Wind tunnel test on the effect of solidity on the wake evolution characteristics of twin counter rotating vertical axis turbines

The wake of a vertical axis turbine is a type of unsteady flow with multi-scale coherent structures formed by a series of complex spatiotemporal evolutions of blade vortex shedding. The evolution characteristics and interaction of multi-scale coherent structures between two turbine wakes are the bases of multi-turbine optimal layout. In this study, the wake time series and turbine aerodynamic output were measured to explore the effect of solidity on the wake evolution characteristics of twin counter rotating vertical axis turbines. The dynamic evolution characteristics of multi-scale coherent structures in a wake were presented by a wavelet analysis. The results show that with the increase in solidity, the wake of the twin turbines gradually merges and is dominated by the shedding instability of the low-frequency large-scale structures. When the solidity remains unchanged and the chord length Reynolds number increases gradually, modulation mode structures appear between the low-frequency large-scale structures and rotation-induced structures. After the rotation-induced structures lose their coherence, energy transfers to a low frequency under the effect of modulation instability, forming self-organized large-scale structures.