• Journal of The Korean Astronomical Society
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• v.36 no.1
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• pp.33-41
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• 2003

Recent observations have shown that coronal magnetic fields in the northern (southern) hemisphere tend to have negative (positive) magnetic helicity. There has been controversy as to whether this hemispheric pattern is of surface or sub-surface origin. A number of studies have focused on clarifying the effect of the surface differential rotation on the change of magnetic helicity in the corona. Meanwhile, recent observational studies reported the existence of transient shear flows in active regions that can feed magnetic helicity to the corona at a much higher rate than the differential rotation does. Here we propose that such transient shear flows may be driven by the torque produced by either the axial or radial expansion of the coronal segment of a twisted flux tube that is rooted deeply below the surface. We have derived a simple relation between the coronal expansion parameter and the amount of helicity transferred via shear flows. To demonstrate our proposition, we have inspected Yohkoh soft X-ray images of NOAA 8668 in which strong shear flows were observed. As a result, we found that the expansion of magnetic fields really took place in the corona while transient shear flows were observed in the photosphere, and the amount of magnetic helicity change due to the transient shear flows is quantitatively consistent with the observed expansion of coronal magnetic fields. The transient shear flows hence may be understood as an observable manifestation of the pumping of magnetic helicity out of the interior portions of the field lines driven by the expansion of coronal parts as was originally proposed by Parker (1974).

• Wind and Structures
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• v.5 no.2_3_4
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• pp.141-150
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• 2002

Numerical simulations based on the ALE finite element method are carried out to examine the aerodynamics of an oscillating circular cylinder when the separated shear flows around the cylinder are stimulated by periodic jet excitation with a shear layer instability frequency. The excitation is applied to the flows from two points on the cylinder surface. The numerical results showed that the excitation with a shear layer instability frequency can reduce the negative damping and thereby stabilize the aerodynamics of the oscillating cylinder. The change of the lift phase seems important in stabilizing the cylinder aerodynamics. The change of lift phase is caused by the merger of the vortices induced by the periodic excitation with a shear layer instability frequency, and the vortex merging comes from the high growth rate, the rapid increase of wave number and decrease of phase velocity for the periodic excitation in the separated shear flows.

• Journal of applied mathematics & informatics
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• v.41 no.5
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• pp.1071-1084
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• 2023

We consider inviscid, incompressible homogeneous shear flows of variable cross section known as extended Rayleigh problem. For this extended Rayleigh problem, we derived instability region which intersect with semi-circle instability region under some condition. Also we derived condition for stability, upper bound for amplification factor and growth rate of an unstable mode.

• Journal of applied mathematics & informatics
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• v.42 no.1
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• pp.105-121
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• 2024

We consider incompressible, inviscid, stratified shear flows in β plane. First, we obtained an unbounded instability region intersect with semi-ellipse region. Second, we obtained a bounded instability regions depending on Coriolis, stratification parameters and basic velocity profile. Third, we obtained a criterion for wave stability. This has been illustrated with standard examples. Also, we obtained upper bound for growth rate.

• 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.

• Macromolecular Research
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• v.17 no.10
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• pp.807-812
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• 2009

In this work, we focus on the tumbling dynamics of rod-like and semi-flexible polymers in mixed flows, which vary from simple shear to pure rotation. By employing a bead-rod model, the tumbling pathways and periods are examined with a focus on the angular distribution of their orientation. Under the mixed flows, the tumbling dynamics agreed well with earlier studies and confirmed the predicted scaling laws. We found that the angular distribution deviates from that of shear flow as the flow type approaches pure rotation. Finally, we investigated the angular distribution of $\lambda$-DNA in a shear flow and found that the present numerical simulations were in quantitative agreement with the previous experimental data.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.2B
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• pp.139-144
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• 2006

Using DNS data for turbulent flows in an open-channel with sidewalls, the mechanisms by which secondary flows are generated and by which Reynolds shear stresses are created, are demonstrated. Near the sidewall, secondary flows invading towards the sidewall are observed in the regions of both lower and upper corners, while secondary flows ejecting from the sidewall towards the center of the channel are created elsewhere. The distributions of Reynolds shear stresses near the sidewall are analyzed, connecting their productions with coherent structures. A quadrant analysis shows that sweeps are dominant in two corner regions where secondary flows invading towards the sidewall are generated, but that ejections are dominant in the region where secondary flows ejecting towards the center of the channel are created. Also, conditional quadrant analyses reveal that the productions of Reynolds shear stresses and the patterns of secondary flows are determined by the directional tendencies of coherent structures.

• Journal of Biomedical Engineering Research
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• v.18 no.2
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• pp.179-186
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• 1997

Steady and physiological flows of a Newtonian fluid and blood in the abdominal gorta/iliac artery bifurcation are numerically simulated to understand the etiology and pathogenesis of atherosclerosis. Distributions of velocity, pressure, and wall shear stress in the bifurcated arterial vessel model are calculated to investigate the differences of flow characteristics between steady and physiological flows and to compare flow characteristics of blood with that of a Newtonian fluid For the given Reynolds number the flow characteristics of physiological flows for a Newtonian fluid and blood in the bifurcated arterial vessel are quite different from thcse of steady flows. No flow separation or flow reversal in the bifurcated region appears downstream of a stenosis during the acceleration phase. However, during the deceleration phase the flow exhibits flow separation in the outer walls of daugtlter branches, which extends to the entire wall region.

• Journal of the Society of Naval Architects of Korea
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• v.53 no.3
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• pp.188-194
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• 2016

Accurate estimation of shear flows in thin-walled beam section is the key issue to evaluate shear stress distribution of ship hull transverse section under the shear forces acting on hull girder. It is regarded that the method using the warping functions obtained by finite element formulation is the state of the art of this field. Recently, however, IACS took effect the new version of CSR in which direct calculation process of shear flow was suggested. In the direct calculation process, shear flow of ship hull section can be obtained by the addition of determinate and indeterminate shear flows calculated respectively. So, in this paper, the shear flow evaluation codes based on the process proposed by IACS CSR and warping function based method were developed respectively. The calculated results of shear flows for the several examples of ship sections were compared with each other and considered in detail.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1607-1612
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• 2004

Numerical simulations are performed to investigate the characteristics of flow past a sphere in uniform shear. The Reynolds numbers considered are Re=300, 425 and 480 based on the inlet center velocity and sphere diameter. The non-dimensional shear rate K of the inlet uniform shear is varied from 0 to 0.15. At Re=300, the head of the hairpin vortex loop always locates on the high-velocity side in uniform shear, and the flow maintains the planar symmetry. At Re=425 and 480, the irregularity in the location and strength of the hairpin vortex appearing in uniform inlet flow is much reduced in uniform shear, but the flows still keep the asymmetry for most inlet shear rates. However, in the cases of K=0.075 and 0.1 at Re=425, the flows become planar symmetric and their characteristics of the evolution of the hairpin vortex loops are different from those of asymmetric flows. A hysteresis phenomenon switching from the planar symmetry to the asymmetry (or vice versa) depending on the initial condition is also observed at Re=425.