Abouzar Moshfegh

Dissipative Particle Dynamics: Effects of Parameterization and Thermostating Schemes on Rheology

We report a sensitivity analysis for dissipative particle dynamics (DPD) method, showing that it is highly sensitive to parameterization, integration method, and thermostating schemes. Practical guidelines are presented to maintain Newtonian characteristics of DPD fluid at a possible range of shear rates over wide-ranging DPD parameters. The effects of cutoff radii and the weight functions exponent in DPD triple forces are studied on viscosity, and optimized ranges are recommended for effective temperature control. Moreover, the advantages and shortcomings of DPD method is manifested using alternative thermostats (Peters, Lowe-Andersen, and Nosé-Hoover-Lowe-Andersen) in simultaneously meeting proper temperature control and Newtonian behavior in moderate to high shear rate regimes. It was shown that improper temperature control may lead to nonphysical shear-thickening. Diverse ranges of simulation settings are identified that are capable of delivering Newtonian viscosities from 0.83 to 46.3 in DPD units. This study also aims to reduce the risk of numerical artefacts giving seemingly accurate results in single-species, suspensions, or other complex systems.

Modified Lees–Edwards boundary condition for dissipative particle dynamics: hydrodynamics and temperature at high shear rates

In simulations of Couette shear flow by dissipative particle dynamics (DPD) method, applying traditional Lees–Edwards boundary condition (LEC) in conjunction with velocity-dependent thermostats may result in artefacts in the form of velocity jumps. This artefact was observed at extreme dissipation rates (), and a modified LEC (M-LEC) was introduced to correct that unconditionally (A. Chatterjee, Modification to Lees–Edwards periodic boundary condition for dissipative particle dynamics simulation with high dissipation rates, Mol Simul, 33:1233–1236, 2007). Here we have studied some unexplored effects of using LEC under high shear rate regimes, not only on velocity profiles but also on temperature control. Given a correct temperature () control, the maximum applicable and effective shear velocity, shear rate, dynamic viscosity and Péclet number achievable under both LEC and M-LEC methods are extracted and discussed rigorously. We also show that despite partial success of M-LEC to impose the intended shear rate to the system, it still has some limitations in certain conditions. Here we have explored the advantages and shortcomings of M-LEC on the functionality of DPD thermostat and the calculated rheological properties in moderate to high shear rates and for various weight function exponents.

Lattice Boltzmann analysis of micro-particles transport in pulsating obstructed channel flow

Dispersion and deposition of microparticles are investigated numerically in a channel in the presence of a square obstacle and inlet flow pulsation. Lattice Boltzmann method (LBM) is used to simulate the flow field and modified Euler method is employed to calculate particles trajectories with the assumption of one-way coupling. The forces of drag, gravity, Saffman lift and Brownian motion are included in the particles equation of motion. The effects of pulsation amplitude ( A M P ), Strouhal number and particles Stokes number ( S t k ) are rigorously studied on particles dispersion and deposition efficiency. Flow vortex shedding and particles dispersion patterns together with the averaged fluid-particle relative velocity and deposition efficiency plots are all discussed thoroughly. The results show that increment of pulsation amplitude enforces the vortices to form closer to the obstacle until their shape deteriorates as Strouhal number ratio (SNR) rises. The average recirculation length shrinks to its minimum at each studied Amp when SNR escalates to 2. Various behaviors are categorized for dispersion pattern of particles when Stokes number changes from 0.001 to 4. Deposition efficiency is indirectly related to Amp for S t k Â? 2 while for higher Stokes numbers ( 2 < S t k Â? 4 ) they show direct relationship. Deposition pattern becomes rather independent of SNR at Amp = 0.1 . The grid independency test was performed for the LBM analysis, and simulation code was successfully verified against credible benchmarks.

Heat Transfer Analysis of a Microspherical Particle in the Slip Flow Regime by Considering Variable Properties

In order to investigate how far the temperature-dependent fluid properties and characteristic length influence the drag coefficient and the heat flux, a three-dimensional simulation study for a slip flow around an unconfined microspherical particle has been performed. Gas properties such as density, viscosity, conductivity, and mean free path were assumed to vary with temperature. Slip velocity and temperature jump at the gas particle interface were both treated numerically by imposition of the slip boundary conditions. The effects of variable gas properties and Knudsen number on momentum and heat transfer were also taken into account. It was concluded that for microflows with high heat transfer rates, the constant fluid properties approximation is very crude. In addition, the slip velocity and temperature jump affect the heat transfer in opposite ways: a large slip on the wall increases the convection along the surface, whereas a large temperature jump decreases the heat transfer by reducing the temperature gradient at the wall. Therefore, neglecting temperature jump will result in the overestimation of the heat transfer coefficient.

The relationship between coronary artery distensibility and fractional flow reserve

Discordance between angiography-based anatomical assessment of coronary stenosis severity and fractional flow reserve (FFR) has been attributed to several factors including lesion length and irregularity, and the myocardial territory supplied by the target vessel. We sought to examine if coronary arterial distensibility is an independent contributor to this discordance. There were two parts to this study. The first consisted of “in silico” models of 26 human coronary arteries. Computational fluid dynamics-derived FFR was calculated for fully rigid, partially distensible and fully distensible models of the 26 arteries. The second part of the study consisted of 104 patients who underwent coronary angiography and FFR measurement. Distensibility at the lesion site (DistensibilityMLA) and for the reference vessel (DistensibilityRef) was determined by analysing three-dimensional angiography images during end-systole and end-diastole. Computational fluid dynamics-derived FFR was 0.67±0.19, 0.70±0.18 and 0.75±0.17 (P<0.001) in the fully rigid, partially distensible and fully distensible models respectively. FFR correlated with both DistensibilityMLA (r = 0.36, P<0.001) and DistensibilityRef (r = 0.44, P<0.001). Two-way ANCOVA analysis revealed that DistensibilityMLA (F (1, 100) = 4.17, p = 0.031) and percentage diameter stenosis (F (1, 100) = 60.30, p < 0.01) were both independent predictors of FFR. Coronary arterial distensibility is a novel, independent determinant of FFR, and an important factor contributing to the discordance between anatomical and functional assessment of stenosis severity.

Thermostatic and rheological responses of DPD fluid to extreme shear under modified Lees-Edwards boundary condition.

Abstract.Thermodynamic, hydrodynamic and rheological interactions between velocity-dependent thermostats of Lowe-Andersen (LA) and Nosé-Hoover-Lowe-Andersen (NHLA), and modified Lees-Edwards (M-LEC) boundary condition were studied in the context of Dissipative Particle Dynamics method. Comparisons were made with original Lees-Edwards method to characterise the improvements that M-LEC offers in conserving the induced shear momentum. Different imposed shear velocities, heat bath collision/exchange frequencies and thermostating probabilities were considered. The presented analyses addressed an unusual discontinuity in momentum transfer that appeared in form of nonphysical jumps in velocity and temperature profiles. The usefulness of M-LEC was then quantified by evaluating the enhancements in obtained effective shear velocity, effective shear rate, Péclet number, and dynamic viscosity. System exchange frequency (

Analysis of Dissipative Particle Dynamics Fluid in Sheared Regimes

\Gamma

Calibration of Dissipative Particle Dynamics Method to Study Rheology of Dense Suspensions

) with Maxwellian heat bath was found to play an important role, in that its larger values facilitated achieving higher shear rates with proper temperature control at the cost of deviation from an ideal momentum transfer. Similar dynamic viscosities were obtained under both shearing modes between LA and NHLA thermostats up to

Effect of magnetic field on haemodynamic perturbations in atherosclerotic coronary arteries

\Gamma = 10

DEVELOPMENT OF A COMPUTATIONAL FLUID DYNAMICS MODEL FOR MYOCARDIAL BRIDGING

, whilst about twice the range of viscosity (