A Study Of Stall Problems On Horizontal Axis Wind Turbine Blades

Designing highly efficient and reliable wind turbine rotor is the most essentialproblem in the research and development of horizontal axis wind turbine (HAWT) whichhas been the most effective wind energy convertor. With the radical increase in windturbine scale, to ensure more optimal designs, more reliable predictions of aerodynamicloads which depend on the use of accurate engineering computational methods,aerodynamic modeling and correction models, are needed. The accuracy of these methodsand models however relies on a correct cognition and a profound understanding in thecomplicated flow mechanisms that are closely related to key aerodynamic problems suchas static stall (stall delay), dynamic stall and dynamic loads etc.Through numerical computations and theoretical analysis, static stall and dynamicstall problems on wind turbines are investigated, a reduced-order model for unsteadyaerodynamic forces prediction based on a surrogate-based recurrence framework (SBRF) isput forward, and a study on the impact of active flow control with synthetic jet on a windturbine airfoil is performed. The research is based on the combined experiment Phase VIrotor and S809airfoil of National Renewable Energy Laboratory (NREL) for which thereare abundant specific and detailed experimental data available. The research achievementshelp establish more accurate loads prediction models and enhance wind turbine designlevel, which provides a strong support for ensuring wind turbine operation reliability andenhancement of its overall performance.The main contents and achievements of the research are as follows:1) Using Menter’s transition corrected k SST turbulence model, numericalsimulations of the flowfield around the typical NREL Phase VI rotor under differentfree stream speeds is carried out, and the result are generally in agreement with the experiments with some discrepancy under high wind speed when the flow separationincreases and the accuracy of simulation is validated. The rotor flow field features areanalyzed in detail with pressure coefficient distribution on different blade radialsections and the limiting streamlines distribution on the blade suction surface.2) Based on the full scale rotor CFD results, the intrinsic flow mechanisms of stall delayis analyzed, and the relations between3D flow field and engineering models isestablished. The axial reduction factors at different blade radial sections are deduced,and the effective angles of attack are obtained. The sectional aerodynamic coefficientswhich include3D rotational effects and blade tip and root effects are extracted.Extensive comparison and analysis is performed among CFD results, those withcorrection by Du-Selig stall delay model and those of Tangler Method, and the stallcharacteristics of different blade radial sections is explored.3) Through numerical simulations, the process of dynamic vortex development,convection and finally shedding into wake for a typical deep stall case are illustratedin detail leading to a further understanding of the mechanism involved in the dynamicstall phenomenon. With the application of Menter’s transition corrected k SSTturbulence model and mesh deformation based dynamic mesh technique, CFDsimulations on the wind turbine airfoil undergoing sinusoidal periodic pitchoscillations about it’s quarter chord point are carried out. The result is compared withthe wind tunnel experimental data, which shows a good agreement, thus the accuracyof CFD simulations is validated. Via streamlines and pressure coefficient distribution,the variation of aerodynamic hysteresis loops is disclosed.4) Depending on the unsteady flow field data of the multiple cases under non-stall, stallonset, light stall and deep stall conditions, the impact of reduced frequency, meanangle of attack and the amplitude of pitch oscillation on the airfoil dynamic stall isinvestigated, which finds that reduced frequency plays a great role in dynamic stall.With increased reduced frequency, lift peak value is to appear at a higher angle ofattack, and the negative aerodynamic damping and hysteresis effect become morepronounced in some cases. Based on the lift drag and moment coefficients, theinfluence of dynamic stall over wind turbine practical operation is analyzed.5) Based on the above computed airfoil unsteady aerodynamic results, with SBRF reduce-order modeling, the unsteady lift, drag and moment is predicted underdynamic stall conditions. The research manifests that the SBRF reduced-ordermodeling approach is ideally suited for a variety of aeroelastics and active/passivedesign optimization studies that require high fidelity aerodynamic response solutionswith efficiency as high as that of semi-empirical models.6) The flow field around static and pitching airfoil is simulated with synthetic jet as anactive flow control method. The study shows that: for static airfoil, under attachedflow with small angle of attack, jet actuation has a negative impact on the airfoilperformance; in prestall regime, the jet enhances the airfoil performance remarkably;and in the poststall regime, the effect diminishes and becomes weak. For pitchingairfoil, the synthetic jet can effectively suppress the hysteresis for a large portion ofthe oscillation cycle. However, for a limited range with large angle of attack in deepstall, strong vortex shedding and aerodynamic oscillation still exist.