New method to capture traveling waves in flow passing a wind turbine

Flow structures in the wake of a wind turbine are analysed by performing spatio-temporal Koopman decomposition (STKD). STKD is a new method to decompose the flow field as a combination of travelling waves and to extract the frequency and wavenumbers (characterizing a specific spatial direction) of the dominant waves. The results indicate that the complex unsteady behaviour of the turbine wake flow is mainly driven by a single wave travelling downstream, combined with smaller amplitude traveling waves representing the flow rotation. This result agrees with the general description of the wake of a wind turbine regarded as the combination of self-sustained oscillations and the development of upstream external noise, which are represented by the dominant traveling wave and the small-magnitude ones, respectively.