Ashraf A. Ibrahim

Developing Pulsatile Flow in a Deployed Coronary Stent

A major consequence of stent implantation is restenosis that occurs due to neointimal formation. This patho-physiologic process of tissue growth may not be completely eliminated. Recent evidence suggests that there are several factors such as geometry and size of vessel, and stent design that alter hemodynamic parameters, including local wall shear stress distributions, all of which influence the restenosis process. The present three-dimensional analysis of developing pulsatile flow in a deployed coronary stent quantifies hemodynamic parameters and illustrates the changes in local wall shear stress distributions and their impact on restenosis. The present model evaluates the effect of entrance flow, where the stent is placed at the entrance region of a branched coronary artery. Stent geometry showed a complex three-dimensional variation of wall shear stress distributions within the stented region. Higher order of magnitude of wall shear stress of 530 dyn/cm2 is observed on the surface of cross-link intersections at the entrance of the stent. A low positive wall shear stress of 10 dyn/cm2 and a negative wall shear stress of -10 dyn/cm2 are seen at the immediate upstream and downstream regions of strut intersections, respectively. Modified oscillatory shear index is calculated which showed persistent recirculation at the downstream region of each strut intersection. The portions of the vessel where there is low and negative wall shear stress may represent locations of thrombus formation and platelet accumulation. The present results indicate that the immediate downstream regions of strut intersections are areas highly susceptible to restenosis, whereas a high shear stress at the strut intersection may cause platelet activation and free emboli formation.

Nonlinear Breakup Model for a Liquid Sheet Emanating From a Pressure-Swirl Atomizer

Predictions of breakup length of a liquid sheet emanating from a pressure-swirl (simplex) fuel atomizer have been carried out by computationally modeling the two-phase flow in the atomizer coupled with a nonlinear analysis of instability of the liquid sheet. The volume-of-fluid (VOF) method has been employed to study the flow field inside the pressure-swirl atomizer. A nonlinear instability model has been developed using a perturbation expansion technique with the initial amplitude of the disturbance as the perturbation parameter to determine the sheet instability and breakup. The results for sheet thickness and velocities from the internal flow solutions are used as input in the nonlinear instability model. Computational results for internal flow are validated by comparing film thickness at exit, spray angle, and discharge coefficient with available experimental data. The predictions of breakup length show a good agreement with semiempirical correlations and available experimental measurements. The effect of elevated ambient pressure on the atomizer internal flow field and sheet breakup is investigated. A decrease in air core diameter is obtained at higher ambient pressure due to increased liquid-air momentum transport. Shorter breakup lengths are obtained at elevated air pressure. The coupled internal flow simulation and sheet instability analysis provides a comprehensive approach to modeling sheet breakup from a pressure-swirl atomizer.

Nonlinear breakup of a coaxial liquid jet in a swirling gas stream

Nonlinear asymmetric breakup of a liquid jet exposed to a swirling gas stream is investigated by a perturbation expansion technique with the initial amplitude of the disturbance as the perturbation parameter. The effects of gas-to-liquid axial velocity ratio and gas swirl number on the liquid jet instability and breakup length have been studied. The breakup length predictions show good agreement with the available empirical correlations for liquid jet breakup in still gas as well as jet breakup in a co-flowing gas stream. Earlier linear analyses have predicted that the gas swirl has a stabilizing influence on the jet and that the axisymmetric disturbance is the most unstable disturbance compared to the helical modes. In contrast, experimental studies report that a swirl imparted on the surrounding gas makes the jet unstable, and at high gas swirl the jet disintegrates through an explosive breakup. The present nonlinear temporal analysis correctly captures the destabilizing effect of the gas swirl. With in...

Modeling of Gas-Liquid Flow in Pressure Swirl Atomizers

Pressure swirl or simplex atomizers are commonly used in a number of industrial applications for liquid atomization, including fuel injection systems for gas turbine engines, spray drying, and paint sprays. Computational modeling of the two-phase flow in the atomizer coupled with a non-linear analysis of instability of liquid sheet exiting from the atomizer has been carried out. The Volume-of-Fluid method is employed to determine the two-phase gas-liquid flow inside the atomizer. Results are validated using available experimental data for film thickness at exit, spray angle, and discharge coefficient. The predictions of breakup length using the non-linear model are compared with available experimental measurements which show excellent agreement. The effect of flow conditions and nozzle geometry on the flow field and sheet breakup are investigated. The coupled internal flow simulation and sheet instability analysis provides a comprehensive approach to modeling atomization from a pressure-swirl atomizer.Copyright

Breakup Model for Annular Liquid Sheets

The instability and breakup of annular liquid sheet is encountered in liquid atomization process used in numerous applications including liquid fuel injection in combustion engines and spray drying of foods. A nonlinear breakup model for annular liquid sheet exposed to both inner and outer air streams by a perturbation expansion technique with the initial amplitude of the disturbance as the perturbation parameter has been developed. The liquid sheet moves at a uniform axial velocity and subjects to axially moving inner and outer gas streams. The temporal evolution of the dimensionless surface deformation at the inner and outer liquid-gas interface has been evaluated until the point of breakup. The breakup length predictions show good agreement with the available experimental data.Copyright

Weakly Nonlinear Instability of an Annular Liquid Sheet Subjected to Unequal Inner and Outer Gas Streams

Instability and breakup of annular liquid sheet are encountered in fuel injectors used in gas turbine engines. A weakly nonlinear stability analysis has been carried out for annular liquid fuel sheet subjected to unequal inner and outer gas velocities by a perturbation expansion technique with the initial amplitude of the disturbance as the perturbation parameter. The liquid sheet is considered to move at a uniform axial velocity and is subjected to inner and outer gas streams of differing axial velocities. The breakup length is calculated, and the effect of the various flow parameters is investigated. It is found that the breakup length is reduced by an increase in the liquid velocity and the gas velocities. The inner air stream is more effective in disintegrating the sheet than the outer air stream.Copyright

Evaluation of Modified Oscillatory Shear Index and Recirculation Zone in a Deployed Coronary Stent

A major consequence of stent implantation is restenosis which occurs due to neointimal formation. There are several factors affecting restenosis among which wall shear stress plays a significant role. The present computational study of developing pulsatile flow through the entrance region of a deployed Palmaz stent in a coronary artery analyzes the local wall shear distribution and its effect on restenosis. A variation from low positive wall shear stress of around 10 dyn/cm2 at the upstream of stent strut intersection to a very high positive wall shear stress of 300 dyn/cm2 at strut intersection and then to a negative wall shear stress of −10 dyn/cm2 at the downstream of strut intersection was observed. Modified oscillatory shear index was calculated which showed persistent recirculation at the downstream of strut intersection indicating it as a highly prone region to restenosis.Copyright