• Wind and Structures
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• 제26권2호
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• pp.89-98
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• 2018

For most full-scale tall buildings the Reynolds number of a flow field around a circular cylinder under strong wind is usually greater than $2{\times}10^7$, which is difficult to achieve in most wind tunnel tests. To explore the wind characteristics of tall cylindrical buildings with equidirectional grooves from subcritical to transcritical flow ($6.6{\times}10^4{\leq}Re{\leq}3.3{\times}10^5$ and $9.9{\times}10^6{\leq}Re{\leq}7.2{\times}10^7$), wind tunnel tests and full-scale large eddy simulations were carried out. The results showed that the rectangular-grooves narrow the wake width due to the downstream movement of the separation point and the deeper grooves cause smaller mean and fluctuating pressure while the peak pressure is little affected. Furthermore, the grooves lead to lower frequency of vortex shedding but the Strouhal number remains at the range from 0.15 to 0.35. The drag coefficient of the cylinders with grooves was found to be 2~3 times as large as that of smooth cylinders.

• 대한기계학회논문집B
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• 제24권6호
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• pp.753-759
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• 2000

Photoelectric charging is a very efficient way of charging small particles. This method can be applied to combustion measurement, electrostatic precipitator, metal separation and control of micro-contamination. To understand the photoelectric charging mechanism, particle charging of silver by exposure to ultraviolet is investigated in this study. Average charges and charge distributions are measured at various conditions, using two differential mobility analyzers, a condensation nucleus counter, and an aerosol electrometer. The silver particles are generated in a spark discharge aerosol generator. After that process, the generated particles are charged in the photoelectric charger using low-pressure mercury lamp that emits ultraviolet having wavelength 253.7 nm. The results show that ultra-fine particles are highly charged by the photoelectric charging. The average charges linearly increase with increasing particle size and the charge distribution change with particle size. These results are discussed by comparison with previous experiments and proposed equations. It is assumed that the coefficient of electron emission probability is affected by initial charge. The results also show that the charge distribution of a particle is dependent on initial charge. Single changed particle, uncharged particle and neutralized particle are compared. The differences of charge distribution in each case increase with increasing particle size.

• 대한기계학회논문집B
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• 제31권4호
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• pp.357-366
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• 2007

The present study investigated local heat/mass transfer characteristics on the surface of the rotating turbine blade with various incidence angles. The experiments are conducted in a low speed annular cascade with a single stage turbine. The blade has a flat tip with the mean tip clearance of 2.5% of the blade chord. A naphthalene sublimation method is used to measure detailed mass transfer coefficient on the blade. At design condition, the inlet Reynolds number is $Re_c=1.5{\times}10^5$ which results in the blade rotation speed of 255.8 rpm. Also, the effect of off-design condition is examined with various incidence angles between $-15^{\circ}$ and $+7{\circ}$. The results indicated that the incidence angle has significant effects on the blade surface heat transfer. In mid-span region, the laminar separation region on the pressure side is reduced and the laminar flow region on the suction side shrinks with increasing incidence angle. Near the tip, the effect of tip leakage flow increases in span wise and axial directions as the incidence angle decreases because the tip leakage flow is formed near the suction side surface. However, the effect of tip leakage flow is reduced with positive incidence angle.

• 대한기계학회논문집B
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• 제22권3호
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• pp.373-385
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• 1998

The local Nusselt numbers have been measured for a round turbulent jet impinging on the convex surface with and without rib. Liquid crystal/transient method was used to determine the Nusselt number distributions along the surface. The temperature on the surface was measured to within .+-.0.25 deg. C accuracy using liquid crystal and a digital color image processing system. The experiments were made for the jet Reynolds number (Re) 23,000, the dimensionless nozzle-to-surface distance (L/d) from 6 to 10, the dimensionless surface curvature (d/D) 0.056, and the various rib types (height(d$_{1}$) from 1 to 2 mm, pitch (p) from 6 to 32 mm). It was found that the average Nusselt numbers on the convex surface with rib are higher than those without rib, mainly due to an increase in the turbulent intensity caused by flow separation, recirculation and reattachment on the wall surface. In addition, we compared the results by the steady-state method using the gold-film Intrex with those by the transient method.

• 유통과학연구
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• 제11권11호
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• pp.5-11
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• 2013

Purpose - Considering corporate companies that are continually growing and bearing in mind the theory of agency, how confident can stakeholders be about their benefits in relation to managers' decisions? Previous research has indicated that the type of corporate governance can have an effective impact on companies' performance. The current study aims to investigate the impact of ownership structure on listed companies on the Tehran Stock Exchange. Research Design, Data, and Methodology - Through use of the correlation coefficient, the results indicate a positive correlation among the percentage of common stock held by board members, the percentage of non-executive board members, and separation of the positions of chairperson of the board of directors and managing director. Results - Based on the return on assets index, only the correlation between the proportion of ownership of the managing director and financial investment company ownership is significant. Conclusion -Managers can potentially make decisions that benefit themselves but are detrimental to shareholders' interests. Corporate governance is a factor that can mitigate agency costs. Corporate governance comprises the laws, regulations, structures, processes, cultures, and systems that lead to the achievement of objectives such as accountability, transparency, justice, and stakeholders' rights.

• 대한원격탐사학회:학술대회논문집
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• 대한원격탐사학회 2007년도 Proceedings of ISRS 2007
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• pp.258-261
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• 2007

Although there have been lack of studies using X-band SAR data particularly for forestry application as compared to C-, and L-band SAR data, it has a potential to distinguish tree species because most signals are backscattered on the top of canopy. This study aimed to compare signal characteristics of multi-band SAR data including X-band for classifying tree species. The data used for the study are SIR-C/X-SAR data (X-, C-, L-band) obtained on Oct. 3, 1994 over the forest area near Seoul, S. Korea. Thirty ground sample plots were collected per each tree species. Initial comparison of backscattering coefficients among three SAR bands shows that X-band data showed better separation of tree species than C- and L-band SAR data irrespective of polarization. The weak penetrating in canopy layer might be possible source of information for X-band data to be useful for the classification of forest species and cover type mapping.

• 한국소음진동공학회논문집
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• 제15권2호
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• pp.206-212
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• 2005

In many machines handling lightweight and flexible media, such as magnetic tape drives, xerographic copiers and sewing machines, the media must transit an open space. It is important to predict the static and dynamic behavior of the sheets with a high degree of reliability. The nonlinear theory of the dynamic elastica has often been used to a nonlinear dynamic deflection model. In this paper, the governing equation is derived and simulated by the finite difference method. The parametric cubic curve is applied for defining the guide shape. The dynamic contact conditions suggested by Klarbring is used to predict the direction of the flexible media according to the initial velocity and the friction coefficient. The analysis is also compared to the conventional model, showing that after contacting a $45^{\circ}$ wall, the directions of flexible media of two models are different.

• 생약학회지
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• 제46권4호
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• pp.352-364
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• 2015

The aim of this study was to quantitatively analyze for quality assessment of eighteen marker compounds, including homogentisic acid, 3,4-dihydroxybenzaldehyde, spinosin, liquiritin, hesperidin, ginsenoside Rg1, liquiritigenin, ginsenoside Rb1, glycyrrhizin, 6-gingerol, atractylenolide III, honokiol, costunolide, dehydrocostuslactone, atractylenolide II, nootkatone, magnolol, and atractylenolide I, in Hyangsayukgunja-tang using an ultra-performance liquid chromatography-electrospray ionization-mass spectrometer. The column for separation of eighteen marker components were used a UPLC BEH $C_{18}$ analytical column ($2.1{\times}100mm$, $1.7{\mu}$) and kept at $45^{\circ}C$ by gradient elution with 0.1% (v/v) formic acid in water and acetonitrile as mobile phase. The flow rate and injection volume were 0.3 mL/min and $2.0{\mu}l$, respectively. The correlation coefficient of all marker compounds was ${\geq}0.9914$, which means good linearity, within the test ranges. The limits of detection and quantification values of the all analytes were in the ranges 0.04-1.11 and 0.13-3.33 ng/mL, respectively. As a result, five compounds, homogentisic acid, 3,4-dihydroxybenzaldehyde, spinosin, liquiritigenin, and atractylenolide I, in this sample were not detected and the amounts of the 13 compounds except for the 5 compounds were $8.10-6736.37{\mu}g/g$ in Hyangsayukgunja-tang extract.

• 한국재료학회지
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• 제19권5호
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• pp.288-292
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• 2009

This article shows various factors that influence the thermal-cycling reliability of semiconductor devices utilizing the lead-on-chip (LOC) die attach technique. This work details how the modification of LOC package design as well as the back-grinding and dicing process of semiconductor wafers affect passivation reliability. This work shows that the design of an adhesion tape rather than a plastic package body can play a more important role in determining the passivation reliability. This is due to the fact that the thermal-expansion coefficient of the tape is larger than that of the plastic package body. Present tests also indicate that the ceramic fillers embedded in the plastic package body for mechanical strengthening are not helpful for the improvement of the passivation reliability. Even though the fillers can reduce the thermal-expansion of the plastic package body, microscopic examinations show that they can cause direct damage to the passivation layer. Furthermore, experimental results also illustrate that sawing-induced chipping resulting from the separation of a semiconductor wafer into individual devices might develop into passivation cracks during thermal-cycling. Thus, the proper design of the adhesion tape and the prevention of the sawing-induced chipping should be considered to enhance the passivation reliability in the semiconductor devices using the LOC die attach technique.

• 설비공학논문집
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• 제17권3호
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• pp.256-261
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• 2005

The objective of the present study is to study the Soret effect of both nanoparticles and solute on the convective instabilities in binary nanofluids. A new stability criterion is obtained based on the linear stability theory. The results show that the Soret effect of solute(${\psi}_{bf}$) makes the binary nanofluids unstable significantly and the convective motion in a binary nanofluid sets in easily as the ratio of Soret coefficient of nanofluid to that of binary basefluid ${\delta}_4$ increases for ${\delta}_4$ > -1. It is also found that as an increase of the volume fraction of nanoparticles, nanofluid becomes stable but at a separation ratio of ${\psi}=-0.3$ the state of fluid changes from stable to unstable.