Interaction of Atmospheric Turbulence with Blade Boundary Layer Dynamics on a 5MW Wind Turbine using Blade-Boundary-Layer-Resolved CFD with hybrid URANS-LES

摘要

Modern commercial megawatt-scale wind turbines occupy the lower 15-20% of the atmospheric boundary layer (ABL), the atmospheric surface layer (ASL). The current trend of increasing wind turbine diameter and hub height increases the interaction of the wind turbines with the upper ASL which contains spatio-temporal velocity variations over a wide range of length and time scales. This work centers on the development of a computational framework to simulate the interaction between the atmospheric and wind turbine blade turbulence dynamical systems using a two step one-way coupled approach. Pseudo-spectral large eddy simulation (LES) is used to generate a true (equilibrium) atmospheric boundary layer over a flat land with specified surface roughness and heating consistent with the stability state of the daytime lower troposphere. Using the data from the precursor simulation as inflow conditions, a second simulation is performed on a smaller domain around the wind turbine using finite volume CFD with a body-fitted grid to compute the unsteady blade loads in response to atmospheric turbulence. The ability of our computational framework to capture blade boundary layer dynamics in response to atmospheric turbulence is intimately associated with the design of our grid and with the development of a new hybrid URANS-LES turbulence model. The new turbulence model blends a 1-equation LES subgrid model in the far field with the k-w-SST-SAS URANS model to the blade boundary layer adjacent to the blade surface. With this computational framework, we simulated a single rotating blade of the NREL 5MW wind turbine in the moderately convective daytime atmosphere using blade-boundary-layer-resolved CFD simulations. The analysis of load fluctuations on a single rotating blade in a daytime atmosphere using blade-boundary-layer-resolved CFD has yielded two key results: (1) Whereas non-steady blade loadings are generally described as the response to non-steadiness in wind speed, our analysis show that time changes in wind vector direction are a much greater contributor to load transients, and strongly impact boundary layer dynamics; (2) largest temporal variations in loadings result from three distinct dynamical responses with disparate time scales: advection of atmospheric eddies through the rotor at the minute time scale, blade response at the rotor rotation time scale (～ 5s) and blade response to turbulence-induced forcings as the blades traverse internal atmospheric eddy structure at sub-blade rotation time scales. In our simulations at rated wind speed, quasi-2D blade boundary layer separation is observed over most of the outer 50% of the blade with chordwise motions, correlated with time changes in relative wind vector angle, which itself is strongly correlated with changes in blade sectional and integrated loads. Thus, tools based on sectional "table lookups" like FAST and Actuator Line Methods, improved using data from high-fidelity simulations and experiment, have the potential to capture the major fluctuations in integrated loads from daytime atmospheric turbulence.