Study on coherent structures in the flow field of single-cell and vortex breakdown tornado-like vortices

The coherent structures at different height planes within a vortex-breakdown tornado-like flow field are investigated using particle image velocimetry. Proper orthogonal decomposition (POD) of the experimental velocity field reveals that, in the flow field of tornadoes with vortex breakdown, the vortex is a single-cell structure at high altitude, and the main large-scale coherent structure is vortex wandering. At lower altitude, in the plane where vortex breakdown occurs, the coherent motion of vortex size variations intensifies significantly. The mechanisms whereby large-scale coherent motions (vortex wandering and vortex size variations) influence the fluctuating velocity and Reynolds stresses in the vortex flow field within different vortex structures are determined through the low-order reconstruction of modes and analysis of modal stresses. An investigation of numerically synthesized vortex POD modes explains the origins of POD spatial modes for vortex wandering and vortex size variations in single-vortex and vortex-breakdown tornado-like flow fields. Furthermore, by identifying fewer modes in the velocity fields simulated by Burgers' theoretical model, a simplified model capable of representing the fluctuating velocity in single-vortex tornado flow fields is constructed and found to be in good agreement with experimental results.