Leok Poh Chua

Calibration ofX-probes for turbulent flow measurements

We compare two methods of calibrating the yaw response of hot-wire probes: (i) the assumption that an effective angle, independent of the flow speed, can be deduced; (ii) the more general approach of determining the yaw response at a number of different speeds. The first, simpler, approach is shown to give surprisingly reasonable results for the usual turbulence statistics, even in high turbulence intensity flows. Some differences in the distribution of the inclination of the instantaneous velocity vector are observed. There is no advantage in using thek2 factor to allow for longitudinal cooling.

Turbulent prandtl number in a circular jet

Abstract Measurements of the Reynolds shear stress and heat flux distributions at a number of streamwise stations in a heated circular jet into still air indicate that the flow is approximately self-preserving at x d = 15 . Measurements made with a 120° X-probe are in closer agreement with calculations obtained by integrating the mean momentum and mean enthalpy equations than measurements made with a 90° X-probe. The turbulent Prandtl number increases near the edge of the jet but, in a region between the axis and the jet half-radius, it is approximately constant (0.81 ± 0.05). This numerical value is in reasonable agreement with that selected by So and Hwang ( ZAMP 37 , 624–631 (1986)) for similarity solutions of non-isothermal round jets. It is suggested that the early attainment of self-preservation in the present flow may be due to the laminar conditions at the jet exit.

Computational model of blood flow in the aorto-coronary bypass graft

BackgroundCoronary artery bypass grafting surgery is an effective treatment modality for patients with severe coronary artery disease. The conduits used during the surgery include both the arterial and venous conduits. Long- term graft patency rate for the internal mammary arterial graft is superior, but the same is not true for the saphenous vein grafts. At 10 years, more than 50% of the vein grafts would have occluded and many of them are diseased. Why do the saphenous vein grafts fail the test of time? Many causes have been proposed for saphenous graft failure. Some are non-modifiable and the rest are modifiable. Non-modifiable causes include different histological structure of the vein compared to artery, size disparity between coronary artery and saphenous vein. However, researches are more interested in the modifiable causes, such as graft flow dynamics and wall shear stress distribution at the anastomotic sites. Formation of intimal hyperplasia at the anastomotic junction has been implicated as the root cause of long- term graft failure.Many researchers have analyzed the complex flow patterns in the distal sapheno-coronary anastomotic region, using various simulated model in an attempt to explain the site of preferential intimal hyperplasia based on the flow disturbances and differential wall stress distribution. In this paper, the geometrical bypass models (aorto-left coronary bypass graft model and aorto-right coronary bypass graft model) are based on real-life situations. In our models, the dimensions of the aorta, saphenous vein and the coronary artery simulate the actual dimensions at surgery. Both the proximal and distal anastomoses are considered at the same time, and we also take into the consideration the cross-sectional shape change of the venous conduit from circular to elliptical. Contrary to previous works, we have carried out computational fluid dynamics (CFD) study in the entire aorta-graft-perfused artery domain. The results reported here focus on (i) the complex flow patterns both at the proximal and distal anastomotic sites, and (ii) the wall shear stress distribution, which is an important factor that contributes to graft patency.MethodsThe three-dimensional coronary bypass models of the aorto-right coronary bypass and the aorto-left coronary bypass systems are constructed using computational fluid-dynamics software (Fluent 6.0.1). To have a better understanding of the flow dynamics at specific time instants of the cardiac cycle, quasi-steady flow simulations are performed, using a finite-volume approach. The data input to the models are the physiological measurements of flow-rates at (i) the aortic entrance, (ii) the ascending aorta, (iii) the left coronary artery, and (iv) the right coronary artery.ResultsThe flow field and the wall shear stress are calculated throughout the cycle, but reported in this paper at two different instants of the cardiac cycle, one at the onset of ejection and the other during mid-diastole for both the right and left aorto-coronary bypass graft models. Plots of velocity-vector and the wall shear stress distributions are displayed in the aorto-graft-coronary arterial flow-field domain. We have shown (i) how the blocked coronary artery is being perfused in systole and diastole, (ii) the flow patterns at the two anastomotic junctions, proximal and distal anastomotic sites, and (iii) the shear stress distributions and their associations with arterial disease.ConclusionThe computed results have revealed that (i) maximum perfusion of the occluded artery occurs during mid-diastole, and (ii) the maximum wall shear-stress variation is observed around the distal anastomotic region. These results can enable the clinicians to have a better understanding of vein graft disease, and hopefully we can offer a solution to alleviate or delay the occurrence of vein graft disease.

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.

Perspective on CFD studies of coronary artery disease lesions and hemodynamics: A review

Coronary artery disease (CAD) is the most common cardiovascular disease. Early diagnosis of CADs physiological significance is of utmost importance for guiding individualized risk-tailored treatment strategies. In this paper, we first review the state-of-the-art clinical diagnostic indices to quantify the severity of CAD and the associated invasive and noninvasive imaging technologies in order to quantify the anatomical parameters of diameter stenosis, area stenosis, and hemodynamic indices of coronary flow reserve and fractional flow reserve. With the development of computational technologies and CFD methods, tremendous progress has been made in applying image-based CFD simulation techniques to elucidate the effects of hemodynamics in vascular pathophysiology toward the initialization and progression of CAD. So then, we review the advancements of CFD technologies in patient-specific modeling, involving the development of geometry reconstruction, boundary conditions, and fluid-structure interaction. Next, we review the applications of CFD to stenotic sites, in order to compute their hemodynamic parameters and study the relationship between the hemodynamic conditions and the clinical indices, to thereby assess the amount of viable myocardium and candidacy for percutaneous coronary intervention. Finally, we review the strengths and limitations of current researches of applying CFD to CAD studies.

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.

The turbulent interaction region of a circular jet

Abstract Measurements are presented for the interaction region of a circular jet with initially laminar boundary layers. These measurements include mean and root mean square velocity and temperature distributions on the jet axis as well as the evolution of the probability density function of the temperature fluctuation. The streamwise extent of the interaction region is considerably shorter than that of the plane jet with nominally similar initial conditions. The strength of the interaction is considerably less for the circular jet than for the plane jet.

A Novel Coronary Artery Bypass Graft Design of Sequential Anastomoses

In this paper, the hemodynamics in a three-dimensional out-of-plane sequential bypass graft model is first investigated. Based on the advantageous flow characteristics observed within the side-to-side (STS) anastomosis in the sequential bypass graft simulation, a new CABG coupled-sequential anastomosis configuration is designed, entailing coupled STS and end-to-side (ETS) anastomotic components. In this new CABG design, the flow fields and distributions of various wall shear stress parameters within the STS and ETS anastomotic regions are studied, and compared to those of the conventional distal anastomosis, by means of computational fluid dynamics simulation of pulsatile Newtonian blood flow. Simulation results demonstrate that the new sequential anastomoses model provides: (i) a more uniform and smooth flow at the ETS anastomosis, without any stagnation point on the artery bed and vortex formation in the heel region of the ETS anastomosis within the coronary artery; (ii) a spare route for the blood flow to the coronary artery, to avoid re-operation in case of re-stenosis in either of the anastomoses; and (iii) improved distribution of hemodynamic parameters at the coronary artery bed and in the heel region of the ETS anastomosis, with more moderate shear stress indices. These advantages of the new design over the conventional ETS anastomosis are influenced by the occlusion ratio of the native coronary artery, and are most prominent when the proximal segment of the coronary artery is fully occluded. By varying the design parameters of the anastomotic angle and distance between the two anastomoses, the superior coupled STS–ETS anastomoses design is found to have the anastomotic angle of 30° and 30 mm distance between the two (STS and ETS) components.

Flow visualization and measurements of a square jet with mixing tabs

Abstract A square jet with mixing tabs was investigated using laser induced fluorescence and laser Doppler anemometer (LDA). The triangular tabs were tilted either downstream (i.e., at an inclination, θ=45°) or upstream (i.e., at θ=135°). With four tabs, the jet flow was bifurcated into a four-finger structure and there was a “mushroom” structure behind each tab for θ=135°. Secondary velocity vector measurements using LDA in the y–z plane for the θ=45° and 135° with tabs at x=0.25DH and 0.075DH respectively showed a vortex pair formed behind each tab. These streamwise vortices spread the jet fluids “outward” along the diagonal direction of the jet cross-section, resulting in the four-finger structure. At the two corners of each finger, a pair of streamwise vortices, which were the legs of weaker horseshoe vortices, were also found. For θ=135°, a third pair of streamwise vortices proposed by Reeder and Samimy [J. Fluid Mech. 311 (1996) 73] was observed at the tip of each tab, which explained the existence of the unique mushroom structure. LDA measurements of mean velocity contours on the two tabbed jets confirmed the flow visualization results that the jet core had bifurcated into a four-finger structure. Furthermore, with tabs at x=0.75DH and 1.25DH, the downstream tilted tab jet produced a larger cross-sectional area of the jet core than the jet with θ=135° and the jet without tab. Downstream tilted tabs were thus more efficient in mixing enhancement than upstream tilted tabs.

Measurements of a confined jet

In this paper we study the flow characteristics of an air jet issuing from a rectangular nozzle of aspect ratio 6.0 into a confined chamber. Hot-wire measurements of mean and root mean square velocities were made in the mixing layers and interaction region of the jet, at two Reynolds numbers. The results showed that the mean velocity profiles achieved approximate self-preservation almost immediately after the end of potential core. However, the self-similarity of root mean square velocity profiles was established only after a distance of 12 nozzle widths downstream of the nozzle exit. The decay rate of the confined jet was found to be almost three times less than that of square and circular free jets but was greater than that of plane free jets by 10–30% with laminar initial conditions and similar Reynolds number (<8000). The spreading rate of the confined jet was found to be quite similar to that of the plane free jets but higher than the circular and square free jets. It is deduced from the results obta...