Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D35: Non-Newtonian Flows: Instability and Turbulence |
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Chair: Irmgard Bischofberger, Massachusetts Institute of Technology Room: Georgia World Congress Center B407 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D35.00001: Disorder suppresses viscoelastic instability Derek Walkama, Nicolas Waisbord, Jeffrey S. Guasto At critically large strain rates or Weissenberg numbers, viscoelastic fluids are known to undergo a transition to a time dependent, chaotic flow(i.e. elastic turbulence), characterized by spatio-temporal velocity fluctuations. In this work, we quantify the flow of a viscoelastic fluid through a microfluidic device consisting of an array of cylindrical pillars, which are tuned from an ordered hexagonal lattice to a disordered medium. Measurement of the temporal fluctuations of the velocity field illustrates that flow through the ordered lattice undergoes a bifurcation at a critical Weissenberg number Wi=0.5. However, the introduction of a finite disorder to the pillar array completely suppresses this critical behavior, as the flow stabilizes along preferential flow paths characterized by a 2-5 fold increase in the velocity field correlation length. Spectral analysis of the Lagrangian strain rate reveals that in addition to a high Wi number, a high quality factor of the strain rate spectrum - observed only in highly ordered systems - is crucial for the onset of the viscoelastic instability. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D35.00002: Elasto-inertial turbulence in straight pipes at low Reynolds numbers George H Choueiri, Bjoern Hof As demonstrated recently, polymers delay transition and can for selected parameters even fully relaminarize turbulent flow. At high polymer concentration however a different fluctuating state emerges, “elasto-inertial turbulence” (EIT), and it is only in this limit that the flow assumes the characteristic friction factor values corresponding to the "maximum drag reduction asymptote". While Newtonian turbulence at onset is spatio-temporally intermittent and requires finite amplitude perturbations, EIT appears globally and sets in at a critical polymer concentration (corresponding to a critical Weissenberg number) regardless of the flow being subject to external perturbations or not. We demonstrate that EIT even occurs in pipes at Reynolds numbers of the order 1 provided that the Weissenberg number is sufficiently large. We additionally discuss possible connections to the case of “purely elastic turbulence”, found at even lower Re in flows with curved streamlines; this has recently been reported also for channel flow however requiring a complicated triggering mechanism. In addition to the nature of the transition scenario we also investigate structures and spectra of EIT and compare them to those found in purely elastic turbulence. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D35.00003: Elasto-inertial turbulence: Reentrant transition and connection to linear mechanisms Ashwin Shekar, Ryan McMullen, Sung-Ning Wang, Beverley J McKeon, Michael David Graham We describe direct numerical simulations (DNS) of channel flow turbulence in a FENE-P fluid. At Reynolds numbers very close to transition, the flow first relaminarizes upon increasing Weisenberg number (Wi) or polymer concentration, but then becomes turbulent again, displaying features of elasto-inertial turbulence (EIT). At higher Reynolds number, the flow evolves as Wi increases from displaying intermittency and streamwise vortex structure characteristic of Newtonian flow to EIT, while at intermediate Wi, a spatiotemporal mixture of the two structures is observed. We tie these observations to the 2D stability of the laminar flow and characterize the observed disturbance amplification which starts to show up at intermediate Wi. Observations point at a bypass transition with turbulent flow structure that resembles that of a discrete eigenmode close to the real axis in the eigenvalue spectrum. Further, we present a tentative phase diagram of polymer drag reduction and draw links to Tollmien-Schlichting modes in the Newtonian limit and Gorodtsov-Leonov modes in the elastic limit. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D35.00004: Nonlinear optimal perturbations in pipe flow of shear thinning fluids Daniel Olvera Cabrera, Chris Pringle Transition to turbulence in a circular pipe remains an active open problem in fluid dynamics. A large body of work in wall-bounded shear flow has been dedicated to find Exact Coherent Structures (ECS) and optimal perturbations that trigger turbulence. Extending the investigation to non-Newtonian fluids is of particular interest for industrial processes. The effects non-Newtonian viscosity have been studied by R. Liu and Q. S. Liu, Phys. Rev. E, 85 (2012). However, recent experimental results (C. Wen, et al, Phys. Rev. Fluids 2, 2017) raise new questions regarding the stability of these fluids. Here we present some numerical results to expand the relatively unexplored study of shear thinning flows. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D35.00005: Taylor-Couette flow of shear-thinning fluids : primary and higher-order instabilities Neil Cagney, Stavroula Balabani Taylor-Couette flow is mathematically well-defined flow that occurs throughout industry and is often used as a mixer in chemical engineering. Many of these applications involve non-Newtonian fluids, which is predicted to have a strong effect on the primary and higher-order instabilities that are essential for effective mixing; however, there is a clear lack of experimental data on this subject. We examine the behaviour of Newtonian and non-Newtonian (shear-thinning) fluids in Taylor-Couette flow for a range of Reynolds number (up to 1000) using a Particle-Image Velocimetry and flow visualisation (for phenomena occurring at short and long time-scales, respectively). Using this combined approach, we identify the variations in flow topography, the critical Reynolds number, frequency and amplitude of various instabilities. We show that shear-thinning causes vorticity to become concentrated in narrow low-viscosity bands and increases the wavelength (vortex size) of the instability. For strongly shear-thinning fluids, the wavelength becomes dependent on Re, with the transitions occurring through the spontaneous merge or splitting of vortices. This process is strongly hysteretic and has not previously been observed in Taylor-Couette flow. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D35.00006: Effects of polymer additives on the energy injection, transfer and dissipation rates in turbulent flow Heng-Dong Xi, Yi-Bao Zhang We report an experimental study of polymer’s effect on the energy transport process in a Von Karman swirling flow system (VKS) with smooth disks. By simultaneous measurement of the torque exerted on the disks and the the three components of the velocity at the center of VKS in a plane along its diameter with high spatial resolution, we determined the energy injection, transfer and dissipation rate independently. It is found that the flow is more isotropic with polymer additives. With increasing poymer concentration, the torque decreases continuously, while the energy transfer and dissipation rate first experience a sharp and then a slow decrease. Besides, the energy dissipation rate is larger than the transfer rate, and their difference is nearly constant. We also studied the Weissenberg number (Wi) effect. It is found that the energy dissipation rate is equal to the transfer rate at small Wi, and becomes larger than the transfer rate at a critical Wi. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D35.00007: Direct Numerical Simulations of Turbulent Planar Viscoelastic Jets Mateus Carvalho Guimaraes, Carlos Bettencourt Da Silva, Nuno Pimentel New direct numerical simulations (DNS) of turbulent planar jets of viscoelastic fluid described by the finitely extensible nonlinear elastic constitutive equation closed with the Peterlin approximation (FENE-P) are carried. The data covers the entire transitional region as well as the fully turbulent jets far-field up to 20 slot widths. The influence of rheological parameters on jets statistical and instantaneous fields are assessed, revealing considerable changes in comparison to the Newtonian case. In particular, the solvent maximum value of the centreline mean kinetic turbulent energy dissipation is reduced by a factor greater than 50% in the most dramatic case, showing clearly the influence of the polymer on the flow turbulence. Significant changes are also found in the transitional region of the flow. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D35.00008: A similarity between the turbulent transport of heat and momentum in drag-reduced flows by polymer additives Kyoungyoun Kim Heat transfer reduction (HTR) in drag-reduced turbulent flows of dilute polymer solutions has been examined using DNS of fully developed viscoelastic turbulent channel flows (Reτ = 125 and Pr = 5) with a constant heat flux boundary condition. The polymer stress is modeled using the finitely extensible nonlinear elastic-Peterlin constitutive model, and low (15%), intermediate (34%), and high drag reduction (DR) (52%) cases are examined. The present results show that the HTR is larger than DR in the viscoelastic flows; the HTRs are 19%, 41%, and 62% for DR = 15%, 34%, and 52% flow, respectively. As DR increases, the wall-normal turbulent heat flux decreases, which leads to an increase in the slope of the mean temperature profile and thus results in the heat transfer reduction. The conditionally averaged fields for the events with a large contribution to the wall-normal turbulent heat flux are very similar to those for the Q2 events in both Newtonian and viscoelastic flows. This suggests that the turbulent transport of momentum and heat are associated with almost the same flow structure in drag-reduced flows as well as Newtonian flow, which is consistent with our recent study (Kim and Sureshkumar, 2018, Phys. Fluids 30). |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D35.00009: Could hairpin structures in the polymer stretch field be the link between elastic turbulence and elasto-inertial turbulence? Yves Dubief, Victor Steinberg, Jacob Page, Rich Kerswell, Vincent E Terrapon For polymer flows, Elasto-inertial turbulence (EIT) and elastic turbulence (ET) are both turbulent states in wall-bounded flows and are driven by polymer dynamics. So far, it has been assumed that ET needs curved streamlines but exists in inertialess flows, whereas EIT may happens in parallel wall-bounded flows for Re>1 to super-critical Reynolds numbers. Using specific initial boundary conditions, 2D simulations of polymer additives in a channel flow based on the FENE-P model reveal peculiar structures in the polymer stretch field. At higher Reynolds numbers, the structures look like hairpin. The pressure signature of the hairpin structure is a bow shock-like structure at the center of the channel. The hairpin shape is a consequence of the flow's inertia. The structure has a very long time scale. At lower Re, the polymer stretch is still structured into thin sheets, where polymers are highly stretched. These sheets, like the hairpin structure, occupy the core of the flow. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D35.00010: Schlieren Imaging of Mixing and Turbulence in Dilute Polymer Solutions Sami Yamanidouzisorkhabi, Gareth H McKinley, Irmgard Bischofberegr Dilute synthetic polymer solutions have the potential to reduce turbulent drag in pipelines and around marine structures. Biopolymeric extracts such as plant mucilage have recently been proposed as alternative material sources for drag reducing polymers due to their cost-effectiveness and lower environmental hazards. The very low concentrations of polymer employed in drag reduction studies makes direct imaging of the new flow structures an outstanding challenge. Here, we employ Schlieren imaging to enable direct flow visualization of the mixing dynamics and recirculating regions that develop in complex flows of dilute aqueous polymer solutions. With this technique, we are able to detect density differences of order 10-3% for dilute aqueous solutions of synthetic polymers and biopolymers. This promising capability is used to understand the vortical structures associated with turbulent drag reduction of polymer solutions. In our experiments, the turbulent mixing of dilute polymer jets is visualized for different values of Reynolds number, polymer concentration, and molecular weight. We compare these visualization results with previous theoretical studies of mixing layer development and experimental reports of the turbulent drag reducing properties of different polymer solutions. |
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