2,085 research outputs found
Mobility of Power-law and Carreau Fluids through Fibrous Media
The flow of generalized Newtonian fluids with a rate-dependent viscosity
through fibrous media is studied with a focus on developing relationships for
evaluating the effective fluid mobility. Three different methods have been used
here: i) a numerical solution of the Cauchy momentum equation with the Carreau
or power-law constitutive equations for pressure-driven flow in a fiber bed
consisting of a periodic array of cylindrical fibers, ii) an analytical
solution for a unit cell model representing the flow characteristics of a
periodic fibrous medium, and iii) a scaling analysis of characteristic bulk
parameters such as the effective shear rate, the effective viscosity,
geometrical parameters of the system, and the fluid rheology. Our scaling
analysis yields simple expressions for evaluating the transverse mobility
functions for each model, which can be used for a wide range of medium porosity
and fluid rheological parameters. While the dimensionless mobility is, in
general, a function of the Carreau number and the medium porosity, our results
show that for porosities less than , the dimensionless
mobility becomes independent of the Carreau number and the mobility function
exhibits power-law characteristics as a result of high shear rates at the pore
scale. We derive a suitable criterion for determining the flow regime and the
transition from a constant viscosity Newtonian response to a power-law regime
in terms of a new Carreau number rescaled with a dimensionless function which
incorporates the medium porosity and the arrangement of fibers
Forchheimer Model for Non-Darcy Flow in Porous Media and Fractures
Imperial Users onl
Erosion of a granular bed driven by laminar fluid flow
Motivated by examples of erosive incision of channels in sand, we investigate
the motion of individual grains in a granular bed driven by a laminar fluid to
give us new insights into the relationship between hydrodynamic stress and
surface granular flow. A closed cell of rectangular cross-section is partially
filled with glass beads and a constant fluid flux flows through the cell.
The refractive indices of the fluid and the glass beads are matched and the
cell is illuminated with a laser sheet, allowing us to image individual beads.
The bed erodes to a rest height which depends on . The Shields
threshold criterion assumes that the non-dimensional ratio of the
viscous stress on the bed to the hydrostatic pressure difference across a grain
is sufficient to predict the granular flux. Furthermore, the Shields criterion
states that the granular flux is non-zero only for . We find
that the Shields criterion describes the observed relationship when the bed height is offset by approximately half a grain diameter.
Introducing this offset in the estimation of yields a collapse of the
measured Einstein number to a power-law function of
with exponent . The dynamics of the bed height relaxation are
well described by the power law relationship between the granular flux and the
bed stress.Comment: 12 pages, 5 figure
Pressure jump interface law for the Stokes-Darcy coupling: Confirmation by direct numerical simulations
It is generally accepted that the effective velocity of a viscous flow over a
porous bed satisfies the Beavers-Joseph slip law. To the contrary, interface
law for the effective stress has been a subject of controversy. Recently, a
pressure jump interface law has been rigorously derived by Marciniak-Czochra
and Mikeli\'c. In this paper, we provide a confirmation of the analytical
result using direct numerical simulation of the flow at the microscopic level.Comment: 25 pages, preprin
Direct numerical simulation of turbulent channel flow over porous walls
We perform direct numerical simulations (DNS) of a turbulent channel flow
over porous walls. In the fluid region the flow is governed by the
incompressible Navier--Stokes (NS) equations, while in the porous layers the
Volume-Averaged Navier--Stokes (VANS) equations are used, which are obtained by
volume-averaging the microscopic flow field over a small volume that is larger
than the typical dimensions of the pores. In this way the porous medium has a
continuum description, and can be specified without the need of a detailed
knowledge of the pore microstructure by indipendently assigning permeability
and porosity. At the interface between the porous material and the fluid
region, momentum-transfer conditions are applied, in which an available
coefficient related to the unknown structure of the interface can be used as an
error estimate. To set up the numerical problem, the velocity-vorticity
formulation of the coupled NS and VANS equations is derived and implemented in
a pseudo-spectral DNS solver. Most of the simulations are carried out at
and consider low-permeability materials; a parameter study is
used to describe the role played by permeability, porosity, thickness of the
porous material, and the coefficient of the momentum-transfer interface
conditions. Among them permeability, even when very small, is shown to play a
major role in determining the response of the channel flow to the permeable
wall. Turbulence statistics and instantaneous flow fields, in comparative form
to the flow over a smooth impermeable wall, are used to understand the main
changes introduced by the porous material. A simulations at higher Reynolds
number is used to illustrate the main scaling quantities.Comment: Revised version, with additional data and more in-depth analysi
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