Payman Jalali | Lappeenranta University of Technology (original) (raw)

Papers by Payman Jalali

Haemodynamic forces applied at the apical surface of vascular endothelial cells (ECs) provide the... more Haemodynamic forces applied at the apical surface of vascular endothelial cells (ECs) provide the mechanical signals at intracellular organelles and through the inter-connected cellular network. The objective of this study is to quantify the intracellular and intercellular stresses in a confluent vascular EC monolayer. A novel three-dimensional, multiscale and multicomponent model of focally adhered ECs is developed to account for the role of potential mechanosensors (glycocalyx layer, actin cortical layer, nucleus, cytoskeleton, focal adhesions (FAs) and adherens junctions (ADJs)) in mechanotransmission and EC deformation. The overriding issue addressed is the stress amplification in these regions, which may play a role in subcellular localization of mechanotransmission. The model predicts that the stresses are amplified 250-600-fold over apical values at ADJs and 175-200-fold at FAs for ECs exposed to a mean shear stress of 10 dyne cm 22 . Estimates of forces per molecule in the cell attachment points to the external cellular matrix and cell-cell adhesion points are of the order of 8 pN at FAs and as high as 3 pN at ADJs, suggesting that direct force-induced mechanotransmission by single molecules is possible in both. The maximum deformation of an EC in the monolayer is calculated as 400 nm in response to a mean shear stress of 1 Pa applied over the EC surface which is in accord with measurements. The model also predicts that the magnitude of the cell-cell junction inclination angle is independent of the cytoskeleton and glycocalyx. The inclination angle of the cell-cell junction is calculated to be 6.68 in an EC monolayer, which is somewhat below the measured value (9.98) reported previously for ECs subjected to 1.6 Pa shear stress for 30 min. The present model is able, for the first time, to cross the boundaries between different length scales in order to provide a global view of potential locations of mechanotransmission.

Physica A-statistical Mechanics and Its Applications, 2007

We introduce a geometric model for unjamming the polydisperse hard-disk packing. The unjamming co... more We introduce a geometric model for unjamming the polydisperse hard-disk packing. The unjamming corresponds to a critical packing density which is assumed to happen uniformly throughout the packing. However, this model can be used for prediction of local dilation in packings. Molecular dynamics (MD) simulations are also performed to create random packings of polydisperse hard disks. In MD simulations, a densification scheme is applied to the event-driven algorithm in which the size of particles increases with controlled rates. The packing densities upon the onset of the critical state are estimated in a series of bidisperse packings by feeding the textural information of the mixtures from MD simulations into the model. These results are relevant for studies of unjamming and shear banding phenomena in granular materials, colloids, metallic glasses and similar systems.

Physica A-statistical Mechanics and Its Applications, 2008

ABSTRACT

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Physica A-statistical Mechanics and Its Applications, 2010

This is a Corrigendum to Physica A 389 (2010) 205-214 to present the correct numerical values of ... more This is a Corrigendum to Physica A 389 (2010) 205-214 to present the correct numerical values of the vertical axis of on page 209. The values represented the pressure drop while they were supposed to stand for the pressure gradient. The correct figure is shown below. .

Physica A-statistical Mechanics and Its Applications, 2010

Hydraulic permeability is studied in porous media consisting of randomly distributed monodisperse... more Hydraulic permeability is studied in porous media consisting of randomly distributed monodisperse spheres by means of computational fluid dynamics (CFD) simulations. The packing of spheres is generated by inserting a certain number of nonoverlapping spherical particles inside a cubic box at both low and high packing fractions using proper algorithms. Fluid flow simulations are performed within the interparticulate porous space by solving Navier-Stokes equations in a low-Reynolds laminar flow regime. The hydraulic permeability is calculated from the Darcy equation once the mean values of velocity and pressure gradient are calculated across the particle packing. The simulation results for the pressure drop across the packing are verified by the Ergun equation for the lower range of porosities (<0.75), and the Stokes equation for higher porosities (∼1). Using the results of simulations, the effects of porosity and particle diameters on the hydraulic permeability are investigated. Simulations precisely specified the range of applicability of empirical or semi-empirical correlations for hydraulic permeability, namely the Carman-Kozeny, Rumpf-Gupte, and Howells-Hinch formulas. The number of spheres in the model is gradually decreased from 2000 to 20 to discover the finite-size effect of pores on the hydraulic permeability of spherical packing, which has not been clearly addressed in the literature. In addition, the scale dependence of hydraulic permeability is studied via simulations of the packing of spheres shrunk to lower scales. The results of this work not only reveal the validity range of the aforementioned correlations, but also show the finite-size effect of pores and the scale-independence of direct CFD simulations for hydraulic permeability.

Physica A-statistical Mechanics and Its Applications, 2006

Local stress fluctuations are measured in annular rapid shear flows of granular medium made of st... more Local stress fluctuations are measured in annular rapid shear flows of granular medium made of steel spheres with 2 and 3 mm in diameter. Both monodisperse packing and bidisperse packing are investigated to reveal the influence of size diversity on intermittent features of granular materials. Experiments are conducted in an annulus that can contain up to 15 kg of the spherical steel balls. Shearing of granular medium takes place via the rotation of the upper plate which compresses the material loaded inside the annulus. Fluctuations of compressive force are locally measured at the bottom of the annulus based on piezoelectric phenomenon. Rapid shear flow experiments are pursued at different compressive forces and shear rates and the sensitivity of fluctuations is then investigated by different means through monodisperse and bidisperse packings. r

Medical Engineering & Physics, 2009

The sensitivity of shear stress over smooth muscle cells (SMCs) to the deformability of media lay... more The sensitivity of shear stress over smooth muscle cells (SMCs) to the deformability of media layer due to pressure is investigated in thoracic aorta wall using three-dimensional simulations. A biphasic, anisotropic model assuming the radius, thickness, and hydraulic conductivity of vessel wall as functions of transmural pressure is employed in numerical simulations. The leakage of interstitial fluid from intima to media layer is only possible through fenestral pores on the internal elastic lamina (IEL). The media layer is assumed a heterogeneous medium containing SMCs embedded in a porous extracellular matrix of elastin, proteoglycan, and collagen fibers. The applicable pressures for the deformation of media layer are varied from 0 to 180 mmHg. The SMCs are cylindrical objects of circular cross section at zero pressure. The cross sectional shape of SMCs changes from circle to ellipse as the media is compressed. The local shear stress over the nearest SMC to the IEL profoundly depends on pressure, SMCs configurations, and the corresponding distance to the IEL. The consideration of various SMC configurations, namely the staggered and square arrays, mimics various physiological conditions that can happen in positioning of an SMC. The results of our simulations show that even the second nearest SMCs to the IEL can significantly change their functions due to high shear stress levels. This is in contrast to earlier studies suggesting the highest vulnerability to shear stress for the innermost layer of SMCs at the intimal-medial interface.

Journal of Porous Media, 2009

Begell House, Contact Authors AZ References Books Journals Digital Library Home. ... Heatline and... more Begell House, Contact Authors AZ References Books Journals Digital Library Home. ... Heatline and Energy-Flux-Vector Visualization of Natural Convection in a Porous Cavity Occupied by a Fluid with Temperature-Dependent Viscosity Kamel Hooman, Hal Gurgenci, Ibrahim ...

Journal of Physics: Conference Series, 2007

Medical & Biological Engineering & Computing, 2008

A biphasic, anisotropic model of the deformable aortic wall in combination with computational flu... more A biphasic, anisotropic model of the deformable aortic wall in combination with computational fluid dynamics is used to investigate the variation of shear stress over smooth muscle cells (SMCs) with transmural pressure. The media layer is modeled as a porous medium consisting of SMCs and a homogeneous porous medium of interstitial fluid and elastin, collagen and proteoglycans fibers. Interstitial fluid enters the media through fenestral pores, which are distributed over the internal elastic lamina (IEL). The IEL is considered as an impermeable barrier to fluid flow except at fenestral pores. The thickness and the radius of aortic wall vary with transmural pressure ranging from 10 to 180 mm Hg. It is assumed that SMCs are cylinders with a circular cross section at 0 mm Hg. As the transmural pressure increases, SMCs elongate with simultaneous change of cross sectional shape into ellipse according to the strain field in the media. Results demonstrate that the variation of shear stress within the media layer is significantly dependent on the configuration and cross sectional shape of SMCs. In the staggered array of SMCs, the shear stress over the first SMC nearest to the IEL is about 2.2 times lower than that of the square array. The shear stress even over the second nearest SMC to the IEL is considerably higher (about 15%) in the staggered array. In addition to configuration and cross sectional shape of SMCs, the variation of structural properties of the media layer with pressure and the sensitivity of the local shear stress to the minimum distance between SMCs and the IEL (reducing with transmural pressure) between SMCs and the IEL are studied. At 180 mm Hg, the ratio of the local shear stress of the nearest SMC to that of the second nearest SMC is 4.8 in the square array, whereas it reduces to about 1.8 in the staggered array. The importance of the fluid shear stress is associated with its role in the biomolecular state of smooth muscle cells bearing the shear stress.

In the present study, we have simulated the pulstile blood flow in the human aorta to investigate... more In the present study, we have simulated the pulstile blood flow in the human aorta to investigate: 1) the flow profiles in the aortic arch and its branches where the maximum disturbed and non-uniform flow pattern are expected, and 2) the wall shear stress profiles on the same areas and determine the probability of atherosclerosis through the human aorta. The aorta geometry with major branching arteries was constructed with available measurements for normal pressure condition and with the help of Computational Fluid dynamics (CFD) code based on Finite Element Method (FEM). This geometry was further modified to mimic hypotension and hypertension pressure condition. The blood flow is assumed as a homogeneous, incompressible, and Newtonian fluid flow. Flow across four cross-sections on the upper section of aortic arch is studied under three different pressure conditions. The wall shear stress profiles are also studied with varying pressure values.

Journal of Physics: Conference Series, 2007

We have used simulations of inelastic hard disks in two-dimensional shear flows to investigate th... more We have used simulations of inelastic hard disks in two-dimensional shear flows to investigate the stability conditions of kinetic energy upon shearing through different circumstances. We study the characteristics of instability via the signals of kinetic energy dissipation and the statistics associated with this quantity. Our results reveal how the flow characteristics of hard-disk assembly can be modelled by a dynamical model for turbulence named as the GOY shell model. Our results clearly show that the behaviour of rapidly sheared hard-disk assemblies is similar to a fluid in highly chaotic state, so called turbulent state. The current results assist us in finding proper modelling ways in simulation of suspension flows, colloids and magnetic fluids.

The aorta, with complex geometry is one of the most vulnerable arteries for creation and developm... more The aorta, with complex geometry is one of the most vulnerable arteries for creation and development of cardiovascular diseases, especially atherosclerosis. Because of the association of disease with region of altered fluid mechanics, the role of blood flow in the localization of atherosclerosis has been extensively studied recently. The diameter of aorta, its branches, and the geometry of branches are parameters which affect the blood flow significantly. However, much less attention has been focused on the effect of these parameters and their association with diseased regions. In the present study, three different geometries of human aorta with three branches are constructed and blood flow inside the arteries is simulated. Flow across longitudinal crosssections of all three geometries is analyzed. The disturbed velocity profiles are observed near branchiocephalic, left common artery and left subclavian artery. On the other hand, the wall shear stress profiles at the roots of branches show significant differences with geometry variation of aorta and branches. The blood flow is considered as homogeneous, incompressible, and Newtonian fluid flow.

Journal of Porous Media, 2009

We have investigated stresses on rough cylindrical objects confronted with a granular stream of i... more We have investigated stresses on rough cylindrical objects confronted with a granular stream of inelastic hard disks. The roughness of cylindrical objects is created via covering their surfaces with hard disks of given size and material. We have employed event-driven simulations using restitution coefficients dependent of the impact velocity in a collision. We report the effect of material property (restitution coefficient) on development of granular shock wave around the object with corresponding stresses exerted on it. The role of the roughness of the object in resulting flow is also studied. Moreover, simulations are performed in two conditions with gravity and without it.

Physica A-statistical Mechanics and Its Applications, 2001

Using computer simulations, the cooling rate of a system of inelastic hard spheres in the Couette... more Using computer simulations, the cooling rate of a system of inelastic hard spheres in the Couette geometry has been investigated after stopping the relative motion of boundaries. The energy of the system is initially balanced between the incoming kinetic energy from the moving boundaries and the dissipated kinetic energy due to the successive collisions of inelastic particles. When the relative motion of moving boundaries is suddenly stopped the pseudo-thermal energy of the system decays. However, the rate of decay is found to be slower than that introduced for a shear-free ensemble. Moreover, the simulations clearly show that the existence of pseudo-thermal energy is a necessary condition for di usion in a particulate system. The simulation results may elucidate the physical origin of some phenomena such as clustering and freezing in hard-sphere systems, and the e ect of velocity gradients on these phenomena.

Medical & Biological Engineering & Computing, 2007

In this study, the shape and the configuration of smooth muscle cells (SMCs) within the arterial ... more In this study, the shape and the configuration of smooth muscle cells (SMCs) within the arterial wall are altered to investigate their influence on molecular transport across the media layer of the thoracic aorta wall. In a 2D geometry of the media layer containing SMCs, the finite-element method has been employed to simulate the diffusion of solutes through the media layer. The media is modeled as a heterogeneous system composed of SMCs having elliptic or circular cross sections embedded in a homogeneous porous medium made of proteoglycan and collagen fibers with an interstitial fluid filling the void. The arrangement of SMCs is in either ordered or disordered fashion for different volume fractions of SMCs. The interstitial fluid enters the media through fenestral pores, which are assumed to be distributed uniformly over the internal elastic lamina (IEL). Results revealed that in an ordered arrangement of SMCs, the concentration of adenosine 5′-triphosphate (ATP) over the surface of SMCs with an elliptic cross section is 5–8% more than those of circular SMCs in volume fractions of 0.4–0.7. The ATP concentration at the SMC surface decreases with volume fraction in the ordered configuration of SMCs. In a disordered configuration, the local ATP concentration at the SMC surface and in the bulk are strongly dependent on the distance between neighboring SMCs, as well as the minimum distance between SMCs and fenestral pores. Moreover, the SMCs in farther distances from the IEL are as important as those just beneath the IEL in disordered configurations. The results of this study lead us to better understanding of the role of SMCs in controlling the diffusion of important species such as ATP within the arterial wall.

ABSTRACT The influences of increased endothelial cell turnover and deformation of the intima on t... more ABSTRACT The influences of increased endothelial cell turnover and deformation of the intima on the transport of low-density lipoprotein (LDL) under hypertension are investigated by applying a multilayered model of aortic wall. The thickness and properties of the endothelium, intima, internal elastic lamina (IEL), and media are affected by the transmural pressure. Navier-Stokes and Brinkman equations are applied for the transport of the transmural flow and the convective-diffusion equation is solved for LDL transport. LDL macromolecules enter the intima through leaky junctions, and then pass through the media layer where they permeate over the surface of smooth muscle cells (SMC). Uptake of LDL by cells is modeled through a uniform reaction evenly distributed in the macroscopically homogeneous media layer. The results show that transmural pressure significantly affects the LDL fluxes across the leaky junction, the intima, fenestral pores in the IEL, and the media layer. Many realistic predictions including the proper magnitudes for the permeability of endothelium and intimal layers, and the hydraulic conductivity of all layers as well as their trends with pressure are predicted by the present model.

ABSTRACT Rapid granular shear flow is a classical example in granular materials which exhibits bo... more ABSTRACT Rapid granular shear flow is a classical example in granular materials which exhibits both fluid-like and solid-like behaviors. Another interesting feature of rapid granular shear flows is the formation of ordered structures upon shearing. Certain amount of granular material, with uniform size distribution, is required to be loaded in the container in order to shear it under stable conditions. This work concerns the experimental study of rapid granular shear flows in annular Couette geometry. The flow is induced by continuous rotation of the plate over the top of the granular bed in an annulus. The compressive pressure, driving torque, instantaneous bed height from three symmetric locations and rotational speed of the shearing plate are measured. The annulus has a capacity of up to 15 kg of spherical steel balls of 3 mm in diameter. Rapid shear flow experiments are performed in one compressive force and rotation rate. The sensitivity of fluctuations is then investigated by different means through monodisperse packing. In this work, we present the results of the experiments showing how the flow properties depend on the amount of loaded granular material which is varied by small amounts between different experiments. The flow can exist in stable (fixed behavior) and unstable (time-dependent behavior) regimes as a function of the loaded material. We present the characteristics of flow to detect the formation of any additional structured layer in the annulus. As a result, an evolution graph for the bed height has been obtained as material is gradually added. This graph shows how the bed height grows when material increases. Using these results, the structure inside the medium can be estimated at extreme stable and unstable conditions.

Physica D-nonlinear Phenomena, 2002

Simulations of bounded dense sheared granular flows were performed to investigate multi-phase flo... more Simulations of bounded dense sheared granular flows were performed to investigate multi-phase flow phenomena, in particular the behavior of the ordered phase. In the absence of gravity, collapse of the granular structures was found to occur for a wide range of shear rate, as evidenced by the disappearance of the signature of the ordered structure in the wall normal stress signal. However, normal stress signals matching those detected in recent experiments were obtained for a system whose dynamics were collision-dominated rather than friction-dominated. Moreover, the system was found to exhibit a similar flow behavior in the presence of gravity. The stress signals were analyzed using wavelet transforms, which indicated the existence of stick-slip dynamics, characterized by harmonic frequencies. It is shown that these observations might elucidate the origin of stick-slip dynamics in the system, which experienced instability due to gravitational compactification at shear rates below a certain critical value.

Haemodynamic forces applied at the apical surface of vascular endothelial cells (ECs) provide the... more Haemodynamic forces applied at the apical surface of vascular endothelial cells (ECs) provide the mechanical signals at intracellular organelles and through the inter-connected cellular network. The objective of this study is to quantify the intracellular and intercellular stresses in a confluent vascular EC monolayer. A novel three-dimensional, multiscale and multicomponent model of focally adhered ECs is developed to account for the role of potential mechanosensors (glycocalyx layer, actin cortical layer, nucleus, cytoskeleton, focal adhesions (FAs) and adherens junctions (ADJs)) in mechanotransmission and EC deformation. The overriding issue addressed is the stress amplification in these regions, which may play a role in subcellular localization of mechanotransmission. The model predicts that the stresses are amplified 250-600-fold over apical values at ADJs and 175-200-fold at FAs for ECs exposed to a mean shear stress of 10 dyne cm 22 . Estimates of forces per molecule in the cell attachment points to the external cellular matrix and cell-cell adhesion points are of the order of 8 pN at FAs and as high as 3 pN at ADJs, suggesting that direct force-induced mechanotransmission by single molecules is possible in both. The maximum deformation of an EC in the monolayer is calculated as 400 nm in response to a mean shear stress of 1 Pa applied over the EC surface which is in accord with measurements. The model also predicts that the magnitude of the cell-cell junction inclination angle is independent of the cytoskeleton and glycocalyx. The inclination angle of the cell-cell junction is calculated to be 6.68 in an EC monolayer, which is somewhat below the measured value (9.98) reported previously for ECs subjected to 1.6 Pa shear stress for 30 min. The present model is able, for the first time, to cross the boundaries between different length scales in order to provide a global view of potential locations of mechanotransmission.

Physica A-statistical Mechanics and Its Applications, 2007

We introduce a geometric model for unjamming the polydisperse hard-disk packing. The unjamming co... more We introduce a geometric model for unjamming the polydisperse hard-disk packing. The unjamming corresponds to a critical packing density which is assumed to happen uniformly throughout the packing. However, this model can be used for prediction of local dilation in packings. Molecular dynamics (MD) simulations are also performed to create random packings of polydisperse hard disks. In MD simulations, a densification scheme is applied to the event-driven algorithm in which the size of particles increases with controlled rates. The packing densities upon the onset of the critical state are estimated in a series of bidisperse packings by feeding the textural information of the mixtures from MD simulations into the model. These results are relevant for studies of unjamming and shear banding phenomena in granular materials, colloids, metallic glasses and similar systems.

Physica A-statistical Mechanics and Its Applications, 2008

ABSTRACT

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Physica A-statistical Mechanics and Its Applications, 2010

This is a Corrigendum to Physica A 389 (2010) 205-214 to present the correct numerical values of ... more This is a Corrigendum to Physica A 389 (2010) 205-214 to present the correct numerical values of the vertical axis of on page 209. The values represented the pressure drop while they were supposed to stand for the pressure gradient. The correct figure is shown below. .

Physica A-statistical Mechanics and Its Applications, 2010

Hydraulic permeability is studied in porous media consisting of randomly distributed monodisperse... more Hydraulic permeability is studied in porous media consisting of randomly distributed monodisperse spheres by means of computational fluid dynamics (CFD) simulations. The packing of spheres is generated by inserting a certain number of nonoverlapping spherical particles inside a cubic box at both low and high packing fractions using proper algorithms. Fluid flow simulations are performed within the interparticulate porous space by solving Navier-Stokes equations in a low-Reynolds laminar flow regime. The hydraulic permeability is calculated from the Darcy equation once the mean values of velocity and pressure gradient are calculated across the particle packing. The simulation results for the pressure drop across the packing are verified by the Ergun equation for the lower range of porosities (<0.75), and the Stokes equation for higher porosities (∼1). Using the results of simulations, the effects of porosity and particle diameters on the hydraulic permeability are investigated. Simulations precisely specified the range of applicability of empirical or semi-empirical correlations for hydraulic permeability, namely the Carman-Kozeny, Rumpf-Gupte, and Howells-Hinch formulas. The number of spheres in the model is gradually decreased from 2000 to 20 to discover the finite-size effect of pores on the hydraulic permeability of spherical packing, which has not been clearly addressed in the literature. In addition, the scale dependence of hydraulic permeability is studied via simulations of the packing of spheres shrunk to lower scales. The results of this work not only reveal the validity range of the aforementioned correlations, but also show the finite-size effect of pores and the scale-independence of direct CFD simulations for hydraulic permeability.

Physica A-statistical Mechanics and Its Applications, 2006

Local stress fluctuations are measured in annular rapid shear flows of granular medium made of st... more Local stress fluctuations are measured in annular rapid shear flows of granular medium made of steel spheres with 2 and 3 mm in diameter. Both monodisperse packing and bidisperse packing are investigated to reveal the influence of size diversity on intermittent features of granular materials. Experiments are conducted in an annulus that can contain up to 15 kg of the spherical steel balls. Shearing of granular medium takes place via the rotation of the upper plate which compresses the material loaded inside the annulus. Fluctuations of compressive force are locally measured at the bottom of the annulus based on piezoelectric phenomenon. Rapid shear flow experiments are pursued at different compressive forces and shear rates and the sensitivity of fluctuations is then investigated by different means through monodisperse and bidisperse packings. r

Medical Engineering & Physics, 2009

The sensitivity of shear stress over smooth muscle cells (SMCs) to the deformability of media lay... more The sensitivity of shear stress over smooth muscle cells (SMCs) to the deformability of media layer due to pressure is investigated in thoracic aorta wall using three-dimensional simulations. A biphasic, anisotropic model assuming the radius, thickness, and hydraulic conductivity of vessel wall as functions of transmural pressure is employed in numerical simulations. The leakage of interstitial fluid from intima to media layer is only possible through fenestral pores on the internal elastic lamina (IEL). The media layer is assumed a heterogeneous medium containing SMCs embedded in a porous extracellular matrix of elastin, proteoglycan, and collagen fibers. The applicable pressures for the deformation of media layer are varied from 0 to 180 mmHg. The SMCs are cylindrical objects of circular cross section at zero pressure. The cross sectional shape of SMCs changes from circle to ellipse as the media is compressed. The local shear stress over the nearest SMC to the IEL profoundly depends on pressure, SMCs configurations, and the corresponding distance to the IEL. The consideration of various SMC configurations, namely the staggered and square arrays, mimics various physiological conditions that can happen in positioning of an SMC. The results of our simulations show that even the second nearest SMCs to the IEL can significantly change their functions due to high shear stress levels. This is in contrast to earlier studies suggesting the highest vulnerability to shear stress for the innermost layer of SMCs at the intimal-medial interface.

Journal of Porous Media, 2009

Begell House, Contact Authors AZ References Books Journals Digital Library Home. ... Heatline and... more Begell House, Contact Authors AZ References Books Journals Digital Library Home. ... Heatline and Energy-Flux-Vector Visualization of Natural Convection in a Porous Cavity Occupied by a Fluid with Temperature-Dependent Viscosity Kamel Hooman, Hal Gurgenci, Ibrahim ...

Journal of Physics: Conference Series, 2007

Medical & Biological Engineering & Computing, 2008

A biphasic, anisotropic model of the deformable aortic wall in combination with computational flu... more A biphasic, anisotropic model of the deformable aortic wall in combination with computational fluid dynamics is used to investigate the variation of shear stress over smooth muscle cells (SMCs) with transmural pressure. The media layer is modeled as a porous medium consisting of SMCs and a homogeneous porous medium of interstitial fluid and elastin, collagen and proteoglycans fibers. Interstitial fluid enters the media through fenestral pores, which are distributed over the internal elastic lamina (IEL). The IEL is considered as an impermeable barrier to fluid flow except at fenestral pores. The thickness and the radius of aortic wall vary with transmural pressure ranging from 10 to 180 mm Hg. It is assumed that SMCs are cylinders with a circular cross section at 0 mm Hg. As the transmural pressure increases, SMCs elongate with simultaneous change of cross sectional shape into ellipse according to the strain field in the media. Results demonstrate that the variation of shear stress within the media layer is significantly dependent on the configuration and cross sectional shape of SMCs. In the staggered array of SMCs, the shear stress over the first SMC nearest to the IEL is about 2.2 times lower than that of the square array. The shear stress even over the second nearest SMC to the IEL is considerably higher (about 15%) in the staggered array. In addition to configuration and cross sectional shape of SMCs, the variation of structural properties of the media layer with pressure and the sensitivity of the local shear stress to the minimum distance between SMCs and the IEL (reducing with transmural pressure) between SMCs and the IEL are studied. At 180 mm Hg, the ratio of the local shear stress of the nearest SMC to that of the second nearest SMC is 4.8 in the square array, whereas it reduces to about 1.8 in the staggered array. The importance of the fluid shear stress is associated with its role in the biomolecular state of smooth muscle cells bearing the shear stress.

In the present study, we have simulated the pulstile blood flow in the human aorta to investigate... more In the present study, we have simulated the pulstile blood flow in the human aorta to investigate: 1) the flow profiles in the aortic arch and its branches where the maximum disturbed and non-uniform flow pattern are expected, and 2) the wall shear stress profiles on the same areas and determine the probability of atherosclerosis through the human aorta. The aorta geometry with major branching arteries was constructed with available measurements for normal pressure condition and with the help of Computational Fluid dynamics (CFD) code based on Finite Element Method (FEM). This geometry was further modified to mimic hypotension and hypertension pressure condition. The blood flow is assumed as a homogeneous, incompressible, and Newtonian fluid flow. Flow across four cross-sections on the upper section of aortic arch is studied under three different pressure conditions. The wall shear stress profiles are also studied with varying pressure values.

Journal of Physics: Conference Series, 2007

We have used simulations of inelastic hard disks in two-dimensional shear flows to investigate th... more We have used simulations of inelastic hard disks in two-dimensional shear flows to investigate the stability conditions of kinetic energy upon shearing through different circumstances. We study the characteristics of instability via the signals of kinetic energy dissipation and the statistics associated with this quantity. Our results reveal how the flow characteristics of hard-disk assembly can be modelled by a dynamical model for turbulence named as the GOY shell model. Our results clearly show that the behaviour of rapidly sheared hard-disk assemblies is similar to a fluid in highly chaotic state, so called turbulent state. The current results assist us in finding proper modelling ways in simulation of suspension flows, colloids and magnetic fluids.

The aorta, with complex geometry is one of the most vulnerable arteries for creation and developm... more The aorta, with complex geometry is one of the most vulnerable arteries for creation and development of cardiovascular diseases, especially atherosclerosis. Because of the association of disease with region of altered fluid mechanics, the role of blood flow in the localization of atherosclerosis has been extensively studied recently. The diameter of aorta, its branches, and the geometry of branches are parameters which affect the blood flow significantly. However, much less attention has been focused on the effect of these parameters and their association with diseased regions. In the present study, three different geometries of human aorta with three branches are constructed and blood flow inside the arteries is simulated. Flow across longitudinal crosssections of all three geometries is analyzed. The disturbed velocity profiles are observed near branchiocephalic, left common artery and left subclavian artery. On the other hand, the wall shear stress profiles at the roots of branches show significant differences with geometry variation of aorta and branches. The blood flow is considered as homogeneous, incompressible, and Newtonian fluid flow.

Journal of Porous Media, 2009

We have investigated stresses on rough cylindrical objects confronted with a granular stream of i... more We have investigated stresses on rough cylindrical objects confronted with a granular stream of inelastic hard disks. The roughness of cylindrical objects is created via covering their surfaces with hard disks of given size and material. We have employed event-driven simulations using restitution coefficients dependent of the impact velocity in a collision. We report the effect of material property (restitution coefficient) on development of granular shock wave around the object with corresponding stresses exerted on it. The role of the roughness of the object in resulting flow is also studied. Moreover, simulations are performed in two conditions with gravity and without it.

Physica A-statistical Mechanics and Its Applications, 2001

Using computer simulations, the cooling rate of a system of inelastic hard spheres in the Couette... more Using computer simulations, the cooling rate of a system of inelastic hard spheres in the Couette geometry has been investigated after stopping the relative motion of boundaries. The energy of the system is initially balanced between the incoming kinetic energy from the moving boundaries and the dissipated kinetic energy due to the successive collisions of inelastic particles. When the relative motion of moving boundaries is suddenly stopped the pseudo-thermal energy of the system decays. However, the rate of decay is found to be slower than that introduced for a shear-free ensemble. Moreover, the simulations clearly show that the existence of pseudo-thermal energy is a necessary condition for di usion in a particulate system. The simulation results may elucidate the physical origin of some phenomena such as clustering and freezing in hard-sphere systems, and the e ect of velocity gradients on these phenomena.

Medical & Biological Engineering & Computing, 2007

In this study, the shape and the configuration of smooth muscle cells (SMCs) within the arterial ... more In this study, the shape and the configuration of smooth muscle cells (SMCs) within the arterial wall are altered to investigate their influence on molecular transport across the media layer of the thoracic aorta wall. In a 2D geometry of the media layer containing SMCs, the finite-element method has been employed to simulate the diffusion of solutes through the media layer. The media is modeled as a heterogeneous system composed of SMCs having elliptic or circular cross sections embedded in a homogeneous porous medium made of proteoglycan and collagen fibers with an interstitial fluid filling the void. The arrangement of SMCs is in either ordered or disordered fashion for different volume fractions of SMCs. The interstitial fluid enters the media through fenestral pores, which are assumed to be distributed uniformly over the internal elastic lamina (IEL). Results revealed that in an ordered arrangement of SMCs, the concentration of adenosine 5′-triphosphate (ATP) over the surface of SMCs with an elliptic cross section is 5–8% more than those of circular SMCs in volume fractions of 0.4–0.7. The ATP concentration at the SMC surface decreases with volume fraction in the ordered configuration of SMCs. In a disordered configuration, the local ATP concentration at the SMC surface and in the bulk are strongly dependent on the distance between neighboring SMCs, as well as the minimum distance between SMCs and fenestral pores. Moreover, the SMCs in farther distances from the IEL are as important as those just beneath the IEL in disordered configurations. The results of this study lead us to better understanding of the role of SMCs in controlling the diffusion of important species such as ATP within the arterial wall.

ABSTRACT The influences of increased endothelial cell turnover and deformation of the intima on t... more ABSTRACT The influences of increased endothelial cell turnover and deformation of the intima on the transport of low-density lipoprotein (LDL) under hypertension are investigated by applying a multilayered model of aortic wall. The thickness and properties of the endothelium, intima, internal elastic lamina (IEL), and media are affected by the transmural pressure. Navier-Stokes and Brinkman equations are applied for the transport of the transmural flow and the convective-diffusion equation is solved for LDL transport. LDL macromolecules enter the intima through leaky junctions, and then pass through the media layer where they permeate over the surface of smooth muscle cells (SMC). Uptake of LDL by cells is modeled through a uniform reaction evenly distributed in the macroscopically homogeneous media layer. The results show that transmural pressure significantly affects the LDL fluxes across the leaky junction, the intima, fenestral pores in the IEL, and the media layer. Many realistic predictions including the proper magnitudes for the permeability of endothelium and intimal layers, and the hydraulic conductivity of all layers as well as their trends with pressure are predicted by the present model.

ABSTRACT Rapid granular shear flow is a classical example in granular materials which exhibits bo... more ABSTRACT Rapid granular shear flow is a classical example in granular materials which exhibits both fluid-like and solid-like behaviors. Another interesting feature of rapid granular shear flows is the formation of ordered structures upon shearing. Certain amount of granular material, with uniform size distribution, is required to be loaded in the container in order to shear it under stable conditions. This work concerns the experimental study of rapid granular shear flows in annular Couette geometry. The flow is induced by continuous rotation of the plate over the top of the granular bed in an annulus. The compressive pressure, driving torque, instantaneous bed height from three symmetric locations and rotational speed of the shearing plate are measured. The annulus has a capacity of up to 15 kg of spherical steel balls of 3 mm in diameter. Rapid shear flow experiments are performed in one compressive force and rotation rate. The sensitivity of fluctuations is then investigated by different means through monodisperse packing. In this work, we present the results of the experiments showing how the flow properties depend on the amount of loaded granular material which is varied by small amounts between different experiments. The flow can exist in stable (fixed behavior) and unstable (time-dependent behavior) regimes as a function of the loaded material. We present the characteristics of flow to detect the formation of any additional structured layer in the annulus. As a result, an evolution graph for the bed height has been obtained as material is gradually added. This graph shows how the bed height grows when material increases. Using these results, the structure inside the medium can be estimated at extreme stable and unstable conditions.

Physica D-nonlinear Phenomena, 2002

Simulations of bounded dense sheared granular flows were performed to investigate multi-phase flo... more Simulations of bounded dense sheared granular flows were performed to investigate multi-phase flow phenomena, in particular the behavior of the ordered phase. In the absence of gravity, collapse of the granular structures was found to occur for a wide range of shear rate, as evidenced by the disappearance of the signature of the ordered structure in the wall normal stress signal. However, normal stress signals matching those detected in recent experiments were obtained for a system whose dynamics were collision-dominated rather than friction-dominated. Moreover, the system was found to exhibit a similar flow behavior in the presence of gravity. The stress signals were analyzed using wavelet transforms, which indicated the existence of stick-slip dynamics, characterized by harmonic frequencies. It is shown that these observations might elucidate the origin of stick-slip dynamics in the system, which experienced instability due to gravitational compactification at shear rates below a certain critical value.