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 B33: Flow Instability: Viscoelastics |
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Chair: Marie-Charlotte Renoult, INSA de Ronen Room: 615 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B33.00001: Experimental investigation Faraday wave onset in viscoelastic materials Xingchen Shao, J. R. Saylor, Joshua Bostwick, Pasquale Ciarletta Herein we explored the onset of parametrically excited surface waves (Faraday waves) on glycerin-water mixtures and agarose gels, ascertaining the effect of viscosity and elasticity on the threshold amplitude and mode selection. Faraday waves were mechanically generated in a circular tank mounted on an electromechanical shaker over a range of driving frequencies. We obtained multiple Faraday instability modes, each of which is characterized by a radial and azimuthal wave number and by a unique stability tongue in the amplitude-frequency space. Glycerin/water mixtures were used to explore a range of viscosities showing that for a given mode the frequency at onset decreases with viscosity, while the amplitude increases. In contrast, for gels, the onset frequency and amplitude both increase with elasticity. Experiments are then compared against theory. This work could potentially be used to develop a diagnostic method for measuring the complex modulus of viscoelastic materials. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B33.00002: Viscoelastic flow instabilities in static mixers: onset and effects on the mixing efficiency Simona Migliozzi, Giovanni Meridiano, Luca Mazzei, Panagiota Angeli Purely elastic instabilities occur in the absence of inertial effects, induced by the combination of strong elastic forces with nonlinearities of the flow. In a laminar mixing process, the onset of these instabilities is likely to occur in the window of applied flow rates, therefore it is of paramount importance to understand the effects of their onset on the mixing efficiency. In this work, we experimentally investigate the onset of elastic instabilities in two in-line mixers with different geometric features, i.e. a Kenics helical mixer and a GFX mixer, characterised by a double X-shaped bars geometry. Concentrations maps were obtained at different mixer lengths by means of Planar Laser Induced Fluorescence. We mapped the onset of the instabilities with Reynolds and Deborah numbers. Three fluids with different rheological behaviour -- i.e. a Boger fluid and two shear-thinning fluids -- were tested to deduce a generalised effect of the fluid elasticity on the mixing patterns. The effect of the instabilities depended on the different kinematics induced by the two distinct geometries: for the helical mixer the typical lamellar structure is not recovered and the two liquid streams remain unmixed, while for the GFX mixer the concentration maps oscillate in time in a quasi-periodic fashion. In both cases, the onset of instabilities worsens the mixing efficiency with respect to the Newtonian case. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B33.00003: Three-dimensional viscoelastic flow instabilities at extensional stagnation points Becca Thomases, Paloma Gutierrez, Adam Kagel Simulations of viscoelastic fluid models in the Stokes limit exhibit instabilities at extensional stagnation points. We present simulations of both the Oldroyd-B and FENE-P models in a 3D periodic domain with cylindrical 4-roll mill background forcing. Beyond a critical Weissenberg number (non-dimensional relaxation time) we find an instability in the z-direction (note that the background force is constant in z). We present both simulations and linear stability analysis and identify criteria for the occurrence of this instability. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B33.00004: Proper orthogonal decomposition of viscoelastic liquid jets. Louise Cottier, Marie-Charlotte Renoult, Christophe Dumouchel Proper Orthogonal Decomposition (POD) is a linear procedure that allows to identify characteristics of a data set by determining an optimized set of function basis. The optimization of that new basis is achieved by maximizing the projection of the data set on it. POD therefore depends on the choice of the scalar product. In the field of fluid dynamics, POD has been mostly applied to single phase turbulent flows, using the classical Euclidean scalar product. Here, we apply POD to a two-phase laminar flow: a low-velocity viscoelastic liquid jet evolving in an inert gas. More precisely, the procedure is applied to the interface between the two phases, and the scalar product is sought in the aim of tracking the jet surface evolution. After a brief presentation of the POD concept, we will describe the operating of our POD procedure on viscoelastic liquid jet images obtained from our experiments. Then, the results regarding pattern identification will be exposed. Finally, we will state what kind of physical information POD could bring about an atomization process involving viscoelasticity. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B33.00005: Analysis for inertial and elastic instabilities in extensional flow and comparisons with cross-slot flow Howard Hu, Ranjiangshang Ran, Paulo Arratia We theoretically investigate the instabilities of a steady planar extensional flow of viscoelastic fluids with the flow vorticity equation. The results of this linear stability analysis indicate two distinct instabilities depending on the values of Reynolds number ($Re$) and Weissenberg number ($Wi$). One instability is an inertia-dominated one occurring at a critical $Re$, in which the vorticity component $\omega_x$ becomes unstable, suggesting an emerging axial vortex in the extensional direction $x$. The other instability is an elasticity-dominated one at high $Wi$, where the vorticity component $\omega_z$ in the direction normal to elongational plane becomes unstable, indicating a symmetry breaking on the elongational $xy$-plane. The predicted critical $Re$ and $Wi$ numbers of these two instabilities by the linear stability theory are critically compared with experimental and numerical results in the cross-slot channel flows. [Preview Abstract] |
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