Investigating the Effects of Helical-shaped Blades on the Wake Characteristics and Performance of Vertical Axis Wind Turbines using Large Eddy Simulations

Date

2023-08

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Abstract

Turbulent wake flows behind helical- and straight-bladed vertical-axis wind tur bines (VAWTs) are studied numerically using the large eddy simulation method com bined with the actuator line model. The effects of blade geometry on turbine wake characteristics are explored at both lab- and field-scale VAWTs differed by their oper ational tip-speed ratios (TSRs). For the lab-scale VAWT, a small-size 5-blade VAWT design is considered, which operates at relatively low TSRs of 0.4-0.6 at the wind speed of 13m/s. For the field-scale VAWT, a commercial 3-blade VAWT is consid ered, which operates at a higher TSR of 1.19 at the wind speed of about 11 − 12m/s. In both cases, the wake flows behind straight- and helical-bladed VAWTs were sim ulated, and the flow characteristics were quantified and compared. At low TSRs, the simulation results show that the wake behind the straight-bladed VAWT expands considerably in the spanwise direction due to the quasi-2D vortex shedding effect from the straight blades. In contrast, the helical-bladed VAWT generates highly 3D wake flow structures to produce a relatively narrow wake with faster decay of turbulent intensity. At high TSRs, the helical-bladed VAWTs generate a screwdriver effect to induce mean vertical flow motions at the spanwise edges of the wake flow, which are balanced by a mean vertical counter-flow (i.e., with reversed direction) near the center of the wake. As a result, the wake flows behind helical VAWTs exhibit vertical tilting that affects the turbulent intensity and the Reynolds transport of momentum in the shear layers around the VAWT wake region, leading to faster wind speed recovery than the wake behind straight-bladed VAWTs. Suppose the different VAWT designs are used in wind farm applications. In that case, the field-scale helical-bladed VAWTs may improve the mean power production rate for the fully developed region of the wind farm by up to about 7.33% compared with the corresponding straight-bladed VAWT. Using the helical-bladed VAWTs also reduces the fatigue load on the structure by significantly reducing the spanwise bending moment (relative to the bottom base), which may improve the longevity of the VAWT system to reduce the long-term maintenance cost.

Description

Keywords

Large Eddy Simulatios, Vertical-Axis Wind Turbines, Boundary Layer, Helical-bladed Vertical-Axis Wind Turbines, Windfarm, Power Performance, Bending Moment.

Citation

Portions of this document appear in: Gharaati, Masoumeh, Shuolin Xiao, Nathaniel J. Wei, Luis A. Martínez-Tossas, John O. Dabiri, and Di Yang. "Large-eddy simulation of helical-and straight-bladed vertical-axis wind turbines in boundary layer turbulence." Journal of Renewable and Sustainable Energy 14, no. 5 (2022)