Pulsatile Boundary Conditions Research Articles

Convergence of Phase-Averaged, Transitional Flow in an Abdominal Aortic Aneurysmal Model.

Abdominal aortic aneurysm can exhibit transitional flow characteristics in laminar flow regimes. To report transitional flow characteristics, we examined the convergence of phase-averaged solutions by executing blood flow simulations of a patient-specific abdominal aortic aneurysmal model for 257 cardiac cycles with periodic, pulsatile boundary conditions. The phase-averaged solutions were computed by averaging the solutions over various numbers of cardiac cycles and compared against the ones averaged over 124 cycles. The phase-averaged solutions reported small differences when they were averaged over a large number of cardiac cycles. The instantaneous solutions, however, failed to exhibit fluctuations reported in the phase-averaged solutions. To study transitional blood flows in the aneurysmal region, we need to report phase-averaged solutions as they exhibit nonperiodic, disturbed flow characteristics. Additionally, when reporting phase-averaged solutions, it is preferred to compute an average over a large number of cardiac cycles to be able to represent flow structures of the converged phase-averaged solutions.

Computational Modeling of Thermodynamical Pulsatile Flow with Uncertain Pressure in Hydrocephalus

The watery cerebrospinal fluid that flows in the subarachnoid space (SAS) surrounds the entire central nervous system via symmetrical thermo-solute flow. The significance of this study was to present a flexible simulation based on theoretical vivo inputs onto a mathematical framework to describe the interaction of cerebrospinal fluid circulation restricted to a pathological disorder. The pathophysiology disorder hydrocephalus is caused by an enormous excess of asymmetric fluid flow in the ventricular region. This fluid imposition increases the void space of its boundary wall (Pia mater). As a result, the dumping effect affects an inertial force in brain tissues. A mathematical model was developed to impose the thermal dynamics of hydrocephalus, in which solute transport constitutes the excess watery CSF fluid caused by hydrocephalus, in order to demonstrate perspective changes in ventricular spaces. This paper investigated brain porous spaces in order to strengthen the acceleration and thermal requirements in the CNS mechanism. To characterize neurological activities, a unique mathematical model that includes hydrodynamics and nutrient transport diffusivity was used. We present the analytical results based on physical experiments that use the novel Laplace method to determine the nutrients transported through permeable pia (brain) parenchyma with suitable pulsatile boundary conditions. This causes high CSF pressure and brain damage due to heat flux over the SAS boundary wall. As a result of the increased Schmidt number, the analysis of the hydrocephalus problem revealed an increase in permeability and drop in solute transport. A high-velocity profile caused a rise in thermal buoyancy (Grashof number). When the CSF velocity reached an extreme level, it indicated a higher Womersley number. Additionally, the present study compared a number of clinical studies for CSF amplitude and pressure. We validated the results by providing a decent justification with the clinical studies by appropriate field references.

Characterizing pulsatile blood flow in a specific carotid bifurcation: insights into hemodynamics and rheology models

<abstract> <p>This study uses laminar and turbulent flow models to investigate the blood flow dynamics in a specific carotid bifurcation. Pulsatile boundary conditions and the rigid carotid artery wall are considered. Three viscosity models describe the non-Newtonian blood behavior. The Fluent solver and the finite volume method solve the equations. Results show a Poiseuille-like flow in the common carotid artery (CCA), unaffected by the flow regime, viscosity model, or boundary conditions. The branching zone exhibits a C-shaped stagnation zone with low velocity and wall shear stress due to the CCA widening and ICA/ECA curvature. Strong secondary flow is observed in the carotid sinus; the flow is directed towards the inner wall with higher velocity in the internal carotid artery. Discrepancies between viscosity models are pronounced in laminar flow, particularly with the natural boundary conditions. The non-Newtonian blood behavior is more apparent in the laminar flow of the external carotid artery, especially with the second set of boundary conditions.</p> </abstract>

Flow ventricular catheters for shunted hydrocephalus: initial clinical results.

The non-homogenous flow of the cerebrospinal fluid within the ventricular catheter is one of the causative factors in shunt obstructions during the treatment of hydrocephalus. Previously, we studied the flow in ventricular catheters under the steady and pulsatile boundary conditions by means of computational fluid dynamics (CFD) in three-dimensional paradigms. Subsequently, several catheter designs with homogeneous flow patterns were developed out of which one prototype was chosen after a validation study. To test the effectiveness of the flow ventricular catheter in a prospective, multicenter, comparative study. Eligible centers were three pediatric hospitals: two with sole adult practice and one a mixed pediatric-adult. Standard silicone material was used to develop a parametric catheter model with homogenous flow characteristics. The flow catheters were inserted in pediatric (n = 30) and adult (n = 10) patients with all types of hydrocephalus. Simultaneously, regular ventricular catheters were inserted in another 43 control patients in the participating centers. Catheter positioning was standardized according to the Schaumann and Thomale classification. All ventricular catheters had a cephalad grade I or II positioning, and caudally, its extension had a peritoneal location. Programmable valves were utilized in 70% and antisiphon devices in 20% of the cases. Regular differential pressure valves were utilized in the remaining. No case of flow catheter obstruction was identified during a mean follow-up period of 2 years at the time of this writing. There were four catheter obstructions in the control cohort, all pediatric cases, during the first year. Shunt infections occurred in two cases in the control group, while there was one recurrent case of adult ventriculitis in the flow catheter group. This prototype model represents the next generation of ventricular catheters with a homogeneous flow pattern. The flow catheter can be inserted safely in hydrocephalic patients, and this preliminary prospective comparative study showed a possible obstruction-free functionality.

Numerical Solution of Blood Flow and Mass Transport in an Elastic Tube with Multiple Stenoses.

The simultaneous effect of flexible wall and multiple stenoses on the flow and mass transfer of blood is investigated through numerical computation and simulations. The solution is obtained using the Marker and Cell technique on an axisymmetric model of Newtonian blood flow. The results compare favorably with physical observations where the pulsatile boundary condition and double stenoses result in a higher pressure drop across the stenoses. The streamlines, the iso-concentration lines, the Sherwood number, and the mass concentration variations along the entire wall segment provide a comprehensive analysis of the mass transport characteristics. The double stenoses and pulsatile inlet conditions increase the number of recirculation regions and effect a higher mass transfer rate at the throat, whereby more mass is expected to accumulate and cause further stenosis.

Fluid–structure interaction between pulsatile blood flow and a curved stented coronary artery on a beating heart: A four stent computational study

This work focuses on fluid–structure interaction (FSI) between a curved (tortuous) coronary artery with an implanted stent, pulsatile blood flow, and heart contractions. The goal is to understand which geometric distribution of stent struts, given by four different, commercially available stent geometries, is least likely to be associated with parameters correlated with pathobiologic responses leading to restenosis, in the case of curved coronary arteries, whose curvature changes significantly with each heart contraction. The stent geometries considered in this study correspond to a Palmaz-like stent, an Express-like stent, a Cypher-like stent, and a Xience-like stent. The biomechanical environment induced by each implanted stent is evaluated in terms of displacement magnitude, Von Mises stress, normal stress experienced by the intimal layer with implanted stent, and wall shear stress. Arterial walls are modeled as multi-layered structures: the intimal layer with the internal elastic laminae is modeled as a nonlinearly elastic membrane, while the media–adventitia complex is modeled as a 3D linearly elastic material. The Navier–Stokes equations for an incompressible, viscous fluid, are used to model the blood flow. Full, two-way coupling between the fluid and the structure, and between the thin and thick structure, is considered. To include the effects of the force exerted by the pericardium and heart muscle contractions, external force is applied to the coronary artery walls. Pulsatile boundary conditions were imposed at the inlet and outlet of the coronary segment, approximating measured diastolic coronary flow. The presence of an implanted stent was modeled by its impact on the mass and elasticity properties of the intimal layer where the stent is located. The stent material is modeled as a 316L stainless steel. A novel, loosely coupled partitioned scheme combined with an ALE approach was used to solve this nonlinear FSI problem. It was found that the Cypher-like stent geometry outperforms the other three stent geometries. The ranking from best to worst is as follows: Cypher-like stent, Express-like stent, Xience-like stent, Palmaz-like stent. It is conjectured that the sinusoidal horizontal stent struts and large cells associated with open-cell design, give rise to a stent geometry that conforms best to the native curved coronary artery, with smallest deviations in Von Mises stress and displacement from the nonstented curved coronary artery both during systole and diastole. To the best of our knowledge, this is the first computational study in which the behavior of different stent geometries implanted in curved coronary arteries is studied using full FSI capturing the behavior of multi-layered, curved, stented coronary arteries contracting on a beating heart.

Numerical Simulation of Dispersed Particle-Blood Flow in the Stenosed Coronary Arteries

A mathematical model of dispersed bioparticle-blood flow through the stenosed coronary artery under the pulsatile boundary conditions is proposed. Blood is assumed to be an incompressible non-Newtonian fluid and its flow is considered as turbulence described by the Reynolds-averaged Navier-Stokes equations. Bioparticles are assumed to be spherical shape with the same density as blood, and their translation and rotational motions are governed by Newtonian equations. Impact of particle movement on the blood velocity, the pressure distribution, and the wall shear stress distribution in three different severity degrees of stenosis including 25%, 50%, and 75% are investigated through the numerical simulation using ANSYS 18.2. Increasing degree of stenosis severity results in higher values of the pressure drop and wall shear stresses. The higher level of bioparticle motion directly varies with the pressure drop and wall shear stress. The area of coronary artery with higher density of bioparticles also presents the higher wall shear stress.

Unsteady Character and Flow Fields in an Axial Blood Pump under Pulsatile Pressure Boundary Condition

Blood pumps have been widely used as a temporary method for heart failure. In recent years, the challenging factors in design have generally concentrated on how to deal with the complicated time-dependent fluid dynamics. Therefore, this research studies the impact to flow field by pulsatile boundary condition with both steady and unsteady numerical simulations. For steady computation, total pressures of 20, 40, 60, 80, 100mmHg and an average pressure of 120mmHg were given at the inlet and outlet respectively, and an unsteady simulation was then conducted with the same fixed pressure of 120mmHg at the outlet but a periodical sinusoidal pressure within 20 and 100mmHg at the inlet. Compared with a single character line drawn through the steady points, the unsteady computation resulted in a closed elliptic character curve. In the whole range of operation of the unsteady condition, flow field showed a well-attached flow in rotor passage but a large area of recirculation in the second row of stator passages. By contrast, flow field in the first row of stator passages and the pressure on the surface of rotor blades were greatly impacted by the acceleration and deceleration of the flow rate, which can be well explained by the change of incidence. Besides, flow rate acceleration or deceleration exaggerated a non-uniform distribution of velocity, which can easily form a larger area of high scalar shear stress than the steady results. A special pressure distribution between rotors and stators was also observed in unsteady flow.

Next generation of ventricular catheters for hydrocephalus based on parametric designs.

The flow pattern of the cerebrospinal fluid is probably the most important factor related to obstruction of ventricular catheters during the normal treatment of hydrocephalus. To better comprehend the flow pattern, we have carried out a parametric study via numerical models of ventricular catheters. In previous studies, the flow was studied under steady and, recently, in pulsatile boundary conditions by means of computational fluid dynamics (CFD) in three-dimensional catheter models. This study aimed to bring in prototype models of catheter CFD flow solutions as well to introduce the theory behind parametric development of ventricular catheters. A preceding study allowed deriving basic principles which lead to designs with improved flow patterns of ventricular catheters. The parameters chosen were the number of drainage segments, the distances between them, the number and diameter of the holes on each segment, as well as their relative angular position. CFD results of previously unreleased models of ventricular catheter flow solutions are presented in this study. Parametric development guided new designs with better flow distribution while lowering the shear stress of the catheters holes. High-resolution 3D printed catheter solutions of three models and basic benchmark testing are introduced as well. The next generation of catheter with homogeneous flow patterns based on parametric designs may represent a step forward for the treatment of hydrocephalus, by possibly broadening their lifespan.

Pulsatile hemodynamics in patient-specific thoracic aortic dissection models constructed from computed tomography angiography.

Thoracic aortic dissection (TAD) is considered one of the most catastrophic and non-traumatic cardiovascular diseases associated with high morbidity and mortality rates in clinical treatment. The purpose of this paper is to investigate the pulsatile hemodynamics changes throughout a cardiac cycle in a Stanford Type B TAD model with the aid of computational fluid dynamics (CFD) method. A patient-specific dissected aorta geometry was reconstructed from the three-dimensional (3D) computed tomography angiography (CTA) scanning. The realistic time-dependent pulsatile boundary conditions were prescribed for our 3D patient-specific TAD model. Blood was considered to be an incompressible, Newtonian fluid. The aortic wall was assumed to be rigid, and a no-slip boundary condition was applied at the wall. CFD simulations were processed using the finite volume (FV) method to investigate the pulsatile hemodynamics in terms of blood flow velocity, aortic wall pressure, wall shear stress and flow vorticity. In the experiments, blood velocity, pressure, wall shear stress and vorticity distributions were analyzed qualitatively and quantitatively. The experimental results demonstrated a high wall shear stress and strong vertical flow at dissection initiation. The results also indicated that wall shear progressed along the false lumen, which is a possible cause of blood flow between aortic wall layers.

Hemodynamic analyses of large intracranial aneurysms

To simulate the computational hemodynamics of large intracranial aneurysms and analyze the hemodynamics of three types of large intracranial aneurysms. A total of 32 patient-specific models of large intracranial aneurysms were constructed with the data of DSA (digital subtraction angiography). According to the location of outflow vessel, plane of main vortex and impact zone, large intracranial aneurysms were classified into type A (outflow vessel in the plane of main vortex), type B1 (outflow vessel out of plane of main vortex, impact zone at the lateral wall of aneurysm) and type B2 (outflow vessel out of plane of main vortex, impact zone at the dome of aneurysm). Blood flow was assumed to be laminar and incompressible and blood Newtonian fluid. The time-dependent pulsatile boundary condition was deployed at inlet. CFD ICEM and Fluent software packages were used to simulate the computational hemodynamics of large intracranial aneurysms. The distributions of hemodynamic variables during the cardiac cycle were analyzed for wall shear stress, velocity and streamlines. The velocity ratio (ratio of aneurysmal flow velocity to parent artery flow velocity) of type A, B1 and B2 was 0.186 ± 0.019, 0.706 ± 0.077 and 0.208 ± 0.041 respectively. The wall shear stress ratio (ratio of aneurysmal wall shear stress to parent artery wall shear stress) of types A, B1 and B2 was 0.081 ± 0.029, 1.019 ± 0.139 and 0.103 ± 0.031 respectively. The flow velocity and wall shear stress were the highest in type B1 group, followed by those in type B2 group and the lowest in type A group. As reflected by the location of impact zone, the location of outflow vessel and inflow-angle can influence the level of blood flow in aneurysm sac.

Hemodynamic study of thoracic aortic aneurysm based on computational fluid dynamics technique

To establish a patient-specific hemodynamics model of thoracic aortic aneurysm (TAA) based on computational fluid dynamics technique and investigate its role in the study of growth and rupture mechanism of TAA. 3D realistic model of thoracic aortic aneurysm was reconstructed from DICOM format computed tomography angiography (CTA) images of a male patient. The geometry was reconstructed using medical image processing software Mimics. The blood flow in aorta was assumed to be laminar and incompressible and the blood Newtonian fluid. A time-dependent pulsatile boundary condition was deployed at inlet. Unsteady blood flow simulation was performed in real geometry of TAA with a finite volume method (FVM) code FLUENT. The hemodynamic parameters related with the growth and rupture of aneurysm were analyzed. Analysis of the distributions of hemodynamic variables during the cardiac cycle such as wall shear stress, streamlines and velocity profiles in TAA were carried out. The numerical simulation results demonstrated that the blood velocity of proximal neck was considerably faster than that of aneurysm body. The outer wall of proximal aneurysm body was hit by blood jet entering aneurysm. Instant streamlines at peak systole showed that the entering blood stream hit the aneurysm wall and right-hand helical vortex was observed in aneurysm body. The distributions of high wall shear stress were observed in the proximal and distal aneurysm neck and the area where the entering blood stream first hit the wall. Large regions of lower wall shear stress occurred in aneurysm body. The growth and rupture mechanisms of TAA may be analyzed based on a constructed patient-specific model and hemodynamic simulation.

Fluid–structure interaction modeling of blood flow and cerebral aneurysm: Significance of artery and aneurysm shapes

Because wall shear stress (WSS) is known to play an important role in initiation, growth and rupture of cerebral aneurysm, predicting the hemodynamic forces near the aneurysmal site helps with understanding aneurysms better. Earlier research reports indicate that the WSS around the aneurysmal site has a significant relationship with the vascular and aneurysm morphology. It was also shown statistically that the aneurysm shape (aspect ratio) is an indicator of rupture risk in cerebral aneurysm. In this study, fluid–structure interaction (FSI) modeling of a ruptured aneurysm, two unruptured aneurysms at the middle cerebral artery (MCA) bifurcation, and a MCA bifurcation without aneurysm is carried out using vascular geometries reconstructed from CT images. We use pulsatile boundary conditions based on a physiological flow velocity waveform and investigate the relationship between the hemodynamic forces and vascular morphology for different arteries and aneurysms. The results are compared with the results obtained for the rigid arterial wall to highlight the role of FSI in the patient-specific modeling of cerebral aneurysm. The results show that the interaction between the blood flow and arterial deformation alters the hemodynamic forces acting on the arterial wall and the interaction strongly depends on the individual aneurysm shapes. Flow impingement on the arterial wall plays a key role in determining the interaction and hemodynamic forces. When the blood flow impinges strongly on the wall, the maximum WSS tends to decrease due to the flow–wall interaction. When the blood flows straight into an aneurysm, the flow and the resulting WSS patterns are altered both qualitatively and quantitatively. When the blood in the aneurysm is nearly stagnant, a slow flow is induced by the wall motion, which raises the minimum WSS on the aneurysmal wall. The results reinforce the importance of FSI in patient-specific analysis of cerebral aneurysms.