Influence of tower shadow on downwind flexible rotors: Field tests and simulations

Abstract

As wind turbine rotors become larger, the blades become more flexible, requiring extra stiffness and cost to avoid the risk of tower strike. Wind turbines in a downwind configuration have a reduced risk of tower strike because the rotor thrust acts away from the tower. However, downwind blades pass through the wake of the tower, and the resulting load variation may contribute to blade fatigue. To date, there have been no field tests to quantify this tower shadow effect on unsteady blade moments. The present study reports on the first field testing of a flexible, downwind, coned rotor and compares the experimental data against simulations run in Open-FAST. The tower shadow effect is simulated using the conventional Powles model and a new Eames model (developed herein), which includes the influence of upstream turbulence. Both models reasonably predict the blade root out-of-plane bending moment data and the tower shadow dip magnitude when compared to field test data in Region 3. Tower shadow was found to increase the short-term Damage Equivalent Loads (DELs) by less than 10% compared to other effects (gravity, shear, and turbulence), and the predictions were consistent with experiments. These results indicate that the tower shadow effect can be reasonably modeled with the simpler Powles model and that the tower shadow effect can be small compared to the effect of turbulence. However, long-term fatigue due to tower shadow should be included in detailed structural analysis and design of the rotor and the tower.

Field tests of a highly flexible downwind ultralight rotor to mimic a 13-MW turbine rotor

“…Thus, based on the results from this field test, tower shadow appears to have minimal impact on flapwise moments of flexible downwind rotors. Additional analysis, including fatigue, can be found in [30]. It is recommended that future downwind turbine designs include a tower shadow model to capture loading and fatigue effects on the rotor.…”

Experimental Study on Aerodynamic Characteristics of Downwind Bionic Tower Wind Turbine

“…In addition, field tests were carried out to quantify the influence of tower shadows on unsteady-state downwind forms. Simpson et al [ 17 ] used field test data and OpenFAST software (AeroDyn V15) to evaluate and model the tower shadow effect of downwind wind turbines. The results verify that a downwind wind turbine was feasible, and the tower shadow effect was relatively small compared with turbulence and wind shear.…”