S.C.M. Yu

Numerical investigation and identification of susceptible sites of atherosclerotic lesion formation in a complete coronary artery bypass model

As hemodynamics is widely believed to correlate with anastomotic stenosis in coronary bypass surgery, this paper investigates the flow characteristics and distributions of the hemodynamic parameters (HPs) in a coronary bypass model (which includes both proximal and distal anastomoses), under physiological flow conditions. Disturbed flows (flow separation/reattachment, vortical and secondary flows) as well as regions of high oscillatory shear index (OSI) with low wall shear stress (WSS), i.e., high-OSI-and-low-WSS and low-OSI-and-high-WSS were found in the proximal and distal anastomoses, especially at the toe and heel regions of distal anastomosis, which indicate highly suspected sites for the onset of the atherosclerotic lesions. The flow patterns found in the graft and distal anastomoses of our model at deceleration phases are different from those of the isolated distal anastomosis model. In addition, a huge significant difference in segmental averages of HPs was found between the distal and proximal anastomoses. These findings further suggest that intimal hyperplasia would be more prone to form in the distal anastomosis than in the proximal anastomosis, particularly along the suture line at the toe and heel of distal anastomosis.

Mixing Characteristics of Forced Mixers with Scalloped Lobes

An experimental investigation has been undertaken to study the mixing characteristics of forced mixers with scalloped lobes using a two-component e ber-optic laser Doppler anemometer at a Reynolds number of 2.27 3 10 4 and at a velocity ratio 1:2 across the lobes. The trailing edge of the mixers, without scalloping, had the shape of a square wave, a semicircular wave, and a triangular wave. Scalloping was achieved by eliminating up to 70% of the sidewall area at the penetration region of each lobe. The aspect ratio of each lobe (lobe height-to-wavelength ratio ) was at unity and the half-angles at the penetration region were 22 and 35 deg for the scalloped and the aggressively scalloped mixers, respectively. The wavelength and lobe height were the same for both types of mixer. The results showed that lobe cone gurations appeared to be more important than the penetration angles for the benee t of scalloping to occur. Strengths of the streamwise circulation near the trailing edge for respective scalloped mixers were higher than the nonscalloping cases, largely because of the formation of two pairs of streamwise vortices at each lobe. The subsequent decay rates with downstream distance were also found to be more rapid, indicating that a faster mixing rate can actually be achieved by the scalloped mixers. Additional production of turbulent kinetic energy appeared at about two to three wavelengths downstream of the trailing edge and then asymptote to lower level in the far-e eld region. Spatial uniformity of the mass e ux at the wake region could be achieved at two wavelengths further downstream of the high turbulent kinetic energy region.

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.

Velocity measurements downstream of a lobed-forced mixer with different trailing-edge configurations

Velocity and turbulence characteristics have been measured downstream of a lobed-forced mixer with three different trailing-edge configurations, using a two-component laser-Doppler anemometer at a Reynolds number of 2.27 x 10 4 (based on the bulk mean velocity of the two streams at 10 m/s, and a nominal wavelength of the lobe at 33 mm), and with velocity ratios of 1:1,1:2, and 1:3 across the lobe. The three trailing-edge configurations under investigation have the shape of a square wave, a semicircular wave, and a triangular wave. The results for the velocity ratio 1:1 indicate that the mixing is not only affected by the strength of the secondary flow shed by a lobe, but also by the boundary-layer thickness grown along the side walls of the lobe penetration and the subsequent shedding of the boundary layer to the wake region. No region of high turbulence was found within six wavelengths downstream of the trailing edge. The results for the higher velocity ratios, 1:2 and 1:3, reveal a very different flow development from those of an equal velocity ratio. A high-turbulence region appeared at around two to three wavelengths downstream of the trailing edge, and was followed by a gradual decay in magnitude. Analyzing the production terms in the Reynolds stresses equations at the corresponding location suggested that positive production of turbulent kinetic energy existed and was a consequence of large mean axial velocity gradients that coincided with shear stresses of the opposite signs. Thus, the present investigations suggest that the high-turbulence region responsible for rapid mixing may not be due to vortex breakdown (at six wavelengths downstream), but may be due to the positive production of turbulent kinetic energy at a location (at about two to three wavelengths downstream) earlier than previously suggested.

A general formulation for the decay of swirling motion along a straight pipe

Abstract This paper describes an analytical approach, based on the conservation of angular momentum flux, to predict the decay of swirling motion due to frictional effects along a straight duct. A combined forced vortex and shear-type velocity profiles were used and the analysis could be applicable to any types of swirling flow. Good agreement with measurements in a straight pipe with different level of inlet swirl intensities (Kitoh O.; J. Fluid Mechanics, 1991) can be obtained with the approximate solution. The present formulation can also demonstrate that the swirling motion along a straight pipe does not always decay exponentially but depending on inlet conditions and the variation of friction coefficient with downstream distance.

Some experimental observations on the single and multi-phase flow patterns in a model flash evaporation chamber

Abstract Flow characteristics inside a model flash evaporation chamber for Multi-stage Flash (MSF) desalination have been investigated using Particle Image Velocimetry (PIV). The back flow region near the inlet gate, the free water surface shape, and the overall velocity distribution were measured so as to identify the characteristics of the flow pattern at different water levels in the chamber. The results show that a large recirculation region with several vortices embedded would be generated at a higher water level (approximately twice the inlet gate height). The water level remained uniform in spite of the presence of the vortices. Three flash evaporation modes at different superheats of the water in the flash chamber were also studied using a high speed CCD camera. The bubble behaviors such as the size, velocity, and growth rate were estimated and analyzed. It was found that the multi-phase flow pattern during the flash evaporation was mainly determined by the liquid superheat and the nuclei distribution.

PIV measurements of proximal models with different anastomotic angles

Abstract The anastomosis glass models are designed and fabricated to investigate the effect of the anastomotic angles (30° to 90° for both forward and backward flow) on the intimal hyperplasia formation. A test rig is designed and built to perform Particle Image Velocimetry (PIV) measurement on the anastomosis models with three Reynolds numbers of 100, 169 and 250. The initial results indicated that there are two low velocity regions, one of them is located at the outer wall of the graft near the joint and the other is located at one diameter down from the joint along the inner wall of the graft. It is found that these low velocity regions are dependent on the Reynolds number and anastomotic angles.

Some measurements downstream of a lobed forced mixer with different trailing edge configurations

Abstract Velocity and turbulence characteristics have been measured downstream of a lobed forced mixer with three different trailing edge configurations, using a two-component laser-Doppler anemometer at a Reynolds number of 2.27 × 10 4 (based on a reference velocity of 10m/s, and a nominal wavelength of the lobe of 33 mm). The three trailing edge configurations under investigation have the shape of a square wave, a semi-circular wave and a triangular wave. Measurements for a velocity ratio of 1:1 indicate that the strength of the secondary flow shed by a lobe is not the only parameter which determines the effectiveness of mixing, as previously suggested. The boundary layer thickness which grows along the side-walls of the lobe penetration and the subsequent shedding of the boundary layer to the wake region are of equal importance. High turbulence regions do not appear downstream of the mixers within six wavelengths downstream of the trailing edge. The normal and shear stresses in two cases (the semi-circular and triangular wave lobes) are shown to have a similar trend of decaying in magnitude.

Flow visualization and preliminary measurements of a confined jet with and without target

Abstract Flow visualization using smoke in a confined jet with and without a target has been investigated in the paper. The results indicated that there are flow oscillations in the confined jet with and without the target. The results also confirm the existence of re-circulation flow when the flow hit the back wall and causing the mean velocity profiles did not reach the zero values at the far end of transverse locations. The flow visualization provides useful qualitative information about the flow physic of the fluidic flowmeter.

Comparison of flow characteristics of enlarged blood pump models with different impeller design

In earlier studies, a 5:1 enlarged pump model of the Kyoto-NTN Magnetically Suspended Centrifugal Blood Pump has been constructed and the gap pressure and wall shear stress have been measured. With the satisfactory results obtained and also as air was the medium used in the previous investigations, another 5:1 enlarged pump model using water as the medium was thus designed and constructed. Five different impeller blade profile designs are used in the present study. By varying (1) the design of the blade profiles i.e. forward, radial and backward, (2) the number of blades used and (3) the rotating speeds, the flow characteristics of the pump are investigated. It is found that the impeller with the higher number of blades used with the forward and straight blade profiles have the best performance.