Estimation of blade loads for a variable pitch Vertical Axis Wind Turbine with strain gage measurements

Abstract

The blade pitch of a Vertical Axis Wind Turbine can have a profound impact on the aerodynamic loading experienced by the turbine. This in turn impacts the structural loads and the performance of the machine. In order to characterize the effects of changing pitch, studies are conducted with fixed pitch offsets from a neutral pitch position in the open jet wind tunnel of TU Delft. Measurements with strain gages bonded to the turbine struts are used to estimate the normal loading of the blades.The measured behavior gives insights into the sensitivity of the turbine loading to the blade pitch angle. Aerodynamic phenomena associated with VAWTs are evident in the data including dynamic stall and blade vortex interaction. Shifting of turbine blade pitch is shown to alter the azimuthally varying normal loading, causing changes in magnitude and direction of rotor thrust. Frequency responses of the turbine and platform mounting structure are presented for rotating and fixed reference frames, respectfully. The effects of stall due to high pitch offsets is shown to excite higher per rev frequencies in both the rotor normal measurements and platform accelerations. The data sets are made available for validation of numerical models.

Blade strain analysis from field measurements on a vertical axis wind turbine

“…This normal load coefficient should be understood as a "flapwise strain coefficient" representing the level of the blade's flapwise bending. It is useful to compare its order of magnitude and its evolution in relation to other experimental parameters, but it is not accurate enough to be used for quanti-220 tative comparison with normal load coefficient obtained through other methods [29,30].…”