The scaling of rotor inertia under dynamic inflow conditions

Abstract Rotor moment of inertia significantly influences the performance of rotating systems, such as wind and tidal turbines, when operating in unsteady environments. However, rotor moment of inertia in itself does not fully define the unsteady behaviour of such rotating systems without taking flow inertia, e.g. the gust acceleration, into consideration. We therefore introduce a normalized inertia number ( I ∗ ), which relates the influence of rotor moment of inertia relative to the flow inertia in order to characterize the dynamic response of a generic-rotor system. Experiments are performed in a towing-tank facility using three geometrically-identical rotor models with weighted tips so as to investigate the influence of I ∗ on the rotor’s ability to adapt to changing inflow conditions. Gust profiles with four gust durations were tested, ranging from slow quasi-steady operation to rapid disturbances. Rotors operating with sufficiently low I ∗ are found to produce higher normalized power during the fastest tested gust when compared to the quasi-steady operation. For instance, the power output of the rotor model with ( I ∗ = 3 . 2 × 1 0 − 3 ) is, at its maximum, 30% higher during the fastest gust versus quasi-steady operation. However, by increasing I ∗ on the same rotor model to 8 . 7 × 1 0 − 3 , by decreasing the flow inertia, the rotor model produced, at maximum, only 19% higher power output. Therefore, we are able to show that our normalization I ∗ is a reasonable way to determine the dynamic performance of a rotating system during a gust.