Coherent turbulent structures in a rapid contraction

The coherent vortical structures in turbulent flow through a strong 16 : 1 three-dimensional contraction are studied using time-resolved volumetric measurements. Visualization using the vorticity magnitude criterion shows the emergence of long, stretched cylindrical vortices aligned with the mean flow. This alignment is quantified by probability density functions (p.d.f.s) 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 p.d.f., both of which reaffirm the strong streamwise alignment. The root mean square streamwise vorticity grows within the contraction to become 4.5 times larger than the transverse component, at the downstream location where the contraction ratio is $C=11$. 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 one or two of these structures present than none.