The vortex model has been extended to account for large changes in power, such as occur with abrupt changes in blade pitch. The wake is treated as a flexible spring, extending from the rotor to the far-field (Trefftz plane), along which the pitch varies according to the connected power history at the rotor. Results of test cases are compared with experimental data available from the Tjaereborg and NREL experiments, indicating that the code gives not only correct power levels asymptotically, but also predicts the transient overshoots and undershoots and recovery time accurately at a very low cost.
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