The Unsteady Characteristics Of Wind Turbine Airfoils With Blunt Trailing Edge

With the development of large-scale wind turbines,wind turbine blades become longer and heavier,leading to the contradiction between aerodynamic performance and structural property,which contributes to the development of wind turbine airfoils with blunt trailing edge.Blunt airfoils can relieve the conflict between aerodynamic performance and structural property effectively.However,blunt airfoils can easily induce periodic shedding vortices due to their trailing-edge thickness,which will bring about aerodynamic noise and aeroelastic problems,and thus affect the operation safety and the service life of a blade.Particularly,very thick blunt airfoils are more prone to flow separation.They are applied in the inboard section of a blade,and thus are restricted by blade root twist angle.As a result,these airfoils are at high angles of attack and facing with flow separation,which makes the flow unsteady.Therefore,it is necessary to investigate the unsteady characteristics of blunt wind turbine airfoils with different relative thickness.The research is beneficial for predicting the aerodynamic performance of blades more accurately and provides a basis for flow control.CFD numerical simulation and wind tunnel experiment are used to explore the unsteady characteristics of a very thick airfoil CAS-W1-450 with a relative thickness of 45%,two moderate-thickness blunt airfoils DU 91-W2-250₆ and DU 91-W2-250₁0,whose relative thicknesses are 25%.These two kinds of blunt airfoils are applied in the root and middle part of a blade respectively.Two-dimensional steady,two-dimensional transient and three-dimensional steady simulation of the flow of the very thick airfoil CAS-W1-450 under experimental conditions are carried out.The aerodynamic coefficients of different simulated conditions and experimental conditions were compared,and the unsteady characteristics of CAS-W1-450 were analyzed.The results show that the lift coefficients of two-dimensional steady,two-dimensional transient and three-dimensional steady simulation are in good agreement with the experimental values in linear region,while there are obvious differences of the lift coefficients between the two-dimensional simulation and the experiment after stall.Moreover,the lift coefficients of two-dimensional transient simulation are significantly larger than the experimental values after stall because the area of separation zone is restrained by trailing edge shedding vortices.Before the angle of attack of 8°,the fluctuation of lift coefficient is extremely small.At this time,the flow is steady.After the angle of attack of 8°,the lift coefficient fluctuates periodically.Besides,the amplitude of lift coefficient increases with the increase of angle of attack,but the frequency and the Strouhal number decrease with the increase of angle of attack.The dominant frequency obtained by spectrum analysis is the same as that obtained by averaging the periodic lift coefficient.In addition,more information about the unsteady lift coefficient can be provided by spectrum analysis.In order to investigate the unsteady characteristics and the near stall instability of blunt airfoils DU 91-W2-250₆ and DU 91-W2-250₁0,wind tunnel experiments were carried out.The surface pressures of these airfoils were measured by PSI-DTC Initium pressure data acquisition system,and then the aerodynamic coefficients were calculated based on the surface pressures.The velocities in the wake were recorded by an IFA 300 anemometer with a two-dimensional hot-wire probe.The dominant frequency of vortex shedding was obtained by applying Fast Fourier Transform on velocity signals.The experimental results reveal that the two airfoils experienced two stalls in the range of 0° to 30°angle of attack.Distinct flow separation on suctioin surface leads to the first stall,while the transition from partial separation to full separation results in the second stall.Spectrum of velocity in the wake is analyzed.The dominant frequencies of different points,velocity directions,angles of attack were thoroughly compared and investigated.In addition,the Strouhal number of the shedding vortices was calculated in this paper.The broadband characteristic of the velocity spectrum near stall is discovered,which illustrates the instability of near stall conditions in frequency domain.Besides,larger trailing-edge thickness will induce more uncertainty of the flow status.The frequency characteristics of velocity in x and y directions are the same,indicating that the velocity in these two directions have correlated and consistent effects on the shedding vortices in the wake.The Strouhal number decreases with the decline of lift coefficient and the increase of angle of attack between the first and the second stall.Otherwise,the Strouhal number is independent with lift coefficient as well as angle of attack.For the sake of studying the unsteady characteristics of blunt airfoils numerically and to make up for the limitation of experimental conditions,numerical simulations of DU 91-W2-250₆ and DU 91-W2-250₁0 airfoils were carried out.The simulation results indicate that the two-dimensional URANS can reasonably predict the average aerodynamic coefficients and surface pressure distributions of these blunt airfoils,but numerical lift coefficients are higher than experimental values after stall.The unsteady characteristic of lift coefficient attributes to the flow separation and shedding vortices in the wake.The lift coefficient begins to fluctuate at 0°angle of attack,and it fluctuates periodically at small angle of attack.With the increase of angle of attack,the fluctuation of lift coefficient will undergo a critical state when the fluctuation amplitude of lift coefficient becomes quite small,which is different from that of CAS-WI-450 airfoil.Elongated vortices and small vorticity magnitude contribute to this phenomenon.The critical status occurs at a=10°and a=15°for DU 91-W2-250₆,at ?=15° for DU 91-W2-250₁0.However,the lift coefficient fluctuates irregularly when the airfoil is severely separated.In addition,the wider the trailing edge is,the larger of the vorticity magnitude and the more significant of the fluctuation of lift coefficient are.