• Structural Engineering and Mechanics
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• v.33 no.4
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• pp.429-446
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• 2009

Three-scale homogenization procedure is proposed in this paper to provide estimates of the effective thermal conductivities of porous carbon-carbon textile composites. On each scale - the level of fiber tow (micro-scale), the level of yarns (meso-scale) and the level of laminate (macro-scale) - a two step homogenization procedure based on the Mori-Tanaka averaging scheme is adopted. This involves evaluation of the effective properties first in the absence of pores. In the next step, an ellipsoidal pore is introduced into a new, generally orthotropic, matrix to make provision for the presence of crimp voids and transverse and delamination cracks resulting from the thermal transformation of a polymeric precursor into the carbon matrix. Other sources of imperfections also attributed to the manufacturing processes, including non-uniform texture of the reinforcements, are taken into consideration through the histograms of inclination angles measured along the fiber tow path together with a particular shape of the equivalent ellipsoidal inclusion proposed already in Sko ek (1998). The analysis shows that a reasonable agreement of the numerical predictions with experimental measurements can be achieved.

• Proceedings of the KIEE Conference
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• 2007.07a
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• pp.864-865
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• 2007

외전형 유도전동기는 내전형에 비해 회전자가 외부에 존재하는 특징을 가지고 있어서 제품의 소형화 및 경량화를 시킬 수 있다는 장점이 있다. 하지만 외전형은 회전자의 직경이 내전형에 비하여 크기 때문에 조립 공정상에 존재하게 되는 회전자와 고정자간의 이심률에 따른 제품 전체의 불균형력이 상대적으로 크며, 이로 인해 모터의 정속적인 운전 방해 및 불규칙한 출력토크로 인한 제품 작동상의 오류발생이 더 크게 나타난다. 본 논문에서는 외전형 유도전동기에서 존재하는 이심률의 크기에 따른 불균형력이 최소화 될 수 있는 슬롯형태를 찾기 위해 회전자 슬롯의 형태를 전폐형, 반폐형, 개구형태의 3가지형태로 변경시킨 후, 이를 전자장 해석을 통해 불균형력을 비교하였다. 또한 이심률이 증가함에 따라 나타나는 불균형력의 증대를 슬롯 형태에 따라 비교 분석하여, 불균형력에 최적화된 외전형 모터의 슬롯형태를 알아보았다.

• Wind and Structures
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• v.29 no.1
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• pp.15-32
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• 2019

Much advancement has been made in wind power due to modern technological developments. The wind energy technology is the world's fastest-growing energy option. More power can be generated from wind energy by the use of new design and techniques of wind energy machines. The geographical areas with suitable wind speed are more favorable and preferred for wind power deployment over other sources of energy generation. Today's wind turbines are mainly the horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs). HAWTs are commercially available in various sizes starting from a few kilowatts to multi-megawatts and are suitable for almost all applications, including both onshore and offshore deployment. On the other hand, VAWTs finds their places in small and residential wind applications. The objective of the present work is to review the technological development, available sizes, efficiencies, structural types, and structural stability of VAWTs. Structural stability and efficiencies of the VAWTS are found to be dependent on the structural shape and size.

• Structural Engineering and Mechanics
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• v.84 no.6
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• pp.707-722
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• 2022

In this study, a new refined hyperbolic shear deformation theory (RHSDT) is developed using an equivalent single-layer shell displacement model for the static bending and free vibration response of cross-ply laminated composite spherical shells. It is based on a new kinematic in which the transverse displacement is approximated as a sum of the bending and shear components, leading to a reduction of the number of unknown functions and governing equations. The proposed theory uses the hyperbolic shape function to account for an appropriate distribution of the transverse shear strains through the thickness and satisfies the boundary conditions on the shell surfaces without requiring any shear correction factors. The shell governing equations for this study are derived in terms of displacement from Hamilton's principle and solved via a Navier-type analytical procedure. The validity and high accuracy of the present theory are ascertained by comparing the obtained numerical results of displacements, stresses, and natural frequencies with their counterparts generated by some higher-order shear deformation theories. Further, a parametric study examines in detail the effect of both geometrical parameters (i.e., side-to-thickness ratio and curvature-radius-to-side ratio), on the bending and free vibration response of simply supported laminated spherical shells, which can be very useful for many modern engineering applications and their optimization design.

• Journal of the Korean Society for Precision Engineering
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• v.11 no.4
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• pp.58-68
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• 1994

An equilibrium approach is suggested as an effective tool for the analysis of sheet metal forming processes on the basis of force balance together with geometric relations and plasticity theroy. In computing a force balance equation, it is required to define a geometric curve approximating the shape of the sheet metal at any step of deformation from the geometric interaction between the die and the deforming sheet. Then the geometic informations for contacting and non-contacting sections of the sheet metal such as the number and length of both non-contact region, contact angle, and die radius of contact section are known from the geometric forming curve and utilized for optimization by force balance equation. In computation, the sheet material is assumed to be of normal amisotropy and rigid-phastic workhardening. It has been shown that there are good agreements between the equilibrium approach and FEM computation for the benchmark test example and auto-body panels whose sections can be assumed in plane-strain state. The proposed equilibrium approach can thus be used as a robust computational method in estimating the forming defects and forming severity rather quickly in the die design stage.

• Design & Manufacturing
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• v.11 no.2
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• pp.1-8
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• 2017

In this study, we applied microcellular foaming injection molding process to improve the performance of system air-conditioner drain fan which had been produced by injection molding process and studied the optimization of process conditions through 6-sigma process and response surface method (RSM) to reduce weight and deformation of products. Additive type, melt temperature, mold temperature, and injection screw shape were selected as the factor affecting the weight and deformation of the products by carrying out analysis of trivial many through ANOVA and design of experiment (DOE) method. Among the effect factor, we set the addictive type to Long G/F and screw shape to foaming screw which had the highest level of weight reduction and deformation reduction. The amount of foaming agent gas was set at 60 ml, which was the limit beyond which the weight of product did not decrease any more. For melt temperature and mold temperature, we studied the conditions where both weight and deformation were minimized using the RSM. As a result, we set the melt temperature to $250^{\circ}C$, fixed mold temperature to $20^{\circ}C$, and moving mold temperature to $40^{\circ}C$. The improvement effect was analyzed by appling the selected optimal conditions to the production process using the microcellular foaming injection molding. The results showed that the mean weight of product was measured to be 1,420g which was 19% lower than that measured in the current process. The standard deviations of the weights were found to be similar to those in the current process and it showed a low dispersion. The mean deformation was measured to be 0.9237mm, which represented a 57% reduction compared to the mean deformation in the current process, and the standard deviation decreased from 0.3298mm to 0.1398mm. Moreover, we analyzed the process capability for deformation, and the results showed that the short-term process capability increased from 2.73 to 6.60 which was even higher than targeted level of 6.0.

• The Journal of the Acoustical Society of Korea
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• v.42 no.6
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• pp.519-528
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• 2023

The aim of this study was to enhance the flow rate and noise performance of a centrifugal pump in dishwashers by designing an optimized impeller shape through numerical and experimental investigations. To evaluate the performance of the target centrifugal pump, experiment was conducted using a pump performance tester and noise experiment was carried out in a semi-anechoic chamber with microphones and a reflecting wall behind the dishwasher. Through the use of advanced computational fluid dynamics techniques, numerical simulations were performed to analyze the flow and aeroacoustics performance of our target centrifugal pump impeller. To achieve this, numerical simulations were carried out using the Reynolds-Average Navier-Stokes equations and Ffowcs-Willliams and Hawkings equations as governing equations. In order to ensure the validity of numerical methods, a thorough comparison of numerical results with experimental results. After having confirmed the reliability of the current numerical method of this study, the optimization of the target centrifugal pump impeller was conducted. An improvement in flow rate was confirmed numerically, and a manufactured proto-type of the optimized model was used for experimental investigation. Furthermore, it was observed that by applying the fan law, we could effectively reduce noise levels without reducing the flow rate.

• Korean Journal of Materials Research
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• v.18 no.10
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• pp.547-551
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• 2008

Crack-free joining of $Si_3N_4\;and\;Al_2O_3$ using 15 layers has been achieved by a unique approach introducing Sialon polytypoids as a functionally graded materials (FGMs) bonding layer. In the past, hot press sintering of multilayered FGMs with 20 layers of thickness $500{\mu}m$ each has been fabricated successfully. In this study, the number of layers for FGM was reduced to 15 layers from 20 layers for optimization. For fabrication, model was hot pressed at 38 MPa while heating up to $1700^{\circ}$, and it was cooled at $2^{\circ}$/min to minimize residual stress during sintering. Initially, FGM with 15 layers had cracks near 90 wt.% 12H / 10 wt.% $Al_2O_3$ and 90 wt.% 12H/10 wt.% $Si_3N_4$ layers. To solve this problem, FEM (finite element method) program based on the maximum tensile stress theory was applied to design optimized FGM layers of crack free joint. The sample is 3-dimensional cylindrical shape where this has been transformed to 2-dimensional axisymmetric mode. Based on the simulation, crack-free FGM sample was obtained by designing axial, hoop and radial stresses less than tensile strength values across all the layers of FGM. Therefore, we were able to predict and prevent the damage by calculating its thermal stress using its elastic modulus and coefficient of thermal expansion. Such analyses are especially useful for FGM samples where the residual stresses are very difficult to measure experimentally.

• Journal of Korean Association for Spatial Structures
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• v.4 no.2 s.12
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• pp.89-97
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• 2004

The objective of this study is the development of a scheme and discrete optimum design algorithm, which is based on the genetic algorithm. The algorithm can perform both scheme and size optimum designs of plane trusses. The developed Scheme genetic algorithm was implemented in a computer program. For the optimum design, the objective function is the weight of structures and the constraints are limits on loads and serviceability. The basic search method for the optimum design is the genetic algorithm. The algorithm is known to be very efficient for the discrete optimization. However, its application to the complicated structures has been limited because of the extreme time need for a number of structural analyses. This study solves the problem by introducing the size & scheme genetic algorithm operators into the genetic algorithm. The genetic process virtually takes no time. However, the evolutionary process requires a tremendous amount of time for a number of structural analyses. Therefore, the application of the genetic algorithm to the complicated structures is extremely difficult, if not impossible. The scheme genetic algorithm operators was introduced to overcome the problem and to complement the evolutionary process. It is very efficient in the approximate analyses and scheme and size optimization of plane trusses structures and considerably reduces structural analysis time. Scheme and size discrete optimum combined into the genetic algorithm is what makes the practical discrete optimum design of plane fusses structures possible. The efficiency and validity of the developed discrete optimum design algorithm was verified by applying the algorithm to various optimum design examples: plane pratt, howe and warren truss.

• Journal of Ocean Engineering and Technology
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• v.26 no.4
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• pp.37-41
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• 2012

Due to global warming, the need to secure alternative resources has become more important nationally. Because of the very strong current on the west coast, with a tidal range of up to 10 m, there are many suitable sites for the application of TCP (tidal current power) in Korea. In the southwest region, a strong current is created in the narrow channels between the numerous islands. A rotor is an essential component that can convert tidal current energy into rotational energy to generate electricity. The design optimization of a rotor is very important to maximize the power production. The performance of a rotor can be determined using various parameters, including the number of blades, shape, sectional size, diameter, etc. There are many offshore jetties and piers with high current velocities. Thus, a VAT (vertical axis turbine) system, which can generate power regardless of flow direction changes, could be effectively applied to cylindrical structures. A VAT system could give an advantage to a caisson-type breakwater because it allows water to circulate well. This paper introduces a multi-layer vertical axis tidal current power system. A Savonius turbine was designed, and a performance analysis was carried out using CFD. A physical model was also demonstrated in CWC, and the results are compared with CFD.