• The Journal of the Acoustical Society of Korea
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• 제25권2E호
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• pp.60-68
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• 2006

High-frequency bistatic scattering measurements from a corrugated surface were made in an acoustic water tank. First the azimuthal scattering pattern was measured from an artificially corrugated surface which has varying impedance. The corrugated surface was installed both transverse to the direction of incident wave and longitudinal to the direction of incident wave. The angle between the corrugated surface and the direction of the incident wave was about $45^{\circ}$. Second, the scattering strengths were measured from the flat sediment and the corrugated sediment. A critical angle of about $37^{\circ}$ was calculated in the acoustic water tank. The measurements were made at three fixed grazing angles: $33^{\circ}$ (lower than critical angle), $37^{\circ}$ (critical angle), and $41^{\circ}$ (higher than critical angle). The scattering angle and the grazing angle are equal in each measurement. Frequencies were from 50 kHz to 100 kHz with an increment of 1 kHz. The corrugated sediment was made transverse to the direction of the incident wave. The first measurement indicates that the scattering patterns depend on the relations between the corrugated surface and the direction of the incident wave. In the second measurement, the data measured from the flat sediment were compared to the APL-UW model and to the NRL model. The NRL model's output shows more favorable comparisons than the APL-UW model. In case of the corrugated sediment, the model and the measured data are different because the models used an isotropic wave spectrum of sediment roughness in the scattering calculations. The isotropic wave spectrum consists of $w_2$ and ${\gamma}_2$. These constants derived from sediment names or bulk size. The model which used the constants didn't consider the effect of a corrugated surface. In order to consider a corrugated surface, the constants were varied in the APL-UW model.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 1999년도 봄 학술발표회 논문집
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• pp.331-338
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• 1999

In the present study, a searching technique for critical slip surface in two dimensional slope stability analysis is proposed. The failure surface generation and analysis has been usually limited to simple geometric shapes. However, more random surfaces need to be examined for some particular ground conditions. For this purpose, random searching technique is developed using genetic algorithm. The generalized limit equilibrium method is employed as the method of stability analysis. Using this technique, the factor of safety is compared with the result by using simplified Bishop's method. In addition, the convergent trend of fitness value is analyzed.

• 소성∙가공
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• 제7권1호
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• pp.66-71
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• 1998

The present study is concerned with the hydrostatic extrusion process of copper-clad aluminium bar to investigate the bonding conditions as well as the basic flow characteristics. Considering the bonding mechanism of bi-metal contact surface as cold pressure welding the normal pressure and the contact surface expansion are selected as process parameters governing the bonding conditions, in this study the critical normal pressure required for the local extrusion-the protrusion of virgin surfaces by the surface expansion at the interface-is obtained using a slip line method and is then used as a criteron for the bonding. A rigid plastic finite element method is used to analyze the steady state extrusion process. The interface profile of bi-metal rod is predicted by tracking the paths of two particles adja-process. The interface profile of bi-metal rod is predicted by tracking the paths of two particles adja-cent to interface surface. The contact surface area ration and the normal pressure along the interface are calculated and compared to the critical normal pressure to check bonding. It is found that the model predictions are generally in good agreement with the experimental observations. The compar-isons of the extrusion pressure and interface profile by the finite element with those by experi-ments are also given.

• 한국군사과학기술학회지
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• 제4권1호
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• pp.207-215
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• 2001

Plastic bonded explosive(PBX) is mainly composed of the nitramine-ploymer compositions. PBX is characterized by high velocity and pressure of detonation, low vulnerability and good thermal stability. Many important applications of PBX require the good adhesion between nitramine crystals and the binder. For PBXs as well as propellants, where good mechanical properties are of great importance, dewetting therefore must be prevented by strong adhesion between filler-binder. Adhesion depends on surface characteristics of filler and binder. In order to design for better adhesion, an understanding of the surface properties of explosive and binder is required. The surface free energies are calculated from contact angle values by the method of Kaelble. Critical surface tension of solids are calculated by Zisman plot. Critical surface tension is a useful parameter for characterizing the wettability of solid surface. In this study, HMX and 3 kinds of copolymers are selected, since they are widely used in many plastic bonded explosives. The technical objective of this investigation is to predict the interaction between filler and binder from their surface free energies.

• 대한화학회지
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• 제16권4호
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• pp.205-207
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• 1972

새로히 합성한 dodecylquinolinium bromide의 수용액에서 표면장력과 전기전도도 측정치로 부터 critical micelle concentration을 결정하였다. 표면장력법으로 결정한 c.m.c.의 값은 $3.34 {\times} 10^{-3}mole/l$이고 전기전도도법으로 결정한 값은 3.41 {\times} $10^{-3}$mole/l이었다. 이값들은 dodecylpyridinium bromide의 그것보다 작은데 이는 dodecylquinolinium bromide의 소수성 group이 큰데서 오는 것으로 생각된다.

• 오토저널
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• 제9권5호
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• pp.41-48
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• 1987

The aerodynamic critical phenomenon of notch-back type automobile-like bodies was investigated experimentally. The aerodynamic forces were measured for the various bodies of different back-light rake angle at R$_{e}$0.8*10$^{5}$ , 1.0*10$^{6}$ and 1.4*10$^{6}$ . Also, surface flow visualization was effected by the oil mixture. It was found that the critical phenomenon for the notch-back type bodies was milder than for the hatch-back type bodies; the drag vs. slant angle curve for the notch-back type bodies exhibiting local maximum was much smoother. Surface oil flow visualization revealed that the flow pattern associated with the critical phenomenon was characterized by the reattachment of the separated flow on the boot. The effect of the forebody on the critical phenomenon was seen to be negligible.e.

• 대한기계학회논문집B
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• 제33권10호
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• pp.729-736
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• 2009

Pool boiling heat transfer and critical heat flux (CHF) of water-based nanofluids with alumina and titania nanoparticles of 0.01% by volume were investigated on a disk heater at saturated and atmospheric conditions. The experimental results showed that the boiling in nanofluids caused the considerable increase in CHF on the flat surface heater. It was revealed by visualization of the heater surface subsequent to the boiling experiments that a major amount of nanoparticles deposited on the surface during the boiling process. Pool boiling of pure water on the surface modified by such nanoparticle deposition resulted in the same CHF increases as what boiling nanofluids, thus suggesting the CHF enhancement in nanofluids was an effect of the surface modification through the nanoparticle deposition during nanofluid boiling. Possible reasons for CHF enhancement in pool boiling of nanofluids are discussed with surface property changes caused by the nanoparticle deposition.

• Journal of Electrical Engineering and Technology
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• 제11권4호
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• pp.961-968
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• 2016

The non-uniform distribution of contamination on insulator surface has appreciable effects on flashover voltage, and corresponding researches are valuable for the better selection of outdoor insulation. In this paper, two typical types of porcelain and glass insulators which are widely used in ac lines were taken as the research subjects, and their corrections of AC flashover voltage under non-uniform pollution were studied. Besides, their flashover characteristics under different ratio (T/B) of top to bottom surface salt deposit density (SDD) were investigated, including the analysis of flashover voltage, surface pollution layer conductivity and critical leakage current. Test results gave the modified formulas for predicting flashover voltage of the two samples, which can be directly applied in the transmission line design. Also, the analysis delivered that, the basic reason why the flashover voltage increases with the decrease of T/B, is due to the decrease of equivalent surface conductivity of the whole surface and the decrease of critical leakage current. This research will be of certain value in providing references for outdoor insulation selection, as well as in proposing more information for revealing pollution flashover mechanism.

• Structural Engineering and Mechanics
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• 제57권1호
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• pp.179-200
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• 2016

In this paper the differential transformation method (DTM) is utilized for vibration and buckling analysis of nanotubes in thermal environment while considering the coupled surface and nonlocal effects. The Eringen's nonlocal elasticity theory takes into account the effect of small size while the Gurtin-Murdoch model is used to incorporate the surface effects (SE). The derived governing differential equations are solved by DTM which demonstrated to have high precision and computational efficiency in the vibration analysis of nanobeams. The detailed mathematical derivations are presented and numerical investigations are performed while the emphasis is placed on investigating the effect of thermal loading, small scale and surface effects, mode number, thickness ratio and boundary conditions on the normalized natural frequencies and critical buckling loads of the nanobeams in detail. The results show that the surface effects lead to an increase in natural frequency and critical buckling load of nanotubes. It is explicitly shown that the vibration and buckling of a nanotube is significantly influenced by these effects and the influence of thermal loadings and nonlocal effects are minimal.

• Nuclear Engineering and Technology
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• 제54권1호
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• pp.61-71
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• 2022

Pool boiling heat transfer is widely applied in nuclear engineering fields. The influence of heating surface orientation on the pool boiling heat transfer has received extensive attention. In this study, the heating surface with different roughness was adopted to conduct pool boiling experiments at different inclination angles. Based on the boiling curves and bubble images, the effects of inclination angle on the pool boiling heat transfer and critical heat flux were analyzed. When the inclination angle was bigger than 90°, the bubble size increased with the increase of inclination angle. Both the bubble departure frequency and critical heat flux decreased as the inclination angle increased. The existing theoretical models about pool boiling heat transfer and critical heat flux were compared. From the perspective of bubble agitation model and Hot/Dry spot model, the experimental phenomena could be explained reasonably. The enlargement of bubble not only could enhance the agitation of nearby liquid but also would cause the bubble to stay longer on the heating surface. Consequently, the effect of inclination angle on the pool boiling heat transfer was not conspicuous. With the increase of inclination angle, the rewetting of heating surface became much more difficult. It has negative effect on the critical heat flux. This work provides experimental data basis for heat transfer and CHF performance of pool boiling.