Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H08: Non-Newtonian Flows: General Flows I |
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Chair: Neil Balmforth, University of British Columbia Room: 212 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H08.00001: Indentation into a plastic fluid layer Thomasina Ball, Neil Balmforth, Ian Hewitt We study the indentation of a rigid object into a layer of a cohesive or non-cohesive plastic material. Existing approaches to this problem using slip-line theory assume that the penetration depth is relatively small, employing perturbation theory about a flat surface. Here, we use two alternative approaches to account for large penetration depths, and for the consequent spreading and uplift of the surrounding material. For a viscoplastic fluid, which reduces to an ideal plastic under the limit of vanishing viscosity, we adopt a viscoplastic version of lubrication theory. For a Mohr-Coulomb material, we adopt an extension of slip-line theory between two parallel plates to account for arbitrary indenter shapes. We compare the theoretical predictions of penetration and spreading with experiments in which a flat plate, circular cylinder or sphere are indented into layers of Carbopol or glass spheres with successively higher loads. There is a clear layer-depth dependence of the indentation and uplift for the viscoplastic material, with a much weaker dependence on layer depth for a Mohr-Coulomb material. Understanding the dynamics of indentation into a viscoplastic layer is of particular importance in the formation of footprints, either by animals or in an industrial process. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H08.00002: Dynamics of viscoelastic ribbons Neil Bamforth, Ian Hewitt A reduced model is presented for the dynamics of slender ribbons of viscoelastic fluid. The model is used to explore the effects of viscoelasticity on the dynamics of a curling ribbon, a drooping cantilever, buckling sheets, snap-through and a falling catenary. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H08.00003: Jeffery orbits in shear-thinning fluids Arman Abtahi, Gwynn Elfring At zero Reynolds number, spheroids and long slender bodies in shear flow undergo a periodic motion. In the absence of inertial and Brownian forces, the motion of a neutrally buoyant ellipsoid of revolution in a simple uniform shear was solved by Jeffery who found that the particle’s axis of revolution rotates on infinitely many degenerate periodic orbits called "Jeffery orbits". Since this classical work, a number of various studies have demonstrated that inertia (both of the particle and the fluid) and viscoelasticity tend to have a dramatic effect on the particle dynamics, but the effects of shear-dependent viscosity has not previously been explored. In this talk we consider the dynamics of a neutrally-buoyant prolate spheroid in a shear flow of a shear-thinning fluid. We model the fluid using a Carreau model and to capture the leading-order effects of shear-thinning rheology on the dynamics we use a perturbation expansion in the weakly shear-thinning regime and use the reciprocal theorem to compute the dynamics. We find that the symmetry of the Jeffery orbits in Newtonian fluids is maintained when the fluid displays shear-thinning, however the changes in viscosity tend to increase the time the particle spends aligned with the flow. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H08.00004: Numerical Study of the Flow of a Shear Thinning Fluid Past a Circular Cylinder Forced to Oscillate Sinusoidally Umang Patel, Jonathan Rothstein, Yahya Modarres-Sadeghi Current study numerically investigates two-dimensional laminar flow of shear-thinning fluids past a circular cylinder forced to oscillate in crossflow direction. Merged PISO-SIMPLE algorithm (PIMPLE) has been used to solve the governing equations on unstructured grid. The sinusoidal oscillations of cylinder were handled by solving cell-centre laplacian for mesh motion displacement. Oscillation frequency has been kept in such a way that reduced velocity remains close to 6. The shear-dependent viscosity has been modelled by the Carreau model where power index is kept below unity to model shear thinning fluid. Reynolds number is defined based on zero shear-rate viscosity. Vortex shedding is observed at very low Reynolds number as compared with the Newtonian fluid since shear-thinning effects are causing the flow to destabilize. For various values of Reynolds number and Carreau model parameters, lift and drag coefficients as well as time- averaged normalized viscosity have been reported. As opposed to Newtonian fluids, we observed the decrease in mean drag value with increasing Reynolds number due to shear thinning effects. The forced oscillations result in shedding of vortices at Reynolds numbers lower than the critical Reynolds number to observe shedding in a fixed cylinder. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H08.00005: Diving into a shear-thickening bath Philippe Bourrianne, Robert E. Cohen, Gareth H. McKinley Shear-thickening fluids, made of suspensions of micro or nanoparticles, react to imposed excitations with a tunable behavior. At low shear-rate, they flow like a Newtonian or weakly shear-thinning liquid, whereas their viscosity rapidly increases following a more rapid perturbation. Due to this enhanced dissipation, shear-thickening fluids are known for their remarkable ability to absorb energy during collisions. When a solid object impacts a bath of shear-thickening fluid, the initial velocity determines the different settling regimes that are observed. We will describe these different regimes with regard to the rheological properties of the shear-thickening liquid and the characteristics of the impacting object. A few surprising observations could be noticed. First, a high velocity is not always the best way to penetrate such suspensions. Under such conditions, an appropriately-shaped fast-moving object can also bounce during the impact due to the shear-thickening behavior. By comparing the deceleration of an object into a viscous Newtonian and a shear-thickening liquid, we will explain the spectacular properties of shear-thickening during a collision. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H08.00006: Dynamic Wetting Failure in Shear-Thinning and Shear-Thickening Liquids Vasileios Charitatos, Wieslaw Suszynski, Marcio Carvalho, Satish Kumar Dynamic wetting failure in shear-thinning and shear-thickening liquids is examined in this work. Flow visualization experiments using a curtain-coating geometry suggest that shear thinning postpones the onset of wetting failure and the resulting air entrainment. To advance fundamental understanding of the underlying physical mechanisms, a hydrodynamic model consisting of liquid displacing air in a rectangular channel in the absence of inertia is developed. Both shear thinning and shear thickening are considered by using Carreau-type models to describe the liquid rheology. Steady-state solutions are calculated using the Galerkin finite-element method and the critical capillary number where wetting failure occurs is identified. Shear thinning is found to postpone the onset of wetting failure whereas shear thickening is found to promote it. The underlying mechanism involves thickening/thinning of the air film as a consequence of shear thinning/thickening of the liquid and the tangential stress balance. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H08.00007: Enhanced speed of a falling sphere in pseudo-plastic fluid with ultrasound irradiation Minoru Iwamuro, Tomoaki Watamura, Kazuyasu Sugiyama We experimentally investigated the effect of ultrasound irradiation on a falling sphere in pseudo-plastic fluid. The falling velocity is measured via an image processing technique. We performed experiments with parametrizing sphere diameter, fluid properties, ultrasound intensity, and its frequency. We found that the falling speed is enhanced by ultrasound irradiation, and the speeding-up in the stronger pseudo-plastic fluid is much greater than that in the weaker one. We conclude that the speeding-up ratio is relevant to the ratio of the viscosity and length scales involved in the system. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H08.00008: The death of the fluid mechanical sewing machine: viscoelastic effect Bernardo Palacios, Stephen W. Morris, Roberto Zenit In this work we study the fall of a viscous filament onto a moving surface. While the problem has been studied extensively for Newtonian fluids, referred to as the ‘fluid-mechanical sewing machine’, the effects of considering other complex fluids have not been explored to date. We replicate the setup used for Newtonian fluids, issuing a fluid filament from a nozzle from certain vertical distance from a moving substrate to observe it coil and stretch simultaneously. The fluid considered is, instead, a Boger fluid (viscoelastic but with constant viscosity). Our experiments show that, for similar conditions, the coiling instability does not appear if the fluid has sufficient elasticity. In most cases, the fluid mechanical sewing machine effect is not observed. Instead, a largely stable fluid catenary is observed. The shape of the catenary is characterized considering a Deborah number and a dimensionless height. A map of the conditions to kill the fluid mechanical sewing machine effect is presented in terms of these two dimensionless groups. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H08.00009: Dynamics of viscoelastic films in reverse squeeze flows Bavand Keshavarz, Erica Lai, Gareth McKinley, Niels Holten-Andersen In many industrial and biological applications thin films of complex fluids act as lubricating layers between solid boundaries. Upon the separation of these boundaries, the kinematics of the flow generates large pressure gradients leading to high values of adhesive forces. We perform a detailed study on the dynamics of these phenomena for a general class of viscoelastic liquids with different relaxation times and elastic moduli. The liquid is initially at rest in the gap between two circular discs. The discs are then separated from each other with an exponentially increasing velocity, ensuring a constant nominal value of stretch rate during the test. Adhesion force measurements show a rate-dependent peak force that scales with the elastic modulus of the liquid followed by an exponentially decaying tail. We show that with a proper scaling all of the measured peak forces for different viscoelastic liquids follow a single master curve. Coupling the simplified kinematics of this “reverse squeeze flow” with the viscoelastic constitutive equation leads to a simple lubrication model. We show that the predictions from the model agree well with the experimental measurements. Results from this study can shed light on the dynamics of liquid adhesion in complex fluids. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H08.00010: A novel non-linear one-dimensional model for viscoelastic lubricants Luca Biancofiore, Humayun Ahmed Lubrication is essential to improve the performance of sliding surfaces. Power transmission in mechanical and biological systems relies on proper lubrication to minimize wear and energy losses. However, most practical applications involve conditions that cause or require the lubricant to exhibit viscoelastic behavior. In this study a novel 1D viscoelastic Reynolds equation is derived based on the Oldroyd-B constitutive relation. It comprises a system of five 1D equations describing the pressure, velocity and shear stress distribution in the film. The model is compared with direct numerical simulations of thin films for different geometries. The results are in good qualitative and quantitative agreement indicating the simplified model is valid within the context of lubrication theory. Firstly, the pressure presents strong variations as the lubricant elasticity becomes significant, but stagnates as the polymer relaxation time becomes slow compared to the characteristic flow time. Secondly, the net film pressure is shown to be a superposition of a Newtonian and viscoelastic component. The viscoelastic component depends on the surface geometry. Surfaces with constant slope exhibit a pressure decrease, whereas the opposite effect is observed in parabolic surfaces. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H08.00011: Effect of viscoelasticity on the stability characteristics of a drying polymer solution George Karapetsas, Athanasios Vadarlis We investigate the stability of an evaporating liquid film which consists of a polymeric solution with a volatile solvent. Besides solutocapillary and thermal Marangoni effects, an important factor affecting the flow, which is often neglected in the literature, is the viscoelastic character of the polymeric solutions typically encoutered in practical applications. During the drying process, the concentration of the solvent continuously decreases thus rendering the non-Newtonian character of the solution increasingly important. Here, we develop a model fully accounting for the viscoelastic behaviour and dynamically varying rheological properties of the liquid film. We use a finite element formulation to solve the time-dependent problem and perform a linear stability analysis employing the quasi-steady state approximation, in the limit of slow evaporation. Our numerical results indicate that the increasingly important effect of viscoelasticity destabilizes the flow and also leads to patterns with smaller wavelengths. We discuss the mechanisms which give rise to these instabilities. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H08.00012: Flow of a Shear Thickening Micellar Fluid Past a Falling Sphere Shijian Wu, Hadi Mohammadigoushki In this work, we present the first quantitative measurements of a dilute shear thickening micellar solution past a falling sphere. The micellar solution consists of cetyltrimethylammonium bromide and 5-methyl salicylate (CTAB/5MS) in de-ionized water and it exhibits shear thickening behavior. This CTAB/5MS micellar solution forms un-entangled rod-like micelles at equilibrium. It is found that the drag coefficient for the falling sphere is similar to that of a Newtonian fluid at a vanishingly small Reynolds number (\textit{Re} $=$ 0.03). However, falling spheres experience a significant drag reduction for conditions that correspond to 0.09 $\le $ \textit{Re} $\le $ 9.86. Moreover, an unusually extended wake which spans over a long distance downstream of the sphere is detected by particle image velocimetry. These unusual results could be rationalized by invoking the phenomenon of flow induced structure formation. We hypothesize that strong shear and/or extensional flows around the falling sphere could trigger the aggregation of rod-like micelles into giant worm-like structures. Such worm-like micelles may induce significant sphere drag reduction and extended elastic wakes in the rear of sphere. This interpretation is consistent with the steady shear and transient extensional flow measurements. [Preview Abstract] |
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