• Journal of computational fluids engineering
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• v.9 no.4
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• pp.34-40
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• 2004

It is known that the non-planar model of bypass is more profitable to suppress the development of intimal hyperplasia that tends to occur preferentially in regions of low time averaged shear stress and rapid temporal changes in wall shear stress. In this study it was numerically simulated the blood flow in an coronary artery grafted by artificial bypass to determine the flow characteristic variations due to the anastomosis angle changing. 5 different non-planar anastomosis angle models such as 45°, 60°, 90°, 120° and 135° were considered. When the anastomosis angle is higher, the backward flow region is spatially extended near the downstream region of the anastomosis because of the development of horseshoes vortex. For the case of the nan-planar 45° and 60° of anastomosis, the area of low-OSI zone was decreased by 26% and 13% respectively and the time averaged wall shear stress was increased by more than 55% as compared with 45° of planar model. However, both of the area of the low-OSI zone and the time averaged wall shear stress of 90°, 120° model were significantly increased.

• Journal of Advanced Marine Engineering and Technology
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• v.20 no.5
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• pp.58-67
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• 1996

In the present study, the pressure distribution, wall shear stress distribution and friction factor of developing turbulent pulsating flows are investigated theoretically and experimentally in the entrance region of a square duct. The pressure distribution for turbulent pulsating flows are in good agreement with the theoretical values. The time-averaged pressure gradients of the turbulent pulsating flows show the same tendency as those of turbulent steady flows as the time-averged Reynolds number $(Re_{ta})$ increase. Mean shear stresses in the turbulent pulsating flow increase more in the inlet flow region than in the fully developed flow region and approach to almost constant value in the fully developed flow region. In the turbulent pulsating flow, the friction factor of the quasi-steady state flow $({\lambda}_{q, tu})$ follow friction factor's law in turbulent steady flow. The entrance length of the turbulent pulsating flow is not influenced by the time-averaged Reynolds number $(Re_{ta})$ and it is about 40 times as large as the hydraulic diameter.

• Journal of Biomedical Engineering Research
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• v.21 no.3 s.61
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• pp.261-271
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• 2000

The present study investigated flow dynamics of a two-dimensional abdominal aortic bifurcation model under sinusoidal flow conditions considering wall motion. impedance phase angle(time delay between pressure and flow waveforms), and non-Newtonian fluid using computational fluid dynamics. The wall shear stress showed large variations in the bifurcated region and the wall motion reduced amplitude of wall shear stress significantly. As the impedance phase angle was changed to more negative values, the mean wall shear stress (time-averaged) decreased while the amplitude (oscillatory) of wall shear stress increased. At the curvature site on the outer wall where the mean wall shear stress approached zero. influence of the phase angle was relatively large. The mean wall shear stress decreased by $50\%$ in the $-90^{\circ}$ phase angle (flow wave advanced pressure wave by a quarter period) compared to the $0^{\circ}$ phase angle while the amplitude of wall shear stress increased by $15\%$. Therefore, hypertensive patients who tend to have large negative phase angles become more vulnerable to atherosclerosis according to the low and oscillatory shear stress theory because of the reduced mean and the increased oscillatory wall shear stresses. Non-Newtonian characteristics of fluid substantially increased the mean wall shear stress resulting in a less vulnerable state to atherosclerosis.

• Journal of Biomedical Engineering Research
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• v.21 no.4
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• pp.363-372
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• 2000

The present study investigated flow dynamics of a straight elastic blood vessel under sinusoidal flow conditions in order to understand influence of wall motion and impedance phase angle(time delay between pressure and flow waveforms) on wall shear stress distribution using computational fluid dynamics. For the straight elastic tube model considered in the our method of computation. The results showed that wall motion induced additional terms in the axial velocity profile and the pressure gradient. These additional terms due to wall motion reduced the amplitude of wall shear stress and also changed the mean wall shear stress. Te trend of the changes was very different depending on the impedance phase angle. As the wall shear stress increased. As the phase angle was reduced from 0$^{\circ}$to -90$^{\circ}$for ${\pm}$4% wall motion case, the mean wall shear stress decreased by 10.5% and the amplitude of wasll shear stress increased by 17.5%. Therefore, for hypertensive patients vulnerable state to atherosclerosis according to low and oscillatory shear stress theory.

• International Journal of Vascular Biomedical Engineering
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• v.1 no.1
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• pp.24-31
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• 2003

As endothelial cells are subject to flow shear stress, it is important to determine the detailed velocity distribution in microvessels in the study of mechanical interactions between blood and endothelium. This paper describes a velocity field of the arteriole in the rat mesentery using an intravital microscope and high-speed digital video system obtained by a highly accurate PIV technique. Red blood cells (RBCs) velocity distributions with spatial resolutions of $0.8{\times}0.8{\mu}m$ were obtained even near the wall in the center plane of the arteriole. By making ensemble-averaged time-series of velocity distributions, velocity profiles over different cross-sections were calculated for comparison. The shear rate at the vascular wall also evaluated on the basis of the ensemble-averaged profiles. It was shown that the velocity profiles were blunt in the center region of the vessel cross-section while they were steep in the near wall region. The wall shear rates were significantly small, compared with those estimated from the Poiseuille profiles.

• Journal of Biomedical Engineering Research
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• v.23 no.4
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• pp.281-286
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• 2002

In this study, blood flow in a carotid artery supplying blood to the human's brain has been numerically simulated to find out how the blood flow affects the genesis and the growth of atherosclerosis and arterial thrombosis. Velocity Profiles and hemodynamic parameters have been investigated for the carotid arteries with three different stenoses under physiological flow condition. Blood has been treated as Newtonian and non-Newtonian fluid. To model the shear thinning properties of blood for non-Newtonian fluid, the Carreau-Yasuda model has been employed. The result shows that the wall shear stress(WSS) increases with the development of stenosis and that the wall shear stress in Newtonian fluid is highly evaluated compared with that in non-Newtonian Fluid. Oscillatory shear index has been employed to identify the time-averaged reattachment point and this point is located farther from the stenosis for Newtonian fluid than for non-Newtonian fluid The wall shear stress gradient(WSSG) along the wall has been estimated to be very high around the stenosis region when stenosis is developed much and the WSSG peak value of Newtonian fluid is higher than that of non-Newtonian fluid.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.10
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• pp.1340-1346
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• 1999

The turbulent shear flow around a surface-mounted vertical fence was investigated using the two-frame PTV system. The Reynolds number based on the fence height(H) was 2950. From this study, it is revealed that at least 400 instantaneous velocity field data are required for ensemble average to get reliable turbulence statistics, but only 100 field data are sufficient for the time-averaged mean velocity information. Various turbulence statistics such as turbulent intensities, turbulence kinetic energy and Reynolds shear stress were calculated from 700 instantaneous velocity vector fields. The fence flow has an unsteady recirculation region behind the fence, followed by a slow relaxation to the flat-plate boundary layer flow. The time-averaged reattachment length estimated from the streamline distribution is about 11.2H. There exists a region of negative Reynolds shear stress near the fence top due to the highly convex (stabilizing) streamline-curvature of the upstream flow. The large eddy structure in the separated shear layer seems to have significant influence on the development of the separated shear layer and the reattachment process.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.2
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• pp.139-152
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• 1998

In the present study, the flow characteristics of developing laminar oscillatory flows in a square -sectional 180 deg. curved duct are investigated experimentally. The experimental study using air in a square-sectional 180 deg. curved duct is carried out to measure velocity distributions with a data acquisition and LDV (Laser Doppler Velocimetry) processing system. In this system, Rotating Machinery Resolver (RMR) and PHASE program are used to obtain the results of unsteady flows. The major flow characteristics of developing oscillatory flows are found by analyzing velocity curves, mean velocity profiles, time-averaged velocity distribution of secondary flow, wall shear stress distributions, and entrance lengths. In a lower dimensionless angular frequency, the axial velocity distribution of laminar oscillatory flow in a curved duct shows a convex shape in a central part and axial symmetry. The maximum value of wall shear stress in a lower dimensionless angular frequency is located in an outside wall, but according to increasing the dimensionless angular frequency, the maximum of wall shear stress is moved to inner wall. The entrance lengths of laminar oscillatory flows in a square-sectional 180 deg. curved duct is obtained to 90 deg. of bended angle of duct in this experimental conditions.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.811-814
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• 2002

Using a mathematical theory, we show that the optimality condition of a turbulent diffuser with maximum pressure recovery at the exit is zero shear stress along the wall. The optimal diffuser shape is designed through iterative procedures by using the $k-{\varepsilon}-{\nu}^{2}-f$ turbulence model for flow simulation. The Reynolds number based on the bulk mean velocity and the channel height at the diffuser entrance is 18,000. We also perform large eddy simulation to validate the shape design results and investigate the flow characteristics near the zero-skin friction wall. Results from large eddy simulation show that the skin friction is slightly higher than zero but is still very small as compared to that of the flat plate boundary layer flow Although the time-averaged wall shear stress is slightly above zero along the diffuser wall, instantaneous flow reversals occur intermittently. The streamwise mein velocity shows an asymptotic behavior of the half-power-law near the wall where the skin friction is close to zero.

• Journal of Biomedical Engineering Research
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• v.17 no.4
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• pp.515-524
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• 1996

The flow characteristics of developing unsteady laminar flow in a square-sectional $180^{\circ}$ curved duct are experimentally investigated by using laser doppler velocimerty (LDV) system with data acquisition and processing system of rotating machinery resolver(RMR) and PHASE software. The major flow characteristics of developing laminar pulsating flows are presented by mean velocity profilel velocity distribution of secondary flow, wall shear stress distributions, entrance lengths according to dimensionless angular frequency($\omega^+$), velocity amplitude ratio($A^1$), and time-averaged Dean number($De_ta$). The velocity profiles and wall shear stress distribution of laminar pulsating flow with dimensionlessangular frequency show the flow characteristics of the quasi-steady laminar flow in a curved duct. The developing region of laminar pulsatile flows in a square-sectional $180^{\circ}$ curved duct is extended to the curved duct angle of approximately $120^{\circ}$ under the present experimental condition.