• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.18 no.4
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• pp.313-320
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• 2005

Stick-slip friction is one of the material removal mechanisms in tribology. It occurs when the static friction force is larger than the dynamic friction force, and make the friction curve fluctuated. In the friction monitoring of chemical mechanical polishing(CMP), the friction force also vibrates just as stick-slip friction. In this paper, an attempt to show the similarity between stick-slip friction and the friction of CMP was conducted. The prepared hard pa(IC1000/Suba400 stacked/sup TM/) and soft pad(Suba400/sup TM/) were tested with SiO₂ slurry. The friction force was measured by piezoelectric sensor. According to this experiment, it was shown that as the head and table velocity became faster, the stick-slip time shortened because of the change of real contact area. And, the gradient of stick-slip period as a function of head and table speed in soft pad was more precipitous than that of hard one. From these results, it seems that the fluctuating friction force in CMP is stick-slip friction caused by viscoelastic behavior of the pad and the change of real contact area.

• Journal of Mechanical Science and Technology
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• v.17 no.6
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• pp.925-934
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• 2003

An experimental study is performed to investigate the characteristics of near wake flow behind a circular cylinder with serrated fins using a constant temperature anemometer and flow visualization. Various vortex shedding modes are observed. Fin height and pitch are closely related to the vortex shedding frequency after a certain transient Reynolds number. The through velocity across the fins decreases with increasing fin height and decreasing fin pitch. Vortex shedding is affected strongly by the velocity distribution just on top of the finned tube. The weaker gradient of velocity distribution is shown as increasing the freestream velocity and the fin height, while decreasing the fin pitch. The weaker velocity gradient delays the entrainment flow and weakens its strength. As a result of this phenomenon, vortex shedding is decreased. The effective diameter is defined as a virtual circular cylinder diameter taking into account the volume of fins, while the hydraulic diameter is proposed to cover the effect of friction by the fin surfaces. The Strouhal number based upon the effective diameters seems to correlate well with that of a circular cylinder without fins. After a certain transient Reynolds number, the trend of the Strouhal number can be estimated by checking the ratio of effective diameter to inner diameter. The normalized velocity and turbulent intensity distributions with the hydraulic diameter exhibit the best correlation with the circular cylinder's data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.6
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• pp.823-832
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• 2002

A modified $textsc{k}$-$\varepsilon$model is proposed for calculation of transitional boundary-layer flows with changing pressure gradient. In order to develop the model for this problem, the flow is divided into three regions; pre-transition region, transition region and fully turbulent region. The effect of pressure gradient is taken into account in stream-wise intermittency factor, which bridges the eddy-viscosity models in the pre-transition region and the fully turbulent region. From intermittency data in various flows, Narashima＇s intermittency function, F(${\gamma}$), has been found to be proportional to $\chi$$^{n}$ according to the extent of pressure gradient. Three empirical correlations of intermittency factor being analyzed, the best one was chosen to calculate three benchmark cases of bypass transition flows with different free-stream turbulence intensity under arbitrary pressure gradient. It was found that the variations of skin friction and shape factor as well as the profiles of mean velocity in the transition region were very satisfactorily predicted.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.1
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• pp.88-96
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• 1987

A method is presented to calculate the fiction factor and velocity distribution of asymmetric flow in rough channels and annuli. This method is derived from an assumption of linearly-varying shear stress which is different from Maubach's constant shear stress assumption. This method gives continuous velocity gradient at the position of maximum velocity and the predicted velocity profiles are in better agreements with experimental data than those obtained by Maubach's method. In this paper, a new correlation of roughness function is proposed and the predictions of friction factors by this correlation agree well with experimental data.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.1
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• pp.87-94
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• 2011

Characteristics of two-phase pressure drop in microchannels were investigated experimentally. The microchannels consisted of 9 parallel trapezoidal channels with each channel having $205\;{\mu}m$ of bottom width, $800\;{\mu}m$ of depth, $3.6^{\circ}$ of sidewall angle, and 7 cm of length. Pressure drops in convective boiling of Refrigerant 113 were measured in the range of inlet pressure 105~195 kPa, mass velocity $150{\sim}920\;kg/m^2s$, and heat flux $10{\sim}100\;kW/m^2$. The total pressure drop generally increased with increasing mass velocity and/or heat flux. Two-phase frictional pressure drop across the microchannels increased rapidly with exit quality and showed bigger gradient at higher mass velocity. A critical review of correlations in the literature suggested that existing correlations were not able to match the experimental results obtained for two-phase pressure drop associated with convective boiling in microchannels. A new correlation suitable for predicting two-phase friction multiplier was developed based on the separated flow model and showed good agreement with the experimental data.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.6
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• pp.632-639
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• 2012

A fully coupled thermo-mechanical model is adopted to study the temperature distribution and the material deformation in friction stir welding(FSW) process. Rotational speed is most important parameters in this research. Three dimension results under different process parameters were presented. Result indicate that the maximum temperature is lower than the melting point of the welding material. The higher temperature gradient occurs in the leading side of the workpiece. The maximum temperature can be increased with increasing the tool angular velocity, rpm in the current numerical modeling. In this research ABAQUS Ver.6.7 is to analyze a fully coupled thermo-mechanical model. ALE(Arbitrary Lagrangian-Eulerian) finite element formulation is used for the large deformation in FSW process and using the Mass scaling for the analysis time efficiency.

• Journal of the Society of Cosmetic Scientists of Korea
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• v.31 no.2 s.51
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• pp.135-140
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• 2005

A solid state emulsion haying high velocity gradient shows two important transition ranges in the plot of storage modulus(G') as a function of shear strain, when the state is changed from solid to liquid. However, a solid state emulsion having low velocity gradient shows only one apparent transition range when the change from solid to liquid state takes place. The result implies the importance of the surface properties in the solid state emulsion. The addition of water phase in the solid state emulsion reduces the modulus in the modulus in the surface transition range by increasing interfacial friction and weakening the matrix. The addition of pigments increases the modulus in the modulus in the surface transition range by reinforcing the matrix, when there is no wafer phase in the solid state emulsion. When the solid state emulsion has water phase, however, the addition of pigments decreases the modulus in the modulus in the surface transition range.

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.6
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• pp.155-165
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• 1998

The resistance coefficient and heat transfer performance are studied for the turbulent water flow in a smooth coiled tube having variable curvature ratios and a corrugated-coiled tube having a ratio of coil to tube diameter of 22. Experiments are carried out for the fully developed turbulent flow of water in tube coils on the uniform wall temperature condition. This work is limited to tube coils of R/a between 22 and 60 and Reynolds numbers from 13000 to 53000. The tube having a ratio of coil to tube diameter of 27 among the 3 smooth tube coils shows the best heat transfer performance. A corrugated-coiled tube(R/a=60) shows more excellent performance than a smooth coiled tub (R/a=60) at a similar curvature ratio. The friction factor f is sensitive to changes in the velocity profile caused by a temperature gradient. Allowance was made for the pressure loss in the short inlet and outlet lengths and due to the presence of the thermocouple inlet and outlet as a result of separate experimental on a straight tube. It is to be expected that the allowance at the exit will be somewhat too low because of secondary flow effects carried over from the coil.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.347-352
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• 2000

The pressure drop characteristics of R-22 and R-410A(a mixture of 50wt% R-32 and 50wt% R-125) flowing in a small diameter tube with 1.77[mm] inner diameter and 3.14[mm] outer diameter was investigated experimentally. the mass fluxes of refrigerants are ranged from 450 to $1050[kg/(m^2{\cdot}s)]$ and the qualites are varied from 0.05 to 0.95. The main experimental results were summarized as follows; The single-phase liquid friction factors for small diameter tubes are higher than those predicted by the Blasius equation. In case of two-phase flow, the pressure gradient of the small diameter tube increases with increasing mass velocity and vapor quality. The experimental data are not well correlated by predictions which were proposed for the large diameter tube.

• Journal of Integrative Natural Science
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• v.6 no.1
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• pp.53-56
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• 2013

We are interested in the rate of convergence of solutions of 2D Navier-Stokes equations in a smooth bounded domain as the viscosity tends to zero under Navier friction condition. If the initial velocity is smooth enough($u{\in}W^{2,p}$, p>2), it is known that the rate of convergence is linearly propotional to the viscosity. Here, we consider the rate of convergence for nonsmooth velocity fields when the gradient of the corresponding solution of the Euler equations belongs to certain Orlicz spaces. As a corollary, if the initial vorticity is bounded and small enough, we obtain a sublinear rate of convergence.