| During Horizontal Axis Wind Turbine (HAWT)'s design and checking process, aerodynamic performance prediction is very important. Calculation of blades'aerodynamic performance after the design of aerodynamic shape is an evaluation for design results; conversely, aerodynamic performance results can be used as feedback for the amendment of aerodynamic shape of blades. But the load prediction is very difficult, because HAWT operates in complex natural environment, such as wind shear, variation of wind speed and direction, tower influence and so on. Currently, wind turbine designers rely on safety factors to compensate for the effect of unknown loads acting on the turbine, which results in components that are overdesigned because precise load level and load paths are unknown. In order to advance wind turbine technology, the forces acting on the turbine structure must by accurately characterized.When HAWT is rotating, wake vortices are released from blades'trailing edge, form an inboard vortex sheet and rolled up to strong tip vortex. The strength of the vortex is determined by the geometry and motion parameters of airfoils, also the aerodynamic loads on blades, conversely; the induced effect of the vortex changes the velocity field around the rotor, consequently effect the strength of vortex, which means that blades and vortex interference each other. The calculation of the vortex wake is the key factor to wind turbine's aerodynamic load analysis, and also carry out wind turbine's aerodynamics research. It also has important scientific and practical significance when in-depth research of vortices wakes. In this paper, a time-marching free wake method was developed to analyze aerodynamic load of HAWT, the major contributions of the author's word are as follows:①The widely used method in aerodynamic airload prediction of HAWT is blade element theory (BEM theory), but which cannot account for yaw inflow's influence. Aiming at this shortcoming, vortex cylinder theory is used to replace momentum theory in BEM. Aerodynamic performance of TUDelft based on the modified BEM model is put forward, which shows large error even turbine works at low wind speed and steady yaw condition. So develop a new method for aerodynamic performance prediction of HAWT is urgent needed.②An integrated HAWT rotor wake model is developed by combination of free vortex wake model and blade aerodynamic model. Free vortex wake model is used to represent the wake behind the rotor, and Vatistas turbulence vortex core model, which is suitable for the turbulent effects of tip vortex, is introduced to eliminate numerical singularity of Biot-Savart law. Then a general method used to deduce multi-step differencing scheme is given, and base upon this method, a new 3-step 3-order accurate predictor-corrector with backward difference algorithm—named PC3B is deduced to improve the accuracy of wake solution. On the other hand, blade aerodynamic model is based on Weissinger-L lifting surface model, which can be used to deduce blades bound circulation distribution, tip vortices strength and release point. The integrated rotor wake model is not only suitable for turbine's aerodynamic performance analysis during steady inflow conditions, but also complex inflow, which is absolutely unsteady.③Using 2-D static airfoil data to calculate airloads on HAWT's blades will underestimate turbine's power generation seriously when blades experiencing dynamic stall, so dynamic stall phenomena's influence must be considered seriously during design and checking process. The Beddoes-Leishman dynamic stall model, which is used for helicopter rotor's dynamic stall analysis originally, is introduced to HAWT's dynamic stall model after some modification. Good agreement is obtained between the predictions and the experimental results for pitch oscillations of S809 at several mean angles of attack and reduced frequencies.④The linear stability, accuracy, nonlinear stability and convergence are analyzed by deduced modified equations, which provide key insight into the nonlinear behavior of the numerical solution. Results shows that 3-step 3-order difference algorithm is stable for all values of time discretization, and which introduces extra implicit dissipation that is independent of the velocity gradients. Finally, numerical experiments were performed for a wind turbine to better understand the concept of the nonlinear stability and wake convergence of the time-marching method.⑤The time-marching free wake model is comprehensively validated against experimental measurements for rotor wake geometry and aerodynamic performance of TUDelft and NREL PhaseⅥ, with which operate under axial and yaw inflow condition. The results shows that: when the wind turbine suffers small axial inflow wind, the wake vortex can not be transported to distant places downstream, vortex-vortex interferences are significant, resulting in the blade tip vortex location and aerodynamic loads calculation error, but with wind speed increases, the vortex induced effects on the blade reduce, and the calculated values are good agreement with experimental data; when turbine suffers steady yaw, the present model needn't any amendments to predict asymmetric wake structure and cyclic loading caused by yaw error, which verify the validity of the free wake model.⑥When wind turbine suffers pitch operation, steady wind shear or extreme dynamic inflow conditions, the aerodynamic performance predicted by the free wake model shows that: there will be instantaneous rotor wake distortion, and the vortex shedding and transmitting to the downstream will take some time to make the wake from one flow pattern into another, resulting in induced velocity response lag, and the free wake model is able to reflect the lagged response of the aerodynamic performance. In additional, wind shear and dynamic inflow will create substantial asymmetries and non-periodicities in the structure of the wake. Conclusions reached by the calculation could support the optimized design and selection of HAWTs. |
PDF Full Text Request