Papers by Mostafa Mahmoudi
J Biomech Eng, 2021
Coronary artery atherosclerosis is a local, multifactorial, complex disease, and the leading caus... more Coronary artery atherosclerosis is a local, multifactorial, complex disease, and the leading cause of death in the US. Complex interactions between biochemical transport and biomechanical forces influence disease growth. Wall shear stress (WSS) affects coronary artery atherosclerosis by inducing endothelial cell mechanotransduction and by controlling the near-wall transport processes involved in atherosclerosis. Each of these processes is controlled by WSS differently and therefore has complicated the interpretation of WSS in atherosclerosis. In this paper, we present a comprehensive theory for WSS in atherosclerosis. First, a short review of shear stress-mediated mechanotransduction in atherosclerosis was presented. Next, subject-specific computational fluid dynamics (CFD) simulations were performed in ten coronary artery models of diseased and healthy subjects. Biochemical-specific mass transport models were developed to study low-density lipoprotein, nitric oxide, adenosine triphosphate, oxygen, monocyte chemoattractant protein-1, and monocyte transport. The transport results were compared with WSS vectors and WSS Lagrangian coherent structures (WSS LCS). High WSS magnitude protected against atherosclerosis by increasing the production or flux of atheroprotective biochemicals and decreasing the near-wall localization of atherogenic biochemicals. Low WSS magnitude promoted atherosclerosis by increasing atherogenic biochemical localization. Finally, the attracting WSS LCS's role was more complex where it promoted or prevented atherosclerosis based on different biochemicals. We present a summary of the different pathways by which WSS influences coronary artery atherosclerosis and compare different mechanotransduction and biotransport mechanisms.
In the present article, the mantle convection is simulated numerically using a temperature depend... more In the present article, the mantle convection is simulated numerically using a temperature dependent non-linear viscoelastic model for the first time. The numerical domain of problem is con sidered as a 4000 km*2000 km rectangular box and the CFD simulation is performed using finite volume method. Unlike the previous works which had been investigated the mantle convection using the linear viscoelastic models or simple nonlinear inelastic viscous equations (such as power law or cross equations), it is solved via the nonlinear Giesekus constitutive equation. Because of large-scale creeping flow in geometry and time, it is shown that the results of Giesekus equation are more reliable for this problem. The main innovative aspects of current study is investigation of temperature dependency of rheological properties of mantle including viscosity, normal stress differences and relaxation time using appropriate equations of state. The variation of gravitational acceleration with depth of Earth and the effect of the work of stress field (viscous dissipation) on mantle convection are also simulated for the first time.
In the present article, a comprehensive study of laminar natural convection heat transfer around ... more In the present article, a comprehensive study of laminar natural convection heat transfer around cylinders of elliptical cross section wrapped with a porous medium is conducted. The presence of a porous layer around a hot cylinder may have different effects on the nature of the heat transfer and can change the heat transfer regime. The aim is to investigate and provide additional insight into the effects of physical and geometrical properties on heat transfer insulation/augmentation using a finite volume method model. Also, the critical porous layer thicknesses for heat reduction/augmentation and its dependency on other factors like conductivity ratio, Darcy number, and the cylinder aspect ratio are obtained, and the optimal design of porous media for insulation/heat transfer augmentation purposes of an elliptical hot cylinder is suggested. In addition, the range of physical properties which is suitable for applying conduction theory is obtained. The results show that there is a heat transfer augmentation/reduction transition point for different conductivity ratios and it depends on the physical properties of the coating. Also, it is shown that for an elliptical cylinder, it is possible to enhance the heat transfer rate up to 150% and to reduce it up to 30% by selecting a suitable porous layer coating.
In this paper, three-dimensional viscoelastic Taylor-Couette instability between concentric rotat... more In this paper, three-dimensional viscoelastic Taylor-Couette instability between concentric rotating cylinders is studied numerically. The aim is to investigate and provide additional insight about the formation of time-dependent secondary flows in viscoelastic fluids between rotating cylinders. Here, the Giesekus model is used as the constitutive equation. The governing equations are solved using the finite volume method (FVM) and the PISO algorithm is employed for pressure correction. The effects of elasticity number, viscosity ratio, and mobility factor on various instability modes (especially high order ones) are investigated numerically and the origin of Taylor-Couette instability in Giesekus fluids is studied using the order of magnitude technique. The created instability is simulated for large values of fluid elasticity and high orders of nonlinearity. Also, the effect of elastic properties of fluid on the time-dependent secondary flows such as wave family and traveling wave and also on the critical conditions are studied in detail.
In the present paper, the three-dimensional numerical investigation of Class 5 medium duty trucks... more In the present paper, the three-dimensional numerical investigation of Class 5 medium duty trucks (based on a production of Volvo Company) is carried out. The aim is to investigate and provide additional insights about the drag reduction methods in medium duty trucks. The flow field and pressure distribution around the truck were simulated using the Finite Volume Method. The Simple algorithm was employed to couple the pressure and momentum. A constant velocity of 30 m/s was set in the inlet, the non-slip condition in conjugation with a wall function were used on the truck and ground surfaces, and the pressure-outlet was applied at the outlet. For the turbulent regime, the well-known standard k-ε model was used to simulate the turbulent flow characteristics. The effects of vortexes around the vehicle on the drag coefficient were studied. Also, some passive devices such as standard and large windbreakers, convex roof, and the axial channel were considered for drag reduction at a high velocity (30 m/s) and standard atmospheric conditions (T=25℃, p=1 atm). The results showed that the large windbreakers and covering the gap between the trailer and the container are not suitable successors for standard windbreakers. Furthermore, it was found that the convex roof is a suitable passive or active device for notable drag reduction (25%). some recommendations for future works might include investigating the effect of combinations of different devices on the drag reduction, studying the different underbody devices like side skirts, and using more sophisticated turbulent models such as large eddy simulation.
In the present article, a study on flow field and particles’ trajectory inside inertial impactors... more In the present article, a study on flow field and particles’ trajectory inside inertial impactors with elliptical impaction plate is conducted. The shape of impaction plate could have various effects on the collection efficiency of an inertial impactor. The aim is to investigate and provide additional insight about the effects of physical and geometrical properties on flow field pattern and collection efficiency curves using finite volume method. Particles’ trajectories were calculated in a Lagrangian reference frame. The effects of aspect ratio of the elliptical plate diameters (AR) on both flow field and collection efficiency curves were investigated numerically. In addition, the effects of Reynolds number on cut-off diameters and on deposition of sub-micron particles were studied. Obtained results revealed that increasing the AR, mainly affect the undersize particles region and did not have a significant effect on the cut-off diameter. Also, using a circular impaction plate with the same surface area as the elliptical ones, leads to a considerable increase in impactor cut-off diameter (about 22%).
In the present article, forced convection heat transfer and pressure drop in helically coiled pip... more In the present article, forced convection heat transfer and pressure drop in helically coiled pipes using TiO2/water nanofluid as working fluid were investigated experimentally and numerically. The aim is to investigate and provide additional insight about the effects of physical and geometrical properties on heat transfer augmentation and pressure drop in helically coiled tubes. The experiments were conducted in the range of Reynolds number from 3000 to 18,000 and in the nanoparticle concentrations of 0.1, 0.2, and 0.5% for five different curvature ratios. In numerical simulations the thermophysical properties of the working fluid were assumed to be a function of nanofluid temperature and concentration. For turbulent regime the standard k − ε model was used to simulate the turbulent flow characteristics. The numerical results were in good agreement with the experimental data. The results showed that utilization of nanofluid instead of distilled water leads to an enhancement in the Nusselt number up to 30%. Also, four formulas were introduced to obtain the average Nusselt number and friction factor in helically coiled tubes under constant wall temperature condition for both laminar and turbulent flow regimes.
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Papers by Mostafa Mahmoudi