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° forward facing, 45° backward facing, and 90° 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° forward facing to 45° backward facing. At the same time, a pair of vortex was found in the cross-sectional planes of the 45° backward facing and 90° 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° 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° 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.

Cyclic flow characteristics in an idealized asymmetric abdominal aortic aneurysm model

The flow characteristics and the corresponding hydrodynamic stability in an idealized asymmetric abdominal aortic aneurysm (AAA) model have been investigated using a laser Doppler anemometer. A rectified sine waveform was used to simulate aortic flow conditions (Re(delta) = 806 and alpha = 12.2). The flow around the distal neck of the AAA model undergoes transition and becomes turbulent for a fraction of time shortly after the commencement of the deceleration phases at every flow cycle while the rest of the flow inside the model stayed laminar throughout the cycle. As a result of non-symmetric vortical structure development inside the model, the distribution of turbulent shear stresses was found to be highly uneven along the radial direction of the model; this is in contrast to results found by the present authors in the symmetrical AAA model. The maximum turbulent shear stress found at the straight side of the distal neck are four times more than the maximum turbulent shear stress measured at the most dilated side of the distal neck. One of the interesting biological implications of the results is that the outward dilation of the arterial wall may be a physiological response to avoid the high turbulent shear load from the momentarily turbulent blood flow.

Effects of scaling on centrifugal blood pumps

Experimental studies on the effects of scaling on the performance of centrifugal blood pumps were conducted in a closed-loop test rig. For the prototype, eight different impellers of the same outer diameter of 25 mm were tested at 1,500, 2,000, and 2,500 revolutions per minute (rpm) using blood analog as fluid medium. This corresponds to Reynolds numbers (Re) of 25,900, 34,500, and 43,200, respectively. The results indicated that the nondimensional pump characteristic is a function of Re. This is understandable since the typical operating Re for centrifugal blood pumps is less than 100,000. Thus, the effects of scaling cannot be ignored for centrifugal blood pumps. Experiments on a 5x scaled-up model have also indicated that the scaled-up model is more efficient than the prototype model. Our results showed that in the range of Re tested, the nondimensional head versus flow curve is a function of Re to the power of approximately 0.25. It is observed that the nondimensional head versus flow is a function of diameter ratio to the power of 0.2.

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.