Tubular limiting stream surface: “tornado” in three-dimensional vortical flow

Abstract

A new physical structure of vortical flow, i.e., tubular limiting stream surface (TLSS), is reported. It is defined as a general mathematical structure for the physical flow field in the neighborhood of a singularity, and has a close relationship with limit cycles. The TLSS is a tornado-like structure, which separates a vortex into two regions, i.e., the inner region near the vortex axis and the outer region further away from the vortex axis. The flow particles in these two regions can approach to (or leave) the TLSS, but never could reach it.