Study Of Wind Turbine Aerodynamic Characteristics Based On Vortex Wake Methods

The operation environment of wind turbine is highly unsteady three-dimensional flow because ofthe complex flow in the atmosphere near the ground. The researchers have focused on the windturbine aerodynamics and the challenges in the unsteady aerodynamic simulations. Considering theaccuracy and the economy, the free vortex wake method, which owns a verticity feature essentially, isperhaps the more suitable tool of simulations.The blade aerodynamic model is based on the Weissinger-L lifting surface model, which has highefficiency and can take into account the three-dimensional effect of blade. An engineering method isdeveloped to consider the nonlinearity of the airfoil lift and drag with angle of attack. A viscos vortexcore model, validated by a vortex ring example, is introduced into induced velocity calculation basedon Biot-Sarvart law to eliminate numerical singularity. On the other hand, the relaxation iterativealgorithm and the time-marching algorithm have been developed for the free wake method. In therelaxation iterative algorithm, a five-point central difference approximation is used for the spatialderivative and the temporal derivative. The dummy period concept is used to improve theconvergence speed and stability of wake iterations with an introduction of the adaptive relaxation. Anew three-step and third-order predictor-corrector time-marching difference algorithm named D3PC isderived using the linear multistep method.Six existing three dimensional rotational effect models, of which the Du-Selig model is chosen tomake simulations for this paper, have been analysed and compared. The three dimensional rotationaleffect models implemented into the vortex wake methods can improve the accuracy for theaerodynamic performance prediction of wind turbines. A new dynamic stall model is developed basedon the two-dimensional Leishman-Beddoes model according to the three dimensional rotational effect.The lift slope and two angles of attack associated with the flow separation point are corrected in thenew model. Comparison between the computation results and experimental data of the NREL phaseⅥ blade demonstrates that the agreement of the lift coefficient is quite good and the accuracy ofthe drag coefficient prediction is improved. A unsteady time-accurate model by coupling of the freevortex wake model and the new dynamic stall model is used to calculate the blade aerodynamic loadsof the NREL phase Ⅵ blade in the yaw. It is showed that the results from the new dynamic stallmodel are closer to the experimental data than the2-D L-B model.The wake geometry and the steady performance of the NREL phase Ⅵ blade are calculated by the free vortex wake method(relaxation iterative algorithm). The distortion and free rolling up of vortexfilaments can be simulated by the free vortex wake model. The wake structure agrees with theexperimental result and the positions of tip vortex cores are consistent with the CFD results. Theaerodynamic loads and performance are also consistent with the experimental results. Avortex-surface/vortex-ring coupled method is developed based on the characteristic of free rolling upof vortex filament. High efficiency and high accuracy of the new coupled method is validated bycomputering the steady performance of the NREL phase Ⅵ blade. The power coefficients of ahigh-performance1.5MW wind turbine blade named NH1500are calculated by the free vortex wakemethod. All the power coefficients from the experiment are lower than that from the free vortex wakemethod generally. This is reasonable because the Reynolds number for the1/16-scale model used inthe wind tunnel test is certainly lower than that for the full-scale blade at the same tip speed, resultingin lower lift and higher drag, and moreover the free vortex wake method take into account theviscostity in the vortex core but not in the whole flow fields.The unsteady aerodynamic characteristics of a large-scale offshore floating wind turbine NREL5-MW are calculated by present unsteady time-accurate model. The load conditions include extremeoperating gust, extreme direction change and the floating platform motions. The response curvesof aerodynamic performance under the disturbance of the flow environment are obtainedsuccessfully. The wake geometries behind the wind turbine at different times are given and thewake reorganising process is observed. The effects due to different floating platform motions arecompared, among which the pitching motion has the greatest influence on the rotor power.A free-wake/panel coupled method is developed to investigate the aerodynamic interactions of thewind turbine rotor and the combination of the nacelle and tower. The low-order panel method is usedin the flow over the combination. The nacelle influence on the power coefficient is slight because ofthe nacelle size is too small to the rotor. It is clear that the combination has obvious influence on theinduced velocity distribution of the rotation plane especially at the blade root in the yaw. The vortexfilaments trailing from the blade root are distorted in the side of the nacelle after wake skewing.