Computational Fluid Dynamics Analysis of Floating Offshore Wind Turbines in Severe Pitching Conditions

Abstract

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.

Multi-fidelity Analyses of Rotor Loads of Floating Offshore Wind Turbines with Wind/Wave Misalignment

“…The maximum differences between BEMT and CFD P estimates are observed with wind misalignment, with a difference of 4.7% of the BEMT and ARCTIC HB4 power for the FBWT. As expected, the FLUENT P estimate for the FBWT at β=0 o is lower than those obtained with the same grid and a steady simulation (no shaft tilt) in [3].…”

“…The physical domain of the ARCTIC simulations is cylindrical, with the rotor center on the cylinder centerline 10 rotor radii from the inlet circular boundary at the front, 20 radii from the outlet circular boundary at the back, and 11 radii from the cylindrical far field boundary. The underlying principles for the choice of rotor domain sizes are discussed in [3]. Characteristic far field boundary conditions are enforced on all far field boundaries.…”

“…The grid adopted for all FLUENT simulations is that used in [3] and [8]; it was shown to provide mesh independent solutions in [3]. It differs from the ARCTIC grid only in that it has 9,879,552 cells.…”

“…The time-dependent inclination of the tower is θp=Θp sin(Ωpt+φp), where Θp is the user-given tower pitching amplitude, and φp is the initial phase between rotor and tower angular positions, which remains constant when Ωr is an integer multiple of Ωp. The mathematical formulation of grid displacements and velocities resulting from the composition of rotor rotation and tower pitching implemented in ARCTIC and COSA is in [3]. All analyses herein use a close-to-rated wind speed of 11 m/s and a rotor speed of 12 RPM, corresponding to a frequency of 0.2 Hz.…”

“…Resolving with adequate reliability the underlying multidisciplinary load-driving physics is also key to FOWT control design. Moreover, the development of FOWT wind farms also requires analyzing the generation process [3] and the propagation characteristics [4] of FOWT wakes, which may differ from those of FBWTs. The study of [3] also investigated the extent of compressible flow effects in FOWT rotor performance, a phenomenon previously found to be negligible in large FBWTs [5].…”