  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.
