Sandor I. Bernad

Numerical model for cavitational flow in hydraulic poppet valves

The paper presents a numerical simulation and analysis of the flow inside a poppet valve. First, the single-phase (liquid) flow is investigated, and an original model is introduced for quantitatively describing the vortex flow. Since an atmospheric outlet pressure produces large negative absolute pressure regions, a two-phase (cavitating) flow analysis is also performed. Both pressure and density distributions inside the cavity are presented, and a comparison with the liquid flow results is performed. It is found that if one defines the cavity radius such that up to this radius the pressure is no larger than the vaporization pressure, then both liquid and cavitating flow models predict the cavity extent. The current effort is based on the application of the recently developed full cavitation model that utilizes the modified Rayleigh-Plesset equations for bubble dynamics.

Hemodynamic parameters measurements to assess severity of serial lesions in patient specific right coronary artery

In this study, effects of serial stenosis on coronary hemodynamics were investigated in the human right coronary artery (RCA) using blood flow analysis. A 3-D model of a serial stenosed RCA was reconstructed based on multislice computerized tomography images. Numerical analysis examined the effect of multiple serial stenoses on the hemodynamic characteristics such as flow separation, pressure drop and FFR. Pressure loss associated with flow expansion after each constriction was large. Overall pressure drop increased from 1700 Pa (12.75 mmHg) at the end diastole to 11000 Pa (82.5 mmHg) at the peak systole. In two stenoses the corresponding pressure gradients werearound 30 mmHg and corresponded to the stenosis with FFR < 0.7 (associated to the sever stenosis). Severe stenosies caused large pressure drops across the throat. Blood flow distal to the stenosis was associated with fluctuations of the wall shear stress and vorticity.

Comparison between experimentally measured flow patterns for straigth and helical type graft

The long-term success of arterial bypass surgery is often limited by the progression of intimal hyperplasia at the anastomosis between the graft and the native artery. The experimental models were manufactured from glass tubing with constant internal diameter of 8 mm, fashioned into a straight configuration and helical configuration. The aim of this study was to determine the three-dimensional flow structures that occur at the proximal anastomosis under pulsatile flow conditions, to investigate the changes that resulted from variations in the anastomosis angle and flow division, and to establishing the major differences between the straight and helical graft. In the anastomosis domain, a strong region of recirculation is observed near the occluded end of the artery, which forces the flow to move into the perfused host coronary artery. The proximal portion of the host tube shows weak counter-rotating vortices on the symmetry plane. The exact locations and strengths of the vortices in this region are only weakly dependent on Re. A detailed comparison of experimentally measured axial velocity patterns for straight and helical grafts confirm the very strong nature of the secondary flows in the helical geometry. The helical configuration promotes the mixing effect of vortex motion such that the flow particles are mixed into the blood stream disal to the anastomotic junction.

Particle Depositions and Related Hemodynamic Parameters in the Multiple Stenosed Right Coronary Artery

Background Blood flow analysis of the human right coronary artery (RCA) has been carried out to investigate the effects of serial stenosis on coronary hemodynamics. A 3-D model of a serial stenosed RCA was reconstructed based on multislice computerized tomography images. Methods A velocity waveform in the proximal RCA and a pressure waveform in the distal RCA of a patient with a severe stenosis were acquired with a catheter delivered wire probe and applied as boundary conditions. The numerical analysis examines closely the effect of a multiple serial stenosis on the hemodynamic characteristics such as flow separation, wall shear stress (WSS) and particle depositions. Results and Conclusions Energy loss associated with such flow expansion after each constriction will be large and consequently the pressure drop will be higher. Overall pressure drop increased from 1700 Pa (12.75 mmHg) at the end diastole to 11000 Pa (82.5 mmHg) at the peak systole. At the peak systole the WSS values reached 110 Pa in the stenosis with 28% diameter reduction and 210 Pa in the stenosis with 54% diameter reduction, which is high enough to damage the endothelial cells. However at the end of one cardiac cycle a percent of 1.4% (15 from 1063 particles release at the inlet section) remain inside the stenosed RCA.

Numerical simulation of the swirl generator discharge cone at lower runner speeds

The paper focuses on numerical analysis of hydrodynamic behavior in a discharge cone from a swirling flow generator when is operated at lower runner speeds. The swirl generator is used in laboratory instead of a turbine model in order to investigate different swirling flow configurations into a straight draft tube. The purpose for reducing the speed of the runner is to increase the investigation regimes. Numerical results have shown that reducing the speed of the runner from the swirl generator, we are able to obtain different flow configurations similar with a turbine model operated in a wide range of operating regimes.

SAPHENOUS VEIN GRAFT PATENCY AFTER GEOMETRY REMODELING

Tortuous saphenous vein graft (SVG) hemodynamics was investigated using computational fluid dynamics (CFD) techniques. Computed tomography (CT) technology is used for noninvasive bypass graft assessment seven days after surgery. CT investigation shows two regions with severe shape remodeling, one is an angle type contortion and the other one is a sharp curvature with tortuous area reduction. The numerical analysis carefully examines the effect of an SVG geometry remodeling through flow separation, particle deposition, and wall shear stress (WSS). During the cardiac cycle, overall pressure drop increases from 2.6mmHg to 4.4mmHg. In the accelerating part of the systolic phase, particles released in the inlet section move downstream toward the first narrowed part (elbow type contortion) with a helical motion. WSS range along the cardiac cycle varies from 2Pa to 42Pa, enough to damage the endothelial cells. Vessel torsion induced helical flow can reduce the flow disturbance and separation. Additionally, in the distal end of the graft, the high particle concentrations can promote the inflammatory processes in the vessels.

Luminal flow alteration in presence of the stent

Luminally protruding struts alter blood flow, creating areas of recirculation, separation, and stagnation. The impact of flow alterations around struts vary as the strut geometrical parameters change. We quantified the influence of the luminal flow alterations due to the presence of the stent struts by performing two-dimensional numerical simulation. Idealized computer models can facilitate understanding of the in-stent restenosis that is impossible to achieve in vivo.

Fluid mechanics in stented arterial model

Local hemodynamic factors are known affect the natural history of the restenosis critically after coronary stenting of atherosclerosis. Stent-induced flows disturbance magnitude dependent directly on the strut design. Strut shape, strut thickness and the distance between consecutive struts have been associated clinically with the with post-intervention clinical outcomes. Hemodynamically favorable designs according to computational modeling can reduced in-stent restenosis after coronary stenting intervention.

Hemodynamics of human placenta

The placenta is responsible for the exchange of oxygen and nutrients from the mother to the fetus. Normal placentation and placental development are critical for a successful pregnancy. In this study the placenta are obtained from normal pregnancy and of normal vaginal delivery (age: 31 years old, parity 1, and gestation 38 weeks). After the delivery of the baby and the placenta commercial polymer mixture was injected into the artery to achieve a placental vascular model. The polymeric cast of the fetal vasculature of full-term placenta is used in order to perform a numerical simulation of the blood perfusion in the placenta. The period for the simulations is 0.5s, which corresponds to a baby heart rate of 120 bpm. Numerical simulation indicated that the velocity profiles in the placental vessels, both immediately upstream and downstream the bifurcation, are close to parabolic profiles.

Particle Motion In Coronary Serial Stenoses

Atherosclerosis creating a constriction can significantly alter the local blood flow dynamics. From a biological aspect, the changes that take place in the flow have a profound effect on the structure and function of the arterial wall and the development of the disease. The purpose of this paper was to non-invasively assess hemodynamic parameters such as wall shear stress, wall pressure and particle depositions with computational fluid dynamics (CFD) in coronary artery serial stenoses. A 3-D model of a serial stenosed RCA was reconstructed based on multislice computerized tomography images. Energy loss associated with such flow expansion after each constriction will be large and consequently the pressure drop will be higher. Pressure drop across the stenoses ST1 and ST3 is lower (4.62 mmHg and 4.81 mmHg respectively) during the time T2 = 0.79s, but is significant during the peak systole T1 = 0.26s. The maximum WSS in the proximal stenosis ST1 is about 254 Pa, and in the distal stenosis ST3 are 232 Pa. One diameter downstream of the each stenosis, the WSS is low because of the formation of the recirculation zone.