• 한국초전도학회:학술대회논문집
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• v.14
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• pp.28-28
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• 2004

• Ocean Systems Engineering
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• v.6 no.4
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• pp.325-344
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• 2016

Using 3D computational fluid dynamics techniques in recent years have shed significant light on the Vortex Induced Vibrations (VIV) encountered by deep-water marine risers. The fatigue damage accumulated due to these vibrations has posed a great concern to the offshore industry. This paper aims to present an algorithm to predict the crossflow and inline fatigue damage for very long (L/D > $10^3$) marine risers using a Finite-Analytical Navier-Stokes (FANS) technique coupled with a tensioned beam motion solver and rainflow counting fatigue module. Large Eddy Simulation (LES) method has been used to simulate the turbulence in the flow. An overset grid system is employed to mesh the riser geometry and the wake field around the riser. Risers from NDP (2003) and Miami (2006) experiments are used for simulation with uniform, linearly sheared and non-uniform (non-linearly sheared) current profiles. The simulation results including inline and crossflow motion, modal decomposition, spectral densities and fatigue damage rate are compared to the experimental data and useful conclusions are drawn.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.5
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• pp.924-932
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• 1992

This paper presents the results of an experimental study on the characteristics of the vortex- induced vibration of an elastically supported circular cylinder in the cross air flow. For a range of velocities, power spectral densities of the signals from a hot-wire anemometer placed in the wake of an oscillating circular cylinder and gap sensors placed in the both ends of a circular cylinder were obtained to determine vortex-shedding frequencies, natural frequencies and vibrating frequencies of a cylinder. The effects of slots in the test section on vortex shedding and cylinder oscillation were investigated. The present study covered the reduced velocity range 1.0 .leg. Ur .leg. 64.6. The response characteristics of the cylinder has been shown to vary extensively, depending on the slots in the test section as well as on the reduced velocity. For an elastically supported cylinder, a purely translation mode oscillation was observed at a low velocity, however a rotation mode oscillation was often superposed for higher velocities. These two oscillating frequencies were equal to their natural frequencies irrespective of the changes of free stream velocities.

• Journal of Magnetics
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• v.15 no.3
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• pp.103-107
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• 2010

We investigated spin-polarized current switching of Pac-man type II (PM-II) nanoelements in Pac-man shaped nanoscale spin-valves (Co/Cu/Py) using micromagnetic simulations. The effects of slot angle and antiferromagnetic (AFM) layer were simulated to obtain optimum switching in less than 2 ns. At a critical slot angle of $105^{\circ}$, the lowest current density for anti-parallel to parallel (AP-P) switching was observed due to no vortex or antivortex formation during the magnetic reversal process. All other slot angles for AP-P formed a vortex or antivortex during the magnetization reversal process. Additionally, a vortex or anti-vortex formed for all slot angles for parallel to anti-parallel (P-AP) switching. The addition of an AFM layer caused the current density to decrease significantly for AP-P and P-AP at slot angles less than $90^{\circ}$. However, at slot angles greater than $90^{\circ}$, the current density tended to decrease by less amounts or actually increased slightly as shape anisotropy became more dominant. This allowed ultra-fast switching with 5.05 and $5.65{\times}10^8\;A/cm^2$ current densities for AP-P and P-AP, respectively, at a slot angle of $105^{\circ}$.

• Journal of Advanced Marine Engineering and Technology
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• v.40 no.2
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• pp.102-106
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• 2016

An experimental study on the flow characteristics under various laminar coflow diffusion flames was conducted with a particular focus on the buoyancy force exerted from gaseous hydrocarbon fuels. Methane ($CH_4$), ethylene ($C_2H_4$), and n-butane ($C_4H_{10}$) were used as the fuels. A coflow burner and the Schlieren imaging technique were used to observe the flow field of each fuel near the nozzle exit as well as the flow characteristics in the flames. The results show that a vortex with a density heavier than air appeared in n-butane near the nozzle exit with a strong negative buoyancy on the fuel steam. As the Reynolds number increased through the control of the fuel velocity of the n-butane flame, the vortices were greater and the vortex tips were moved up from the nozzle exit. In addition, the heated nozzle affected the flow fields of the fuel steam near the nozzle exit.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2005.11a
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• pp.445-449
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• 2005

The mixed acoustic-convective mode combustion instability and the possibility of combustion control using a loudspeaker to these instabilities were studied. By changing inlet velocity, combustor length and equivalence ratio, the dynamic pressure signals and the flame structures were simultaneously taken. The results showed that as the combustor length increased and the inlet velocity decreased, the instability frequency decreased and the maximum power spectral densities of the dynamic pressures generally decreased. The instability frequency could be affected by an equivalence ratio over the operating conditions. From the data of close-loop control, as the loudspeaker may work out-of-phase with the natural instability, the optimum time-delay controller was confirmed to be able to reduce the vortex shedding from the mixed acoustic-convective mode combustion instability.

• Progress in Superconductivity
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• v.3 no.2
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• pp.135-139
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• 2002

We have fabricated high-quality c-axis-oriented $MgB_2$ thin films by using a pulsed laser deposition technique. The thin films grown on (1 1 0 2) $Al_2$$O_3$ substrates show an onset transition temperature of 39.2 K with a sharp transition width of ~0.15 K. X-ray diffraction patterns indicate a c-axis-oriented crystal structure perpendicular to the substrate surface. We observed high critical current densities ($J_{c}$) of ~ 16 $MA/\textrm{cm}^2$ at 15 K and under self-field, which is comparable to or exceeds those of cuprate high-temperature superconductors. The extrapolation $J_{c}$ at 5 K was estimated to be ~ 40 MA/$\textrm{cm}^2$, which is the highest record for $MgB_2$ compounds. At a magnetic field of 5 T, the $J_{c}$ of~ 0.1 $MA/\textrm{cm}^2$ was detected at 15 K, suggesting that this compound is very promising candidate for the practical applications at high temperature with lower power consumption. As a possible explanation for the high current-carving capability, the vortex-glass phase will be discussed.d.d.d.