Turbulent Structures of Non-Newtonian Solutions in Taylor-Couette Flow
Creators: Li, Haozhe
Publisher: name: University of Alberta Library
Year: 2024

<p>The turbulent structures of Taylor-Couette (T-C) flow of a Polyacrylamide (PAM) solution<br>
(200 and 400 ppm) is experimentally investigated and compared with the same flow with<br>
water at Re = 1.50 × 104, to understand the effect of nonlinear shear-thinning viscosity on<br>
the flow regime, in terms of turbulence statistics, coherent structures, turbulence production,<br>
and viscous dissipation. The turbulent Taylor-Couette (TTC) flow regime is achieved and<br>
driven by inner cylinder rotation (outer cylinder stays stationary). Velocity measurements<br>
are conducted by employing Stereoscopic-Particle Image Velocimetry (SPIV) to obtain three<br>
velocity components within a rectangular field of view (FOV) in radial-axial (r − z) (merid-<br>
ional) plane, and the torque acted on the inner cylinder is recorded to obtain skin friction<br>
coefficient Cf,l.<br>
The torque measurements show that with the addition of shear-thinning polymer, 41%<br>
and 52% drag reduction is achieved for 200 ppm and 400 ppm solutions both under Re =<br>
1.50 × 104, respectively. The velocity measurement of the in-plane velocity shows that the<br>
motions of non-Newtonian Taylor vortices are more flexible compared to the Newtonian case<br>
in both the axial and radial direction induced by the shear-thinning effect. The turbulence<br>
statistics show that wall-normal (r) and spanwise (z) Reynolds normal stresses are signif-<br>
icantly attenuated and smeared out across the bulk domain as the concentration of shear-<br>
thinning polymers increases. The radial jet (both inner and outer jets) intensity is reduced<br>
with the addition of shear-thinning polymer. Snapshot Proper Orthogonal Decomposition<br>
(POD) of in-plane velocity fluctuations field shows that vortical-shape fluctuations are the<br>
dominant energetic structures for both Newtonian and non-Newtonian T-C flows that sta-<br>
tistically contribute to most of the turbulence kinetic energy (TKE) for both Newtonian and<br>
non-Newtonian fluids. The shear layer structure changes due to the shear-thinning effect,<br>
which is reflected by the second POD mode. The Snapshot POD of the azimuthal veloc-<br>
ity fluctuation field captures energetic structures at locations where turbulence is generated<br>
and the shear-thinning effect is pronounced. With the addition of shear-thinning polymer,<br>
the energetic structures are compressed closer to the wall, and the bulk flow becomes less<br>
important in terms of contributing to turbulence production.<br>
The spatial correlation analysis of the in-plane velocity fluctuations shows that the shear-<br>
thinning polymer enhances the space correlation compared to the Newtonian fluid. Anti-<br>
correlation is found more pronounced than the Newtonian case in the axial direction, which<br>
leads to a reduced magnitude of axial integral lengthscale L33. The 2D energy spectrum<br>
shows that the Kolmogorov -5/3 law does not hold for T-C turbulence for both Newtonian<br>
and non-Newtonian fluids. Non-Newtonian energy spectrum based on radial and azimuthal<br>
fluctuations shows an increased importance of large-scale structures and a decreased im-<br>
portance of motions at small lengthscales in terms of energy containment. The dissipation<br>
spectrum shows increased contribution for dissipation at smaller lengthscales as concentra-<br>
tion increases.</p>