Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session LT: Non-Newtonian Flows I |
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Chair: Roberto Zenit, Universidad Nacional Autonoma de Mexico Room: 204A |
Monday, November 24, 2008 3:35PM - 3:48PM |
LT.00001: Mesoscopic dynamics of polymer chains in high strain rate extensional flows Demosthenes Kivotides, Theo Theofanous The study of high speed aerobreakup of polymeric liquids is obstructed by the lack of standard viscoelastic constitutive laws valid for high strain rate extensional flows. In order to reliably estimate polymer induced elastic stresses in these processes, we perform Brownian dynamics calculations of a bead-spring polymer model at high Deborah numbers. The predictions of our computational chain model match experimental largest relaxation time and elastic stress levels. By kinematically prescribing the solvent flow, we study polymer response in dilute, high strain rate extensional flows typical of aerobreakup experiments clarifying the physical mechanisms of chain stretching and computing transient extensional viscosities. In the context of confined systems, we investigate the dynamical effects of topological chain entanglement. [Preview Abstract] |
Monday, November 24, 2008 3:48PM - 4:01PM |
LT.00002: A microfluidic study of the rheology, slippage and flow instabilites of wormlike micelles Philippe Nghe, Guillaume Degre, Patrick Tabeling, Armand Ajdari We characerize by Particle Image Velocimetry the Poiseuille flow semi-dilute solutions of wormlike micelles (a CTAB and sodium nitrate aqueous solution and a CpCl solution) in pressure resistant microchannels. Thanks to the high aspect ratio of our channels, we can measure the local rheology of the solution, independantly from the slippage at the wall, according to a method already validated on non-newtonian polymer solutions. As the pressure driving the flow is increased, the velocity profiles reveal first a newtonian phase, then apparition of a dramatically lower viscosity second phase at the walls, which is the so called shear banding regime. First we deduce the local rheology of the solution from these velocity profiles. This method gives access to the stress versus shear rate relation over a domain unexplored in classical Couette geometries, characterizing more than a decade of deformation rates for the high shear phase. Then we measure the slip length to be below 1.5 microns in these flows. Finally we study the development of an instability at the interface between the two phases, similarly to what has already been found in Couette like geometries. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LT.00003: Numerical simulations of time-dependent, fully 3-D viscoelastic flows past bluff bodies David Richter, Eric Shaqfeh, Gianluca Iaccarino With the goal of creating a robust numerical method for simulating three dimensional, time dependent non-Newtonian flows, we have developed an unstructured, finite-volume code to compute a wide variety of viscoelastic flows over a large range of Reynolds ($Re$) and Weissenberg ($Wi$) numbers. Our method is based on the FENE-P constitutive model to describe the flow of dilute polymeric solutions, and an implicit time-stepping technique is utilized that properly maintains boundedness of the polymer stresses and extensions even at high flow strengths. We will present the time-dependent, viscoelastic flow past a circular cylinder at moderate $Re$ ($Re \sim O(100)$). Within this range, regular vortex shedding occurs, and the characteristic frequency of this shedding was found to decrease with increasing fluid elasticity. Furthermore, the coefficients of both friction drag and form drag are reduced with increasing $Wi$, and new qualitative effects have been observed at large polymer lengths where the cylinder drag rises dramatically due to a rapid increase in form drag. Physical mechanisms for this behavior will be proposed and discussed. [Preview Abstract] |
Monday, November 24, 2008 4:14PM - 4:27PM |
LT.00004: Vertical structures in vibrated wormlike micellar solutions Tamir Epstein, Robert Deegan Vertically vibrated shear thickening particulate suspensions can support a free-standing interfaces oriented parallel to gravity. We find that shear thickening worm-like micellar solutions also support such vertical interfaces. Above a threshold in acceleration, the solution spontaneously accumulates into a labyrinthine pattern characterized by a well-defined vertical edge. The formation of vertical structures is of interest because they are unique to shear-thickening fluids, and they indicate the existence of an unknown stress bearing mechanism. [Preview Abstract] |
Monday, November 24, 2008 4:27PM - 4:40PM |
LT.00005: A Mixing Transition in a Viscoelastic Fluid Becca Thomases, Michael Shelley Dynamical behavior in low Reynolds number viscoelastic flows is investigated numerically in the Oldroyd-B model. For low Weissenberg number, flows are ``slaved" to the four-roll mill geometry of the body forcing. For sufficiently large Weissenberg number, such slaved solutions are unstable and under perturbation transit in time to a structurally dissimilar flow state dominated by a single large vortex, rather than four vortices of the four-roll mill state. The transition to this new steady-state also leads to regions of well-mixed fluid, and may be related to a recently discovered transition in cross-channel flows of a viscoelastic fluid. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LT.00006: Filament break up, drop size and non- Newtonian borate esters in jet flows Suresh Ahuja Study and analysis of jet flows has found application in such industrial applications as spray coating and inkjet printers. Length-scales and timescales in controlling the dynamics of the thinning and break-up process is found to depend on gravitational forces, surface forces, and mechanical forces shear and extensional forces acting on a fluid. If the gravitational effects are not important, midpoint radius of the viscous filament for Newtonian fluids has been analyzed to depend on the ratio of surface tension to viscosity of the fluid and the process time. The ratio of time to breakup for the visco-capillary and inertio-capillary processes is related to a dimensionless number known as the Ohnesorge\textit{ number }In non-Newtonian and visco-elastic fluids, filament radius is dependent on the ration of relaxation modulus to surface tension and exponentially decays with the ratio of process time to the fluid (polymer) relaxation time. Analogous to Ohnesorge number, time scale of break up, in non-Newtonian and visco-elastic fluids, time scale of break up is Deborah number, the ratio of relaxation time to process time. Using fluids of glycol, polyethylene oxide and borate esters, torsion strain experiments were used to determine viscosity and visco-elastic parameters (relaxation modulus and relaxation time) and applied to inkjet process. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LT.00007: Non-modal energy amplification in channel flows of viscoelastic fluids Mihailo Jovanovic, Nazish Hoda, Satish Kumar Energy amplification in channel flows of Oldroyd-B fluids is studied from an input-output point of view by analyzing the responses of the velocity components to spatio-temporal body forces. These inputs into the governing linearized equations are assumed to be harmonic in the streamwise and spanwise directions and stochastic in the wall-normal direction and in time. Such inputs enable the use of powerful tools from linear systems theory that have recently been applied to analyze Newtonian fluid flows. It is found that the energy amplification increases with a decrease in viscosity ratio and increase in Reynolds number and elasticity number. In most of the cases, streamwise constant perturbations are most amplified and the location of maximum energy amplification shifts to higher spanwise wavenumbers with an increase in Reynolds number and elasticity number and decrease in viscosity ratio. For streamwise constant perturbations, an explicit Reynolds number scaling of energy amplification from different forcing to different velocity components is developed, showing the same $Re$-dependence as in Newtonian fluids. At low Reynolds numbers, the energy amplification decreases monotonically when the elasticity number is sufficiently small, but shows a maximum when the elasticity number becomes sufficiently large, suggesting that elasticity can amplify disturbances even when inertial effects are weak. [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LT.00008: Fluid Dynamics and Rupture of Polymeric Solutions at Ultra-High Strain Rates Vladimir Mitkin, Alexey Rozhkov, Theo Theofanous We create inertially-driven, nearly free, expanding rings to access rheology and flow phenomena of polymeric solutions at ultra-high strain rates. For a given fluid, with increasing initial velocity, three regimes are identified: expansion followed by elastic rebound, steady expansion, expansion interrupted by ruptures (cohesiveness failure). From expansion histories we deduce rheology (relaxation time and elasticity modulus) in the frame of the Oldroyd B model, and show that regime transitions can be captured consistently over a wide range of fluid constitutions by a single dimensionless group; that is the product of the Deborah number and an elasticity number---the elasticity modulus scaled by the initial flow kinetic energy. Moreover in this manner we find that: (a) the onset of rupture is determined by a specific value of tension (a critical rupture stress) which is characteristic on the solvent-polymer involved, and (b) the critical rupture stress scales in proportion to polymer concentration just as is the elasticity modulus. The method and results complement low strain-rate rheology, as in the well-known filament-thinning method, and is particularly well-suited for informing the micromechanics of rupture at high strain rates. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LT.00009: Controllable adhesion using field-responsive fluids Randy Ewoldt, Gareth McKinley, A.E. Hosoi Viscous Newtonian fluids confined in sufficiently small gaps can provide strong resistance to the separation of two parallel rigid surfaces, a phenomenon known as Stefan adhesion. However, the resistance to a shear load is considerably lower than for normal loads in such confined geometries. In principle, a field- responsive ``smart'' fluid, which exhibits a field-dependent microstructure with dramatically increased resistance to shear loading, can be used in place of a Newtonian fluid enabling externally-tunable adhesion. We report experimental results for both normal and shear loading of field-responsive, non- Newtonian fluids confined between rigid surfaces as the external magnetic field, the geometry of the adhesive contact pad, and the roughness of the adherent are varied. The peak adhesive force, the ``work of adhesion'' and the mode of failure are all controlled by the field-responsive nature of the magnetorheological fluid forming the adhesive layer. [Preview Abstract] |
Monday, November 24, 2008 5:32PM - 5:45PM |
LT.00010: ABSTRACT WITHDRAWN |
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