• Journal of the Korean Society of Visualization
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• v.16 no.1
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• pp.30-36
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• 2018

In the present study, we investigate the flow characteristics of a quadrotor UAV in a hovering mode by measuring multiple two-dimensional velocity fields in the wake. The experiment is conducted at Re = 24,000 in a chamber large enough to neglect the ground effect, where Re is the Reynolds number based on the rotor chord length and the rotor tip speed. The rotational speed of the rotor is determined by an optical tachometer so that the lift force can be balanced with the weight of the UAV. The velocity field measured on the center plane of the rotor shows that the vortices are shedding from the tip of the rotor, inducing large fluctuations in the streamwise velocity along the wake shear layer. The strength of the rotor-tip vortex shedding is asymmetric with respect to the rotor axis due to the interaction between the rotor and the wake centerline of each rotor is inclined to the center of the UAV due to the pressure difference caused by the induced velocity. The wake from each rotor moves closer to each other while traveling in the streamwise direction, and then is merged together inducing large fluctuations in the transverse velocity. Due to the wake merging, on the center plane of the UAV, the velocity increases in the streamwise direction showing two-peak structure in the streamwise velocity contours.

• Clean Technology
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• v.14 no.1
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• pp.35-39
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• 2008

In this study, we successfully fabricated a super-hydrophobic polymer layer on the surface of slide glass using the solubility difference of a polymer at two different solvents. After dissolving polystyrene (PS) resins in tetrahydrofuran (THF) as a good solvent and then adding ethanol(EtOH) as a poor solvent, we were able to fabricate super-hydrophobic surface. We also investigated the effect of EtOH addition, coating methods, solution mixing time and speed, and other alcohols on the surface hydrophobicity. The water contact angle (WCA) of the fabricated surface revealed $WCA>150^{\circ}$ and the microporous surface structure composed of microparticles with the size less than $5\;{\mu}m$.

• Korean Journal of Materials Research
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• v.12 no.11
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• pp.856-859
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• 2002

To investigate effects of heat treatments including grain size control in substrate aluminum on nanopore arrays in anodic alumina template, aluminum was heat treated at $500^{\circ}C$ for 1h. The heat treated aluminum was anodized by two successive anodization processes in oxalic solution and the nanopore arrays in anodic alumina layer were studied using TEM and FE-SEM. The highly ordered porous alumina templates with 110 nm interpore distance and 40 nm pore diameter have been observed and the pore array of the anodic alumina has a uniform and closely-packed honeycomb structure. In the case of alumina template obtained from heat treated aluminum substrate, the well- ordered nanopore region in anodic alumina increased and became more homogeneous compared with that from non-heattreated one.

• Proceedings of the KIEE Conference
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• 2008.05a
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• pp.169-170
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• 2008

The heterolayered PZT/PT thick films were fabricated by two different methods - thick films of the PZT by screen printing method on alumina substrates electrodes with Pt, thin films of $PbTiO_3$ by the spin coating method on the PZT thick films and once more thick films of the PZT by the screen printing method on the $BaTiO_3$ layer The structural and the dielectric properties were investigated for effect of various stacking sequence of sol-gel prepared $PbTiO_3$ coating solution at interface of the PZT thick films. The insertion of $PbTiO_3$ interlayer yielded the PZT thick films with homogeneous and dense grain structure with the number of $PbTiO_3$ layers. The leakage current density of the PZT/$PbTiO_3-1$ film is less that $4.41{\times}10^{-9}\;A/cm^2$ at 5 V.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.35 no.3
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• pp.289-296
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• 2002

Using results obtained from field surveys in the outflow region of the Keum River in winter (February 22), spring (May 9), summer (July 31) and autumn (October 17) in 2000, it was described the nutrient front relating to the hydrography. It showed that the horizontal gradients was sharp in the distributions not only in salinity but also in nutrients in the region of the mouth of estuary far away about 20 km from the dyke of the Keum River during the year. The two layer structure was a ubiquitous feature in the inner estuarine side of the region showing the maximum horizontal gradients in salinity and nutrients. Although the absolute values in salinity and nutrient concentrations were distinguished by season, the nutrient front seperates the saltier and nutrients poor water in the open sea from the brackish and nutrients rich water of the plume. These results suggest that the nutrient front coincides with the esturine front in the region of freshwater influence (ROFI) of the Keum River.

• Nuclear Engineering and Technology
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• v.42 no.2
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• pp.193-202
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• 2010

In order to study the cladding properties of zirconium after a loss-of-coolant accident (LOCA)-simulation oxidation and water quenching test, commercial Zircaloy-4 and two kinds of HANA claddings were oxidized at temperatures ranging from $900^{\circ}C$ to $1250^{\circ}C$ and exposed for 300 s, and then cooled to $700^{\circ}C$ before quenching. Microstructural observations were made to evaluate the matrix characteristics with the chemical compositions after the LOCA-simulation test. Ring compression testing was then performed to compare the ductile behaviour of the HANA and Zircaloy-4 claddings. An X-ray diffraction (XRD) analysis was carried out for temperatures ranging from room temperature to $1250^{\circ}C$ for the oxide layer to verify the oxide crystal structure at each oxidation temperature.

• Particle and aerosol research
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• v.5 no.4
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• pp.159-170
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• 2009

A depth filter medium was newly designed in order to achieve high collection of dust and low pressure drop in this work. Multilayer depth filter media consist of an upstream layer of highly porous structure which allows particles to pass through and to follow by one or more downstream layers to hold the particles inside the media. For each filter media, flat sheet and pleated module were made of newly developed depth filter media and filter media of commercial products. Commercial depth filter cartridge for gas turbine air intake cleaning were used as reference for filtration area and pleat geometry of pleated modules. This work attempts to evaluate and compare the newly developed depth filter medium and two commercial filter media in terms of filtration parameters such as air permeability, initial pressure drop, particle retention and pressure drop variation with dust loading. According to the close examination the newly developed depth filter showed better performance compared to the commercial depth filter media.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.10
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• pp.2675-2685
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• 1994

A study has been conducted to provide the experimental information for the velocity and temperature profiles of steam-air mixutre and to investigate their roles on the film condensation with wavy interface. Saturated gas mixture of steam-air was made to flow through the nearly horizontal$(4.1^{\circ})$ square duct of 0.1m width and 1.56m length at atmospheric pressure, and was condensated on the bottom cold plate. The air mass fraction in the gas mixture was changed from zero(W =0, pure steam) to one(W =1, pure air), and the bulk velocity was varied from 2 to 4 m/s. Water film was injected concurrently to investigate the effect of wavy interface on the condensation. The velocity and temperature profiles were measured by LDA system and thermocouples along the three parameters ; air mass fraction, mixture velocity and film flow rate. The profiles moved toward the interface with increasing steam mass fraction, mixture velocity and film flow rate. The Prandtl and Schmidt numbers were near one in the present experimental range, however there was no complete similarity between the velocity and temperature profiles of gas mixture. And the heat transfer characteristics and interfacial structure were coupled with each other.

• Proceedings of the KIEE Conference
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• 2006.10a
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• pp.123-124
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• 2006

The sandwiched PZT/$BaTiO_3$/PZT thick films were fabricated by two different methods thick films of the PZT by screen printing method on alumina substrateselectrodes with Pt, thin films of $BaTiO_3$ by the spin-coating method on the PZT thick films and once more thick films of the PZT by the screen printing method on the $BaTiO_3$ layer. The structural and the dielectric properties were investigated for effect of various stacking sequence of sol-gel prepared $BaTiO_3$ coating solution at interface of the PZT thick films, The insertion of BaTi03 interlayer yielded the PZT thick films with homogeneous and dense grain structure with the number of $BaTiO_3$ layers. The leakage current density of the $PZT/BaTiO_3-1$ film is less that $4.41{\times}10^{-9}A/cm^2$ at 5 V.

• Journal of Surface Science and Engineering
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• v.13 no.4
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• pp.221-227
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• 1980

When anodizing the Al in the acid electrolyte, it is well known that the parallel pores grow continuously perpendicular to the surface. This fact can be used for the manufacturing of the porous membrane, if thc pores pass through the anodized foil. Anodizing both surfaces of the Al-foil spontaneously in 20$^{\circ}C$, 2% oxalic acid under tile potentiostatic condition, it is found that the harrier layer remaining in the midst of the foil finally disappears and thc pores pass through the foil. And examined the porous structure change when the voltage is changed during the anodizing treatment. From the result, it is revealed that the new pores and cell grow, adjusting themselves to the final voltage. The characteristic of the porous membrane is greatly dependent upon the diameter of the pore and the cell. So studied the relationship between the voltage and the diameter of the pore and the cell quantitatively with the aid of field-assisted dissolution concept. And derived the following two equation, Pi = 8.32Vi, Ci = 26.80Vi. These equations are in good accord with the experimental data above 30V, but do not accord nuder 30V.