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.
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.
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.
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.
The unsteady aerodynamics of floating offshore wind turbine (FOWT) rotors is more complex than that of fixed–bottom turbine rotors, and the uncertainty of low-fidelity aerodynamic predictions, such as those of the blade element momentum theory (BEMT) codes, is higher for the former rotors. Navier-Stokes CFD can improve the understanding of FOWT rotor and wake aerodynamics, and help improve lower-fidelity models. To highlight this potential, blade–resolved analyses of the in-house compressible CFD COSA code and the commercial incompressible CFD code FLUENT were used to investigate the unsteady flow of the NREL 5 MW rotor subjected to prescribed harmonic pitching past the tower base. CFD results were compared to the predictions of the FAST wind turbine code, using BEMT for rotor aerodynamics. Improved tuning of the COSA numerical set-up enabled close matching of the FAST rotor power and loads in fixed–tower mode, and high resolution of the near field wake dynamics; similar agreement levels were obtained with the FLUENT simulations. A novel user-defined function approach, enforcing an additional rigid body motion of the rotor grid conformal to the tower motion, enabled the FLUENT FOWT rotor simulations of this study, and provided new general-purpose FLUENT functionalities for FOWT analyses. All predicted periodic patterns of rotor power and thrust were found to be qualitatively similar, but the power peaks of both CFD predictions were significantly higher than those of FAST. Inspection of the CFD profiles of blade static pressure highlighted and quantified significant compressible flow effects on FOWT rotor power and loads. The blade–resolved analyses of the rotor downstream flow field revealed wake features unique to pitching turbine rotors, primarily the space- and time–dependence of the wake generation, highlighted by the intermittency of the tip vortex shedding.
The flow field of a non-premixed industrial gas burner is analysed with Reynolds-averaged Navier Stokes computational fluid dynamics validated against velocity and pressure measurements. Combustion is not modeled because the aim is optimizing the predictive capabilities of the cold flow before including chemistry. The system's complex flow physics, affected by a 90° turn, backward and forward facing steps, and transversal jets is investigated at full and partial load. The sensitivity of the computed flow field to inflow boundary condition setup, approach for resolving/modeling wall bounded flows, and turbulence closure are assessed. In the first sensitivity analysis, the inflow boundary condition is prescribed using measured total pressure or measured velocity field, and the boundary layers are resolved down to the wall or modeled with wall functions. In the second sensitivity analysis, the turbulence closure uses the k-ω shear stress transport eddy viscosity model or two variants of the Reynolds stress model. The agreement between the predictions of most simulation set-ups among themselves and with the measurements is good. For given type of inflow condition and wall flow treatment, the ?-based Reynolds stress model gives the best agreement with measurements among the considered turbulence models at full load. At partial load, the comparison with measured data highlights some scatter in the predictions of different patterns of the flow measurements. Overall, the findings of this study provide insight into the fluid dynamics of industrial gas burners, and guidelines for their simulation-based analysis.
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