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Near wall hemodynamics: Modelling the glycocalyx and the endothelial surface
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.In this paper a coarse-grained model for blood flow in small arteries is presented. Blood is modelled as a two-component incompressible fluid: the plasma and corpuscular elements dispersed in it. The latter are modelled as deformable liquid droplets having greater density and viscosity. Interfacial surface tension and membrane effects are present to mimic key properties and to avoid droplets’ coalescence. The mesoscopic model also includes the presence of the wavy wall, due to the endothelial cells and incorporates a representation of the glycocalyx, covering the vessel wall. The glycocalyx is modelled as a porous medium, the droplets being subjected to a repulsive elastic force when approaching it, during their transit. Preliminary simulations are intended to show the influence of the undulation on the wall together with that of the glycocalyx
A modified lattice Bhatnagar-Gross-Krook model for convection heat transfer in porous media
The lattice Bhatnagar-Gross-Krook (LBGK) model has become the most popular
one in the lattice Boltzmann method for simulating the convection heat transfer
in porous media. However, the LBGK model generally suffers from numerical
instability at low fluid viscosities and effective thermal diffusivities. In
this paper, a modified LBGK model is developed for incompressible thermal flows
in porous media at the representative elementary volume scale, in which the
shear rate and temperature gradient are incorporated into the equilibrium
distribution functions. With two additional parameters, the relaxation times in
the collision process can be fixed at a proper value invariable to the
viscosity and the effective thermal diffusivity. In addition, by constructing a
modified equilibrium distribution function and a source term in the evolution
equation of temperature field, the present model can recover the macroscopic
equations correctly through the Chapman-Enskog analysis, which is another key
point different from previous LBGK models. Several benchmark problems are
simulated to validate the present model with the proposed local computing
scheme for the shear rate and temperature gradient, and the numerical results
agree well with analytical solutions and/or those well-documented data in
previous studies. It is also shown that the present model and the computational
schemes for the gradient operators have a second-order accuracy in space, and
better numerical stability of the present modified LBGK model than previous
LBGK models is demonstrated.Comment: 38pages,50figure
Comparison of multiphase SPH and LBM approaches for the simulation of intermittent flows
Smoothed Particle Hydrodynamics (SPH) and Lattice Boltzmann Method (LBM) are
increasingly popular and attractive methods that propose efficient multiphase
formulations, each one with its own strengths and weaknesses. In this context,
when it comes to study a given multi-fluid problem, it is helpful to rely on a
quantitative comparison to decide which approach should be used and in which
context. In particular, the simulation of intermittent two-phase flows in pipes
such as slug flows is a complex problem involving moving and intersecting
interfaces for which both SPH and LBM could be considered. It is a problem of
interest in petroleum applications since the formation of slug flows that can
occur in submarine pipelines connecting the wells to the production facility
can cause undesired behaviors with hazardous consequences. In this work, we
compare SPH and LBM multiphase formulations where surface tension effects are
modeled respectively using the continuum surface force and the color gradient
approaches on a collection of standard test cases, and on the simulation of
intermittent flows in 2D. This paper aims to highlight the contributions and
limitations of SPH and LBM when applied to these problems. First, we compare
our implementations on static bubble problems with different density and
viscosity ratios. Then, we focus on gravity driven simulations of slug flows in
pipes for several Reynolds numbers. Finally, we conclude with simulations of
slug flows with inlet/outlet boundary conditions. According to the results
presented in this study, we confirm that the SPH approach is more robust and
versatile whereas the LBM formulation is more accurate and faster
Pore-scale modeling of fluid-particles interaction and emerging poromechanical effects
A micro-hydromechanical model for granular materials is presented. It
combines the discrete element method (DEM) for the modeling of the solid phase
and a pore-scale finite volume (PFV) formulation for the flow of an
incompressible pore fluid. The coupling equations are derived and contrasted
against the equations of conventional poroelasticity. An analogy is found
between the DEM-PFV coupling and Biot's theory in the limit case of
incompressible phases. The simulation of an oedometer test validates the
coupling scheme and demonstrates the ability of the model to capture strong
poromechanical effects. A detailed analysis of microscale strain and stress
confirms the analogy with poroelasticity. An immersed deposition problem is
finally simulated and shows the potential of the method to handle phase
transitions.Comment: accepted in Int. Journal for Numerical and Analytical Methods in
Geomechanic
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