Kavli Affiliate: Feng Wang
| First 5 Authors: Feng Wang, Yi-Bao Zhang, Ping-Fan Yang, Heng-Dong Xi,
| Summary:
Investigations into the effects of polymers on small-scale statistics and
flow patterns were conducted in a turbulent von Karman swirling (VKS) flow. We
employed the tomographic particle image velocimetry (Tomo-PIV) technique to
obtain full information on three-dimensional velocity data, allowing us to
effectively resolve dissipation scales. Under varying Reynolds numbers
($R_lambda=168 – 235$) and polymer concentrations ($phi=0 -25~rm ppm$), we
measured the velocity gradient tensor (VGT) and related quantities. Our
findings reveal that the ensemble average and probability density function
(PDF) of VGT invariants, which represent turbulent dissipation and enstrophy
along with their generation terms, are suppressed as polymer concentration
increases. Notably, the joint PDFs of the invariants of VGT, which characterize
local flow patterns, exhibited significant changes. Specifically, the
third-order invariants, especially the local vortex stretching, are greatly
suppressed, and strong events of dissipation and enstrophy coexist in space.
The local flow pattern tends to be two-dimensional, where the eigenvalues of
the rate-of-strain tensor satisfy a ratio $1:0:-1$, and the vorticity aligns
with the intermediate eigenvector of the rate-of-strain tensor while is
perpendicular to the other two. We find that these statistics observations can
be well described by the vortex sheet model. Moreover, we find that these
vortex sheet structures align with the symmetry axis of the VKS system and
orient randomly in the horizontal plane. Further investigation, including flow
visualization and conditional statistics on vorticity, confirms the presence of
vortex sheet structures in turbulent flows with polymer additions. Our results
establish a link between single-point statistics and small-scale flow topology,
shedding light on the previously overlooked small-scale structures in polymeric
turbulence.
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