• Proceedings of the KSRS Conference
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• v.1
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• pp.21-24
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• 2006

Processes controlling the interannual variation of mixed layer temperature (MLT) averaged over the NINO3 domain ($150-90^{\circ}W$, $5^{\circ}N-5^{\circ}S$) are studied using an ocean data assimilation product that covers the period of 1993 to 2003. Advective tendencies are estimated here as the temperature fluxes through the domain's boundaries, with the boundary temperature referenced to the domain-averaged temperature to remove the dependence on temperature scale. The overall balance is such that surface heat flux opposes the MLT change but horizontal advection and subsurface processes assist the change. The zonal advective tendency is caused primarily by large-scale advection of warm-pool water through the western boundary of the domain. The meridional advective tendency is contributed mostly by Ekman current advecting large-scale temperature anomalies though the southern boundary of the domain. Unlike many previous studies, we explicitly evaluate the subsurface processes that consist of vertical mixing and entrainment. In particular, a rigorous method to estimate entrainment allows an exact budget closure. The vertical mixing across the mixed layer (ML) base has a contribution in phase with the MLT change. The entrainment tendency due to temporal change in ML depth is negligible comparing to other subsurface processes. The entrainment tendency by vertical advection across the ML base is dominated by large-scale changes in wind-driven upwelling and temperature of upwelling water. Tropical instability waves (TIWs) result in smaller-scale vertical advection that warms the domain during La Ni? cooling events. When the advective tendencies are evaluated by spatially averaging the conventional local advective tendencies of temperature, the apparent effects of currents with spatial scales smaller than the domain (such as TIWs) become very important as they redistribute heat within the NINO3 domain. However, such internal redistribution of heat does not represent external processes that control the domain-averaged MLT.

• Journal of Korea Water Resources Association
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• v.47 no.10
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• pp.907-919
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• 2014

This study presents scour processes and its characteristics around the pile groups using the large eddy simulation (LES) coupled with sediment transport and morphodynamic models. The scour and deposition around pile groups were significantly influenced by the pile interval. In case the non-dimensional pile interval was less than 3.75, the local scours as well as the contraction scour were observed around the pile group. On the other hand, in case the non-dimensional pile interval was more than 3.75, the contraction scour disappeared and only local scours were developed at individual piles. Change in the scour depth at piles located in the upstream was similar with the case of single pile, but the scour depth around piles located in the downstream was lower and showed a significantly different tendency due to the presence of piles in the upstream. The non-dimensional maximum scour depth around the pile group decreased as the pile interval increased.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.11a
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• pp.363-366
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• 2006

The effectiveness of two passive control techniques for alleviating the pressure oscillation generated in a supersonic cavity flow is investigated numerically. The passive devices suggested in the present research include a triangular bump and a sub-cavity installed near the upstream edge of a rectangular cavity. The supersonic cavity flow characteristics are examined by using the three-dimensional, unsteady Wavier-Stokes computation based on a finite volume scheme. Large eddy simulation (LES) is carried out to properly predict the turbulent features of cavity flow. The results show that the pressure oscillation near the downstream edge dominates overall time-dependent cavity pressure variations. Such an oscillation is attenuated more considerably using the sub-cavity compared with other methods, and a larger sub-cavity leads to better control performance.

• Journal of the Korean Society of Propulsion Engineers
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• v.21 no.5
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• pp.1-9
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• 2017

The cryogenic nitrogen spray of a swirl injector has been numerically investigated using three dimensional LES turbulence model to analyze the dynamic characteristics under supercritical condition. To predict the precise nitrogen properties under supercritical condition, SRK equation of state, Chung's method for viscosity and thermal conductivity and Takahashi's correlation based on Fuller's theory for diffusion coefficient are implemented. The complex flow structures due to interaction between flow field and acoustic field are observed inside and outside the injector under supercritical condition. FFT, POD, and DMD techniques are employed to understand the coherent structures. By implementing the FFT, the dominant frequencies are identified inside and outside the injector. The coherent flow structures related to the dominant frequencies are visualized using the POD and DMD techniques. In addition, the DMD provides the damping coefficient which is related with the instability prediction.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.19 no.9
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• pp.1051-1057
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• 2008

In this paper, we analyzed the characteristics of the EM wave absorber which was fabricated by using flake sendust (soft metal magnetic powder). The flake sendust was made of 3 different granularity by attrition mill. First, we have fabricated 3 kind of EM wave absorbers using the flake sendust and CPE(Chlorinated Polyethylene) and measured the S-parameters of the EM wave absorber. The complex relative permittivity and permeability were calculated from the measured data and the variations according to a change of granularity were researched. As a result, it was confirmed that the EM wave absorber using flake sendust with the $140{\mu}m$ average granularity has outstanding absorption ability in high frequency range(C band) for the reduction of eddy current loss(increase of permeability) and the increase of space charge polarization(increase of permittivity).

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.3
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• pp.194-198
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• 2010

Recently as the electronic devices are getting to be more and more smaller, transformers are needed to be micro fabricated using MEMS technology. In this paper transformers have been fabricated and measured by depositing insulation layer to reduce the loss of eddy current and in the middle core a high permeability permalloy was designed based on the turns ratio between primary coil and secondary coil which are 1:1 transformers. (the number of turns of primary coil and secondary coil: 3/3, 5/5, 7/7). The size of the transformers including ground shield are $1mm{\times}1.5mm$, $1mm{\times}1.95mm$, $1mm{\times}2.35mm$ respectively. The line width, pitch and the height of post are 50um. Based on the measured data from the micro fabricated transformers, the 3/3 turns in the primary coil and secondary coil showed the lowest insertion loss with 1.5 dB at 480 MHz and the 7/7 turns in the primary coil and secondary coil showed the highest insertion loss with 2.5 dB at 280 MHz. Also confirmed that the bandwidth goes up as the number of turns goes down. There was some difference between the actual measured data and the HFSS simulation result. It looks as if it is an error of the difference between oxidation of copper or the permeability of SU-8.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.5
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• pp.1319-1332
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• 1995

Numerical analysis was performed to characterize the particle deposition behavior on a horizontal free-standing wafer with thermophoretic effect under the turbulent flow field. A low Reynolds number k-.epsilon. turbulence model was used to analyze the turbulent flow field around the wafer, and the temperature field for the calculation of the thermophoretic effect was predicted from the energy equation introducing the eddy diffusivity concept. The deposition mechanisms considered were convection, diffusion, sedimentation, turbulence and thermophoresis. For both the upper and lower surfaces of the wafer, the averaged particle deposition velocities and their radial distributions were calculated and compared with the laminar flow results and available experimental data. It was shown by the calculated averaged particle deposition velocities on the upper surface of the wafer that the deposition-free zone, where the deposition velocite is lower than 10$^{-5}$ cm/s, exists between 0.096 .mu.m and 1.6 .mu.m through the influence of thermophoresis with positive temperature difference of 10 K between the wafer and the ambient air. As for the calsulated local deposition velocities, for small particle sizes d$_{p}$<0.05 .mu.m, the deposition velocity is higher at the center of the wafer than at the wafer edge, whereas for particle size of d$_{p}$ = 2.0 .mu.m the deposition takes place mainly on the inside area of the wafer. Finally, an approximate model for calculating the deposition velocities was recommended and the calculated deposition velocity results were compared with the present numerical solutions, those of Schmidt et al.'s model and the experimental data of Opiolka et al.. It is shown by the comparison that the results of the recommended model agree better with the numerical solutions and Opiolka et al.'s data than those of Schmidt's simple model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.9
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• pp.1149-1164
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• 1997

In this study, the applicability of the RNG k-.epsilon. model to the analysis of the complex flows is studied. The governing equations based on a non-orthogonal coordinate formulation with Cartesian velocity components are used and discretized by the finite volume method with non-staggered variable arrangements. The predicted results using the RNG k-.epsilon. model of three complex flows, i.e., the flow over a backward-facing step and a blunt flat plate, the flow around a 2D model car are compared to these from the standard k-.epsilon. model and experimental data. That of the unsteady axisymmetric turbulent flow within a cylinder of reciprocating model engine including port/valve assembly and the spray characteristics within a chamber of direct injection model engine are compared to these from the standard k-.epsilon. model and experimental data. The results of reattachment length, separated eddy size, average surface pressure distribution using the RNG k-.epsilon. model show more reasonable trends comparing with the experimental data than those using the modified k-.epsilon. model. Although the predicted rms velocity using the modified k-.epsilon. model is lower considerably than the experimental data in incylinder flow with poppet valve, predicted axial and radial velocity distributions at the valve exit and in-cylinder region show good agreements with the experimental data. The spray tip penetration predicted using the RNG k-.epsilon. model is more close to the experimental data than that using the modified k-.epsilon. model. The application of the RNG k-.epsilon. model seems to have some potential for the simulations of the unsteady turbulent flow within a port/valve-cylinder assembly and the spray characteristics over the modified k-.epsilon. model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.2
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• pp.75-83
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• 2016

Since a turbomachine has complex flow characteristics, which are caused by adverse pressure gradient and high speed motion, an elaborate turbulence model is needed to accurately predict the flow. Some turbulence models such as an algebraic or a two-equation eddy viscosity model have been used for in-house RANS-code, but it is difficult to obtain good result for several complex flows. In this study, Durbin's V2-F turbulence model, which has been known for better prediction for severe flow separation, is applied to T-Flow. It was validated for simple cases such as channel and compressor cascade, and its applicability to turbomachinery was shown by analyzing internal flow of a single rotor. As a result, the V2-F turbulence model shows better blade surface pressure distribution than the one-and-two equation turbulence model.

• Atmosphere
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• v.26 no.3
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• pp.401-412
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• 2016

A 3D sonic anemometer has been installed at Yongpyong alpine slope since Oct. 23th 2014 to observe the slope winds and to analyze turbulent characteristics with the change in surface cover (grass and snow) and the synoptic wind strength. Eddy covariance method has been applied to calculate the turbulent quantity after coordinate transformation of a planar-fit rotation. We have carefully selected 3 good episodes in the winter season (23 October 2014 to 28 February 2015) for each category (9 days in total), such as grass and snow covers in case of weak synoptic wind condition, and grass cover of strong synoptic wind. The diurnal variations of the slope winds were well developed like the upslope wind in the daytime and downslope wind in the nighttime for both surface covers (grass and snow) in the weak synoptic forcing, when accordingly both heat and momentum fluxes significantly increased in the daytime and decreased in the nighttime. Meanwhile, diurnal variation of heat flux was not present on the snow cover probably in associated with significant fraction of sunlight reflection due to high albedo especially during the daytime in comparison to those on the grass cover. In the strong synoptic regime, the most dominant feature at Yongpyong, only the southeasterly downslope winds were steadily generated irrespective of day and night with significant increases in momentum flux and turbulent kinetic energy as well, which could suggest that local circulations are suppressed by the synoptic scale forcing. In spite of only one season analysis applied to the limited domain, this kind of an observation-based study will provide the basis for understanding of the local wind circulation in the complex mountain domain such as Gangwon in Korea.