Tadashi Sawada

Numerical Study on the Flow of a Non-Newtonian Fluid Through an Axisymmetric Stenosis

The laminar steady flow of non-Newtonian fluid (biviscosity fluid) through an axisymmetric stenosis is calculated using the finite element methods. The flow pattern, the separation and reattachment points, and the distributions of pressure and shear stress at the wall are obtained. Then, the axial force acting on the stenosis is evaluated. It is suggested by the authors that this force can become one of the causes of post-stenotic dilatation. Calculated results show that the non-Newtonian property of blood weakens the distortion of flow pattern, pressure and shear stress at the wall associated with the stenosis and that the non-Newtonian property of blood decreases the axial force acting on the stenosis.

Numerical Study on the Unsteady Flow of Non-Newtonian Fluid

First, the unsteady flows of biviscosity fluids in a straight pipe without a stenosis are analyzed numerically. Second, the unsteady flows of biviscosity fluids through an axisymmetric stenosis are analyzed numerically, and the separation phenomena and the distributions of wall shear stress are investigated. On the basis of these results, the role of the non-Newtonian property of blood in the development of vascular lesions in investigated qualitatively

Numerical study on the laminar pulsatile flow of slurries

Abstract The laminar pulsatile flow of slurries is studied numerically. A slurry is usually described as a Bingham plastic fluid, but since the apparent viscosity coefficient of a Bingham plastic fluid diverges if the velocity gradient becomes zero, a modified Bingham model (bi-viscosity model) is used in this calculation. As one of the characteristics of the pulsatile flow of non-Newtonian fluids is the flow enhancement (the increase in the mean flow rate due to pulsation), the calculated flow enhancement rates are compared with the experimental results of Kajiuchi and Saito and good agreements are obtained. Finally, the extra power required to pulsate the flow and the characteristics of the calculated velocity distribution and the shear stress at the wall are investigated.

Effects of surface roughness on gas flow conductance in channels estimated by conical roughness model

Passages with rough walls allow for smaller conductance for rarefied gas flow than passages with smooth walls. For the purpose of clarifying the effects of surface roughness on rarefied gas flow through channels, surface roughness is modeled by circular cones with the same base radius. The cones are located in a statistical manner consistent with the measured angle distributions of actual surface roughness. Molecules are assumed to scatter on the cone surfaces according to the cosine law. The calculated results predict well the measured reduction of free-molecule and near-free molecule flow conductance through the passages composed of two flat plates.

Spiral grooved visco-vacuum pumps with various groove shapes

Abstract A method of analyzing the pumping performance of spiral grooved visco-vacuum pumps with arbitrary groove cross-sections is presented. Numerical calculations are performed on triangular and semicircular, as well as on rectangular, grooves. The flow in the groove region is connected with the flow in the ridge region by means of Boon and Tals ‘Narrow groove theory’ in order to obtain the pumping performance. The zero-flow performances of the three groove shapes are almost the same if optimum helix angles are chosen. The results suggest that the radial clearance can be enlarged by factors of 20–100, compared with that of a conventional Holweck-type pump.

Experimental verification of theory for the pumping mechanism of a dry-scroll vacuum pump

Abstract The ultimate pressure predicted by the theory that the authors have proposed agrees closely with the values from an actual machine at the rated speed, but shows smaller values than the actual machine for lower orbiting speeds as compared to the rated speed. In order to clarify this contradiction and examine the validity of the theory, the following three experiments were performed: (1) separation of the direct effect of orbiting speed on the ultimate pressure from the effect of clearance change with temperature change accompanying a change in orbiting speed, (2) measurement of the pressure inside the pump and (3) measurement of the pumping speed throughout the working pressure range. The validity of the theory was confirmed by a comparison of the theoretical predictions with the measurements.

Improvement in the performance of spiral grooved visco-vacuum pumps in the coarse vacuum region

This study is concerned with a kind of oil-free mechanical vacuum pump with spiral grooved rotors. For improving the performance of the pumps at pressures above 1000 Pa, sleeves which have tringular or rectangular grooves were tested instead of the ordinarily used smooth ones. The pressure difference between the suction and discharge sides was measured for various pairs of rotors and sleeves. The measured values were normalized and plotted against the Reynolds number defined as Re = 4ϱUm/μ (U: peripheral speed, m: hydraulic mean groove depth, ϱ: density, μ: viscosity). Grooves on the sleeves serve as the paths for a leakage in the laminar regime (Re < 1000), but they activate the generation and conveyance of vortices when the flow becomes turbulent, and thus the performance is improved by a factor of 1.5–3, compared with the ordinary pumps with smooth sleeves.

Blade force measurement and flow visualization of Savonius rotors.

For the purpose of clarifying the mechanism of rotation of Savonius rotors, ones which have two semicylindrical blades are studied experimentally. The force acting on a blade is measured in a water two tank for both cases where a rotor is at rest and rotated. The flow around a rotor is observed by using aluminum powder floating on the water surface. Although the Savonius rotor is classified into a resistance type, the lift produces the torque in a pretty wide range of angle relative to the flow.

Model study on the strain and stress distributions in the vicinity of an arterial stenosis

By the evaluation of the strain and stress distributions in the vicinity of a stenosis, it is suggested that the bending moment generated by the axial force acting on a stenosis is one of the causes of the post-stenotic dilatation. The conditions which enhance this bending moment are investigated and it is expected that the present mechanism is specially effective for the artery where the ratio of wall thickness to radius is very small. Lastly, the concrete numerical value of this bending moment is evaluated and it is shown that the bending moment generated by this mechanism is large enough to cause the post-stenotic dilatation.

Pumping Performance Analysis of Turbine Blade with a Narrow Inter-stage Clearance

The performance of turbine blades under a medium vacuum was investigated using the Stokes equation in a previous paper, where we could predict the performance of the turbo molecular pump in the pressure range of under 10 Pa. For further improvement of the pumping performance of turbine blades under a medium vacuum, experiments were carried out with a pump which has a narrow clearance between the rotor and the stator blade compared with the conventional pump. As a result, we found that the narrow clearance pump had a high performance under a medium vacuum compared with the conventional pump. Therefore, we analyzed the flow field in the turbo molecular pump by the two dimensional Finite Element Method (FEM). In order to examine the influence of the interstage clearance, the model was changed from the single stage of turbine blade to the combination of stator blade rotor blade stator blade. The comparison between the measurements and the predicted values shows a good agreement under 10 Pa.