1,618 research outputs found
Roughness induced boundary slip in microchannel flows
Surface roughness becomes relevant if typical length scales of the system are
comparable to the scale of the variations as it is the case in microfluidic
setups. Here, an apparent boundary slip is often detected which can have its
origin in the assumption of perfectly smooth boundaries. We investigate the
problem by means of lattice Boltzmann (LB) simulations and introduce an
``effective no-slip plane'' at an intermediate position between peaks and
valleys of the surface. Our simulations show good agreement with analytical
results for sinusoidal boundaries, but can be extended to arbitrary geometries
and experimentally obtained surface data. We find that the detected apparent
slip is independent of the detailed boundary shape, but only given by the
distribution of surface heights. Further, we show that the slip diverges as the
amplitude of the roughness increases.Comment: 4 pages, 6 figure
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Calibration of lubrication force measurements by lattice Boltzmann simulations
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Many experiments explore the hydrodynamic boundary of a surface by approaching a colloidal sphere and measuring the occurring lubrication force. However, in this case many different parameters like wettability and surface roughness influence the result. In the experiment these cannot be separated easily. For a deeper understanding of such surface effects a tool is required that predicts the influence of different surface properties. Here computer simulations can help. In this paper we present lattice Boltzmann simulations of a sphere submerged in a Newtonian liquid and show that our method is able to reproduce the theoretical predictions. In order to provide high precision simulation results the influence of finite size effects has to be controlled. We study the influence of the required system size and resolution of the sphere and demonstrate that already moderate computing ressources allow to keep the error below 1%.This study is funded by DFG priority program SPP 1164
Simulation of fluid flow in hydrophobic rough microchannels
Surface effects become important in microfluidic setups because the surface
to volume ratio becomes large. In such setups the surface roughness is not any
longer small compared to the length scale of the system and the wetting
properties of the wall have an important influence on the flow. However, the
knowledge about the interplay of surface roughness and hydrophobic
fluid-surface interaction is still very limited because these properties cannot
be decoupled easily in experiments.
We investigate the problem by means of lattice Boltzmann (LB) simulations of
rough microchannels with a tunable fluid-wall interaction. We introduce an
``effective no-slip plane'' at an intermediate position between peaks and
valleys of the surface and observe how the position of the wall may change due
to surface roughness and hydrophobic interactions.
We find that the position of the effective wall, in the case of a Gaussian
distributed roughness depends linearly on the width of the distribution.
Further we are able to show that roughness creates a non-linear effect on the
slip length for hydrophobic boundaries.Comment: 10 pages, 5 figure
Lattice Boltzmann simulations of apparent slip in hydrophobic microchannels
Various experiments have found a boundary slip in hydrophobic microchannel
flows, but a consistent understanding of the results is still lacking. While
Molecular Dynamics (MD) simulations cannot reach the low shear rates and large
system sizes of the experiments, it is often impossible to resolve the needed
details with macroscopic approaches. We model the interaction between
hydrophobic channel walls and a fluid by means of a multi-phase lattice
Boltzmann model. Our mesoscopic approach overcomes the limitations of MD
simulations and can reach the small flow velocities of known experiments. We
reproduce results from experiments at small Knudsen numbers and other
simulations, namely an increase of slip with increasing liquid-solid
interactions, the slip being independent of the flow velocity, and a decreasing
slip with increasing bulk pressure. Within our model we develop a semi-analytic
approximation of the dependence of the slip on the pressure.Comment: 7 pages, 4 figure
Aspectos hormonais, bioquĂmicos e hematolĂłgicos de prĂ© e pĂłs-parto de suĂnos.
bitstream/item/84751/1/DCOT-045.pd
Simulations of slip flow on nanobubble-laden surfaces
On microstructured hydrophobic surfaces, geometrical patterns may lead to the
appearance of a superhydrophobic state, where gas bubbles at the surface can
have a strong impact on the fluid flow along such surfaces. In particular, they
can strongly influence a detected slip at the surface. We present two-phase
lattice Boltzmann simulations of a flow over structured surfaces with attached
gas bubbles and demonstrate how the detected slip depends on the pattern
geometry, the bulk pressure, or the shear rate. Since a large slip leads to
reduced friction, our results allow to assist in the optimization of
microchannel flows for large throughput.Comment: 22 pages, 12 figure
The Viewing Angles of Broad Absorption Line Versus Unabsorbed Quasars
It was recently shown that there is a significant difference in the radio
spectral index distributions of broad absorption line (BAL) quasars and
unabsorbed quasars, with an overabundance of BAL quasars with steeper radio
spectra. This result suggests that source orientation does play into the
presence or absence of BAL features. In this paper we provide more quantitative
analysis of this result based on Monte-Carlo simulations. While the
relationship between viewing angle and spectral index does indeed contain a lot
of scatter, the spectral index distributions are different enough to overcome
that intrinsic variation. Utilizing two different models of the relationship
between spectral index and viewing angle, the simulations indicate that the
difference in spectral index distributions can be explained by allowing BAL
quasar viewing angles to extend about 10 degrees farther from the radio jet
axis than non-BAL sources, though both can be seen at small angles. These
results show that orientation cannot be the only factor determining whether BAL
features are present, but it does play a role.Comment: Accepted for publication in Ap
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