Reliable predictions of the aero-and hydrodynamic loads acting on floating offshore wind turbines are paramount for assessing fatigue life, designing load and power control systems, and ensuring the overall system stability at all operating conditions. However, significant uncertainty affecting both predictions still exists. This study presents a cross-comparative analysis of the predictions of the aerodynamic loads and power of floating wind turbine rotors using a validated frequency-domain Navier-Stokes Computational Fluid Dynamics solver, and a state-of-the-art Blade Element Momentum theory code. The considered test case is the National Renewable Energy Laboratory 5 MW turbine, assumed to be mounted on a semi-submersible platform. The rotor load and power response at different pitching regimes is assessed and compared using both the high-and low-fidelity methods. The overall qualitative agreement of the two prediction sets is found to be excellent in all cases. At a quantitative level, the high-and low-fidelity predictions of both the mean rotor thrust and the blade out-of-plane bending moments differ by about 1 percent, whereas those of the mean rotor power differ by about 6 percent. Part of these differences at high pitching amplitude appear to depend on differences in dynamic stall predictions of the approaches.
A multi-scale computational fluid dynamics analysis of wind turbine blade leading edge erosion is presented. The test case is a large set of eroded blade sections. These are obtained by fitting the resolved eroded leading edge geometry of the outboard part of a multi-megawatt offshore wind turbine to the NACA633-618 airfoil. The erosion geometry measured by a blade laser scan is geometrically resolved in the aerodynamic simulations, whereas the aerodynamic effects of unresolved lower-amplitude scales are accounted for by using distributed surface roughness models. The simulations also account for the laminar-to-turbulent transition of the blade boundary layers with and without distributed roughness. An existing semi-empirical model and simulations of the nominal airfoil enable one to estimate the roughness level needed to trip leading edge boundary layer transition at the considered Reynolds number of 9 million. It is found that a) the mean roughness heights of the observed geometry perturbations are well above the critical roughness height, and b) consideration of either large or small erosion scales in isolation results in underestimating the airfoil performance impairment.
The unsteady aerodynamics of floating offshore wind turbines is more complex than that of fixed--bottom turbines, and the uncertainty of low-fidelity predictions is higher for floating turbines. Navier--Stokes CFD can improve the understanding of rotor and wake aerodynamics of floating turbines, and help improving lower-fidelity models. Here, blade--resolved simulations of the compressible CFD COSA code and the incompressible CFD FLUENT code are used to investigate the unsteady flow of the NREL 5 MW rotor subjected to prescribed harmonic pitching past the tower base. CFD results are compared to predictions of the FAST wind turbine code, which uses blade element momentum theory. The rotor power and loads in fixed--tower mode predicted by both CFD codes and FAST are in very good agreement. For the floating turbine, all predicted periodic profiles of rotor power and thrust are qualitatively similar, but the power peaks of both CFD predictions are significantly higher than those of FAST. Cross--comparisons of the COSA and FLUENT CFD profiles of blade static pressure also highlights significant compressible flow effects on rotor power and loads. The CFD analyses of the downstream rotor flow field reveals wake features unique to pitching turbines, primarily the space- and time--dependence of the wake generation, highlighted by the intermittency of the tip vortex shedding. The FLUENT pitching rotor analyses use a novel user--defined function, enforcing an additional rigid body motion of the grid conformal to the tower motion, providing new functionalities for floating turbine analyses.
Reliable predictions of the aero- and hydrodynamic loads of fixed-bottom and floating offshore wind turbines are paramount for assessing fatigue life and designing load and power control systems. However, significant uncertainty affecting aerodynamic predictions still exists. This study presents cross-comparative analyses of the predictions of aerodynamic loads and power of fixed-foundation and floating wind turbine rotors with and without yaw errors using time- and frequency-domain Navier-Stokes Computational Fluid Dynamics, and the Blade Element Momentum theory. The considered test case is the National Renewable Energy Laboratory 5 MW reference turbine, assumed to be mounted in the floating case on a semi-submersible platform and undergoing pitching motion about the tower base. Although the overall qualitative agreement of the low- and high-fidelity predictions is found to be fair in all cases, for the considered regimes the agreement between the two methods is better for the pitching rotor in aligned wind than for the yawed flows regardless of the tower motion.
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