• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.10 s.241
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• pp.1092-1100
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• 2005

In the present study, characteristics of steady state laminar flows of a straight duct connected to a 180$^{o}$ curved duct were examined in the entrance region through experimental and numerical analyses. For the analysis, the governing equations of laminar flows in the Cartesian coordinate system were applied. Flow characteristics such as velocity profiles and secondary flows were investigated numerically and experimentally in a square cross-sectional straight duct by the PIV system and a CFD code(STAR CD). For the PIV measurement, smoke particles produced from mosquito coils. The experimental data were obtained at 9 points dividing the test sections by 400 3m. Experimental and numerical results can be summarized as follows. 1) Reynolds number, Re was increased, dimensionless velocity profiles at the outer wall were increased due to the effect of the centrifugal force and secondary flows. 2) The intensity of a secondary flow became stronger at the inner wall rather than the outer wall regardless of Reynolds number. Especially, fluid dynamic phenomenon called conner impact were observed at dimensionless axial position, x/D$_{h}$=50.

• Nuclear Engineering and Technology
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• v.23 no.3
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• pp.275-284
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• 1991

It is important to predict the main feature of fully developed turbulent secondary flow through infinite triangular arrays of parallel rod bundles. One-equation turbulence model which include anisotropic eddy viscosity model was applied to predict the exact velocity field. For a constant properties, Reynolds equations were solved by the finite element method. Mean axial velocity near the wall is simulated by the law of the wall. The numerical results showed good agreement with avaiable experimental data. The strength of the secondary flow increased with Reynolds number but decreased with rod spacing, P/D (pitch-to-diameter). The secondary flow affects remarkably the distribution of the axial velocity, wall shear stress and turbulent kinetic energy in the closely packed rod array bundles.

• 한국전산유체공학회:학술대회논문집
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• 1999.05a
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• pp.155-161
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• 1999

The three-dimensional turbulent cascade flows with and without endwall fences are numerically investigated by solving the incompressible Navier-Stokes equations with a high-Reynolds number $k-{\varepsilon}$ turbulence closure model. A projection method based algorithm is used in the finite-volume formulation, with the second order upwind-differencing scheme for the convective terms. First, assessments on accuracy of the present method are made by comparing the static pressure distributions at the mid-span of the cascade with measured data, and also by confirming the experimental observations on the choice of an optimal fence height for the secondary flow control. In understanding the three-dimensional nature of the secondary flow in turbine cascade, the limiting streamline patterns and the static pressure contours at the suction surface of the blade as well as on the cascade endwall are employed to visualize the effectiveness of the endwall fence for the secondary flow control. Analysis on the streamwise vorticity contour maps along the cascade with the three-dimensional representation of their iso-surfaces reveals the strucuture of the complicated vortical flow in the turbine cascade with endwall fence, and also leads to an understanding on formation of the counter-rotating streamwise vortex over the endwall fence, in explaining the mechanisms of controlling the secondary flow and also for the proper selection of an optimal fence height.

• ETRI Journal
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• v.45 no.1
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• pp.28-44
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• 2023

We propose short packet communication in an underlay cognitive radio network assisted by an intelligent reflecting surface (IRS) composed of multiple reconfigurable reflectors. This scheme, called the IRS protocol, operates in only one time slot (TS) using the IRS. The IRS adjusts its phases to give zero received cumulative phase at the secondary destination, thereby enhancing the end-to-end signal-to-noise ratio. The transmitting power of the secondary source is optimized to simultaneously satisfy the multi-interference constraints, hardware limitations, and performance improvement. Simulation and analysis results of the average block error rates (BLERs) show that the performance can be enhanced by installing more reconfigurable reflectors, increasing the blocklength, lowering the number of required primary receivers, or sending fewer information bits. Moreover, the proposed IRS protocol always outperforms underlay relaying protocols using two TSs for data transmission, and achieves the best average BLER at identical transmission distances between the secondary source and secondary destination. The theoretical analyses are confirmed by Monte Carlo simulations.

• IEIE Transactions on Smart Processing and Computing
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• v.3 no.4
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• pp.173-180
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• 2014

Random access protocols feature inherent sensing functionality and distributed coordination, making them suitable for cognitive radio communication environments, where secondary users must detect the white space of the primary spectrum and utilize the idle primary spectrum efficiently without centralized control. These characteristics have led to the adoption of carrier-sensing-multiple-access/collision-avoidance (CSMA/CA) in cognitive radio. This paper proposes a new analytical framework for evaluating the performance of a CSMA/CA protocol that considers the characteristics of idle periods based on the primary traffic behavior in cognitive radio systems. In particular, the CSMA/CA-based secondary network was analyzed in the terms of idle period utilization, which is the average effective data transmission time portion in an idle period. The use of the idle period was maximized by taking its statistical features into consideration.

• Journal of Korean Home Economics Education Association
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• v.8 no.2
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• pp.15-28
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• 1996

The purpose of this study is to investigate the family life participation and to analysis its related variables focusing on comparisons of male and female in the secondary students. The questonaire included school environmental variables, family environmental variables, the preference degree of Home Economics, sex role attitudes and the family life participation. The participants were 600 middle and high school students but the final data used on this study were 559. For the statistical analysis of this study, fequency, mean, Cronbach’s ${\alpha}$ coefficient, one way ANOVA, t-test and multiple regression were calculated. The results of this study were summarized as follows；1) The degree of family life participation was a little higher than middle level. While the highest participation degree was family activities participation, the participation degree of household work, communication and decision-making, leisure activities were low. 2) The variables that influence on the family life participation were the level of living and preference degree of Home Economics for both male and female.

• International Journal of Air-Conditioning and Refrigeration
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• v.9 no.2
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• pp.44-50
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• 2001

The Sirocco fan performance and its three-dimensional flow characteristics are numerically prediction by STAR-CD. Turbulent flow computations are performed using approximately 500,000 mesh points, and the performance results of tow computational methods, transient and quasi-static flow analyses are compared with experimental data. In the present study, our attention is focused on the three-dimensional flow characteristics of the Sirocco fan blades and the secondary flow structure in the scroll. For a design optimization study, the scroll shape is titled by $10^\circ$ to modify the secondary flow structure, which yields some improvement of the fan performance.

• Journal of Fisheries and Marine Sciences Education
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• v.17 no.3
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• pp.305-323
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• 2005

To reveal secondary teachers' in-service education needs, in-depth interviews were conducted with 15 individual teachers based on semi-structured questions and qualitative data analysis procedures were adopted. They wanted to have; a. more knowledge of one's subject, subject related information, culture and common sense, counseling psychology, foreign language, and the 7th curriculum, b. better skills of student-centered teaching-learning methods, guidance and class management, good use of computers, and educational evaluation, c. improved attitudes regarding democratic problem solution processes, conversation with and service to students from their viewpoints, commitment to preparations for teaching, educational innovation and improvement, elimination of improperness, and pride of teaching job.

• Solar Energy
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• v.19 no.4
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• pp.21-31
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• 1999

This paper presents a quasi-three-dimensional calculation method considered a spanwise mixing effect in a diagonal flow impeller. The effect of this spanwise mixing caused by spanwise distribution of blade loading is evaluated by a secondary flow theory. In order to verify the validity of this method, it is applied to the analysis of a diagonal flow fan designed under a vortex type of constant circumferential velocity and that of a free vortex. The comparison of the calculated result with experimental data shows a good agreement except the regions near the casing where the flow field is affected by the tip leakage flow.

• Proceedings of the Korean Vacuum Society Conference
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• 1996.02a
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• pp.161-161
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• 1996

Recently, the role of the surface analysis on the development of advanced materials has become larger and larger as the surface compositions of these materials is the key of their performances. Especially three techniques, Auger electron spectroscopy, X- ray photoelectron spectroscopy and secondary ion mass spectroscopy are widely used in technology fields. However, because of the relatively short history of these techniques(thirty years or so), there has been no accumulation of data commonly available, physical parameters for analysis have not been established and there has been no standard data. With these background, the VAMAS projects which aims to standardize the manner in the field of these techniques has started in 1982 at Versailles Summit. Along the projects, we have conducted the international collaborating study on the sharing of spectral data. In 1994, the Science and Technology Agency of Japan began the project on computer network, on which our fruits from the study on spectral data sharing is boarded.(omitted)mitted)