Bulk Viscosity (bulk + viscosity)

Distribution by Scientific Domains

Selected Abstracts

Pt-catalyzed chemical modification of ,,,-bis(hydrido)polydimethylsiloxanes and copoly(methylhydridosiloxane/dimethylsiloxane) with vinylheptaphenylcyclotetrasiloxane

Thomas M. Gädda
Abstract Pt-catalyzed hydrosilylation between vinylheptaphenylcyclotetrasiloxane and a series of ,,,-bis(hydrido)polydimethylsiloxanes and copoly(methylhydridosiloxane/dimethylsiloxane) was used to prepare chemically modified materials. These modified polymers were characterized by IR, UV, and 1H, 13C, and 29Si NMR spectroscopy and gel permeation chromatography (GPC). The molecular weights, determined by GPC, UV, and NMR end-group analysis, showed the anticipated increases. The thermal properties of the polymers were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The crystallinity, determined by DSC, was either reduced or completely eliminated for the modified polymers. The thermal stabilities, measured in both air and nitrogen by TGA, were slightly higher than the thermal stability of ,,,-bis(trimethylsiloxy)polydimethylsiloxane. Significantly increased bulk viscosities were observed for all the modified polymers. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3007-3017, 2005 [source]

Progress in the Modelling of Air Flow Patterns in Softwood Timber Kilns

T.A.G. Langrish
Progress in modelling air flow patterns in timber kilns using computational fluid dynamics (CFD) is reviewed in this work. These simulations are intended to predict the distribution of the flow in the fillet spaces between boards in a hydraulic model of a timber kiln. Here, the flow regime between the boards is transitional between laminar and turbulent flow, with Reynolds numbers of the order of 5000. Running the simulation as a transient calculation has shown few problems with convergence issues, reaching a mass residual of 0.2% of the total inflow after 40 to 100 iterations per time step for time steps of 0.01 s. Grid sensitivity studies have shown that non-uniform grids are necessary because of the sudden changes in flow cross section, and the flow simulations are insensitive to grid refinement for non-uniform grids with more than 300,000 cells. The best agreement between the experimentally-measured flow distributions between fillet spaces and those predicted by the simulation have been achieved for (effective) bulk viscosities between the laminar viscosity for water and ten times that value. This change in viscosity is not very large (less than an order of magnitude), given that effective turbulent viscosities are typically several orders of magnitude greater than laminar ones. This result is consistent with the transitional flows here. The effect of weights above the stack can reduce the degree of non-uniformity in air velocities through the stack, especially when thick weights are used, because the stack may then be separated from the eddy at the top of the plenum chamber. [source]

A unified continuum representation of post-seismic relaxation mechanisms: semi-analytic models of afterslip, poroelastic rebound and viscoelastic flow

Sylvain Barbot
SUMMARY We present a unified continuum mechanics representation of the mechanisms believed to be commonly involved in post-seismic transients such as viscoelasticity, fault creep and poroelasticity. The time-dependent relaxation that follows an earthquake, or any other static stress perturbation, is considered in a framework of a generalized viscoelastoplastic rheology whereby some inelastic strain relaxes a physical quantity in the material. The relaxed quantity is the deviatoric stress in case of viscoelastic relaxation, the shear stress in case of creep on a fault plane and the trace of the stress tensor in case of poroelastic rebound. In this framework, the instantaneous velocity field satisfies the linear inhomogeneous Navier's equation with sources parametrized as equivalent body forces and surface tractions. We evaluate the velocity field using the Fourier-domain Green's function for an elastic half-space with surface buoyancy boundary condition. The accuracy of the proposed method is demonstrated by comparisons with finite-element simulations of viscoelastic relaxation following strike-slip and dip-slip ruptures for linear and power-law rheologies. We also present comparisons with analytic solutions for afterslip driven by coseismic stress changes. Finally, we demonstrate that the proposed method can be used to model time-dependent poroelastic rebound by adopting a viscoelastic rheology with bulk viscosity and work hardening. The proposed method allows one to model post-seismic transients that involve multiple mechanisms (afterslip, poroelastic rebound, ductile flow) with an account for the effects of gravity, non-linear rheologies and arbitrary spatial variations in inelastic properties of rocks (e.g. the effective viscosity, rate-and-state frictional parameters and poroelastic properties). [source]

Viscosity effects in cobaloxime-mediated catalytic chain-transfer polymerization of methacrylates

G. Evan Roberts
Abstract The effect of bulk viscosity on the cobaloxime-mediated catalytic chain-transfer polymerization of methacrylates at 60 °C was investigated by both the addition of high molecular weight poly(methyl methacrylate) to methyl methacrylate polymerization and the dilution of benzyl methacrylate polymerization by toluene. The results indicate that the bulk viscosity is not directly linked to the chain-transfer activity. The previously measured relationship between chain-transfer-rate coefficient and monomer viscosity therefore probably reflects changes at the molecular level. However, the results in this article do not necessarily disprove a diffusion-controlled reaction rate because cobaloxime diffusion is expected to scale with the monomer friction coefficient rather than bulk viscosity. Considering the published data, to date we are not able to distinguish between a diffusion-controlled reaction rate or a mechanism directly affected by the methacrylate substituent. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 782,792, 2002; DOI 10.1002/pola.10152 [source]

Multi-Scale Study of Sintering: A Review

Eugene A. Olevsky
An integrated approach, combining the continuum theory of sintering with a kinetic Monte-Carlo (KMC) model-based mesostructure evolution simulation is reviewed. The effective sintering stress and the normalized bulk viscosity are derived from mesoscale simulations. A KMC model is presented to simulate microstructural evolution during sintering of complex microstructures taking into consideration grain growth, pore migration, and densification. The results of these simulations are used to generate sintering stress and normalized bulk viscosity for use in continuum level simulation of sintering. The advantage of these simulations is that they can be employed to generate more accurate constitutive parameters based on most general assumptions regarding mesostructure geometry and transport mechanisms of sintering. These constitutive parameters are used as input data for the continuum simulation of the sintering of powder bilayers. Two types of bilayered structures are considered: layers of the same particle material but with different initial porosity, and layers of two different materials. The simulation results are verified by comparing them with shrinkage and warping during the sintering of bilayer ZnO powder compacts. [source]