On the Breakdown of Helical Wake behind a Rotating Blade in Thermally Stratified Atmosphere

Abstract

In this thesis, a fundamental study on the breakdown mechanisms of a helical flow is conducted. The helical flow shares common traits with the wind turbine wake flow. The simulation data generated by a Direct Numerical Simulation (DNS) of flow around a rotating blade in a thermally stratified atmosphere were analysed in the physical space before two modal analysis techniques which are Dynamic Mode Decomposition (DMD) and Proper Orthogonal Decomposition (POD) were employed to the dataset. The DMD and POD analyses were able to determine the dynamics of the wake behind the rotating blade, i.e., the dominant mode of the helical flow structures and the corresponding frequency spectrum. As a result, the identification of the coherent structures and dynamics of the helical vortices behind the rotating blade in the stratified atmosphere is presented with the discussion on how the stratified temperature field influenced the deformation and breaking down of the helical wake structure. Various modes showed different characteristics of the flow fields. Focuses on the energy of flow; the POD technique could capture large-scale vortex structures and their organized behaviour, whereas the DMD method focuses on the frequency, and it represented the perturbation dynamics. Overall, the helical wake structure behind a rotating blade was proved to be remarkably influenced by the variation of the thermal stratification in terms of their characteristics, dynamics, and stability. Among all, the most affected is the one from the weakly stable stratified atmosphere.