• Journal of the Korean Society of Visualization
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• v.6 no.2
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• pp.14-19
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• 2008

In this paper, the recirculation flow motion and mixing characteristics driven by air bubble flow in a rectangular water tank is studied. The Time-resolved PIV technique is adopted for the quantitative visualization and analysis. 532 nm Diode CW laser is used for illumination and orange fluorescent particle images are acquired by a PCO 10bit high-speed camera. To obtain clean particle images, 545 nm long pass optical filter and an image intensifier are employed and the flow rates of compressed air is changed from 2 l/min to 4 l/min at 0.5 MPa. The recirculation and mixing flow field is further investigated by the POD analysis technique. It is observed that the large scale counterclockwise rotation and main vortex is generated in the upper half depth from the free surface and one quarter width from the sidewall. When the flow rates are increased, the main vortex core is moved to the side and bottom wall direction.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.172-172
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• 2010

$MgB_2$ doped with $TiO_2$ was prepared by the in-situ solid state reaction to study the effects of $TiO_2$ dopant on the flux pinning behavior of $MgB_2$ superconductor. From the field-cooled and the zero-field-cooled temperature dependences of magnetization, the realms of vortex-glass and vortex-liquid states of $TiO_2$-doped $MgB_2$ were determined in the H-T diagram (the temperature dependence of upper critical magnetic field and irreversibility line). The critical current density was estimated from the width of hysteresis loops in the framework of Beam's model at different temperatures. The results indicate that nano-scale $TiO_2$ inclusions play a role of the effective pinning centers and lead to the enhanced upper critical field and critical current density. It is suggested that the grain-boundary pinning mechanism is realized in $TiO_2$-doped $MgB_2$ superconductor.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.8
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• pp.790-797
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• 2010

Mode transition of the axi-symmetric screech tone in the low supersonic Mach number range from 1.0 to 1.20 is numerically analyzed. The axi-symmetric Navier-Stokes equations and the k-e turbulence model are solved in the cylindrical coordinate system. The dispersion-relation-preserving(DRP) scheme is applied for space discretization and the optimized four levels marching method are used for time integration. At low supersonic Mach numbers with an axi-symmetric A1 mode in the simulation, it is shown that acoustic propagation due to the nonlinear effects is seen in the lateral direction and the screech tone frequency is the same as the vortex passing frequency due to the generation of intense large-scale vortical motions.

• International Journal of Naval Architecture and Ocean Engineering
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• v.4 no.3
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• pp.241-255
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• 2012

Three types of 100 kW-class tidal stream turbines are proposed and their performance is studied both numerically and experimentally. Following a wind turbine design procedure, a base blade is derived and two additional blades are newly designed focusing more on efficiency and cavitation. For the three designed turbines, a CFD is performed by using FLUENT. The calculations predict that the newly designed turbines perform better than the base turbine and the tip vortex can be reduced with additional efficiency increase by adopting a tip rake. The performance of the turbines is tested in a towing tank with 700 mm models. The scale problem is carefully investigated and the measurements are compared with the CFD results. All the prediction from the CFD is supported by the model experiment with some quantitative discrepancy. The maximum efficiencies are 0.49 (CFD) and 0.45 (experiment) at TSR 5.17 for the turbine with a tip rake.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.12-12
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• 2010

By using scanning tunneling microscopy/spectroscopy (STM/S), we can make images of various physical properties in nanometer-scale spatial resolutions. Here, I demonstrate imaging of two electron-correlated subjects; screening and superconductivity by STM/S. The electrostatic potential around a charge is described with the Coulomb potential. When the charge is located in a metal, the potential is modified because of the free electrons in the host. The potential modification, called screening, is one of the fundamental phenomena in the condensed matter physics. Using low-temperature STM we have developed a method to measure electrostatic potential in high spatial and energy resolutions, and observed the potential around external charges screened by two-dimensional surface electronic states. Characteristic potential decay and the Friedel oscillation were clearly observed around the charges [1]. Superconductivity of nano-size materials, whose dimensions are comparable with the coherence length, is quite different from their bulk. We investigated superconductivity of ultra-thin Pb islands by directly measuring the superconducting gaps using STM. The obtained tunneling spectra exhibit a variation of zero bias conductance (ZBC) with a magnetic field, and spatial mappings of ZBC revealed the vortex formation [2]. Size dependence of the vortex formation will be discussed at the presentation.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.5 s.248
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• pp.438-445
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• 2006

We performed the simulation of the unsteady three dimensional flow over a square cylinder in a wind tunnel in moderate Reynolds number range, $100{\sim}2500$ by using LBM. SGS model was applied for the turbulent flow. Frist of all we compared LBM(Lattice Boltzmann Method) solution of Poiseuille flow applied Farout and bounce back boundary conditions with the analytical and FOAM solutions to verify the applicability of the boundary conditions. For LBM simulation the calculation domain was formed by structured grids and prescribed uniform velocity and density inlet and Farout boundary conditions were imposed on the in-out boundaries. Bounceback and wind tunnel boundary conditions were applied to the cylinder walls and the boundaries of calculation domain respectively. The maximum Strouhal number of the vortex shedding is 0.2025 at Re = 750. and the number maintains the constant value of 0.18 when Re>1000. We also predicted that the critical reynolds number of the turbulent flow is in the range of $250{\sim}500$.

• Journal of Power System Engineering
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• v.22 no.6
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• pp.28-35
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• 2018

The large-eddy simulation (LES) was carried out to evaluate the instantaneous vector and vorticity profiles of a small-size axial fan (SSAF) at the operating point of full-flowrate. The downstream flow of the SSAF exhibits a shorter axial flow when not fully developed, especially the stronger vortex appears at the edge near the flow end. On the other hand, the downstream flow of the SSAF exhibits a longer axial flow, and the weaker vortex appears at the edge near the flow end when the flow is sufficiently developed. Moreover, in the downstream of the SSAF, a periodic and intermittent flow pattern appears at the edge showing the axial flow, and the instantaneous vorticity contour lines showing the form of a circle group are distributed at specific intervals from the downstream region of the blade tip, which is considered to be the result of the intermittency phenomenon influenced by the number of blades and the number of revolutions.

• Wind and Structures
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• v.32 no.5
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• pp.453-469
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• 2021

Snow accumulation on the road frequently induces a big traffic problem in the cold snowy region. Accurate prediction on snow distribution is fundamental for solving drifting snow disasters on roads. The present study adopts the transient method to simulate the wind-induced snow distribution on embankment based on the mixture multiphase model and dynamic mesh technique. The simulation and field measurement are compared to confirm the applicability of the simulation. Furthermore, the process of snow accumulation is revealed. The effects of friction velocity and snow concentration on snow accumulation are analyzed to clarify its mechanism. The results show that the simulation agrees well with the field measurement in trends. Moreover, the snow accumulation on the embankment can be approximately divided into three stages with time, the snow firstly deposited on the windward side, then, accumulation occurs on the leeward side which induced by the wake vortex, finally, the snow distribution reaches an equilibrium state with the slope of approximately 7°. The friction velocity and duration have a significant influence on the snow accumulation, and the vortex scale directly affected the snow deposition range on the embankment leeward side.

• Nuclear Engineering and Technology
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• v.55 no.6
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• pp.1999-2010
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• 2023

Undesirable flapping motion of discs can cause the failure of swing check valves in nuclear passive safety systems. Time-resolved particle image velocimetry (PIV) was employed to investigate the flow characteristics around a free-to-rotate plate and the motion response, with the Reynolds numbers, based on the hydraulic diameter of the channel, from 1.32 × 10⁴ to 3.95 × 10⁴. Appreciable flapping motion (±3.52°) appeared at the Reynolds number of 2.6 × 10⁴ with the frequency of 5.08 Hz. In the low-Reynolds-number case, the plate showed negligible flapping. In the high-Reynolds-number case, the deflection angle increased with reduced flapping amplitude. The torque from the fluid determined the flapping amplitude. In the low-Reynolds-number case, Karman vortices were absent. With increasing Reynolds numbers, Karman vortices developed behind the plate with larger deflection angles. Strong interaction between the wake flow from the leading and trailing edge of the plate was observed. Based on power spectrum density (PSD) analysis, the vortex shedding frequency coincided with the flapping frequency, and the amplitude was positively correlated to the strength of the vortices. Proper orthogonal decomposition (POD) modes evince that, in the case of appreciable motion, coherent structures exhibited a larger spatial scale, enhancing the magnitude of the external torque on the plate.

• Journal of the Korean earth science society
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• v.39 no.3
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• pp.211-227
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• 2018

This study evaluates the prediction skills of stratospheric polar vortex intensification events (VIEs) in Global Seasonal Forecasting System (GloSea5) model, an operational subseasonal-to-seasonal (S2S) prediction model of Korea Meteorological Administration (KMA). The results show that the prediction limits of VIEs, diagnosed with anomaly correlation coefficient (ACC) and mean squared skill score (MSSS), are 13.6 days and 18.5 days, respectively. These prediction limits are mainly determined by the eddy error, especially the large-scale eddy phase error from the eddies with the zonal wavenumber 1. This might imply that better prediction skills for VIEs can be obtained by improving the model performance in simulating the phase of planetary scale eddy. The stratospheric prediction skills, on the other hand, tend to not affect the tropospheric prediction skills in the analyzed cases. This result may indicate that stratosphere-troposphere dynamic coupling associated with VIEs might not be well predicted by GloSea5 model. However, it is possible that the coupling process, even if well predicted by the model, cannot be recognized by monotonic analyses, because intrinsic modes in the troposphere often have larger variability compared to the stratospheric impact.