Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session H30: Non-Newtonian Flows: Instability and Turbulence |
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Chair: Michael Allshouse, Northeastern Room: North 229 B |
Monday, November 22, 2021 8:00AM - 8:13AM |
H30.00001: Elastic turbulence generates anomalous flow resistance in porous media Christopher A Browne, Sujit S Datta Polymer solutions are often injected in porous media for applications such as oil recovery and groundwater remediation. In many cases, the macroscopic flow resistance abruptly increases above a threshold flow rate in a porous medium, but not in bulk solution. The reason why has been a puzzle for over half a century. Here, by directly visualizing the flow in a transparent 3D porous medium, we demonstrate that this anomalous increase is due to the onset of an elastic instability in which the flow exhibits strong spatio-temporal fluctuations reminiscent of inertial turbulence, despite the vanishingly small Reynolds number. We quantitatively establish that the energy dissipated by unstable pore-scale fluctuations generates the anomalous increase in flow resistance through the entire medium. Moreover, we show that this finding applies in porous media with different mean grain sizes, and show how it can be generalized to media that are more structurally heterogeneous, akin to those that arise in many natural settings. Thus, by linking the onset of unstable flow at the pore scale to transport at the macroscale, our work provides generally-applicable guidelines for predicting and controlling polymer solution flows in a variety of porous media. |
Monday, November 22, 2021 8:13AM - 8:26AM Not Participating |
H30.00002: The Origin of Elastic Turbulence in the Taylor-Couette Flow: A Direct Numerical Simulation Study Bamin Khomami, NanSheng Liu, Jiaxing Song Almost two decades ago, Groisman & Steinberg experimentally discovered an essentially smooth and temporally random flow state with broad temporal frequency spectra in curvilinear flows of dilute polymeric fluids in the limit of vanishing Re and Wi>>1 , i.e., very large elasticity number, E=Wi/Re, dubbed “purely elastic turbulence” (ET). Since the discovery of ET, three-dimensional Direct Numerical Simulation (DNS) of purely elastic turbulence has remained a grand challenge problem for the research community engaged in developing first-principal models and simulations that can predict faithfully the complex spatio-temporal dynamics of polymeric flows. To this end, lack of fundamental understanding of the flow–microstructure coupling that leads to production of turbulent kinetic energy in an essentially spatially smooth flow has hampered mechanistic understanding of how ET is realized. In this presentation, results of the first three-dimensional DNS simulation of ET in the Taylor-Couette flow of dilute polymeric solutions will be discussed. Specifically, it will be demonstrated that purely elastic instabilities lead to a chaotic flow composed of two distinct flow structures, namely, large-scale diwhirls that span the entire gap and travelling wave like flow patterns near the rotating inner cylinder. In turn, the influence of the polymeric body force associated with these flow structures on the enhancement of angular momentum transport and mixing, as well as production of turbulent kinetic energy in this inertialess flow will be discussed. |
Monday, November 22, 2021 8:26AM - 8:39AM |
H30.00003: Transition to turbulence against various perturbations and concentrations in viscoelastic flows Alexia Martinez Ibarra, Jae Sung Park The addition of small amounts of long-chain polymers to a flow has been well-studied thanks to their observed benefits, such as turbulent drag reduction. However, the study of the laminar-to-turbulent transition in the flow of polymer solutions flows is currently lacking. For this study, direct numerical simulations of viscoelastic flows are performed to investigate the turbulence transition of polymer solutions subject to various magnitudes of disturbances and polymer concentrations. Simulations are performed at friction Reynolds numbers up to 132. At a low friction Reynolds number of 94, viscoelastic flows start to transition at low polymer concentrations even against small perturbations, whereas the Newtonian counterpart and high concentration solutions still remain laminar. When subjected to higher disturbance magnitudes, higher concentration polymer solutions eventually undergo a transition at a similar time to lower concentrations but display reduced bursting magnitudes. For the highest Reynolds number of 132 studied, the transition seems delayed as the concentration is increased when subjected to small perturbations. As stronger perturbations are applied, the transition time becomes almost the same across all concentrations considered. More interestingly, there is no significant difference in the transition time between viscoelastic and Newtonian flows. The dependence of Reynolds number and detailed transition dynamics will be further discussed. |
Monday, November 22, 2021 8:39AM - 8:52AM |
H30.00004: Analysis of spectral energy transfer in connection with turbulent drag reduction by polymer Miralireza Nabavi, Alexia Martinez Ibarra, Jae Sung Park, Jeonglae Kim Wavelet multiresolution analysis is used to investigate physical connections between polymer drag reduction and spectral energy transfer in viscoelastic turbulent flows. Direct numerical simulation of viscoelastic channel flow is performed at the friction Reynolds number of 145 at dilute polymer concentrations. Polymers are modeled by the finitely extensible nonlinear elastic model and assumed highly flexible. Newtonian counterpart is also simulated for comparison. Variations of turbulence kinetic energy (TKE) due to interactions between polymer solutions and turbulence are evaluated locally in space and in scale, parameterized by wall-normal locations. Larger structures become more energetic in viscoelastic flow, while smaller eddies are weakened regardless of the distance from the wall. In the viscous sublayer, the net result of these two opposing factors leads to smaller TKE for viscoelastic flow, resulting in drag reduction. Interactions between turbulence and polymers are a strong function of scale and wall-normal location $y^+$; polymers absorb TKE from near-wall regions and store it as elastic energy at $y^+ \lesssim 20$, while they enhance TKE in the log layer. |
Monday, November 22, 2021 8:52AM - 9:05AM |
H30.00005: Numerical study of vortex-induced vibrations of a cylinder in shear-thinning and shear-thickening power-law fluids Umang Patel, Jonathan P Rothstein, Yahya Modarres-Sadeghi Vortex-induced Vibration (VIV) of a cylinder in Newtonian fluid is a model problem in Fluid-Structure Interactions (FSI) and has been studied extensively. In this work, we study the influence of shear-thinning and shear-thickening fluids on the VIV response of a 1DOF flexibly-mounted cylinder with a mass ratio of m^{*} = 2 at Re_{0} = 15 and Re_{0} = 200, respectively, defined based on the zero-shear-rate viscosity. We investigate how the VIV amplitude and frequency, flow forces, and vorticity contours change as the reduced velocity, U*, and fluid’s time constant, λ, change. When the results are compared based on Re_{0}, shear-thinning fluids enhance the oscillations while shear-thickening fluids suppress them. If, however, we define a characteristic Reynolds number, Re_{char}, based on a viscosity evaluated at the characteristic shear rate, U/D, then at a constant Re_{char}, the amplitude of response stays very similar for the shear-thinning, shear-thickening, and Newtonian fluids. Despite this similarity, the observed far wake is different: shear thinning amplifies the vorticity generation and reduces the extent of the wake, whereas shear thickening limits the vorticity generation and extends the wake. |
Monday, November 22, 2021 9:05AM - 9:18AM |
H30.00006: Tollmien-Schlichting route to elastoinertial turbulence in channel flow Ashwin Shekar, Ryan McMullen, Beverley J McKeon, Michael D Graham Direct simulations of two-dimensional channel flow of a viscoelastic fluid have revealed the existence of a family of Tollmien-Schlichting (TS) attractors that is nonlinearly self-sustained by viscoelasticity. Here, we describe the evolution of this branch in parameter space and its connections to the Newtonian TS wave and to elastoinertial turbulence (EIT). At Reynolds number Re=3000, there is a solution branch with TS-wave structure but which is not connected to the Newtonian solution branch. At fixed Weissenberg number, Wi and increasing Reynolds number from 3000-10000, this attractor goes from displaying a striation of weak polymer stretch localized at the critical layer to an extended sheet of very large polymer stretch. This transition can be attributed to a coil-stretch transition when the local Weissenberg number at the hyperbolic stagnation points of the Kelvin cat's eye structure of the TS wave exceed 1/2. At Re=10000, unlike 3000, the Newtonian TS attractor evolves continuously into EIT as Wi is increased. We describe how the structure of the flow and stress fields changes, highlighting a ``sheet-shedding" process by which the individual sheets associated with the critical layer structure break up to form the layered multisheet structure characteristic of EIT. |
Monday, November 22, 2021 9:18AM - 9:31AM |
H30.00007: Mode regulation of elastic instability from non-Brownian suspension Sijie Sun, Nan Xue, Stefano Aime, Howard A Stone, Gareth H McKinley, David A Weitz Introduction: Viscoelastic fluid carrying suspension is ubiquitous in engineering applications. When the shear rate is high, the visco-elastic flow tends to be unstable. And secondary flows will emerge. This phenomenon is named elastic instability. In this experimental work, we combined rheo-microscope, rheo-flow visualization, and dynamic mode decomposition. We investigated the influence of finite volume fraction non-Brownian suspension on the elastic instability of visco-elastic shear flow. We found that, in the visco-elastic flow, suspension assembles to large ordered structures, even under a volume fraction far lower than the random close pack. Further, such structures will regulate the secondary flow, which suppresses higher-order modes of the secondary flow. This research highlights the connections between suspension structures and the elastic instabilities of a viscoelastic flow and provides guidelines for the use of suspensions in viscoelastic fluid in industrial applications. |
Monday, November 22, 2021 9:31AM - 9:44AM |
H30.00008: Exact coherent state in purely elastic pressure-driven channel flow Alexander N 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. The mechanism of this new type of chaotic motion is poorly understood. |
Monday, November 22, 2021 9:44AM - 9:57AM |
H30.00009: Lubricating layer in drag-reducing solutions of rigid polymers Lucas N Warwaruk, Sina Ghaemi Existing theories on the mechanism for polymer drag-reduction (DR), imply that the phenomenon typically relies on the solution being viscoelastic. While most solutions of flexible polymers exhibit pronounced viscoelasticity, rigid polymer solutions do not. Solutions of rigid polymers have been shown to demonstrate larger viscosities and a noticeable shear-thinning trend, well approximated by the Generalized Newtonian models. The aim of the present investigation was to explore how the spatial gradient of mean viscosity in the wall-normal direction affects DR. Experiments in a turbulent channel flow were performed using a solution of xanthan gum with varying Reynolds number (Re). Quantities of DR grew from 20% to 32% with increasing Re. Planar particle image velocimetry and steady shear viscosity measurements were used to characterize the velocity and shear rheology of the fluid. Normalized mean velocity profiles overlapped in the inner layer for the different Re scenarios, despite unique DR values. We observed different mean viscosity profiles across the channel for each DR case. All profiles demonstrated a thin low-viscosity layer in the vicinity of the wall. This "lubricating layer" is proposed to play a major role in drag-reduction using rigid polymers. |
Monday, November 22, 2021 9:57AM - 10:10AM |
H30.00010: Revisiting decaying turbulence K. R Sreenivasan, John Panickacheril John, Diego A Donzis Inspection of available data on the decay exponent for the kinetic energy of homogeneous and isotropic turbulence (HIT) shows that it varies by as much as 100%. Measurements and simulations often show no correspondence with theoretical arguments, which are themselves varied. This situation is unsatisfactory given that HIT is a building block of turbulence theory and modeling. We take recourse to a large base of direct numerical simulations and study decaying HIT for a variety of initial conditions. We show that the Kolmogorov decay exponent and the Birkhoff-Saffman decay are both readily observed, albeit approximately, for long periods of time if the initial conditions are appropriately arranged. We also present, for both cases, other turbulent statistics such as the velocity derivative skewness, energy spectra and dissipation, and show that the decay and growth laws are approximately as expected theoretically, though the wavenumber spectrum near the origin begins to change relatively quickly, suggesting that the invariants do not strictly exist. We comment briefly on why the decay exponent has varied so widely in past experiments and simulations. |
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