Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Q19: Non-Newtonian Flows: Instability and Turbulence I |
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Chair: Louison Thorens, Tufts University; Yves Dubief, University of Vermont Room: 205 |
Monday, November 21, 2022 1:25PM - 1:38PM |
Q19.00001: Lagrangian stretching reveals polymeric stress field Arezoo M Ardekani, Manish Kumar, Jeffrey Guasto Viscoelastic flows are common in many natural and industrial applications such as biofilm transport, drug delivery, and enhanced oil recovery. The stretching of polymeric chains in viscoelastic flows induces elastic instability, which manifests into symmetry-breaking, time-dependent flows and anomalous transport properties. The knowledge of the polymeric stress field is essential for understanding transport in viscoelastic flows because the topology of the polymeric stress field controls the flow states and dynamics in viscoelastic flows. However, the experimental measurements of the stress field are challenging. Through analytical and numerical analyses, we obtain a relationship between the polymeric stress field and the Lagrangian stretching field. The Lagrangian stretching field depends solely on the flow kinematics, which is relatively easy to measure in the experiment. Thus, our result establishes a simple framework to unveil the topology of the polymeric stress field directly from readily measurable flow field data, even for strongly viscoelastic and unstable flows. |
Monday, November 21, 2022 1:38PM - 1:51PM |
Q19.00002: Measured Lagrangian stretching reveals stress topology and transport barriers in viscoelastic flows Louison Thorens, Maliheh Teimouri, Manish Kumar, Arezoo M Ardekani, Jeffrey Guasto Viscoelastic flows through confined geometries cause large polymeric stresses and elastic instabilities at large Weissenberg number (Wi), which lead to chaotic flow and enhanced mixing in a multitude of natural and industrial processes. Determining the polymeric stress distribution is key to understanding the onset of instabilities and material transport in these systems, but direct measurements remain challenging. In this work, we experimentally demonstrate that the Lagrangian stretching field of viscoelastic flows strongly correlates with the polymeric stress field. The stretching field is quantified from micro-PIV measurements of viscoelastic flows through a host of canonical microfluidic geometries for a broad range of Wi. Across both steady and unstable flow regimes, we show that the measured stretching field topology mirrors the polymeric stress field, determined from simulations. Furthermore, we investigate the mixing properties of these flows across a range of Wi and Peclet numbers, and show that the regions of high stress serve as local transport barriers. This work helps to establish a new Lagrangian framework to analyze viscoelastic flows and directly illustrates the link between stress, stretching, and transport. |
Monday, November 21, 2022 1:51PM - 2:04PM |
Q19.00003: On the role of stretched polymer sheets in the dynamics of elasto-inertial turbulence Yves C Dubief, Fuqian Yin Elasto inertial turbulence (EIT) is a chaotic state of flow that is driven by complex interactions between polymers, boundary conditions, and flow dynamics. EIT may occur over a range of Reynolds numbers, including subcritical and supercritical Reynolds numbers from the perspective of the inertial turbulence (flows without polymers). We investigate the transition from elastic dominated regime (EDR) to EIT in two-dimensional natural convection flows at low Rayleigh numbers. The flow are simulated using spectral methods and domain of short dimensions bounded by walls in the vertical direction and with periodic boundary condition in the horizontal direction. Isothermal boundary conditions at the walls impose a negative temperature gradient acting on the momentum equations via the Boussinesq approximation for buoyancy. The focus is on the dynamics of sheets formed by highly-stretched polymers and the mechanism that control the onset of EIT. Other numerical experiments are also considered to further isolate the physics at play and the influence of inertia, and the polymer parameters of the viscoelastic models. The presentation will discuss the similarities that exist in the dynamics of EIT in natural convection flows and channel flows, as well as possible connections with elastic turbulence. |
Monday, November 21, 2022 2:04PM - 2:17PM |
Q19.00004: High Weissenberg asymptotics of the centre-mode instability in viscoelastic channel flow Rich R Kerswell, Jacob Page Simple (Newtonian) fluids exhibit fascinating new phenomena when small amounts of long-chain polymers are added to them. However, until the work of Garg et al. (Phys. Rev Lett. 121, 024502, 2018), the rectilinear flow of polymer-enriched Newtonian solvents was believed only to be linearly unstable if the corresponding Newtonian flow was. That is, there was no elastically-generated linear instability without curved streamlines. Garg et al., however, found a centre-mode instability in pipe flow which was later confirmed also to exist in channel flow by Khalid et al. (J. Fluid Mech. 915, A43, 2021) at large Weissenberg numbers (Wi) and finite Reynolds numbers (Re). By studying the ultra-dilute limit (β → 1), this instability could be tracked down to Re = 0 in channel flow (Khalid et al., Phys. Rev. Lett. 127, 134502, 2021). We will discuss the asymptotics of the instability in the distinquished limit Wi → ∞, β → 1 such that Wi(1-β) = O(1) and Re = 0 in the hope of revealing the fundamental mechanism for this interesting new elastic instability. |
Monday, November 21, 2022 2:17PM - 2:30PM |
Q19.00005: Time-dependent 3D dynamics in viscoelastic pressure-driven channel flow Martin Lellep, Moritz Linkmann, Alexander Morozov Dilute polymer solutions do not flow like Newtonian fluids. Their flows exhibit instabilities at very low Reynolds numbers that are driven not by inertia, but rather by anisotropic elastic stresses. Further increase of the flow rate results in a chaotic flow, often referred to as purely elastic turbulence (PET). The mechanism of this new type of chaotic motion is poorly understood. |
Monday, November 21, 2022 2:30PM - 2:43PM |
Q19.00006: Inertio-elastic instabilities in channel flow Sami Yamanidouzisorkhabi, Yashasvi Raj, Tamer A Zaki, Gareth H McKinley, Irmgard Bischofberger The addition of small amounts of long-chain polymers to a Newtonian solvent can lead to frictional drag reduction in inertial shear flows. The interplay of viscoelasticity and inertia in a dilute polymer solution results in the emergence of inertio-elastic instabilities. The nonlinear evolution of these instabilities engenders a state of turbulence with significantly different spatiotemporal features compared to Newtonian counterpart, termed elasto-inertial turbulence (EIT). Recent numerical simulations suggest the emergence of two distinct instabilities as pathways to EIT. The first route is an inertio-elastic wall-mode instability that appears as amplified Tollmien-Schlichting waves, while the second route is an elastic center-mode instability that continues to exist at infinitesimally small levels of inertia. Despite these advancements, there is a lack of experimental work on viscoelastic channel flows. We explore the emergence and evolution of these elastic and inertio-elastic instabilities in a channel flow using schlieren imaging. The spatio-temporal structures of a viscoelastic channel flow are visualized at the centerline and in proximity to the wall in a Lagrangian manner and are compared to the turbulent structures present in the corresponding Newtonian channel flow. |
Monday, November 21, 2022 2:43PM - 2:56PM |
Q19.00007: Multistability of elasto-inertal two-dimensional channel flow Miguel Beneitez, Jacob Page, Yves C Dubief, Rich R Kerswell Elasto-inertial turbulence (EIT) is a recently discovered, chaotic flow state observed in dilute polymer solutions. The dynamical origin of EIT has been hypothesised to be linked to a centre-mode instability [Garg et al. PRL 121, 2018], which gives rise to an arrowhead-shaped travelling wave. Two-dimensional direct numerical simulations (DNS) have shown evidence of various dynamical regimes, including stable arrowheads, chaotic arrowheads and full elasto-inertial chaos [Samanta et al. PNAS 110, 2013; Dubief et al. PRF 7, 2022], with the preferred dynamics appearing to depend in a non-trivial way on the flow parameters. In this talk we show that these regimes do not succeed each other (e.g. in bifurcations as the Weissenberg number is varied) but rather coexist in parameter space. In fact, two-dimensional viscoelastic channel flow is a multistable system with up to four different coexisting attractors: the laminar state, a steady arrowhead, a chaotic arrowed and EIT. We use DNS to explore the effect of Weissenberg and Reynolds numbers, polymer concentration and finite extensibility (using the FENE-P model) on the existence of the four attractors. |
Monday, November 21, 2022 2:56PM - 3:09PM |
Q19.00008: On the turbulent drag reduction in viscoelastic flows: effects of elasticity number and polymer characteristics Alexia Martinez Ibarra, Jae Sung Park The effects of introducing small amounts of flexible long-chain polymers to a Newtonian liquid have been well-studied since the discoveries of Toms in 1948 for a pipe flow. In some cases, friction drag can be reduced up to 80%, making polymer additives a promising drag-reducing technique in industrial pipe flow systems. In practice, studying the effect of the elasticity number (El) becomes challenging since changing El (which is independent of the velocity) constitutes changing the fluid properties or flow geometry. In this study, direct numerical simulations of viscoelastic flows using the FENE-P model are performed. Dimensionless parameters such as Reynolds number (Re) and Weissenberg number (Wi), and polymer characteristics are evaluated for the effects of El = Wi/Re. The preliminary results show that for the low-El regime studied, higher drag reduction (DR) is achieved for increasing Wi at a constant Re. Furthermore, increasing polymer concentration increases DR due to a resulting increase in Wi, but the magnitude of DR increase between polymer concentrations appears to depend on Re. The effects of other polymer characteristics will be further discussed. |
Monday, November 21, 2022 3:09PM - 3:22PM |
Q19.00009: Elasto-inertial turbulence and the nature of the maximum drag reduction asymptote Sarath S. Suresh, Bjoern Hof
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