• Tribology and Lubricants
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• v.32 no.2
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• pp.56-60
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• 2016

Sapphire has a high hardness and strength and chemical stability as a superior material. It is used mainly as a material for a semiconductor as well as LED. Recently, the cover glass industry used by a sapphire is getting a lot of attention. The sapphire substrate is manufactured through ingot sawing, lapping, diamond mechanical polishing (DMP) and chemical mechanical polishing (CMP) process. DMP is an important process to ensure the surface quality of several nm for CMP process as well as to determine the final form accuracy of the substrate. In DMP process, the material removal is achieved by using the mechanical energy of the relative motion to each other in the state that the diamond slurry is disposed between the sapphire substrate and the polishing platen. The polishing platen is one of the most important factors that determine the material removal characteristics in DMP. Especially, it is known that the geometric characteristics of the polishing platen affects the material removal amount and its distribution. This paper investigated the material removal characteristics and the effects of the polishing platen groove in sapphire DMP. The experiments were preliminarily carried out to evaluate the sapphire material removal characteristics according to process parameters such as pressure, relative velocity and so on. In the experiment, the monitoring apparatus was applied to analyze process phenomena in accordance with the processing conditions. From the experimental results, the correlation was analyzed among process parameters, polishing phenomena and the material removal characteristics. The material removal equation based on phenomenological factors could be derived. And the experiment was followed to investigate the effects of platen groove on material removal characteristics.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.7
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• pp.535-543
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• 2014

A computational simulation for a nonslender BWB UCAV configuration with rounded leading edge and span of 1.0m was performed to analyze its aerodynamic characteristics. The freestream is 50m/s over -4 to 26 degree A.o.A.s. Reynolds number based on the mean chord length is $1.25{\times}10^6$. 3D multi block hexahedral grids are used which allow good grid quality and ease to capture boundary layer. ${\gamma}-Re_{\theta}$ model as well as $k-{\omega}$ SST model is employed to assess the effect of transition for flow behavior. Drag and lift of the UCAV were well predicted while $C_M$ is under predicted at high angle of attacks and influenced by the turbulence models strongly. After assessing pressure distribution, skin friction lines and velocity field around the UCAV configuration, it was found that transition effect should be considered to enhance the prediction of aerodynamic behavior by a vortical flowfield.

• Tunnel and Underground Space
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• v.14 no.5
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• pp.373-380
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• 2004

In this study, the emission rate of pollutant was modified according to the published standards, and the distribution of pollutant concentration was analyzed for each vehicle velocity. This modified emission rate was applied to a model tunnel and it was proved that the required air quantity was reduced to 49%, compared to the PIARC method. From the simulation result, it was proved by using statistics that the most sensitive factor among them is the friction coefficient and it was modified to the value in the range of 0.018 to 0.021. It is also expected that the required air quantity can be decreased form 14.4% to 19.2% according as the coefficient is applied to the domestic model tunnels. In conclusion, it is proposed that the number of jet fans can be reduced and the annual operating cost can be curtailed as well.

• Journal of Ship and Ocean Technology
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• v.1 no.1
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• pp.35-47
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• 1997

The consequences associated with ships running aground depend very much on the soil characteristics of the sea bed and the geometrical shape of the ship bow. The penetration into the sea bed depends on these factors and the penetration is an important factor for the ship motion because it influences the ship heave and pitch motions as well as the friction between the ship and the soil. In this paper a rational calculation model is presented for the sea bed soil reaction forces on the ship bottom. The model is based on the assumption that the penetration of the ship bow generates a flow of pore water through the grain skeleton of the soil. The flow is governed by Darcy\`s law and it is driven by the pressure of the pore water at the bow. In addition to this pore water pressure, the bow is subjected to the effective stresses in the grain skeleton at the bow surface. These stresses are determined by the theory of frictional soils in rupture. Frictional stresses on the bow surface are assumed to be related to the normal pressure by a simple Coulomb relation. The total soil reaction as a function of velocity and penetration is found by integration of normal pressure and frictional stresses over the surface of the bow. The analysis procedure is implemented in a computer program for time domain rigid body analysis of ships running aground and it is verified in the paper through a comparison of calculated stopping lengths, effective coefficients of friction, and sea bed penetrations with corresponding experimental results obtained by model tests as well as large, scale tests.

• Transactions on Electrical and Electronic Materials
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• v.8 no.1
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• pp.5-10
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• 2007

Using a reference slurry, ammonium dodecyl sulfate (ADS), an anionic and environmentally friendly surfactant, was investigated as an alternative to BTA for its inhibition and lubrication characteristics. Results demonstrated that the inhibition efficiency of ADS was superior to that of BTA. Coefficient of friction (COF) was the lowest when the slurry contained ADS. This suggested that adsorbed ADS on the surface provided lubricating action thereby reducing the wear between the contacting surfaces. Temperature results were consistent with the COF and removal rate data. ADS showed the lowest temperature rise again confirming the softening effect of the adsorbed surfactant layer and less energy dissipation due to friction. Spectral analysis of shear force showed that increasing the pad-wafer sliding velocity at constant wafer pressure shifted the high frequency spectral peaks to lower frequencies while increasing the variance of the frictional force. Addition of ADS reduced the fluctuating component of the shear force and the extent of the pre-existing stick-slip phenomena caused by the kinematics of the process and collision event between pad asperities with the wafer. By contrast, in the case of BTA, there were no such observed benefits but instead undesirable effects were seen at some polishing conditions. This work underscored the importance of real-time force spectroscopy in elucidating the adsorption, lubrication and inhibition of additives in slurries in CMP.

• 한국해양학회지
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• v.25 no.1
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• pp.26-35
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• 1990

Sound velocities and attenuation coefficients of marine surface sediments were calculated from insitu acoustic experiments on 4 nearshore areas off Pohang, Pusan Yeosu, and Kunsan around the Korean Peninsula. The relationship between these values and physical properties of sediments was examined and attenuation mechanism was analysed using the estimated gas content. Sound velocities and attenuation coefficients ranging from 1470 to 1616 m/sec and 0.0565 to 0.6604 dB/kHz-m, respectively, are well related to sediment types. The attenuation coefficient is maximum in coarse silts, and the sound velocity increases with density. The gas content estimated less than 8 ppm increases with the decreasing sediment grain size. When the sediment size is greater than fine sand, sound attenuation is mostly due to friction losses, and probably negligible viscous loss remains unchanged with the varying physical properties of sediments. The maximum attenuation in coarse silts result from both friction loss and cohesion of finer sediments between the contacts of silt grains. The cohesion begins to be the dominant dissipative process with decreasing grain size from medium and fine silts.

• Steel and Composite Structures
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• v.35 no.1
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• pp.61-76
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• 2020

In this paper, the dynamic behaviour of an inclined functionally graded material (FGM) beam with different boundary conditions under a moving mass is investigated based on the first-order shear deformation theory (FSDT). The material properties vary continuously along the beam thickness based on the power-law distribution. The system of motion equations is derived by using Hamilton's principle. The finite element method (FEM) is adopted to develop a general solution procedure. The moving mass is considered on the top surface of the beam instead of supposing it on the mid-plane. In order to consider the Coriolis, centrifugal accelerations and the friction force, the contact force method is used. Moreover, the effects of boundary conditions, the moving mass velocity and various material distributions are studied. For verification of the present results, a comparative fundamental frequency analysis of an FGM beam is conducted and the dynamic transverse displacements of the homogeneous and FGM beams traversed by a moving mass are compared with those in the existing literature. There is a good accord in all compared cases. In this study for the first time in dynamic analysis of the inclined FGM beams, the Coriolis and centrifugal accelerations of the moving mass are taken into account, and it is observed that these accelerations can be ignored for the low-speeds of the moving mass. The new provided results for dynamics of the inclined FGM beams traversed by a moving mass can be significant for the scientific and engineering community in the area of FGM structures.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.12
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• pp.1351-1358
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• 2014

This study numerically simulates brake squeal and validates it experimentally by using a lab-scaled brake dynamometer. The system frequencies of the disc brake are traced with respect to the brake pressure by using a modal test and FEM. Then, the squeal frequencies measured from the brake dynamometer are found to correspond to the brake system mode with the dominant displacement of the caliper and pad. Furthermore, a complex eigenvalue analysis conducted using the finite element model confirms that the caliper mode generating the rotational displacement of the pad becomes unstable owing to the negative friction-velocity slope.

• The KSFM Journal of Fluid Machinery
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• v.11 no.1
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• pp.9-17
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• 2008

A numerical study for three-dimensional laminar flow in the entrance region of helical tubes connected with straight ones is carried out to investigate the effects of Reynolds number, pitch and curvature ratio on the oscillation periods of the flow. The fully elliptic governing equations were solved by means of a finite volume method. The fully developed laminar flow boundary condition was applied at the straight tube inlet. This results cover a curvature ratio range of 1/10${\sim}$1/320, a pitch range of 0.0${\sim}$3.2, and a Reynolds number range of 62.5${\sim}$2000. A comparison is made with previous experimental correlations and numerical data. The developments of velocity, local and average friction factors are discussed. The average friction factors are oscillatory in the entrance region of helical pipes. It has been found that the angle required for the flow to be similarly developed is most affected by the curvature ratio. The pitch and Reynolds number do not have any significant effect on the angle. The characteristic angle ${\phi}_c(={\phi}/sqrt{\delta})$, or the characteristic length to diameter ratio $s_c(=l\sqrt{\delta} cos(atan{\lambda})/d)$, can be useful to represent the development of flow in helical tubes. As the pitch increases and as the curvature ratio and Reynolds number decrease, the amplitude and the number of flow oscillations along the main streamwise direction decrease.

• International Journal of Fluid Machinery and Systems
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• v.2 no.4
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• pp.353-362
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• 2009

In the process of turbine modernizations, the investigation of the influences of water passage roughness on radial flow machine performance is crucial and validates the efficiency step up between reduced scale model and prototype. This study presents the specific losses per component of a Francis turbine, which are estimated by CFD simulation. Simulations are performed for different water passage surface roughness heights, which represents the equivalent sand grain roughness height. As a result, the boundary layer logarithmic velocity profile still exists for rough walls, but moves closer to the wall. Consequently, the wall friction depends not only on roughness height but also on its shape and distribution. The specific losses are determined by CFD numerical simulations for each component of the prototype, taking into account its own specific sand grain roughness height. The model efficiency step up between reduced scale model and prototype value is finally computed by the assessment of specific losses on prototype and by evaluating specific losses for a reduced scale model with smooth walls. Furthermore, surveys of rough walls of each component were performed during the geometry recovery on the prototype and comparisons are made with experimental data from the EPFL Laboratory for Hydraulic Machines reduced scale model measurements. This study underlines that if rough walls are considered, the CFD approach estimates well the local friction loss coefficient. It is clear that by considering sand grain roughness heights in CFD simulations, its forms a significant part of the global performance estimation. The availability of the efficiency field measurements provides an unique opportunity to assess the CFD method in view of a systematic approach for turbine modernization step up evaluation. Moreover, this paper states that CFD is a very promising tool for future evaluation of turbine performance transposition from the scale model to the prototype.