Showing papers by "Young I. Cho published in 1993"

The Rheology and Hydrodynamic Analysis of Grease Flows in a Circular Pipe

Abstract: The shear-rate dependent viscosities of two NLGI grade No. 2 greases, Grease A and Grease B, were measured using a capillary tube viscometer within a temperature range of 5°−35°C. The yield stresses of these greases were directly measured without extrapolation process as a function of temperature. Two types of the yield stresses were found to exist: one was a start-up yield stress below which grease does not start flowing, and the other was a stopping yield stress at which the grease stopped flowing. The former is substantially larger than the latter and should play an important role in the design of an automatic grease pumping system, a system normally operated in an intermittent mode rather than in a continuous mode. In general, the yield stress of Grease A increased with increasing temperatures whereas that of Grease B decreased with increasing temperatures. Continuous pumping tests with Grease A were conducted in four different diameter pipes. A new friction coefficient-Reynolds number correlation was...

Numerical studies of three-dimensional arterial flows in reverse curvature geometry: Part I--Peak flow.

Abstract: A three-dimensional flow simulation at Repeak = 192 and 580 was made in a smooth reverse curvature model that conformed to the gentle "S" shape from a human left femoral artery angiogram. The objective of this numerical investigation was to find the changes in pressure, shear stress, velocity profile, and particle path occurring in the double-curved arterial vessel. Due to the impingement of blood at the outer wall in the first bend region, the wall shear stress approached 40 dyne/cm2--a value over twice as large as in the straight upstream segment. Conversely, at the inner wall in the first bend, a low shear stress region was found where the value of the shear stress was consistently smaller than that in the straight section. The initiation of centrifugal effects caused by the first bend could clearly be seen at Repeak = 580, but due to the close proximity of the reverse curvature segment, the momentum effect due to the second bend overshadowed the centrifugal effect. Hence, only near the end of the second bend did the centrifugal effect due to the second bend result in a double-spiral-secondary motion. In addition, the numerically calculated pressure drop data were in agreement with prior experimental values.

Two-dimensional pulsatile hemodynamic analysis in the magnetic resonance angiography interpretation of a stenosed carotid arterial bifurcation

Abstract: A two‐dimensional pulsatile hemodynamic analysis based on the finite‐element technique was performed on a minimally stenosed carotid artery to identify the possible explanation for the differences in the x‐ray and magnetic resonance carotid angiograms of a patient. The magnetic resonance angiogram was obtained by applying the maximum intensity projection algorithm to axial slices, acquired using the time‐of‐flight principle. The differences in the x‐ray and magnetic resonance depictions were interpreted based on velocity profile, wall shear stress, and streamline data provided by the hemodynamic analysis. The specific contribution of the stenosis was further isolated from that of the bifurcation by comparing the flow patterns within the stenotic artery with those of its normal counterpart. The Doppler spectral velocity wave form of the patient constituted the basis for the pulsatile flow velocity specification. The analysis took into consideration the non‐Newtonian viscosity of blood. The numerical procedure was validated through different convergence criteria and through shear stress comparisons. The importance of hemodynamic analyses in relation to magnetic resonance angiography was further discussed along with possible shortcomings of the technique.

Numerical studies of non·newtonian pulsed flow in a femoral branched vessel

Abstract: A numerical investigation was carried out to acquire an understanding of local flow phenomena in an arterial branch model similar to the femoral anery of man v.ith a branch angle of 90 degree. Flow separations were not only found along the main lumen wall opposite to the branch during the early decelerating phase of the sy" . I;C part of the pulse cycle, and also at the proximal wall of the , Due to the tri-phasic nature of the femoral pulse, milder j, • .l1ating flows were observed at the wall of the main lumen located near the branch. Recirculating zones were functions of both temporal and spatial parameters. The distal wall of the branch and the wall of the main lumen near the branch orifice showed elevated shear rate and shear stress values. During a cardiac cycle. enhanced fl!ctuation of shear rate and shear stress from positive to negative values were observed near the branch region. The changes in hydrodynamic flow parameters near branches may be associated v.ith site-specific lesion formation regions that have been observed.