Particle image velocimetry measurements of three proximal anastomosis models under a pulsatile flow condition.

This study was designed to examine the effects of the anastomotic angle on the flow and haemodynamic parameter distribution patterns of the proximal anastomoses, with emphasis on identifying site-specific haemodynamic features that could reasonably be expected to trigger the initiation and further development of anastomotic intimal hyperplasia. Particle image velocimetry measurements were carried out with three simplified glass proximal models under a physiological flow condition. The results revealed that the disturbed flow and the induced shear stress patterns including low recirculation flow, stagnation point, high wall shear stress, high temporal wall shear stress gradient, low time-averaged wall shear stress (TAWSS), and high oscillating shear index (OSI) occurred around the anastomotic joints and the flow field at proximal anastomosis was strongly affected by the anastomotic angle. Among the three models investigated, the 45 degrees backward anastomosis is found to have a smaller low-recirculation-flow region along the graft inner wall, non-stationary stagnation, and separation points, a higher TAWSS and smaller high-OSI low-TAWSS and low-OSI high-TAWSS regions.

Flow studies in three-dimensional aorto-right coronary bypass graft system.

This paper presents the fluid dynamics of blood flow in a coronary bypass model of the aorto-right coronary bypass system. Three-dimensional computational fluid dynamic simulations are developed of the blood flow in coronary artery-bypass systems, using the computational fluid dynamics software (FLUENT 6.0.1). These blood flow simulations are performed within small intervals of the cardiac cycle, using input data consisting of physiological measurements of flow rates in the aorta, obtained from earlier studies. We have calculated the flow-field distributions of the velocity and the wall shear stress at four typical instants of the cardiac cycle, two during systole and two during the diastole phase. Plots of velocity vector and the wall shear stress are displayed in the aorto-graft-coronary arterial flow-field domain, providing an insight into the link between fluid dynamics and arterial diseases. The prime regions of disturbed flow patterns are at the entrance into the graft from the aorta and at the exit from the graft into the right coronary artery. Our objective is to obtain an understanding of how the coronary artery is perfused by the graft, and thereby into the factors affecting graft patency.

Wavelet multiresolution analysis of the three vorticity components in a turbulent far wake.

The main objective of the present study is to examine the characteristics of the vortical structures in a turbulent far wake using the wavelet multiresolution technique by decomposing the vorticity into a number of orthogonal wavelet components based on different central frequencies. The three vorticity components were measured simultaneously using an eight-wire probe at three Reynolds numbers, namely 2000, 4000, and 6000. It is found that the dominant contributions to the vorticity variances are from the intermediate and relatively small-scale structures. The contributions from the large and intermediate-scale structures to the vorticity variances decrease with the increase of Reynolds number. The contributions from the small-scale structures to all three vorticity variances jump significantly when Reynolds number is changed from 2000 to 4000, which is connected to previous observations in the near wake that there is a significant increase in the generation of small-scale structures once the Reynolds number reaches about 5000. This result reinforces the conception that turbulence "remembers" its origin.

Comparisons between different approximations to energy dissipation rate in a self-preserving far wake.

By using a four-hot-wire probe and an eight-hot-wire probe, different approximations to energy dissipation rate have been made in the far field of a cylinder wake. The appropriateness of the various approximations is evaluated by examining their mean values, spectra, conditional analysis, and scaling range exponents. It is found that there are significant differences between the instantaneous values of E(iso), the isotropic dissipation rate, and other approximations. The present measurements also allow the examination of the spatial correlation between the energy dissipation rate and the enstrophy Omega. While the correlation between E(iso) and Omega is low, there is a strong correlation between the other approximations to energy dissipation rate and the enstrophy Omega. The scaling range exponents show that the substitutes to the energy dissipation rate and enstrophy based on isotropy are more intermittent than their corresponding true values. The present results suggest that using E(iso) as a substitute of should be re-examined, especially for the instantaneous values.

Numerical study on the pulsatile flow characteristics of proximal anastomotic models.

Haemodynamics was widely believed to correlate with anastomosis restenosis. Utilizing the haemodynamic parameters as indicator functions, distal anastomosis was redesigned by some researchers so as to improve the long-term graft patency rate. However, there were few studies upon the proximal anastomosis. Therefore, in this study, flow characteristics and distributions of the haemodynamic parameters in proximal anastomosis under physiological flow condition have been investigated numerically for three different grafting angles: namely, 45 degrees forward facing, 45 degrees backward facing, and 90 degrees anastomotic joints. The simulation results showed a flow separation region along the graft inner wall immediately after the heel at peak flow phase and it decreased in size with the grafting angle shifting from 45 degrees forward facing to 45 degrees backward facing. At the same time, a pair of vortex was found in the cross-sectional planes of the 45 degrees backward facing and 90 degrees grafts. In addition, stagnation point was found along the graft outer wall with small shifting during the physiological cycle. High spatial and temporal wall shear stresses gradients (WSSG) were observed around the anastomotic joint. Low time-averaged wall shear stress (WSS) with elevated oscillation shear index (OSI) was found near the middle of anastomosis at the aorta wall and along the graft inner wall respectively, while high time-averaged WSS with low OSI was found at the heel, the toe, and the region downstream of the toe. These regions correlated to early lesion growth. Elevated time-averaged WSSG was found at the same region, where the elevated low-density lipoprotein (LDL) permeability was observed as reported in the literature. The existence of nearly fixed stagnating location, flow separation, vortex, high time-averaged WSS with low OSI, low time-averaged WSS with elevated OSI, and high time-averaged WSSG may lead to graft stenosis. Moreover, the simulation results obtained were consistent with those of experimental measurements. Based on the validated simulation results, the 45 degrees backward-facing graft was found to have the lowest variation range of time-averaged WSS and the lowest segmental average of WSSG among the three models investigated. The 45 degrees backward-facing graft is thus recommended for the bypass operation with expected higher patency rate.

Scaling of longitudinal and transverse velocity increments in a cylinder wake.

Longitudinal and transverse velocity increments are measured both temporally and spatially using two X-wire probes in the intermediate region of a cylinder wake over Taylor microscale Reynolds numbers in the range of 100-300. The scaling exponents of both the spatial and temporal longitudinal velocity increments agree favorably with the predictions of Kolmogorov and She and Leveque. The scaling exponents of the transverse velocity increments are considerably smaller than those of the longitudinal ones, with the values for spatial transverse velocity increments being slightly larger than the temporal ones. The difference between the scaling exponents of the longitudinal and transverse velocity increments is examined against the refined similarity hypotheses for transverse velocity increments (RSHT) proposed by Chen It is found that the RSHT can account for the difference between the scaling exponents of the longitudinal and spatial transverse velocity increments at all Reynolds numbers considered.

Numerical investigation of the effect of blade geometry on blood trauma in a centrifugal blood pump.

Fluid dynamic forces in centrifugal blood pump impellers are of key importance in destruction of red blood cells (RBCs) because high rotational speed leads to strong interaction between the impeller and the RBCs. In this paper, three-dimensional models of five different blade geometries are investigated numerically using the commercial software CFX-TASCflow, and the streaklines of RBCs are obtained using the Lagrangian particle tracking method. In reality, RBCs pass through the pump along complicated paths resulting in a highly irregular loading condition for each RBC. In order to enable the prediction of blood damage under the action of these complex-loading conditions, a cumulative damage model for RBCs was adopted in this paper. The numerically simulated percent hemoglobin (%HB) released as RBCs traversed the impeller and volute was examined. It was observed that the residence time of particles in the blade passage is a critical factor in determining hemolytic effects. This, in turn, is a function of the blade geometry. In addition, it was observed that the volute profile is an important influence on the computed HB% released.

A computational study of the effects of inlet guide vanes on the performance of a centrifugal blood pump.

This article presents computational studies on the effects of inlet guide vanes (IGVs) on the flow pattern and shear stress in a centrifugal blood pump. The effect of IGVs is to introduce a pre-swirl to fluid particles entering the impeller with the intention that the fluid particles will travel along the blade profile. Currently, most commercial centrifugal blood pumps employ straight radial impeller blades that are not hydrodynamically ideal for a good flow pattern within the blade passage. Flow separation and formation of vortices within the blade passage are believed to increase the degree of hemolysis and thrombosis. These are causes for blood clotting that will lead to malfunctioning of ventricular assist devices. Four IGVs of different geometrical profiles have been numerically investigated using a commercial software program CFX-Tascflow. The pump is operated at 2,000 rpm, and the results revealed that the relative flow patterns in the blade passage have been dramatically altered. The size of the vortices was reduced, and the pressure contours indicated a gradual rise from the impeller leading edge to the trailing edge. However, inclusion of IGV causes a drop in the pressure head generated. Higher frictional losses are incurred as fluid particles passed through the IGV. In addition, the IGV modifies the inlet velocity triangles, and this also contributes to a drop in the pressure head generated that is consistent with Euler's pump theory. The change in the flow patterns and the gradual variation of the pressure contours have led to lower shear stress within the blade passages as compared to the case without IGVs.

Proportional assist ventilation system based on proportional solenoid valve control.

A new proportional assist ventilation (PAV) method using a proportional solenoid valve (PSV) to control air supply to patients suffering from respiratory disabilities, was studied. The outlet flow and pressure from the proportional solenoid valve at various air supply pressures were tested and proven to be suitable for pressure and flow control in a PAV system. In vitro tests using a breathing simulator, which has been proven to possess the general characteristics of human respiratory system in spontaneous breathing tests, were conducted and the results demonstrated the viability of this PAV system in normalizing the breathing patterns of patients with abnormally high resistances and elastances as well as neuromuscular weaknesses. With a back-up safety mechanism incorporated in the control program, pressure "run-away" can be effectively prevented and safe operation of the system can be guaranteed.

The flow patterns within the impeller passages of a centrifugal blood pump model.

The effects of impeller geometry on the performance of a centrifugal blood pump model [the MSCBP design of Akamatsu and Tsukiya (The Seventh Asian Congress of Fluid Mechanics (1997), 7-10) at a 1:1 scale] have been investigated both experimentally and computationally. Four impeller designs were tested for pump hydraulic performance at the operating point (i.e. 2000 rpm), using blood analog as the working fluid. Each impeller has seven blades with different configurations including the radial straight blade and backward swept blade designs. The results show that both designs can achieve a stable head of about 100 mm Hg at the operating point. Subsequent investigations involved the visualization of the relative flow field within the impeller passages via the image de-rotation system coupled with a 2.5 W argon ion laser. Flow structures in all sectors of each impeller were examined and discussed. To further quantify the possible effects of blade geometry to thrombus formation and hemolysis, computational fluid dynamics (CFD) was used to simulate a simplified two-dimensional blade-to-blade flow analysis so as to estimate the shear stress levels. The results indicate that the stress levels found within the blade passages are generally below the threshold level of 150 N/m(2) for extensive erythrocyte damage to occur. There are some localized regions near the leading edge of the blades where the stress levels are 60% above the threshold level. However, given such a short residence time for the fluid particles to go through these high shear stress regions, their effects appear to be insignificant.

Performance of enlarged blood pump models with five different impellers.

In earlier studies, a 5:1 enlarged pump model of the Kyoto-NTN Magnetically Suspended Centrifugal Blood Pump had been constructed and the flow characteristics investigated. Although the results obtained were satisfactory, the medium used was air. A 5:1 enlarged pump model using water as the medium thus was designed and constructed. Five different impeller blade profile designs were used in the present study. By varying (1) the blade profile design: forward, radial, and backward, (2) the number of blades used, and (3) the rotating speed, the flow characteristics of the pump were investigated. It was found that the impeller with the higher number of blades, used in the forward and straight blade profiles, have the best performance.

Measurements of enlarged blood pump models using Laser Doppler Anemometer.

In an earlier study (Chua et al., 1998, 1999a), a 5:1 enlarged model of the Kyoto-NTN Magnetically Suspended Centrifugal Blood Pump (Akamatsu et al., 1995) with five different impeller blade profiles was designed and constructed. Their respective flow characteristics with respect to (1) the three different blade profile designs: forward, radial, and backward, (2) the number of blades used, and (3) the rotating speed were investigated. Among the five impeller designs, the results obtained suggested that impellers A and C designs should be adopted if higher head is required. Impellers A and C therefore were selected for the flow in between their blades to be measured using Laser Doppler Anemometer (LDA), so as to have a better understanding of the flow physics with respect to the design parameters.

Measurements of gap pressure and wall shear stress of a blood pump model.

The centrifugal blood pump with a magnetically suspended impeller has shown its superiority as compared to other artificial hearts. However, there is still insufficient understanding of fluid mechanics related issues in the clearance gap. The design nature of the pump requires sufficient washout in the clearance between the impeller and stationary surfaces. As the gap is only 0.2 mm in width, it is very difficult to conduct measurements with present instrumentation. An enlarged model with 5:1 ratio of the pump has been designed and constructed according to specifications. Dimensionless gap pressure measurements of the model are very close to the prototype. The measurements of wall shear stress of the fluid flow in the clearance gap between the impeller face and inlet casing of a blood pump model were accomplished through hot-wire anemometry and rotating disk apparatus. Regions of relatively high and low shear stresses are identified. These correspond to spots where the likelihood of hemolysis and thrombus formation is high. With the use of dimensional analysis, it is found that the highest wall shear stress is equivalent to 146 Pa which is much lower than the threshold value of 400 Pa for hemolysis reported in the literature.

A numerical investigation on the steady and pulsatile flow characteristics in axi-symmetric abdominal aortic aneurysm models with some experimental evaluation.

Steady and pulsatile flow characteristics in rigid abdominal aortic aneurysm (AAA) models were investigated computationally (using Fluent v. 4.3) over a range of Reynolds number (from 200 to 1600) and Womersley number (from 17 to 22). Some comparisons with measurements obtained by particle image velocimetry under the pulsatile flow conditions are also included. A sinusoidal inlet flow waveform 1 + sin omega t with thin inlet boundary layers was used to produce the required pulsatile flow conditions. The bulk features of the mean flow as well as some detailed features, such as wall shear stress distributions, are the foci of the present investigation. Recirculating vortices appeared at different phases of a flow cycle causing significant spatial and temporal variations in wall shear stresses and static pressure distributions. A high level of shear stresses usually appeared at the upstream and downstream ends of the bulge. Effects of pressure rise caused by the increase in cross-sectional area were transmitted into the downstream tube. Further simulation studies were conducted using simulated physiological waveforms under resting and exercise conditions so as to determine the possible implication of vortex dynamics inside the AAA model.