The Physics of H Darrieus Turbine Starting Behavior

Abstract

This paper provides a resolution to the contradictory accounts of whether or not the Darrieus turbine can selfstart. The paper builds on previous work proposing an analogy between the aerofoil in Darrieus motion and flappingwing flow mechanisms. This analogy suggests that unsteadiness could be exploited to generate additional thrust and that this unsteady thrust generation is governed by rotor geometry. Rotors which do not exploit this unsteadiness will not selfstart. To confirm the hypothesis, unsteady effects were measured and then incorporated into a timestepping rotor analysis and compared to experimental data for selfstarting wind turbines. When unsteady effects were included, the model was able to predict the correct starting behavior. The fundamental physics of starting were also studied and parameters that govern the generation of unsteady thrust were explored, namely, chordtodiameter and blade aspect ratios (ARs). Further simulation showed that the Darrieus rotor is prone to be locked in a deadband where the thrust is not continuous around a blade rotation. This discrete thrust is caused by the large variation in incidence angle during startup, making the Darrieus blade ineffective during part of the rotation. The results show that unsteady thrust can be promoted through an appropriate selection of blade aspect and chordtodiameter ratios, therefore selfstarting rotors may be designed. A new definition of selfstarting is also proposed.