• Wind and Structures
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• v.26 no.5
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• pp.305-315
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• 2018

A numerical study based on a delayed detached eddy simulation (DDES) is conducted to investigate the aerodynamic mechanism behind the suppression of vortex-induced vibrations (VIVs) of twin box girders by central grids, which have an inhibition effect on VIVs, as evidenced by the results of section model wind tunnel tests. The mean aerodynamic force coefficients with different attack angles are compared with experimental results to validate the numerical method. Next, the flow structures around the deck and the aerodynamic forces on the deck are analyzed to enhance the understanding of the occurrence of VIVs and the suppression of VIVs by the application of central grids. The results show that shear layers are separated from the upper railings and lower overhaul track of the upstream girder and induce large-scale vortices in the gap that cause periodical lift forces of large amplitude acting on the downstream girder, resulting in VIVs of the bridge deck. However, the VIVs are apparently suppressed by the central grids because the vortices in the central gap are reduced into smaller vortices and become weaker, causing slightly fluctuating lift forces on the deck. In addition, the mean lift force on the deck is mainly caused by the upstream girder, whereas the fluctuating lift force is mainly caused by the downstream girder.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.3
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• pp.373-381
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• 2000

The flow and heat transfer characteristics on the impingement surface can be controlled by the change of vortex with the acoustic excitation, because the flow characteristics of an impinging jet are affected strongly by the vortices formed at the jet exit. To investigate the effects of acoustic excitation, we measured the velocity, turbulent intensity distributions for the free jet and local heat transfer coefficients on a impingement surface. As the acoustic excitation, subharmonic frequency of excited frequency plays an important role to the control of the jet flow. If the vortex pairings are promoted by the acoustic excitation, turbulence intensity of the jet flow is increased quickly. On the other hand if the vortex pairings are suppressed, the jet flow has low turbulence intensity at the center of the jet. Therefore, the low heat transfer rates are obtained on the impingement plate for a small nozzle-to-plate distance. However, it has high heat transfer rates at a large distance between the nozzle and plate due to the increasing of potential-core length.

• Journal of Mechanical Science and Technology
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• v.18 no.3
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• pp.513-525
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• 2004

This paper reports an experimental study on flow boiling of pure refrigerants R l34a and R l23 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10㎜ located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5-50㎾/㎡, vapor quality 0-100 percent and mass velocity of 150-600㎏/㎡s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen's superposition model, a new correlation is presented for heat transfer coefficients of mixture.

• Journal of the Korean Vacuum Society
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• v.11 no.2
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• pp.108-112
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• 2002

We investigated the secondary electron emission characteristics of MgO layer used as the protecting material in a.c. plasma display panel(a.c.-PDPs) using a pulsed ion beam technique, where the surface charging can be effectively suppressed during the measurement. The measurement of the secondary electron emission coefficients ($\gamma$) on the surface of $SiO_2$ was carried out and then, it was found that the yields were dependent on incident ion energies. In addition, it was clearly demonstrated that the sputtering on MgO surface leads to lower yields, which suggests that the surface plays a key role on the operating conditions, such as life time, fast response, and etc.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06a
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• pp.522-522
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• 1996

An investigation of the effects of transverse magnetic field and Peltier effect on melt convection and macrosegregation in vertical Bridgman crystal grosth of Te doped InSb was been carried out by means of microstructure observation, Hall measurement, electrical resistivity measurement and X-ray analysis. Before the experiments, Interface stability, convective instability and suppression of convection by magnetic field were calculated theoretically. After doping 1018, 1019 cm-3 Te in InSb, the temperature of Bridgman furnace was set up at $650^{\circ}C$. The samples were grown in I.D. 11mm, 100mm high quartz tube. The velocity of growth was about 2${\mu}{\textrm}{m}$/sec. In order to obtain the suppression of convection by magnetic field in the middle of growth, 2-4KG magnetic field was set on the melt. For searching of the shape of solid-liquid interface and the actual velocity of crystal growth, let 2A current flow from solid to liquid for 1second every 50seconds repeatedly (Peltier effect). The grown InSb was polycrystal, and each grain was very sharp. There was no much difference between the sample with and without magnetic field at a point of view of microstructure. For the sample with Peltier effect, the Peltier marks(striation) were observed regularly as expected. Through these marks, it was found that the solid-liquid interface was flat and the actual growth velocity was about 1-2${\mu}{\textrm}{m}$/sec. On the ground of theoretical calculation, there is thermosolutal convection in the Te doped InSb melt without magnetic field in this growth condition. and if there is more than 1KG magnetic field, the convection is suppressed. Through this experiments, the effective distribution coefficients, koff, were 0.35 in the case of no magnetic field, and 0.45 when the magnetic field is 2KG, 0.7 at 4KG. It was found that the more magnetic field was applied, the more convection was suppressed. But there was some difference between the theoretical calculation and the experiment, the cause of the difference was thought due to the use of some approximated values in theoretical calculation. In addition to these results, the sample with Peltier effect showed unexpected result about the Te distribution in InSb. It looked like no convection and no macrosegregation. It was thought that the unexpected behavior was due to Peltier mark. that is, when the strong current flew the growing sample, the mark was formed by catching Te. As a result of the phenomena, the more Te containing thin layer was made. The layer ruled the Hall measurement. The values of resistivity and mobility of these samples were just a little than those of other reference. It was thought that the reason of this result was that these samples were due to polycrystal, that is, grain boundaries had an influence on this result.

• Journal of Biomedical Engineering Research
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• v.25 no.6
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• pp.589-598
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• 2004

We propose a new algorithm to extract the fetal electrocardiogram (FECG) from a multi-channel electrocardiogram (ECG) recorded at the chest and abdomen of a pregnant woman. To extract the FECG from the composite abdominal ECG, the classical time-domain method based on singular value decomposition (SVD) has been generally used. However, this method has some disadvantages, such as its high degree of computational complexity and the necessary assumption that vectors between the FECG and the maternal electrocardiogram (MECG) should be orthogonal. The proposed algorithm, which uses SVD in a discrete cosine transform (DCT) domain, compensates for these disadvantages. To perform SVD with lower computational complexity, DCT coefficients corresponding to high-frequency components were eliminated on the basis of the properties of the DCT coefficients and the frequency characteristics of the FECG. Moreover, to extract the pure FECG with little influence of the direction of the vectors between the FECG and MECG, three new channels were made out of the MECG suppressed in the composite abdominal ECG, and the new channels were appended to the original multi-channel ECG. The performance of the proposed algorithm and the classical time-domain method based on SVD were compared using simulated and real data. It was experimentally verified that the proposed algorithm can extract the pure FECG with reduced computational complexity.

• Wind and Structures
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• v.23 no.2
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• pp.109-125
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• 2016

Flow interference is investigated between two tripped cylinders of identical diameter D at stagger angle ${\alpha}=0^{\circ}{\sim}180^{\circ}$ and gap spacing ratio $P^*$ (= P/D) = 0.1 ~ 5, where ${\alpha}$ is the angle between the freestream velocity and the line connecting the cylinder centers, and P is the gap width between the cylinders. Two tripwires, each of diameter 0.1D, were attached on each cylinder at azimuthal angle ${\beta}={\pm}30^{\circ}$, respectively. Time-mean drag coefficient ($C_D$) and fluctuating drag ($C_{Df}$) and lift ($C_{Lf}$) coefficients on the two tripped cylinders were measured and compared with those on plain cylinders. We also conducted surface pressure measurements to assimilate the fluid dynamics around the cylinders. $C_D$, $C_{Df}$ and $C_{Lf}$ all for the plain cylinders are strong function of ${\alpha}$ and $P^*$ due to strong mutual interference between the cylinders, connected to six interactions (Alam and Meyer 2011), namely boundary layer and cylinder, shear-layer/wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex interactions. $C_D$, $C_{Df}$ and $C_{Lf}$ are very large for vortex and cylinder, vortex and shear layer, and vortex and vortex interactions, i.e., the interactions where vortex is involved. On the other hand, the interference as well as the strong interactions involving vortices is suppressed for the tripped cylinders, resulting in insignificant variations in $C_D$, $C_{Df}$ and $C_{Lf}$ with ${\alpha}$ and $P^*$. In most of the (${\alpha}$, $P^*$ ) region, the suppressions in $C_D$, $C_{Df}$ and $C_{Lf}$ are about 58%, 65% and 85%, respectively, with maximum suppressions 60%, 80% and 90%.

• Journal of Korea Water Resources Association
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• v.42 no.5
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• pp.375-383
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• 2009

This study examined hydraulic characteristics on diagonal weirs with hydraulic experiment and presented a discharge coefficient equation utilizing multiple regression analysis for various design conditions. This study had a object in designing efficiently diagonal weirs utilizing this equation. Diagonal weirs maintained uniformly upstream water level than rectangular suppressed weirs. Also, as installation degrees of diagonal weirs increased, diagonal weirs increased maintenance effects of a upstream water level. Because of these characteristics, diagonal weirs were suitable to canal system. This study presented discharge coefficient equations for diagonal weirs utilizing simple regression analysis. But, these equations are some restrictions on degrees. Therefore, this study presented an equation to estimate directly discharge coefficients to various degrees utilizing multiple regression analysis. This equation was verified by making use of analyses of $R^2$, the sum of residuals, MAPE. Therefore, this equation is enable to make good use of a design in diagonal weirs.

• Natural Product Sciences
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• v.25 no.3
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• pp.238-243
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• 2019

In this study, the marker compounds of Curcumae Rhizoma (CR) were simultaneously quantified by high-performance liquid chromatography equipped with a photodiode array detector and the anti-inflammatory effects of CR extract and marker compounds in human benign prostatic hyperplasia epithelial-1 (BPH-1) cell lines were investigated. The marker components (4S,5S)-(+)-germacrone-4,5-epoxide, furanodienone, and germacrone, were separated on Gemini $C_{18}$ columns ($250mm{\times}4.6mm$, $5{\mu}m$) at $40^{\circ}C$ by using a gradient of two mobile phases eluting at 1.0 mL/min. Prostaglandin $E_2$ ($PGE_2$) levels in Human BPH-1 cells were determined with an ELISA kit. The coefficients of determination in a calibration curve of each analyte were all 0.9997. The limits of detection and quantification of the three compounds were $0.10-0.32{\mu}g/mL$ and $0.30-0.98{\mu}g/mL$, respectively. The content of three compounds, (4S,5S)-(+)-germacrone-4,5-epoxide, furanodienone, and germacrone, in the CR sample were found to be 5.79 - 5.92 mg/g, 4.72 - 4.86 mg/g, and 1.06 - 1.09 mg/g, respectively. Regarding pharmacological activity against benign prostatic hyperplasia, CR and its components significantly suppressed $PGE_2$ levels of BPH-1 cells. The established analysis method will help to improve quality assessment of CR samples and related products. In addition, CR and its components exhibit antiinflammatory activity in BPH-1 cells, suggesting the inhibitory efficacy of these compounds against the pathogenesis of BPH.

• Tribology and Lubricants
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• v.38 no.6
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• pp.235-240
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• 2022

Atomically-thin 2D nanomaterials can be easily deformed and have surface corrugations which can influence the frictional characteristics of the 2D nanomaterials. Chemical vapor deposition (CVD) graphene can be grown in a wafer scale, which is suitable as a large-area surface coating film. The CVD growth involves cooling process to room temperature, and the thermal expansion coefficients mismatch between graphene and the metallic substrate induces a compressive strain in graphene, resulting in the surface corrugations such as wrinkles and atomic ripples. Such corrugations can induce the friction anisotropy of graphene, and therefore, accurate imaging of the surface corrugation is significant for better understanding about the friction anisotropy of CVD graphene. In this work, the combinatorial analysis using friction force microscopy (FFM) and transverse shear microscopy (TSM) was implemented to unveil the friction anisotropy of CVD bi-layer graphene. The periodic friction anisotropy of the wrinkles was measured following a sinusoidal curve depending on the angles between the wrinkles and the scanning tip, and the two domains were observed to have the different friction signals due to the different directions of the atomic ripples, which was confirmed by the high-resolution FFM and TSM imaging. In addition, we revealed that the atomic ripples can be easily suppressed by ironing the surface during AFM scans with an appropriate normal force. This work demonstrates that the friction anisotropy of CVD bilayer graphene is well-correlated with the corrugated structures and the local friction anisotropy induced by the atomic ripples can be controllably removed by simple AFM scans.