• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.11a
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• pp.481-481
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• 2007

The characteristics of slow wave structure employed for backward wave oscillator expected to be a high power microwave source are studied analytically. The slow wave structure is a sinusodially corrugated wall waveguide. The waveguide is designed and transmitted characteristics for microwave are measured in the air. There exist literatures on high efficiency of enhansed radiation from backward wave oscillators involving plasmastudied experimentally another results.

• Journal of the Korean Society of Propulsion Engineers
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• v.21 no.3
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• pp.49-55
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• 2017

In this study, gas turbine engine inlet duct was designed to satisfy uniform flow at aerodynamic interface plane (AIP). Haack-series was selected as nose cone profile and duct outer radius($r_o$) was designed to satisfy to match with area change rate between the nose cone and outer duct wall by the 1-D sizing. The design object of the inlet duct wall profile which has the gradual area change rate was uniform Mach number in the core flow region and minimum boundary later thickness at the both inner nose wall and outer duct wall. The flow characteristics inside the inlet duct was evaluated using CFD. The static pressure distribution at the AIP showed uniform pattern within 0.16%. Based on Mach number profile, the boundary layer thickness was 2% of channel height. Kiel temperature rake location was decided less than 100 mm in front of nose cone where the Mach number is less than 0.1 in order to maximize the temperature probe recovery rate.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.25 no.3
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• pp.150-159
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• 2015

An experimental study on frequency characteristics of the microphone array covered with Kevlar sheet was conducted in the closed test section. Microphones that were flush-mounted in the wall of wind tunnel were subjected to very high flow noise resulting from the turbulence in the wall boundary layer. This noise interference by the boundary layer was referred as 'a microphone self-noise' and various approaches were studied to reduce this interference. Recessed microphone array with high tensioned cover was one of the good approaches to reduce this self-noise. But, the array cover could cause an unexpected interference to the measuring results. In this paper the frequency characteristics of the microphone array with Kevlar cover was experimentally studied. The white noise was used as a reference noise source. Three kinds of tensions for the Kevlar cover were tested and those results were compared with the test results without the Kevlar cover. The gap effect between the cover and microphone head was also tested to find out the proper position of microphone in the array module. Test results show that the mid-tension and 10mm gap was the best choice in the tested cases.

• Journal of the Society of Naval Architects of Korea
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• v.50 no.6
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• pp.399-406
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• 2013

In the present study, the flat plate model test method is developed to evaluate the skin friction of the marine coating in the cavitation tunnel. Six-component force balance is used to measure the profile drag of the flat plate and strut. LDV(laser Doppler velocimetry) technique is also employed to evaluate the drag and to figure out the reason of the drag reduction. The flow velocities above the surface can be used to assess the skin friction, combined with direct force measurement. Since the vortical structure in the coherent turbulence structure influences on the skin friction in the high Reynolds number regime, the interaction between the turbulence structure and the surface wall is paying more attention. This sort of thing is important in the passive control of the turbulent boundary layer because the skin friction can't be determined only by wall condition. As complicated flow phenomena exist around a paint film, systematic measurement and analysis are necessary to evaluate the skin friction appropriately.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.11
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• pp.1496-1508
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• 1997

The effects of porous wind fence on the pressure characteristics around a 2-dimensional prism model of triangular cross-section were investigated experimentally. The fence and prism model were embedded in a neutral atmospheric surface boundary layer over the city suburb. In this study, various fences of different porosity, back fence, inclination angle of prism and location of additional back prisms were tested to investigate their effects on the pressure and wall shear stress of the prism surface. The fence and prism had the same height of 40 mm and Reynolds number based on the model height was Re=3.9*10$^{4}$. The porous fence with porosity 40% was found to be the best wind fence for decreasing the mean and pressure fluctuations on the prism surface. By installing the fence of porosity 40%, the wall shear stress on the windward surface of prism was largely decreased up to 1/3 of that without the fence. This indicates that the porous fence is most effective to abate the wind erosion. Pressure fluctuations on the model surface were decreased more than half when a back fence was located behind the prism in addition to the front fence. With locating several back prisms and decreasing the inclination angle of triangular prism, the pressure fluctuations on the model surface were increased on the contrary.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.193-196
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• 2008

The vortical structures in a turbulent boundary layer (TBL) developed over a flat plate have been investigated experimentally. The flow conditions tested in this study were Re$_{\theta}$ = 3700, Re$_{\delta}$ = 11${\times}$105 and the shape factor H = 1.3. Instantaneous velocity fields in the streamwise-wall-normal planes were measured by using a dynamic PIV system. A trip-wire and sandpapers were placed behind the leading edge to promote the turbulent transition. 1000 velocity fields were obtained consecutively with a time interval of 1 millisecond. Streamwise u-velocity components were temporally averaged in the measuring plane. In addition, 2000 velocity fields were obtained randomly and ensemble-averaged to get the fully-developed turbulent characteristics. Profiles of the normalized u-component, turbulent intensities and Reynolds shear stress were evaluated. The structures of spanwise vortices were extracted from the instantaneous velocity fields by determining the swirling strength, ${\lambda}_{ci}$. The wall-normalized locations of vortices were temporally averaged in the measuring plane with respect to their rotational direction. The correlations between the temporally averaged u and the temporally averaged $y^+$ of vortices were evaluated. For the case of positive vortices, the correlation is not significant. However, the negative vortices show a strong negative correlation. The y-location of negative vortices tends to increase, as the averaged u decreases and vice versa. These findings indicate that the number of negative vortices in the outer layer increases during the outward bursting events.

• Journal of Advanced Navigation Technology
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• v.18 no.4
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• pp.304-311
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• 2014

In this paper, we propose a ultra sound inspection technique for automatic defect detection and classification in aircraft composite materials. Using local maximum values of ultra sound wave, we choose peak values for defect detection. Distance data among peak values are used to construct histogram and to determine surface and back-wall echo from the floor of composite materials. C-scan image is then composed through this method. A threshold value is determined by average and variance of the peak values, and defects are detected by the values. PCA(principal component analysis) and QDA(quadratic discriminant analysis) are carried out to classify the types of defects. In PCA, 512 dimensional data are converted into 30 PCs(Principal Components), which is 99% of total variances. Computational cost and misclassification rate are reduced by limiting the number of PCs. A decision boundary equation is obtained by QDA, and defects are classified by the equation. Experimental result shows that our proposed method is able to detect and classify the defects automatically.

• Restorative Dentistry and Endodontics
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• v.23 no.1
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• pp.163-174
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• 1998

The purpose of this study is to evaluate the distribution of stress in the root end resected teeth. The finite element method was used to compare stresses along the root and retrograde filling material in seven two-dimensional models of mandibular 2nd premolar. Each model was endodontic treatment and gold crown' restoration. Each model divided with amagam core restoration or gold casting post restoration. Thus each model divided with shape of root end resection, depth of retropreparation and exposure length of root in the bony cavity. The seven models were classified as in the table 1 below. A load of 500N was applied $45^{\circ}$ diagonally on the lingual slope of the buccal cusp. These mode were analyzed with two dimensional finite element methods. The results of this study were as follows : 1. The maximum tensile stress along the inner canal wall was shown on the model 7. 2. When the model 1 was compared with the model 5, the maximum tensile stress along the inner canal wall showed the model 1. 3. Less equivalent stress was shown on the model 6 and more equivalent stress was shown on the model 4. 4. More shear stress was shown on the retrograde filling material of the model 7. 5. The models with increased length of exposed root in the bony cavity demonstrated a gradual increase to the tensile stress in X direction which occurred approximately a boundary between the bone and exposed root in' the bony cavity. 6. The model which had a case of matching the apex of post and a boundary between the bone and exposed root in the bony cavity demonstrated more increase tensile stress in X direction than other models.

• Steel and Composite Structures
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• v.23 no.4
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• pp.473-481
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• 2017

Steel plate shear wall (SPSW) system has been increasingly used for lateral loads resisting system since 1980s when the utilization of post-buckling strength of SPSW was realized. The structural response of SPSWs largely depends on the behavior of the surrounded beams. The beams are normally required to behave in the elastic region when the SPSW fully buckled and formed the tension field action. However, most modern design codes do not specify how this requirement can be achieved. This paper presents theoretical investigation and design procedures of manually calculating the plastic flexural capacity of the beams of SPSWs and can be considered as an extension to the previous work by Qu and Bruneau (2011). The reduction in the plastic flexural capacity of beam was considered to account for the presence of shear stress that was altered towards flanges at the boundary region, which can be explained by Saint-Venant's principle. The reduction in beam web was introduced and modified based on the research by Qu and Bruneau (2011), while the shear stress in the web in this research is excluded due to the boundary effect. The plastic flexural capacity of the beams is given by the superposition of the contributions from the flanges and the web. The developed equations are capable of predicting the plastic moment of the beams subjected to combined shear force, axial force, bending moment, and tension fields induced by yielded infill panels. Good agreement was found between the theoretical results and the data from previous research for flexural capacity of beams.

• Journal of the Korean Geotechnical Society
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• v.24 no.11
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• pp.121-131
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• 2008

In constructing a slurry trench cutoff wall, bentonite-water slurry is used to secure the stability of sidewalls during excavation before the wall is completed by backfilling. Unexpectedly, a thin but relatively impermeable layer called filter cake can be formed on the excavation surface, which significantly influences the result of slug test analysis in the cutoff wall if not considered. This study is to examine the effect of filter cake on evaluating hydraulic conductivity of the vertical cutoff wall through slug test analysis with the aid of the verified numerical program Slug_3D. The no-flux boundary conditions were adopted in Slug_3D to simulate the filter cake on the interface between the wall and the natural soil. A new set of type curves were built for applying the type curve method. New modification factors were obtained for using the modified line-fitting method. With consideration of filter cake, the type curve method and the modified line-fitting method were adopted to reanalyze the case study taken from EMCON (1995). The previous results achieved by Choi and Daniel (2006) without consideration of filter cake were compared with the present results obtained in this paper. The comparison emphasizes the necessity of considering filter cake when analyzing slug test results in vertical cutoff walls.