Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A1: Non-Newtonian Flows: General |
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Chair: Nicholas T. Ouellette, University of Chicago Room: 3000 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A1.00001: Finite Amplitude Stability Analysis of Polymer Fiber Spinning Karan Gupta , Paresh Chokshi The spinning of polymeric fibers suffers from draw resonance instability, manifested by periodic variation of fiber diameter. This occurs when the draw ratio exceeds a certain critical value above which the extensional is unstable. In the present study, weakly nonlinear analysis is performed for polymer fiber spinning to estimate the nature of bifurcation and to construct the finite amplitude branch near critical point. For entangled polymers, we employ the eXtended Pom-Pom model which describes nonlinear rheology of polymer melt. The linear stability analysis provides the critical draw ratio as a function of fluid elasticity represented by Deborah number. In the unstable regime, the nonlinearities saturate the disturbance amplitude to an equilibrium value. Weakly nonlinear analysis is carried out to obtain the equilibrium amplitude along the neutral stability curve. The dynamical equation for the amplitude is the Landau equation with a Landau constant representing nonlinear growth rate. For flows at small \textit{De}, the Landau constant is found to be negative, indicating supercritical bifurcation. The amplitude branch constructed shows a limit cycle behavior. As the fluid elasticity is increased, initially the equilibrium amplitude is found to decrease and reaches the lowest value when the strain hardening is maximum. Further increase in elasticity, the material undergoes strain softening behavior which leads to increase in amplitude of the oscillations. At very high \textit{De}, the flow becomes subcritically unstable meaning the flow can become unstable below critical draw ratio. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A1.00002: Impact of drag reducing polymers on the onset of instability in a pipe with reverse flow H.J. Shashank , K.R. Sreenivas The objective of this study is to understand the mechanism by which drag reducing polymer (DRP) additives modify turbulent flow, so as to reduce turbulent drag. Reverse flow in a pipe occurs when the fluid close to the wall moves in an opposite direction to that of the core fluid. Reverse flow is established by using a piston-cylinder mechanism, the programmed motion of which imparts a known impulse to the fluid. When the piston is stopped at the end of the stroke, fluid inertia makes the core of the flow to continue in the same direction. In order to conserve mass, reverse flow is established close to the wall. An inflection point is thus formed, leading to flow instability above a critical Reynolds number \footnote{Das \& Arakeri, \textbf{J Fluid Mech} / Volume 374 / November 1998, pp 251-283}. Dye and streak flow visualization experiments are performed to highlight the impact of DRP additives (polyethylene oxide, PEO, dissolved in water). The time of onset of the instability and the wavelength of the observed instability are studied in systems with and without DRP additives. This study will provide further insight into the phenomenon of turbulent polymer drag reduction. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A1.00003: New insights into the nature of the asymmetrical flow of shear-thinning polymer solutions in transitional pipe flow Chaofan Wen , Robert Poole , David Dennis Previous studies of shear-thinning fluids in pipe flow discovered that, although the time-averaged velocity profile was axisymmetric when the flow was laminar or fully turbulent, contrary to expectations it was asymmetric in the laminar-turbulent transition regime. The general consensus of these previous experiments was that the location of the peak velocity remained at a fixed point in space. We present new experimental data which demonstrates that this is in fact not the case. The experiment was performed using an aqueous solution of Xanthan Gum (0.15wt\%), a shear-thinning polymer solution. Stereoscopic particle image velocimetry (SPIV) was used to measure the 3C velocity vectors over the entire circular cross-section of the pipe, 220 pipe diameters downstream of the inlet. The exhibition of significant departures from axisymmetry in transitional flows of shear-thinning fluids was observed and in addition it was discovered that the asymmetric flow pattern is not stationary, although the peak velocity does preferentially arise at certain azimuthal locations. The ensemble average of all the SPIV data results in the recovery of the velocity profile measured using laser Doppler velocimetry in previous studies: still asymmetric but to a lesser extent than the instantaneous flow. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A1.00004: Microstructure change of shear-bands in concentric cylinder flow of wormlike miceller solutions observed by birefringence profile Masatoshi Ito , Yumiko Yoshitake , Tsutomu Takahashi The shear-bands formation process of wormlike micellar solutions in start-up shear has been examined by flow-birefringence observation. A concentric cylinder flow cell is used as a platform to generate the start-up shear and a birefringence observation system using the crossed Nichol polarizes with a white light source is built on it together. In this system, the entire flow field along both radial and circumferential direction can be observed. The magnitude of the birefringence is evaluated by the hue profile calculated from the color profile. The orientation angle at each band is estimated by the extinction angle. CTAB/NaSal aqueous solutions were used as a test fluid and it is known that they generate the shear-induced structure (SIS) at a certain shear rate. The birefringence appears homogeneously in the entire area of the flow field at low shear rate. At a certain shear rate, a thin highly oriented band is generated near the driven wall. With increasing the shear rate, this band changes to the SIS state gradually. When the extreme shear-hardening phenomenon appears at start region of high shear rate flow, the birefringence changes to homogeneous. In this case, the stress-optical coefficient keeps a constant that is the almost same value at the low shear. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A1.00005: On the rise velocity discontinuity of a deformable bubble in unbounded viscoelastic solutions John Tsamopoulos , Dimitris Fraggedakis , Yiannis Dimakopoulos It is well-documented experimentally, but not well-understood that a bubble steadily rising in a viscoelastic solution exhibits a negative wake and a jump discontinuity in its rise velocity, when its radius exceeds a critical value. In all experiments, the bubble shape forms a cusp in its back side and in some experiments it loses axial symmetry forming a wedge. Some authors have related the velocity jump with the existence of the negative wake or even the wedge formation. We have undertaken a computational study to explore the mechanisms behind these phenomena. To this end, we have used the ePTT model and determined its rheological parameters by fitting it to experiments. Then we developed an FE code (using elliptic grid generation and the SUPG and EVSS methods) and calculated the bubble rise and deformation as its radius increases. This simultaneously affects all parameters: Bond, Archimedes and Deborah numbers. Our predictions reproduce very accurately bubble shapes and the results up to the velocity jump or, in certain cases, beyond it using arc-length continuation. The discontinuity is attributed to a hysteresis loop, but does not require the presence of a wedge in the bubble shape and the negative wake is predicted even before this jump. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A1.00006: Polymer Stress-Gradient Induced Migration in Thin Film Flow Over Topography Sophia Tsouka , Yiannis Dimakopoulos , John Tsamopoulos We consider the 2D, steady film flow of a dilute polymer solution over a periodic topography. We examine how the distribution of polymer in the planarization of topographical features is affected by flow intensity and physical properties. The thermodynamically acceptable, Mavrantzas-Beris two-fluid Hamiltonian model is used for polymer migration. The resulting system of differential equations is solved via the mixed FE method combined with an elliptic grid generation scheme. We present numerical results for polymer concentration, stress, velocity and flux of components as a function of the non-dimensional parameters of the problem (Deborah, Peclet, Reynolds and Capillary numbers, ratio of solvent viscosity to total liquid viscosity and geometric features of the topography). Polymer migration to the free surface is enhanced when the cavity gets steeper and deeper. This increases the spatial extent of the polymer depletion layer and induces strong banding in the stresses away from the substrate wall, especially in low polymer concentration. Macromolecules with longer relaxation times are predicted to migrate towards the free surface more easily, while high surface tension combined with a certain range of Reynolds numbers affects the free surface deformations. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A1.00007: Turbulent Fluctuations in Dilute Polymer Solutions Alexandre de Chaumont Quitry , Nicholas T. Ouellette The interaction of complex fluids with turbulent flows presents challenges illustrated in many natural and industrial phenomena. In this study, we report experiment measurements of the modification of turbulence in the presence of long-chain polyacrylamide in water. We use Lagrangian Particle Tracking to study the central region of a Von Karman swirling flow, generated by placing counter-rotating impellers in a cylindrical container. While it has been shown that concentrations as low as 1p.p.m. by weight can affect turbulent fluctuations, it remains theoretically challenging to identify a physical mechanism distinct from an increase in effective viscosity observed at higher concentrations. We attempt to characterize such a mechanism with measurements of spatial and temporal correlations of the velocity and acceleration fields. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A1.00008: Lattice Boltzmann simulations of liquid crystal particulate flow in a channel with finite anchoring boundary conditions Rui Zhang , Tyler Roberts , Juan de Pablo Liquid crystals (LC) posses anisotropic viscoelastic properties, and, as such, LC flow can be incredibly complicated. Here we employ a hybrid lattice Boltzmann method (pioneered by Deniston, Yeomans and Cates) to systematically study the hydrodynamics of nematic liquid crystals (LCs) with and without solid particles. This method evolves the velocity field through lattice Boltzmann and the LC-order parameter via a finite-difference solver of the Beris-Edwards equation. The evolution equation of the boundary points with finite anchoring is obtained through Poisson bracket formulation. Our method has been validated by matching the Ericksen-Leslie theory. We demonstrate two applications in the flow alignment regime. We first investigate a hybrid channel flow in which the top and bottom walls have different anchoring directions. By measuring the apparent shear viscosity in terms of Couette flow, we achieve a viscosity inhomogeneous system which may be applicable to nano particle processing. In the other example, we introduce a homeotropic spherical particle to the channel, and focus on the deformations of the defect ring due to anchorings and flow. The results are then compared to the molecular dynamics simulations of a colloid particle in an LC modeled by a Gay-Berne potential. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A1.00009: Brownian Dynamics Simulation of two-dimensional nanosheets under extensional flow Yueyi Xu , Micah Green We investigated the morphology change of two-dimensional nanosheets under extensional flow using a coarse-grained model. Nanosheets such as graphene are promising materials for a variety of materials and electronics applications; extensional flow fields are used to cast or process liquid nanosheet dispersions in several processing techniques, including spin coating and compression molding. Process parameters, including bending stiffness and Weissenberg numbers can have a significant impact on the nanosheet morphology and the physical properties of the finished products. We use Brownian Dynamics simulations to study the impact of external flow field on a two-dimensional bead-rod lattice model. Our model was previously demonstrated for steady shear flow. Here we studied the change of morphology of graphene over time and varied the sheet size, bending stiffness and Weissenberg number. Our results showed a flattening behavior that increases with Weissenberg number. Our results also showed significant differences between nanosheets as a function of bending stiffness, with contrasting ``plate'' and ``washrag'' results under extension. The intrinsic viscosity first experiences a drop with Weissenberg number followed by a plateau associated with maximum extension. [Preview Abstract] |
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