Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session H33: Mini-Symposium: Complex Fluid Flows in Memory of Daniel D. Joseph II |
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Chair: Howard Hu, University of Pennsylvania Room: 29A |
Monday, November 19, 2012 10:30AM - 10:43AM |
H33.00001: Linear Instabilities in Simple Shear Flow of Polymer Solutions Driven by Stress-gradient/Concentration Coupling Gary Leal, Michael Cromer, Michael Villet, Glenn Fredrickson Inhomogeneities in the flow of polymer solutions have been observed in experiment over the last 40 years, e.g. including the well studied formation of shear bands in a linear shear flow device. In this study we investigate the role of concentration non-uniformities in causing hydrodynamic instabilities in a linear shear flow. To incorporate a stress-concentration coupling we follow a two-fluid model formalism, in which the polymer conformation is described by the Rolie-Poly model. In a shear flow, the model may exhibit linear instability due to perturbations in the gradient direction, even for a monotonic constitutive curve, provided there is a sufficient separation between the characteristic polymer relaxation times. We show that the mechanism driving the instability is the motion of polymer up stress gradients, leading to concentration nonuniformities in the flow. [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H33.00002: Brownian Swimming via Taylor Dispersion Joe Goddard, Eric Lauga We show that the theory of generalized Taylor dispersion can be employed to analyze a model of a low-Re swimmer undergoing Brownian tumbling coupled with systematic translation along a preferred axis. The resulting formula for translational diffusivity confirms a previous analysis\footnote{Lauga, E., PRL 106, 178101 (2011)} based on Langevin dynamics. This present approach may provide a useful method for treating more complex stochastic swimmers. [Preview Abstract] |
Monday, November 19, 2012 10:56AM - 11:09AM |
H33.00003: Active Nematic Flows Greg Forest, Qi Wang, Ruhai Zhou The recent flurry of activity in swimming particle suspensions is extended to macromolecular rods by incorporating polarity, active stress, and density gradients into the kinetic theory of nematic polymers. Simulations predict phenomena unique to nano-rod swimmers at dilute and semi-dilute concentrations. [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H33.00004: Apparent viscosity during unyielding of a thixotropic yield stress fluid Yuriko Renardy, Kara Maki We present a mathematical interpretation of a thixotropic yield stress fluid, based on a viscoelastic constitutive law in the limit of large relaxation time, together with a Newtonian solvent. The dynamics is initiated by a step-up or step-down in prescribed shear stress. There is no presumption of a yield stress, but nevertheless, we obtain yield stress behavior. The thixotropic behavior of the model arises from the multiple time scales which emerge in the limit of large relaxation time. These give rise to fast dynamics (elastic deformation) and slow dynamics (unyielding), in addition to yielded dynamics for shear flow. We present how the model predicts the evolution of apparent viscosity during unyielding. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H33.00005: A universal constraint-based formulation for freely moving immersed bodies in fluids Neelesh A. Patankar Numerical simulation of moving immersed bodies in fluids is now practiced routinely. A variety of variants of these approaches have been published, most of which rely on using a background mesh for the fluid equations and tracking the body using Lagrangian points. In this talk, generalized constraint-based governing equations will be presented that provide a unified framework for various immersed body techniques. The key idea that is common to these methods is to assume that the entire fluid-body domain is a ``fluid'' and then to constrain the body domain to move in accordance with its governing equations. The immersed body can be rigid or deforming. The governing equations are developed so that they are independent of the nature of temporal or spatial discretization schemes. Specific choices of time stepping and spatial discretization then lead to techniques developed in prior literature ranging from freely moving rigid to elastic self-propelling bodies. To simulate Brownian systems, thermal fluctuations can be included in the fluid equations via additional random stress terms. Solving the fluctuating hydrodynamic equations coupled with the immersed body results in the Brownian motion of that body. The constraint-based formulation leads to fractional time stepping algorithms a la Chorin-type schemes that are suitable for fast computations of rigid or self-propelling bodies whose deformation kinematics are known. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H33.00006: Computationally and experimentally assessed base-flow, stability, and sensitivity differences between shear dominated (negligible gravity) and gravity assisted internal condensing flows Amitabh Narain, Ranjeeth Naik, Soumya Mitra, Michael Kivisalu Annular regimes for internal condensing flow are desirable for high heat transfer rates out of a condenser. Predominantly shear driven flows typically occur in horizontal channels (with condensation on the bottom horizontal-surface), zero gravity flows, and in milli-meter to micro-meter scale hydraulic diameter ducts. This talk presents steady and unsteady computational results obtained from the numerical solutions of the full two-dimensional governing equations for annular internal condensing flows in a channel. Results obtained for inclined, horizontal, and zero-gravity cases (with and without surface-tension) bring out the differences between shear driven and gravity assisted/driven flows. The results highlight the differences between steady solutions, their stability, and their noise-sensitivity. It is shown that annular flows are more stable and easily realized for gravity driven or gravity assisted flows than for primarily shear driven flows. Besides stability, extreme-sensitivity of shear driven flows to typically present persistent fluctuations is also demonstrated. This sensitivity is beneficially exploited to achieve significant heat-transfer rate enhancements. The talk also highlights conditions for which surface tension forces become important. The computational results have been validated by good comparisons with condensing flow experimental results for the annular regimes. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H33.00007: A New Model for Instantaneous Coal and Gas Outbursts Kangping Chen An instantaneous coal and gas outburst is a sudden and violent simultaneous ejection of large amounts of coal and gas from the working coalface during underground mining. Existing theories are incapable of explaining many precursors of an outburst, which include the occurrence of distinct audible noises originating close to the mine opening and a decrease in the temperature in the solid of the coalface and the nearby atmosphere. Nor can they explain the increased proneness to outbursting with increased rate-of-advance of the coalface. They are incapable of predicting a failure of explosive and catastrophic nature which characterizes an instantaneous outburst. A new model combining fracture mechanics, gas dynamics and rock mechanics is presented to elucidate the physical mechanisms leading to instantaneous coal and gas outbursts. This model suggests a domino effect that causes a catastrophic failure of the coal and an instantaneous outburst; it identifies a critical condition for the onset of an outburst, and it successfully predicts all of the observed phenomena preceding outbursts. The model also predicts a fracture aperture size effect which is confirmed by observations. [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H33.00008: Transient Flow due to the Adsorption of Particles Pushpendra Singh, Naga Musunuri, Bhavin Dalal, Ian Fischer, Daniel Codjoe When small particles, e.g., glass, flour, pollen, etc., come in contact with a fluid-liquid interface they disperse so quickly to form a monolayer on the surface that it appears explosive, especially on the surface of mobile liquids like water. This is a consequence of the fact that a particle coming in contact with a liquid surface is pulled into the surface by capillary force causing the particle to accelerate to a relatively-large velocity in the direction normal to the surface. This vertical motion of the particle gives rise to a lateral flow on the surface away from the particle. PIV measurements show that the adsorption of a spherical particle causes a transient axisymmetric flow about the vertical line passing through the center of the particle. The flow develops in a fraction of second after the adsorption of the particle and persists for several seconds. The fluid directly below the particle rises upwards and near the surface it moves away from the particle. [Preview Abstract] |
Monday, November 19, 2012 12:14PM - 12:27PM |
H33.00009: Thermal Dielectrophoretic (T-DEP) Force Howard Hu, Barukyah Shararenko, Haim Bau When subjected to a non-uniform electric field, a dielectric particle in a dielectric medium experiences a dielectrophoretic (DEP) force. For some applications in microfluidic systems, thermal effects due to Joule heating are quite important. In this study, we examine the additional dielectrophoretic force due to the thermal effect, which we termed as thermal dielectrophoretic (T-DEP) force. A thermal gradient may be established in the fluid due to Joule heating, which leads to the spatial variations in conductivity and permittivity of the fluid. With the gradients in the conductivity and permittivity, an electric field (even a uniform field) will induce electric forces in the fluid (and in the particle), and cause a flow (electrothermal flow). We have derived an expression for the net thermal dielectrophoretic (T-DEP) force acting on a particle suspended in a medium with a temperature gradient. This extra T-DEP force has never been discussed in literature, could be important in predicting the particle trajectories in such flow systems, and explain the discrepancy observed between the theoretical prediction and experimental measurements. [Preview Abstract] |
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