlled wind turbines subject to regular operation under very low temperatures. The three rotors have a nominal power of more than 500 kW and are installed on three different sites. Two of the rotors are identical and constructed by the same manufacturer. Data recorded by turbine are completed by data recorded by separate mast towers or nearby meteorological stations. Parameters recorded are: electrical power output, wind velocity at hub height, wind velocity standard deviation, air temperature, atmospheric pressure, wind direction, and generator rotational speed. To provide a quantitative estimate of density and atmospheric turbulence effects on power output, a procedure recommended in the IEC 61400-12 international standard for elaboration of a wind turbine power curve is used. To perform the parametric analysis, specific groups of data, called Conditional Groups are formed. For the first turbine analyzed, it has been shown that power outputs are on the average proportional to density increases. While continuous acquisition of data is estimated to be necessary to formulate definitive conclusions, the two other turbines analyzed have shown to produce power increases significantly higher than increases of density (30% more on the average). Regarding the influence of turbulence intensity, it has been observed that for constant hub height incoming wind velocity and density, power output increases with turbulence intensity at low winds (15% maximum

Wind Turbines Operating in Cold Climates: Reynolds Number and Turbulence Effects on Performances

The present study is motivated by several observations of unexpected, recurring, high levels of power for stall-regulated wind turbines operating under very low temperatures. As power levels recorded largely exceed design levels of the rotor, operation in such conditions can cause dramatic damage to turbine. This study aims to understand the origin of such phenomenon by analyzing experimental data gathered from a stall-controll ed wind turbine, having a nominal power of more than 500 kW, and comparing the experimental behaviour with numerical simulations. To provide a quantitative estimate of density and atmospheric turbulence effects on power output, a procedure based on the IEC 61400-12 international standard for elaboration of a wind turbine power curve is used. The numerical simulations is based on the solution of the time-aver aged, steady-state, incompressible Navier-Stokes equations with an appropriate turbulence closure model. The actuator disk model, together with blade element theory, are used to model the turbines. The stall-regulated turbine analyzed has shown to produce measured power increases significantly higher than increases of density. Regarding the influence of turbulence intensity, it has been observed that for constant hub height incoming wind velocity and density, power output increases with turbulence intensity at low winds, the opposite being true at higher winds. The numerical simulations show a good agrement with the measurements.