  The coherent vortical structures in turbulent flow through a strong 16:1 3-D
contraction, are studied using time-resolved volumetric measurements.
Visualization using vorticity magnitude criterion shows the emergence of long,
stretched cylindrical vortices aligned with the mean flow. This alignment is
quantified by PDFs of the direction cosines. We propose two measures to
quantify the alignment, the peak height in the probability and a coefficient
from the moment of the PDF, both of which reaffirm the strong streamwise
alignment. The r.m.s. streamwise vorticty grows within the contraction to
becoming 4.5 times larger than the transverse component, at the downstream
location where the contraction ratio C=11. The characteristic vortices become
as long as the measurement volume, or more than 4 times the integral scale at
the entrance to the contraction. We also characterize the vorticity enhancement
along individual vortices, measuring 65% strengthening over the distance where
C goes from 4 to 11. The prevalence of these coherent structures is estimated
from 700,000 measured volumes, showing that near the outlet it is more likely
to have 1 or 2 of these structures present than none.
