• Journal of Mechanical Science and Technology
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• v.14 no.8
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• pp.861-866
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• 2000

Flow induced shape change is important for spatial interpretation of vascular response and for understanding of mechanotransduction in a single cell. We investigated the possible shapes of endothelial cell (EC) in a mathematical model and compared these with experimental results. The linearized analytic solution from the sinusoidal wavy wall and Stokes flow was applied with the constraint of EC volume. The three dimensional structure of the human umbilical vein endothelial cell was visualized in static culture or after various durations of shear stress (20 $dyne/cm^2$ for 5, 10, 20, 40, 60, 120min). The shape ratio (width: length: height) of model agreed with that of the experimental result, which represented the drag force minimizing shape of stream-lining. EC would be streamlined in order to accommodate to the shear flow environmented by active reconstruction of cytoskeletons and membranes through a drag force the sensing mechanism.

• Journal of Mechanical Science and Technology
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• v.17 no.10
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• pp.1543-1551
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• 2003

The present study investigates turbulent flows subject to strong wall injection in a channel through a Direct Numerical Simulation technique. These flows are pertinent to internal flows inside the hybrid rocket motors. A simplified model problem where a regression process at the wall is idealized by the wall blowing has been studied to gain a better understanding of how the near-wall turbulent structures are modified. As the strength of wall blowing increases, the turbulence intensities and Reynolds shear stress increase rapidly and this is thought to result from the shear instability induced by the injected flows at the wall. Also, turbulent viscosity grows rapidly as the flow moves downstream. Thus, the effect of wall-blowing modifies the state of turbulence significantly and more sophisticated turbulence modeling would be required to predict this type of flows accurately.

• Journal of computational fluids engineering
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• v.21 no.4
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• pp.96-101
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• 2016

A two-dimensional Stokes flow past a circular disk in a circular tube is analyzed. The circular disk is located coaxially with the circular tube and the Hagen-Poiseuille flow exists at upstream and downstream far from the circular disk. The Stokes approximation is used and the flow is investigated analytically by using the method of eigenfunction expansion and the method of least square. From the analysis, the stream function and the pressure of the flow field are obtained, and the streamlines and pressure distribution are shown. Also, the pressure and shear stress distributions on the circular disk and circular tube wall are calculated, and shown for some typical radii of the circular disk. The additional pressure drop induced by the disk and the drag force exerted on the disk are compared as functions of the radius of the circular disk, and it is shown that the shear force on the wall of the tube increases due to the disk.

• Proceedings of the KSR Conference
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• 2004.10a
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• pp.1101-1106
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• 2004

Rainfall induced landslides are disasters causing sever damage on the human life and the infrastructures. In this paper, a simplified procedure to evaluate the slope stability problems induced by rainfall by modifying the Iverson's pressure head dispersion model. The proposed approach extends the applicability of the Iverson's model in to the cases of higher rainfall intensity than the permeability of the soil by incorporating the existence of overland flow. In addition, the Manning equation is applied to calculated the depth of overland flow. From the calculated depth of overland flow, shear stress acting on the surface is included for the driving component triggering the landslides. From the analysis of a case study, the long term rainfall alters the stability of slope.

• Korea-Australia Rheology Journal
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• v.19 no.2
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• pp.61-66
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• 2007

The aggregation characteristics of red blood cells (RBCs) are known as important factors in the microvascular flow system, and increased RBC aggregation has been observed in various pathological diseases, such as thrombosis and myocardial infarction. This paper describes a simple microfluidic device for measuring the RBC aggregation by integrating a microfluidic slit rheometry and laser-backscattering technique. While a decreasing-pressure mechanism was applied to the microfluidic rheometry, a syllectogram (the light intensity versus time) showed an initial increase and a peak caused by the high shear stress-induced disaggregation, immediately followed by a decrease in the light intensity due to RBC aggregation. The critical shear stress (CST) corresponding to the peak intensity was examined as a new index of the RBC aggregation characteristics. The CST of RBCs increased with increasing aggregation-dominating protein (fibrinogen) in the blood plasma. The essential feature of this design was the combination of the rheometric-optic characterization of RBC aggregation with a microfluidic chip, which may potentially allow cell aggregation measurements to be easily carried out in a clinical setting.

• Korea-Australia Rheology Journal
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• v.16 no.3
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• pp.141-146
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• 2004

Shear-induced damage of Red Blood Cell (RBC) is an imminent problem to be solved for the practical application of artificial organs in extra corporeal circulation, as it often happens and affects physiological homeostasis of a patient. To design and operate artificial organs in a safe mode, many investigations have been set up to correlate shear and shear-induced cell damage. Most studies were focused on hemolysis i.e. the extreme case, however, it is important as well to obtain a clear understanding of pre-hemolytic mechanical damage. In this study, the change in deformability of RBC was measured by ektacytometry to investigate the damage of RBC caused by shear. To a small magnitude of pre-shear, there is little difference, but to a large magnitude of pre-shear, cell damage occurs and the effect of shear becomes significant depending on both the magnitude and imposed time of shearing. The threshold stress for cell damage was found to be approximately 30 Pa, which is much less than the threshold of mechanical hemolysis but is large enough to occur in vitro as in the extra corporeal circulation during open-heart surgery or artificial heart. In conclusion, it was found and suggested that the decrease of deformability can be used as an early indication of cell damage, in contrast to measuring plasma hemoglobin. As cell damage always occurs during flow in artificial organs, the results as well as the approach adopted here will be helpful in the design and operation of artificial organs.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.1C
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• pp.53-62
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• 2008

A constitutive model, which can simulate the effect of principal stress rotation associated with direct simple shear test, is proposed in this study. The model is based on two mobilized planes. The plastic strains occur from the two mobilized planes, and depend on stress state, and they are added. The first plane is a plane of maximum shear stress, which rotates about the horizontal axis, and the second plane is a horizontal plane which is spatially fixed. The second plane is used to consider the effect of principal stress rotation on simple shear tests under different stress states. The soil skeleton behavior observed in drained simple shear tests is captured in the model. This constitutive model is incorporated into the dynamic coupled stress-flow finite difference program FLAC. The model is first calibrated with drained simple shear tests on loose Fraser River sand. The measured shear stress and volume change are partially induced by principal stress rotation and compared with model calculations. The model is verified by comparing predicted and measured settlements due to rigid footing resting on loose sands. Settlements predicted by the proposed model were very similar to measured settlements. Mohr-Coulomb model can not consider the effect of principal stress rotation and its prediction was only 20% of measured settlements.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.9
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• pp.1073-1084
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• 1999

The effects of secondary flow on the structure of a turbulent wake generated by a flat plate was investigated experimentally. The secondary flow was induced In a $90^{\circ}$ curved duct in which the flat plate wake generator was installed. The wake generator was installed in such a way that the wake velocity gradient exists in the span wise direction of the curved duct. Measurements were made in the plane containing the mean radius of curvature where pressure gradient and curvature effects were small compared with the secondary flow effect. All six components of the Reynolds stresses were measured in the curved duct. Turbulence intensities in the curved wake are higher than those in the straight wake due to an increase of the turbulent kinetic energy production by the secondary flow. In the inner wake region, shear stress and strain in the plane containing the velocity gradient of the wake show opposite signs with respect to each other, so that eddy viscosity Is negative in this region. This indicates that gradient-diffusion type turbulence models are not appropriate to simulate this type of flow.

• Wind and Structures
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• v.19 no.5
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• pp.523-540
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• 2014

This paper presents a comprehensive study of pressure developed on different faces of a 'Y' plan shape tall building using both numerical and experimental means. The experiment has been conducted in boundary layer wind tunnel located at Indian Institute of Technology Roorkee, India for flow condition corresponding to terrain category II of IS:875 (Part 3) - 1987, at a mean wind velocity of 10 m/s. Numerical study has been carried out under similar condition using computational fluid dynamics (CFD) package of ANSYS, namely ANSYS CFX. Two turbulence models, viz., $k-{\varepsilon}$ and Shear Stress Transport (SST) have been used. Good conformity among the numerical and experimental results have been observed with SST model yielding results of higher magnitude. Peculiar pressure distribution on certain faces has been observed due to interference effect. Furthermore, flow pattern around the model has also been studied to explain the phenomenon occurring around the model.

• Korea-Australia Rheology Journal
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• v.17 no.4
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• pp.157-164
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• 2005

Hydrodynamic characteristic of surfactant drag-reducing flows is still not fully understood. This work investigated the temperature and diameter effect on hydrodynamic characteristic of cationic surfactant drag reducing flows in pipes. Solution of oleyl bishydroxyethyl methyl ammonium chloride (Ethoquad O/12), 900 ppm, as a cationic surfactant and sodium salicylate (NaSal), 540 ppm, as a counter-ion was tested at 12, 25, 40, and $50^{\circ}C$ in pipes with diameter of 13, 25, and 40 mm. Drag reduction effectiveness of this surfactant solution was evaluated in 25 mm pipe from 6 to $75^{\circ}C$. Rheological characteristic of this solution was measured by stress control type rheometer with cone-and-plate geometry. Scale-up laws proposed by previous investigators were used to evaluate the flow characteristic of the solution. It was found that this surfactant solution has clear DR capability until $70^{\circ}C$. Result of this work suggested that temperature has a significant influence in changing the hydrodynamic entrance length of surfactant drag reducing flows. From rheological measurement, it was found that the solution exhibits Shear Induced Structure at all temperatures with different degree of peak viscosity and critical shear rate.