# Alteration of helical vortex core without change in flow topology

## TL;DR

This paper reports experimental observations of a helical vortex core enlarging abruptly in turbulent flow without altering the flow's topological structure, highlighting a new form of vortex core alteration.

## Contribution

It introduces the novel finding that vortex core size can change abruptly without topological transition, under the influence of non-uniform pressure gradients.

## Key findings

- Vortex core size can increase abruptly without topology change.
- Helical symmetry is preserved despite core growth.
- Pressure gradients influence helical pitch without reversing flow topology.

## Abstract

The abrupt expansion of the slender vortex core with changes in flow topology is commonly known as vortex breakdown. We present new experimental observations of an alteration of the helical vortex core in wall bounded turbulent flow with abrupt growth in core size, but without change in flow topology. The helical symmetry as such is preserved, though the characteristic parameters of helical symmetry of the vortex core transfer from a smooth linear variation to a different trend under the influence of a non-uniform pressure gradient, causing an increase in helical pitch without changing its sign.