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ABSTRACT: Mass transfer in an oscillatory electro-osmotic flow (EOF) is theoretically studied, for the case of a cylindrical tube with
a reactive wall. An expression for the dispersion coefficient, reflecting the time-averaged mass flux of an electrically neutral
solute, is derived analytically. Under the influence of a reversible solute-wall mass exchange, the dispersion coefficient
exhibits a complex dependence on the various parameters representing the effects of the electric double-layer thickness, oscillation
frequency, solution transport properties, solute partitioning, and reaction kinetics. Our results suggest that, in the presence
of a reversible mass exchange at the wall, an oscillatory EOF may be used for separation of species. It is found that optimal
conditions for separation are achieved for a thin double-layer, where an inert solute, or one with slow exchange kinetics,
experiences virtually no dispersion while the dispersion is maximized for the reactive solute exhibiting fast kinetics.
KeywordsOscillatory electro-osmotic flow–Taylor–Aris dispersion–Enhanced mass transfer–Solute separation
Microfluidics and Nanofluidics 05/2012; 10(1):97-106. · 3.37 Impact Factor
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ABSTRACT: Spectral methods are very efficient and powerful for solving periodic problems. A new spectral method is developed for problems with no spatial periodicity, and demonstrated for water waves. The method splits the potential into the sum of a prescribed non-periodic component and an unknown periodic component. Computed results are compared with experiments by Shemer et al (1998).
European Journal of Mechanics - B/Fluids.
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ABSTRACT: The effect of lung morphology on the heterogeneity of regional ventilation and particle deposition in the bronchial airways is studied using Horsfield's regular-asymmetric lung model. Flow distribution among the airways is calculated by solving the whole tree network, assuming laminar flow hydrodynamic resistances without accounting for gravitationally enhanced preferential airflow distribution. The variation of morphological properties, such as the lung volume and surface area distal to any airway generation, and physiological properties, such as ventilation and particle deposition, is calculated, and fractal dimensions that characterize these properties and processes are computed. The close agreement between the model fractal dimension characterizing ventilation and those found from clinical data assess the validity of the model. It is shown that the fractal dimensions that characterize the morphological properties and the physiological processes are similar, suggesting that all are related and stem from a common underlying attribute—the lung morphology. The variation of particle deposition in the lung, as well as the variation of ventilation and morphological attributes, increases moderately with the lung tree asymmetry. The deposition density, regarded as a key exposure metric or therapeutic index, does not follow a spatial scale-free distribution.
Journal of Aerosol Science.
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ABSTRACT: A two-dimensional, boundary layer model is presented, for describing the heat transfer in the feed channel of a vacuum membrane distillation (VMD) module. The model formulation allows for variations of viscosity with temperature, and introduces an ‘effective’ slip coefficient so as to account for the possible deviation of the flow and heat transfer characteristics over a liquid–gas interface, from those at a solid surface. The model solution provides the temperature field in the feed channel and its dependence on the bulk velocity and temperature, as well as the vapor mass flux across the membrane. While the effect of a temperature variable viscosity becomes increasingly apparent at higher temperatures, its effect on the evaporation mass flux is not substantial. The relative contribution of transverse convection to the evaporation mass flux is shown to be insignificant, within the range of parameters examined. In the presence of an ‘effective’ velocity slip, the degree of temperature polarization is reduced and a corresponding increase in the evaporation mass flux is observed. Calculated results are in good agreement with experimental data. The presented results suggest that the effect of alternating solid and gas interfaces, encountered by the fluid, on the flow field and resulting heat transfer, should be accounted for if accurate predictions are to be made.
Journal of Membrane Science.
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ABSTRACT: A Boussinesq method is derived that is fully dispersive, in the sense that the error of the approximation is small for all 0⩽kh<∞ (k the magnitude of the wave number and h the water depth). This is made possible by introducing the generalized (2D) Hilbert transform, which is evaluated using the fast Fourier transform. Variable depth terms are derived both in mild-slope form, and in augmented mild-slope form including all terms that are linear in derivatives of h. A spectral solution is used to solve for highly nonlinear steady waves using the new equations, showing that the fully dispersive behavior carries over to nonlinear waves. A finite-difference–FFT implementation of the method is also described and applied to more general problems including Bragg resonant reflection from a rippled bottom, waves passing over a submerged bar, and nonlinear shoaling of a spectrum of waves from deep to shallow water.
European Journal of Mechanics - B/Fluids.
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ABSTRACT: Two iterative solutions of the Helmholtz equation for a scalar field in R3 above a rough surface that admits the Dirichlet boundary condition are derived. The bases for the two iterative methods are two different boundary integral equations that represent the solution. The first integral equation is classified as a Fredholm integral equation of the first kind. The second is classified as a Fredholm integral equation of the second kind. This classification suggests that it is easier to find stable solution methods to the second equation. In both methods, the boundary integral was separated into a major part which is easy to calculate and a local residual part. The major part is a convolution and thus can be calculated using FFT in complexity O(N log N), where N is the number of surface points in which the surface height and its first derivatives together with the incoming wave and its normal derivative are all known. The residual element of the equations can be approximated efficiently only for surfaces where their amplitude is less than the wavelength of the incoming wave. The iterative schemes were tested numerically against a reference solution in order to examine the applicability range, the error estimation and the stability of the schemes. All tests supported the superiority of the second method. In particular the error estimation and stability tests indicated good performance for surfaces with slope up to 1. Yet, being an equation in the scattered field alone, makes the first method useful as a benchmark solution in its domain of applicability.
Wave Motion.
