Dynamic mode structure analysis of the near-wake region of a Savonius-type hydrokinetic turbine

Hydrokinetic turbine is a clean technology that can effectively use tidal energy, the Savonius-type runner is very suitable for natural flows, and has been widely used in tidal energy power stations. The dynamic structure of the flow field in the near-wake region downstream has strong impact on the dense layout of the turbine, but there has been no in-depth exploration till now. To explore the potential for beneficial interactions between turbines, this study conducted numerical simulations of Savonius-type turbine, combining fast Fourier transformation and Dynamic mode decomposition to conduct in-depth analysis of the velocity fluctuation frequency distribution and dynamic mode in near-wake region, and compared the advantages and disadvantages of the two methods. This research obtained the distribution of two main frequencies in the near-wake region and clarified their excitation sources. The frequencies of turbines are mixed in the far field, greatly affects the far-field flow state in the near-wake region. The DMD method has great advantages in flow field analysis, as it does not require prior knowledge of flow field compared to FFT method. The conclusions of this study provide new considerations for large-scale arrangement of turbine units, and provide a new method for the flow state analysis.