• Journal of the korean Society of Automotive Engineers
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• v.2 no.1
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• pp.32-38
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• 1980

In the case of a automobile marking skid on road for a period of braking, in general, the automobile velocity just before rapid braking can be obtained by evaluating the coefficient of friction between tires and road. Up to now, the coefficient of friction has been derived from mean velocity measured by a time watch, but the automobile velocity obtained in this manner would be deviated from actual value considerably, due to errors arising from not only measuring time but other various factors. In this paper the presumption method of automobile velocity by accelerometer is presented so as to improve the accuracy of measurement, and to determine the velocity readily. The results obtained in this experiment show that the frictional coefficients between tires and road under the given experimental conditions are considered to take linear relation over the fixed velocity limits 30km/h to 50km/h while for the same limits of velocities the coefficients of friction by the time watch method are not valid ar low velocity range. It will be seen that the former is simple and reliable whilst the latter is cumbersome and unreliable.

• Journal of the Korean Society of Food Science and Nutrition
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• v.17 no.4
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• pp.371-375
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• 1988

Tubular ultrafiltration membranes were used to investigated mass transfer characteristics of waste cheese whey. The effects of bulk concentration and flow velocity on permeat flux, mass transfer coefficient and apparent rejection coefficient were measured. Mass transfer coefficient was increased linearly with increasing flow velocity, and following relationship between mass transfer coefficient(k) and linear velocity(u) was obtained. $k=0.87{\times}10^{-5}u^{1-1}$ It is interjecting to note that plots for all linear velocity tend to converge to the same point for zero permeating flux, and the maximum bulk concentration that can be achieved with cheese whey extracts was 38(w/v %). In general, membrane rejection coefficient increased with increasing flow velocity and the rejection coefficients of cheese whey solution and that of lactose in cheese whey solution were obtained $0.40{\sim}0.65$, $0.15{\sim}0.30$, respectively.

• Journal of Korean Tunnelling and Underground Space Association
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• v.20 no.2
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• pp.405-421
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• 2018

In most of cases, the wall friction coefficients applied for local tunnel design are quoted directly from foreign data or local design guideline. In the previous studies, the wall friction coefficient was estimated using the velocity decay method. However, it is difficult to estimate the wall friction coefficient when the convergence wind velocity in the tunnel is negative (-) or if there is a change in the natural wind. Therefore, in this study, the wall friction coefficient is estimated by applying the dynamic simulation technique in addition to the conventional the velocity decay method. As a result of the analysis, the coefficient of wall friction in the tunnels for the total of 9 tunnels (18 tubes both directions) was 0.011~0.025, and the mean value was estimated to be 0.020. In addition, the wall friction coefficient obtained quantitatively through this study was compared with the current design criteria.

• Korean Journal of Materials Research
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• v.31 no.5
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• pp.286-295
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• 2021

This study attempts to find optimal conditions of the friction coefficient using a discrete element method (DEM) simulation with various friction coefficient conditions and three different grinding media with various ball sizes in a traditional ball mill (TBM). Using ball motion of the DEM simulation are obtained using the optimal friction coefficient compared with actual motion; photographs are taken by the digital camera and the snapshot images are analyzed. In the simulation, the rotation speed of the mill, the materials and velocity of the grinding media, and the friction coefficient between the balls and the wall of the pot are fixed as the actual experimental conditions. We observe the velocity according to the friction coefficient from the DEM simulation. The friction coefficient is found to increase with the velocity. Milling experiments using a traditional ball mill with the same experimental conditions as those of the DEM simulation are conducted to verify the simulated results. In addition, particle morphology change of copper powder is investigated and analyzed using scanning electron microscopy (SEM) for the milling experiment.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.21 no.4
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• pp.677-682
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• 2012

For the product with small diameter, long column, and parabolic shape, the forging formability of the high-carbon steel wire rod was investigated in this study. By using the three-dimensional finite element method, the formability of wire was reviewed by forming analysis for the desired parabolic shape of local part. Analysis results due to forging direction, forging velocity, friction coefficient and constraint location were also investigated. On the basis of these results, it is noted that the forging direction has the big influence when the product with long column is forged. As the forging velocity increases, buckling tends to be limited and formability of parabolic shape is improved. By constraining the lower parabolic shape part to suppress plastic strain, the effect depending on friction coefficient is not almost appeared. And good parabolic shape is obtained at the region of the forging velocity of more than 0.5 m/s.

• Journal of the Korea institute for structural maintenance and inspection
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• v.3 no.2
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• pp.177-183
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• 1999

The major object of this study is to investigate experimentally the experimental equation by the non-destructive testing methods of ultrasonic pulse velocity, rebound number, combined method of ultrasonic pulse velocity and rebound number, maturity which are applicable to the evaluation of compressive strength of concrete at early ages. Also test result of mix are statistically analyzed to infer the correlation coefficient between the maturity and the compressive strength of concrete. The results show good application of Logistic curve for estimating strength development under various curing temperature. The relation between ultrasonic pulse velocity, rebound number, combined method of ultrasonic pulse velocity and rebound number and compressive strength of concrete have low correlation coefficient, but maturity method show good correlation coefficient.

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.5
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• pp.86-96
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• 1998

A numerical study has been carried out of three-dimensional turbulent flows around a MIRA reference vehicle model both with and without wheels in computation. Two convective difference schemes with two k-$\varepsilon$ turbulence models are evaluated for the performance such as drag coefficient, velocity and pressure fields. Pressure coefficients along the surfaces of the model are compared with experimental data. The drag coefficient, the velocity and pressure fields are found to change considerably with the adopted finite difference schemes. Drag forces computed in the various regions of the model indicate that design change decisions should not rely just on the total drag and that local flow structures are important. The results also indicate that the RNG model with the QUICK scheme predicts fairly well the tendency of velocity and pressure fields and gives more reliable drag coefficient rather than the other cases.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.05a
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• pp.148-151
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• 2004

The hardening of concrete after setting is accompanied with nonlinear temperature distribution caused by development of hydration heat of cement. Especially at early ages, this nonlinear distribution has a large influence on the tensile cracking. As a result, in order to predict the exact temperature distribution in concrete structures it is required to examine thermal properties of concrete. In this study, the coefficient of air convection for concrete mix of nuclear power plant, which presents thermal transfer between surface of concrete and air, was experimentally investigated with variables such as velocity of wind and types of form. The coefficient of air convection obtained from experiment increases with velocity of wind, and its dependance on wind velocity is varied with types of form. This tendency is due to a combined heat transfer system of conduction through form and convection to air. The coefficient of air convection for concrete mix of nuclear power plant obtained from this study was well agreed with the existing models.

• The Journal of the Acoustical Society of Korea
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• v.22 no.4E
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• pp.155-160
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• 2003

Both ultrasonic velocity at 3 MHz and absorption coefficient for the frequency range of 0.2-2 MHz were measured in an aqueous solution of polyacrylamide for the concentration range of 0.5% to 2.5% by weight. Pulse echo overlap method was taken for measuring the ultrasonic velocity over the temperature range of 10-90℃ and the high-Q ultrasonic resonator method was used for the absorption coefficient at 30℃. The velocity exhibited a maximum value at approximately 70℃, 71℃, 72℃, 73℃ and 74℃ in 2.5%, 2.0%, 1.5%, 1.0%, and 0.5% solutions, respectively. The velocity increased with the concentration at a given temperature. The ultrasonic absorption (a/f²) at a given temperature increased linearly with the concentration for the concentration below 1.5%, but suddenly increased for the concentration above 1.5% concentration. The value of a/f² at 1MHz was entirely due to the classical Stoke's viscous effect. The ultrasonic relaxation in polyacrylamide aqueous solutions, which may be the result of structural fluctuations of polymer molecules such as the segmental motion of the polymer chains, was observed, and at 2.5%, the value of a/f² was found to suddenly increase as frequency decreased.

• Journal of Power System Engineering
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• v.17 no.4
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• pp.72-78
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• 2013

Compacted graphite iron 340 was carried out the heat treatment from 873 to 1,273 K. Compacted graphite iron 340 was evaluated relationship between the sound velocity, the attenuation coefficient and the tensile strength. The obtained results are as following. The signal strength of C scan images were weak according to increasing of heat treatment temperature and time. The amplitude of A scan and B scan was also low. This can be cause that the graphite was grown into the type of vermicular, and the many of grain boundary with ultrasound scattering were increase. The sound velocity was depend upon the heat treatment temperature and time, the attenuation coefficient had nothing to do with the temperature and time. The higher the heat treatment temperature, the tensile strength and the sound velocity were decreased. However, the tensile strength was proportional to the sound velocity. The higher tensile strength, the faster the sound velocity.