• Transactions of the Korean Society of Mechanical Engineers B
• /
• 제26권7호
• /
• pp.911-918
• /
• 2002

Evaporation heat transfer coefficients of carbon dioxide($CO_2$), R-22, and R-134a in a horizontal smooth tube were measured and analyzed as a function of heat flux, mass flux, and evaporating temperature. The experiments were carried out by varying heat flux from 10 to 20 $kW/m^2$, mass flux from 170 to 340 $kg/m^2s$, and saturation temperatures of 5 and $10^{\circ}C$. It was found that the heat transfer coefficient of $CO_2$ decreased with a rise of quality due to an earlier liquid-film dryout as compared to R-22 and R-134a. Averaged heat transfer coefficients of $CO_2$ were 22-63% higher than those of R-22 and R-134a at all test conditions. The effects of mass flux and heat flux on averaged heat transfer coefficients were much greater in $CO_2$ than in R-22 and R-134a. When comparing $CO_2$ test results with the correlations in the literature, the existing models yielded large deviations at medium and high qualities. Therefore, a generalized correlation for $CO_2$evaporation heat transfer needs to be developed by including the effects of dryout phenomenon.

• Wind and Structures
• /
• 제31권4호
• /
• pp.287-298
• /
• 2020

This paper presents an experimental investigation on the surface pressures on the CAARC standard tall building model concerning the effects of freestream turbulence. Two groups of incidence turbulence are generated in the wind tunnel experiment. The first group has an approximately constant turbulence intensity of 10.3% but different turbulence integral scale varying from 0.141 m to 0.599 m or from 0.93 to 5.88 in terms of scale ratio (turbulence integral scale to building dimension). The second group presents similar turbulence integral scale but different turbulence intensity ranging from 7.2% to 13.5%. The experimental results show that the mean pressure coefficients on about half of the axial length of the side faces near the leading edge slightly decrease as the turbulence integral scale ratio that is larger than 4.25 increases, but respond markedly to the changes in turbulence intensity. The root-mean-square (RMS) and peak pressure coefficients depend on both turbulence integral scale and intensity. The RMS pressure coefficients increase with turbulence integral scale and intensity. As the turbulence integral scale increases from 0.141 m to 0.599 m, the mean peak pressure coefficient increases by 7%, 20% and 32% at most on the windward, side faces and leeward of the building model, respectively. As the turbulence intensity increases from 7.2% to 13.5%, the mean value of peak pressure coefficient increases by 47%, 69% and 23% at most on windward, side faces and leeward, respectively. The values of cross-correlations of fluctuating pressures increase as the turbulence integral scale increases, but decrease as turbulence intensity increases in most cases.

• Journal of the Korean Geotechnical Society
• /
• 제26권11호
• /
• pp.99-110
• /
• 2010

A fundamental study of the pile-soil systems subjected to negative skin friction in soft soil was conducted using the long-term field measurements. The emphasis was on the identification of the magnitude and distribution of skin frictions ($\alpha$ and $\beta$ coefficients) in bitumen coated and uncoated piles. A skin friction coefficient of instrumented piles is back-calculated by varying degrees of consolidation (U) of surrounding soils. It is shown that the bitumen coated pile is capable of reducing the negative skin friction up to almost 50 to 90 percents. Through comparisons with the existing friction coefficient values ($\alpha$ and $\beta$ coefficients), the calculated coefficients are within the appropriate range, and thus we can suggest basic materials to estimate the realistic pile behavior in the short-term and long-term analysis.

• Journal of the Korean Statistical Society
• /
• 제29권1호
• /
• pp.45-62
• /
• 2000

Rotation design is a sampling technique to reduce response burden and to estimate the population characteristics varying in time. Park and Kim(1999) discussed a generation of one-level rotation design which is called as {{{{r_1^m ~-r_2^m-1}}}} design has more applicable form than existing before. In the structure of {{{{r_1^m ~-r_2^m-1}}}} design, we derive the exact variances of generalized composite estimators for level, change and aggregate level characteristics of interest, and optimal coefficients minimizing their variances. Finally numerical examples are shown by the efficiency of alternative designs relative to widely used 4-8-4 rotation design. This is continuous work of Part Ⅰ studied by Park and Kim(1999).

• Journal of Institute of Control, Robotics and Systems
• /
• 제21권12호
• /
• pp.1160-1166
• /
• 2015

This paper proposes a decentralized robust adaptive control scheme for robot manipulators with input torque saturation in the presence of uncertainties. The control system should consider the practical problems that the controller gain coefficients of each joint may be nonlinear time-varying and the input torques applied at each joint are saturated. The proposed robot controller overcomes the various uncertainties and the input saturation problem. The proposed controller is comparatively simple and has no robot model parameters. The proposed controller is adjusted by the adaptation laws and the stability of the control system is guaranteed by the Lyapunov function analysis. Simulation results show the validity and robustness of the proposed control scheme.

• Bulletin of the Korean Chemical Society
• /
• 제35권5호
• /
• pp.1469-1478
• /
• 2014

Use of acetonitrile in electrolytes promotes better operation of supercapacitors. Recent efforts show that electrolytes containing acetonitrile can also function in a wide range of operating temperatures. Therefore, this paper addresses the dielectric relaxation processes, structure and dynamic properties of the bulk acetonitrile at various temperatures. Systems of acetonitrile were modeled using canonical ensemble and simulated by employing Molecular Dynamics method. Results show that interactions among the molecules were correlated within a cut-off radius while parallel and anti-parallel arrangements are observed beyond this radius at relatively high and low temperatures respectively. Furthermore, effects of C-C-N and C-H bending modes were greatly appreciated on the power spectral density of time rate change of dipole-dipole correlations whereas frequency shifts were observed on all modes at the lowest temperature under consideration. Linear variations with temperature were depicted for reorientation times and self-diffusion coefficients. Shear viscosity was also computed with a good accuracy within a certain range of the temperature as well.

• Journal of the Korean Institute of Telematics and Electronics B
• /
• 제31B권7호
• /
• pp.16-25
• /
• 1994

The major reason that reduces the performance of satellite communication channels is known as the nonlinearities of high power amplifiers(HPA's) which are usually modeled by memoryless nonlinear systems, in the channels. This paper proposes a nonlinear adaptive predistorter,which predistorts the input symbols such that the symbol passing through the nonlinear HPA preserves the desired constellation, and derives the region of convergence for the predistorter. The predistorter has capability of coping adaptively with any time-varying environment such as changes of operating points if HPA since the coefficients of the predistorter which has a form of truncated Tayler series are obtained by recursive optimization procedure utilizing QAM data sequence directely. Simulation results show that the proposed one with 16 QAM input has better performance than the conventional fixed predistorter with the same input.

• Proceedings of the KSME Conference
• /
• 대한기계학회 2001년도 춘계학술대회논문집D
• /
• pp.483-488
• /
• 2001

In the present study a numerical simulation is performed on a natural convection inside a square cavity with a vibrating wall. The study has been conducted varying the heat transfer rate, wall excitation frequency and also the orientation of the cavity. The temperature and velocities inside the cavity was observed and also, the heat transfer coefficients on the heating wall was seen. From the results, it can be seen that the temperature inside the cavity decreases when excited with the proper frequency and the heat transfer coefficient increased with cavity inclination angle, ${\theta}$. It is also found from the results that flow resonance is occurred near the inclination angle ${\theta}=90^{\circ}$.

• Transactions of the Korean Society of Mechanical Engineers
• /
• 제17권9호
• /
• pp.2162-2171
• /
• 1993

Geneal Structure optimization is utilized to minimize the weight of structures while satisfying constraints imposed on stress, displacements and natural frequencies, etc. Sandwich structures consist of inside core and outside face sheets. The selected sandwich structures are isotropic sandwich beams and isotropic sandwich plate. The face sheets are treated as membrane and assumed to carry only tensions, while the core is assumed to carry only transverse shear. The characteristic of the varying area are considered by adding the projected component of the tension to the transverse shear. The bending theory and energy method are adopted for analyzing sandwich beams and plates, respectively. In the optimization process, the cost function is the weight of a structure, and a deflection and stress constraints are considered. Design variable are thickness and tapering coefficients which determine the shape of a structure. An existing optimization code is used for solving the formulated problems.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• 제33권10호
• /
• pp.42-50
• /
• 2005

Based upon the derived analytical model and equation of motion for the general rotor system with anisotropic stator and asymmetric rotor treated as a periodically time-varying system, the series of equations are structured by associating with the time modulated coefficients. The frequency response functions (FRFs) expressed by physical parameters are derived in such a convenient way from the direct inverse matrices of the Fourier transformation of those series of equations, from which the characteristics are analyzed and the properties are suggested.