Browsing by Author "Pasquali, M."

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    Couette flows of a thixotropic yield-stress material: Performance of a novel fluidity-based constitutive model
    (AIP Publishing, 2020) Siqueira, I.R.; Pasquali, M.; de Souza Mendes, P.R.
    We present a theoretical and computational study of thixotropic yield-stress materials in cylindrical Couette flows using a novel fluidity-based constitutive model introduced by de Souza Mendes et al. [J. Nonnewtion. Fluid Mech. 261, 1–8 (2018)]. The model relies on measurable rheological properties to couple the equations of motion with an additional equation for the evolution of the material fluidity (i.e., the reciprocal of viscosity). The fluidity itself is used as a structure parameter to assess the material structuring state without the introduction of phenomenological functions or additional parameters. Our simulations parallel rheological tests with a stress-controlled rheometer and are carried out with the material properties obtained experimentally for the laponite suspension from which the model was originally developed. The results reveal that the processes of breakdown and buildup of the microstructure as well as the position of the yield surface in the flow essentially depend on the applied stress and on two material properties associated with distinct thixotropic time scales, namely, the avalanche time and the construction time. The model predictions also capture many features observed in the flow of yield-stress materials with thixotropy, such as the avalanche effect and transient shear banding. We also show that the steady-state flow is uniquely determined by the imposed stress and does not depend on the material initial structuring state. This contrasts with previous reports for nonthixotropic elastoviscoplastic materials, suggesting that nonunique steady flows of structured materials are probably associated with the transient evolution of elastic stresses from a given initial condition.
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    Parallel Solution of Large-Scale Free Surface Viscoelastic Flows Via Sparse Approximate Inverse Preconditioning
    (2006-08) Castillo, Z.; Xie, X.; Sorensen, D.C.; Embree, M.; Pasquali, M.
    Though computational techniques for two-dimensional viscoelastic free surface flows are well developed, three-dimensional flows continue to present significant computational challenges. Fully coupled free surface flow models lead to nonlinear systems whose steady states can be found via Newton's method. Each Newton iteration requires the solution of a large, sparse linear system, for which memory and computational demands suggest the application of an iterative method, rather than the sparse direct methods widely used for two dimensional simulations. The Jacobian matrix of this system is often ill-conditioned, resulting in unacceptably slow convergence of the linear solver; hence preconditioning is essential. In this paper we propose a variant sparse approximate inverse preconditioner for the Jacobian matrix that allows for the solution of problems involving more than a million degrees of freedom in challenging parameter regimes. Construction of this preconditioner requires the solution of small least squares problems that can be simply parallelized on a distributed memory machine. The performance and scalability of this preconditioner with the GMRES solver are investigated for two- and three-dimensional free surface flows on both structured and unstructured meshes in the presence and absence of viscoelasticity. The results suggest that this preconditioner is an extremely promising candidate for solving large-scale steady viscoelastic flows with free surfaces.
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    Transport mechanism in granular Ni deposited on carbon nanotubes fibers
    (American Physical Society, 2012) Salvato, M.; Lucci, M.; Ottaviani, I.; Cirillo, M.; Tamburri, E.; Orlanducci, S.; Terranova, M.L.; Notarianni, M.; Young, C.C.; Behabtu, N.; Pasquali, M.; Richard E. Smalley Institute for Nanoscale Science and Technology
    We investigate the transport properties of granular nickel electrodeposited on carbon nanotube fibers by measuring the electrical resistance and the current voltage characteristics as a function of the temperature. The bare fiber is governed by a three-dimensional variable range hopping transport mechanism, however, a semiconducting to metallic transition is observed after the Ni deposition as a consequence of the evolution from weak to strong coupling between the deposited nickel grains. The experimental results indicate that the charge transport in the Ni-coated fiber develops from hopping governed by the Coulomb blockade in the case of small grains dimensions to a metallic electron phonon interaction mechanism for large grains dimensions. Tunneling enhanced by thermal fluctuation is responsible for the transport in the intermediate conductivity range. The role of the fiber and the effects due to the magnetic nature of the nickel grains are also discussed.