99,331 research outputs found
Modeling the Flow of Yield-Stress Fluids in Porous Media
Yield-stress is a problematic and controversial non-Newtonian flow
phenomenon. In this article, we investigate the flow of yield-stress substances
through porous media within the framework of pore-scale network modeling. We
also investigate the validity of the Minimum Threshold Path (MTP) algorithms to
predict the pressure yield point of a network depicting random or regular
porous media. Percolation theory as a basis for predicting the yield point of a
network is briefly presented and assessed. In the course of this study, a
yield-stress flow simulation model alongside several numerical algorithms
related to yield-stress in porous media were developed, implemented and
assessed. The general conclusion is that modeling the flow of yield-stress
fluids in porous media is too difficult and problematic. More fundamental
modeling strategies are required to tackle this problem in the future.Comment: 27 pages and 5 figure
Microrheology with optical tweezers: data analysis
We present a data analysis procedure that provides the solution to a long-standing issue in microrheology studies, i.e. the evaluation of the fluids' linear viscoelastic properties from the analysis of a finite set of experimental data, describing (for instance) the time-dependent mean-square displacement of suspended probe particles experiencing Brownian fluctuations. We report, for the first time in the literature, the linear viscoelastic response of an optically trapped bead suspended in a Newtonian fluid, over the entire range of experimentally accessible frequencies. The general validity of the proposed method makes it transferable to the majority of microrheology and rheology techniques
The Flow of Newtonian and power law fluids in elastic tubes
We derive analytical expressions for the flow of Newtonian and power law
fluids in elastic circularly-symmetric tubes based on a lubrication
approximation where the flow velocity profile at each cross section is assumed
to have its axially-dependent characteristic shape for the given rheology and
cross sectional size. Two pressure-area constitutive elastic relations for the
tube elastic response are used in these derivations. We demonstrate the
validity of the derived equations by observing qualitatively correct trends in
general and quantitatively valid asymptotic convergence to limiting cases. The
Newtonian formulae are compared to similar formulae derived previously from a
one-dimensional version of the Navier-Stokes equations.Comment: 15 pages, 4 figure
Applications of recurrent neural networks in batch reactors. Part I: NARMA modelling of the dynamic behaviour of the heat transfer fluid
This paper is focused on the development of nonlinear models, using artificial neural networks, able to provide appropriate predictions when acting as process simulators. The dynamic behaviour of the heat transfer fluid temperature in a jacketed chemical reactor has been selected as a case study. Different structures of NARMA (Non-linear ARMA) models have been studied. The experimental results have allowed to carry out a comparison between the different neural approaches and a first-principles model. The best neural results are obtained using a parallel model structure based on a recurrent neural network architecture, which guarantees better dynamic approximations than currently employed neural models. The results suggest that parallel models built up with recurrent networks can be seen as an alternative to phenomenological models for simulating the dynamic behaviour of the heating/cooling circuits which change from batch installation to installation.Publicad
Probing the link between residual entropy and viscosity of molecular fluids and model potentials
This work investigates the link between residual entropy and viscosity based
on wide-ranging, highly accurate experimental and simulation data. This link
was originally postulated by Rosenfeld in 1977, and it is shown that this
scaling results in an approximately monovariate relationship between residual
entropy and reduced viscosity for a wide range of molecular fluids (argon,
methane, CO2, SF6, refrigerant R-134a (1,1,1,2-tetrafluoroethane), refrigerant
R-125 (pentafluoroethane), methanol, and water), and a range of model
potentials (hard sphere, inverse power, Lennard-Jones, and
Weeks-Chandler-Andersen). While the proposed "universal" correlation of
Rosenfeld is shown to be far from universal, when used with the appropriate
density scaling for molecular fluids, the viscosity of non-associating
molecular fluids can be mapped onto the model potentials. This mapping results
in a length scale that is proportional to the cube root of experimentally
measureable liquid volume values
Performance of CSMA in Multi-Channel Wireless Networks
We analyze the performance of CSMA in multi-channel wireless networks,
accounting for the random nature of traffic. Specifically, we assess the
ability of CSMA to fully utilize the radio resources and in turn to stabilize
the network in a dynamic setting with flow arrivals and departures. We prove
that CSMA is optimal in ad-hoc mode but not in infrastructure mode, when all
data flows originate from or are destined to some access points, due to the
inherent bias of CSMA against downlink traffic. We propose a slight
modification of CSMA, that we refer to as flow-aware CSMA, which corrects this
bias and makes the algorithm optimal in all cases. The analysis is based on
some time-scale separation assumption which is proved valid in the limit of
large flow sizes
Solving the flow fields in conduits and networks using energy minimization principle with simulated annealing
In this paper, we propose and test an intuitive assumption that the pressure
field in single conduits and networks of interconnected conduits adjusts itself
to minimize the total energy consumption required for transporting a specific
quantity of fluid. We test this assumption by using linear flow models of
Newtonian fluids transported through rigid tubes and networks in conjunction
with a simulated annealing (SA) protocol to minimize the total energy cost. All
the results confirm our hypothesis as the SA algorithm produces very close
results to those obtained from the traditional deterministic methods of
identifying the flow fields by solving a set of simultaneous equations based on
the conservation principles. The same results apply to electric ohmic
conductors and networks of interconnected ohmic conductors. Computational
experiments conducted in this regard confirm this extension. Further studies
are required to test the energy minimization hypothesis for the non-linear flow
systems.Comment: 11 pages, 2 figures, 1 tabl
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