Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session X37: NonNewtonian Flows: Turbulence & Instabilities 
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Chair: Prabir Daripa, Texas A&M University Room: 203AB 
Tuesday, November 21, 2023 8:00AM  8:13AM 
X37.00001: Influence of shearthinning fluid on vortex formation in pulsatile, turbulent flow Moira R Barnes, David E Rival Vortex formation in shearthinning fluids is an important research topic due to their ubiquity in biological and engineering contexts. Vortices formed in shearthinning fluids have been shown to possess lower circulation, propagate a smaller distance, and have reduced local strain rates compared to those formed in Newtonian fluids (PalaciosMorales and Zenit, 2013). The presence of vortical structures affects the overall macro characteristics of the flow such as fluid mixing and drag. In the present work, we have investigated these structures using timeresolved PIV measurements in a pulsatile flow loop experiment, comparing both a xanthan gum shearthinning fluid and a Newtonian fluid. The field of view is downstream a stenosis model designed to create vortical structures when the flow loop is operated in a pulsatile manner to produce unsteady and very unsteady flows. The parameter space includes transitional to turbulent mean Reynolds numbers and nonnegligible Strouhal numbers. Steady cases at the respective mean Reynolds numbers are tested for comparison. Flow field analysis has been carried out to elucidate the differences between the two fluids where it is predicted that the viscoelastic properties of the xanthan gum affects the flow physics and could reduce mixing. 
Tuesday, November 21, 2023 8:13AM  8:26AM 
X37.00002: Lundgrens Infinite Hierarchy of Probability Density Functions for Linear Viscoelastic Materials Johannes H Conrad, Simon Görtz, Martin Oberlack The statistical theory of inertial turbulence at high Reynolds numbers has been wellestablished and thoroughly studied for the better part of a century. The treatment in terms of multipoint probability density functions, governed by Lundgren's infinite hierarchy of transport equations, is particularly tractable. 
Tuesday, November 21, 2023 8:26AM  8:39AM 
X37.00003: Abstract Withdrawn

Tuesday, November 21, 2023 8:39AM  8:52AM 
X37.00004: Fracturing instability in interfacial HeleShaw flows involving complex fluids Prabir Daripa

Tuesday, November 21, 2023 8:52AM  9:05AM 
X37.00005: The polymer diffusive instability in high concentration polymer solutions Theo Lewy, Rich R Kerswell Numerous mechanisms have previously been proposed to explain `melt fracture' instabilities that are seen during plastic extrusion, many of which were made on the basis that no linear instability exists. The polymer diffusive instability (PDI) is a recently discovered linear instability that is seen in OldroydB and FENEP fluids in rectilinear geometries when the effects of polymer stress diffusion are included in the model. We consider PDI in an OldroydB fluid in the concentrated polymer solution limit. We present analytic results, where we identify the boundary layer structure of PDI, and we predict its eigenvalues. We show that PDI could influence the plastic extrusion process due to its large growth rate, and it bears many similarities with the `sharkskin' instability. Inertia is shown to have a destabilising effect, with it lowering the critical Weissenberg number required for the system to become unstable. 
Tuesday, November 21, 2023 9:05AM  9:18AM 
X37.00006: Efficiency loss in viscoelastic turbulent flows due to polymer degradation Alexia Martinez Ibarra, Jae Sung Park Introducing small amounts of longchain polymers to a viscous liquid has been shown to dramatically reduce skinfriction drag in turbulent flows. One of the main challenges in polymerinduced drag reduction is the mechanical degradation of polymers due to strong elongational strain and shear stresses generated by turbulence. During this process, polymer chains are likely to split, which significantly diminishes drag reduction (DR) effects. For this study, we perform direct numerical simulations of viscoelastic flows using the FENEP model to investigate the effects of polymer degradation. The simulations are performed at a friction Reynolds number of 100 while other parameters such as Weissenberg number (Wi) and polymer length/concentration are adjusted based on reported experimental observations to model the degradation process. As expected, the degradation contributes to a significant loss of DR effects, where DR% decreases by a factor of at least 3.8 after the first degradation pass. It is also found that the more flexible the polymers (high Wi number), the more loss of DR%. However, there seems to be an apparent saturation reached at sufficiently high Wi numbers. During the second degradation pass, the loss of DR effects becomes less dramatic due to the resulting very lowWi regime. We will also discuss the effects of the initial polymer length and concentration on the loss of DR effects. 
Tuesday, November 21, 2023 9:18AM  9:31AM 
X37.00007: Vortex ring interaction with a layer of polymeric fluid Rabia Sonmez, Robert A Handler, David B Goldstein, Saikishan Suryanarayanan, Ryan Kelly Numerical simulations of laminar vortex rings interacting with a layer of polymeric fluid were performed. The results show that when ringpolymer interaction occurs, the flow kinetic energy decreases while enstrophy and dissipation increase. Detailed analysis of the flow reveals that elastic forces act to counter the motion of the ring. Future work will focus on identifying the key nondimensional parameters, such as the Weissenberg number, that govern this phenomenon. 
Tuesday, November 21, 2023 9:31AM  9:44AM 
X37.00008: Connecting polymer dynamics and Lagrangian flow structures in viscoelastic flows Louison Thorens, Arezoo M Ardekani, Jeffrey S Guasto In viscoelastic flows, the distribution of polymeric stress dictates the onset of elastic instabilities and material transport properties in a wide range of biological and industrial processes. However, measurements of the stress are often nontrivial even in steady flows, due to the intrinsic dependence of the polymer conformation upon the Lagrangian history of fluid deformation. In this work, we employ microPIV measurements of viscoelastic flows through a variety of canonical microfluidic geometries to elucidate the link between Lagrangian flow structure and polymer conformation. The measured Lagrangian fluid deformation is directly correlated with realtime imaging of single polymer kinematics in flow across a range of Weissenberg numbers. Our results are additionally compared to the theoretical polymer conformation and to stress fields from numerical simulations. Taken together, this work refines our Lagrangian perspective of viscoelastic flows by illustrating the relationship between fluid stretch and polymeric stress, and has implications for understanding their mixing and transport properties. 
Tuesday, November 21, 2023 9:44AM  9:57AM 
X37.00009: 3D measurements of flow topology in a polymer dragreduced turbulent boundary layer Lucas N Warwaruk, Sina Ghaemi Threedimensional particle tracking velocimetry and the invariants of the velocity gradient tensor (VGT), rate of deformation tensor (RDT) and rate of rotation tensor (RRT) are used to characterize the fine scale flow motions in a Newtonian and polymer dragreduced turbulent boundary layer (TBL). Relative to the Newtonian TBL, the polymerladen flow has a similar momentum thickness Reynolds number Re_{θ} of 2300, but a 33% lower skin friction coefficient. Joint probability density functions (JPDFs) of the VGT invariants (Q and R) for the Newtonian TBL produce the familiar teardrop pattern, commonly seen in direct numerical simulations of Newtonian turbulence. Compared to the Newtonian TBL, the polymerladen flow has significantly attenuated values of R, implying a reduction in the tendency for the flow to exhibit extension. Invariants in RDT (Q_{D} and R_{D}) imply that straining motions within the polymeric flow are more twodimensional compared to the Newtonian TBL. Moreover, JPDFs of Q_{D} and the invariant in the RRT (Q_{W}) suggest that the flow consists of fewer biaxial extensional events and more sheardominate, sheetlike motions. Few, if any, experimental investigations have measured the fine scale motions in a Newtonian and polymer dragreduced TBL. This investigation provides the first experimental evidence in support of the notion that an attenuation of biaxial straining motions is central to the mechanism of polymer drag reduction. 
Tuesday, November 21, 2023 9:57AM  10:10AM 
X37.00010: Bubble migration in a viscoelastic duct flow Metin Muradoglu, Hafiz U Naseer, Daulet Izbassarov Interfaceresolved direct numerical simulations have been performed to study the bubble migration in a viscoelastic pressuredriven duct flow using the Giesekus model. Flow parameters are varied to reveal the effects of bubble deformability, fluid elasticity, shear thinning and inertia on the bubble migration. The rate of bubble migration is found to be much higher than a solid particle mainly due to the slip condition. The bubbleinduced secondary flow velocity is also found to be an order of magnitude higher than the one induced by a solid particle under similar flow conditions. At a higher Weissenberg number, the flow becomes elastically unstable due to the curvature of streamlines across the bubble. It is found that at the lower values of β (polymeric to solvent viscosity ratio), shear thinning effect suppresses this path instability whereas at the higher value of β, shear thinning effect reverses its role and promotes path instability. During the flow build up, the fluctuations are only observed in the migration velocity of the bubble when the dominant lift force is due to elasticity. Simulations are also performed to investigate onset of a chaotic flow by injecting multiple bubbles into the viscoelastic duct flow. 
Tuesday, November 21, 2023 10:10AM  10:23AM 
X37.00011: The nature of bubbleinduced turbulence in polymeric fluids for Re ~ Ο(1) Mithun Ravisankar, Roberto Zenit

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