Spacial characteristics of vortices in a channel flow – vortex generation with shear independence

The present study investigates the topological characteristics of vortices in the generation and development in terms of the local topology, with relating to the spacial or shear condition of a channel flow. The generation process of vortices can be followed in the pre-swirl coordinate system and vortex space coordinate system that are Galilei invariant associated with (predicted) swirl plane. Then directions of the swirl planes of vortices in the generation and after development are analyzed, relating them to the primary directions of the turbulent flow. The analysis clarifies that a vortex near the viscous sublayer is generated with a swirl plane parallel to the wall of the channel flow, which indicates that the intense shear in this region does not support in the vortex generation in terms of the local flow, or rather prevent it. A vortex in a flow region with less shear is generated with random direction that may be similar to the vortex generation in homogeneous turbulence. On the other hand, if a vortex is developed, an effect of shear on the direction of the swirl plane decreases. These results indicate that the shear does not always support the vortex generation and that the contribution of the generation depends on its characteristics.