The vorticity field and coherent structures in turbulent flow through rapid contractions*

Following the work of Mugundhan et al. [1] we measure the three-dimensional velocity and vorticity r.m.s in four different contractions: Three different 2-D, 4:1-contractions with different lengths and a more rapid 3-D 16:1-contraction. An active-grid generates turbulence which is advected through the contractions in a vertical gravity-driven water tunnel.  Grid-rotation is at 240 rpm, Re_l of 220 at the inlet. We use Tomo-PIV to obtain velocities in 1 to 3 regions along the streamwise direction inside contractions. Up to 50,000 time-resolved volumes of 3-D velocities are obtained by tracking ~140,000 particles with shake-the-box PTV algorithm. This allows us to compute velocity and vorticity through these contractions. This shows decay in streamwise velocity r.m.s, u_rms and increase in transverse r.m.s, v_rms.  In the strongest contraction, u_rms after an initial decay, increases tending towards isotropy. This has been characterized as an “anomaly” at high-contraction ratios in point measurements [2]. The streamwise vorticity r.m.s gets amplified by ~3 in the 3-D contraction. Visualization of coherent structures using vorticity magnitude criteria shows the existence of long, stretched tubular structures aligned with the mean flow. This alignment is quantified by PDFs of cosine of alignment angles reaffirmed their preferential alignment. With the strongest-straining 3-D contraction, the relavite peak in the PDF values grows from  ~5 near the contraction inlet to ~50 near its exit.