A study on separate learning algorithm using support vector machine for defect diagnostics of gas turbine engine

A separate learning algorithm with support vector machine (SVM) has been studied for the development of a defect-diagnostic algorithm applied to the gas turbine engine. The system using only an artificial neural network (ANN) falls in a local minima and its classification accuracy rate becomes low in case it is learning nonlinear data. To make up for this risk, a separate learning algorithm combining ANN with SVM has been proposed. In the separate learning algorithm, a sequential ANN learns selectively after classification of defect patterns and discrimination of defect position using SVM, resulting in higher classification accuracy rate as well as the rapid convergence by decreasing the nonlinearity of the input data. The results have shown this suggested method has reliable and suitable estimation accuracy of the defect cases of the turbo-shaft engine. This paper was recommended for publication in revised form by Associate Editor Dongsik Kim Tae-Seong Roh received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1984 and 1986. He then went on to receive his Ph.D. degree from Pennsylvania State University in 1995. Dr. Roh is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of combustion instabilities, rocket and jet propulsions, interior ballistics, and gasturbine engine defect diagnostics. Dong-Whan Choi received his B.S. degree in Aeronautical Engineering from Seoul National University in 1974. He then went on to receive his M.S. and Ph.D. degrees from University of Washington in 1978 and 1983. Dr. Choi had served three years as a President of Korea Aerospace Research Institute since 1999. He is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of turbulence, jet propulsions, and gasturbine defect diagnostics.     

Defect diagnostics of SUAV gas turbine engine using hybrid SVM-artificial neural network method

A hybrid method of an artificial neural network (ANN) combined with a support vector machine (SVM) has been developed for the defect diagnostic system applied to the SUAV gas turbine engine. This method has been suggested to overcome the demerits of the general ANN with the local minima problem and low classification accuracy in case of many nonlinear data. This hybrid approach takes advantage of the reduction of learning data and converging time without any loss of estimation accuracy because the SVM classifies the defect location and reduces the learning data range. The results of test data have shown that the hybrid method is more reliable and suitable algorithm than the general ANN for the defect diagnosis of the gas turbine engine. This paper was recommended for publication in revised form by Associate Editor Tong Seop Kim Tae-Seong Roh received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1984 and 1986. He then went on to receive his Ph.D. degree from Pennsylvania State University in 1995. Dr. Roh is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of combustion instabilities, rocket and jet propulsion, interior ballistics, and gas turbine engine defect diagnostics. Dong-Whan Choi received his B.S. degree in Aeronautical Engineering from Seoul National University in 1974. He then went on to receive his M.S. and Ph.D. degrees from University of Washington in 1978 and 1983. Dr. Choi served three years as a President of the Korea Aerospace Research Institute from 1999. He is currently a professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of turbulence, jet propulsion, and gas turbine defect diagnostics.     

Flow instability due to cryogenic cavitation in the downstream of orifice

Flow instability in LRE (liquid rocket engine) occurs due to various reasons such as flow interactions with valve, orifice and venturi, etc. The inception of cavitation, especially in the propellant feeding system, is the primary cause of mass and pressure oscillations because of the cyclic formation and depletion of cavitation. Meanwhile, the main propellant in a liquid rocket engine is the cryogenic fluid, which properties are very sensitive to temperature variation. And the change of propellant properties to temperature variation by thermodynamic effect needs to be properly taken into account in the flow analysis in order to understand basic mechanisms for cryogenic cavitation. The present study focuses on the formation of cryogenic cavitation by using the IDM model suggested by Shyy and coworkers. The flow instability was also numerically investigated in the downstream of orifice with a developed numerical code. Calculation results show that cryogenic cavitation can be a primary source of flow instability, leading to mass fluctuations accompanied by pressure oscillations. The prediction of cavitation in cryogenic fluid is of vital importance in designing a feeding system of an LRE. This paper was recommended for publication in revised form by Associate Editor Jun Sang Park Changjin Lee received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1983 and 1985. He then went on to receive his Ph.D. degree from University of Illinois at Urbana- Champaign in 1992. Dr. Lee is currently a Professor at the department of Aerospace Engineering at Konkuk University in SEOUL, Korea. His research interests are in the area of combustion instabilities of hybrid, liquid rocket and jet propulsions. Tae-Seong Roh received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1984 and 1986. He then went on to receive his Ph.D. degree from Pennsylvania State University in 1995. Dr. Roh is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of combustion instabilities, rocket and jet propulsions, interior ballistics, and gas turbine engine defect diagnostics.     

Computational study on aerodynamics of long-span bridges

A numerical procedure for aerodynamic load analysis of long span bridges is presented. The preconditioned Reynolds averaged Navier-Stokes equations are adopted to compute flows over the bridges. To capture the turbulent characteristics of the flows, two equation turbulence models, Coakley’s q − ω model and Menter’s k − ω SST model, are used to compute the turbulent viscosity. A dual time stepping method in conjunction with the AF-ADI method is used to advance the solution in time. A loosely coupled method of the preconditioned RANS equations with the turbulence model equations is employed for fast computation without losing numerical stability. The numerical method for the aerodynamic load analysis is verified against well-known benchmark problems. Aerodynamic loads of two real bridges are computed with the method to demonstrate the usefulness of the method. This paper was recommended for publication in revised form by Associate Editor Kyung-Soo Yang Ilyong Yoo is a Ph.D. candidate in Aerodynamic Analysis and Design Laboratory at Inha University. He received his B.S. and M.S. degrees in Aerospace Engineering from Inha University in 2004 and 2006, respectively. His research area includes computational fluid dynamics, and its application to active flow control using MEMS devices. Einkeun Kwak is a Ph.D. candidate in Aerodynamic Analysis and Design Laboratory at Inha University. He holds B.S. and M.S. degrees in Aerospace Engineering from Inha University. His research area includes computational fluid dynamics, and its application to supersonic inlet analysis and design. Seungsoo Lee is a professor in Aerospace Engineering at Inha University. Prior to joining the faculty at Inha University, he was a senior research engineer at the Agency for Defense Development. He earned his Ph.D. degree from the Pennsylvania State University in 1990. He also holds B.S. and M.S degrees in Aeronautical and Astronautical Engineering from Seoul National University. Dr. Lee’s research interests are in the area of computational fluid dynamics, overset grid method, and applied aerodynamics. Beom Soo Kim received his B.S. and M.S degrees in Aeronautical and Astronautical Engineering from Seoul National University in 1974 and 1977, respectively. He earned his Ph.D. degree from University of Oklahoma in 1983. Dr. Kim is currently a Professor at the Department of Aerospace Engineering at Inha University. Dr. Kim’s research interests are in the area of hypersonic aerodynamics, and wind tunnel testing. Si Hyong Park is a developer in the applied analysis team of MidasIT Co. Ltd, Korea. He received the Bachelor, the Master and the Ph.D degree in Aerospace Engineering from Seoul National University in 1996, in 1998 and in 2003, respectively. His research interest is currently development of CAE software including FEM, CFD and Multi-physics simulation.     

NOx emission characteristics in turbulent hydrogen jet flames with coaxial air

The characteristics of NOx emissions in pure hydrogen nonpremixed jet flames with coaxial air are analyzed numerically for a wide range of coaxial air conditions. Among the models tested in simple nonpremixed jet flame, the one-half power scaling law could be reproduced only by the Model C using the HO₂/H₂O₂ reaction, implying the importance of chemical nonequilibrium effect. The flame length is reduced significantly by augmenting coaxial air, and could be represented as a function of the ratio of coaxial air to fuel velocity. Predicted EINOx scaling showed a good concordance with experimental data, and the overall one-half power scaling was observed in coaxial flames with Model C when flame residence time was defined with flame volume instead of a cubic of the flame length. Different level of oxygen mass fraction at the stoichiometric surface was observed as coaxial air was increased. These different levels imply that the coaxial air strengthens the nonequilibrium effect. This paper was recommended for publication in revised form by Associate Editor Haecheon Choi Hee-Jang Moon received his B.S. degree in Aeronautical Engineering from Inha University, Korea in 1986. He then received his M.S. and Doctoral degrees from Universite de Rouen, France in 1988 and 1991, respectively. Dr. Moon is currently a Professor at the School of Aerospace and Mechanical Engineering at Korea Aerospace University in Koyang, Korea. He serves on the Editorial Board of the Korean Society of Propulsion Engineers. His research interests are in the area of turbulent combustion, hybrid rocket combustion and nanofluids. Youngbin Yoon received his B.S. and M.S. degrees in Aerospace Engineering from Seoul National University, Korea in 1985 and 1987, respectively. He received a Ph.D. degree from the University of Michigan in 1994. Dr. Yoon is currently a professor at the School of Mechanical and Aerospace Engineering in Seoul National University, Korea. He is currently on the Editorial board and executive of ILASS-KOREA. The research areas of Dr. Yoon are liquid rocket injectors, combustion instability and control, ram and gas turbine combustor and laser diagnostics.     

Analysis of performance deterioration of a micro gas turbine and the use of neural network for predicting deteriorated component characteristics

Deteriorated performance data of a micro gas turbine were generated and the artificial neural network was applied to predict the deteriorated component characteristics. A program to simulate operation of a micro gas turbine was set up and deterioration of each component (compressor, turbine and recuperator) was modeled by changes in the component characteristic parameters such as compressor and turbine efficiency, their flow capacities and recuperator effectiveness and pressure drop. Single and double faults (degradation of single and two parameters) were simulated. The neural network was trained with a majority of the generated deterioration data. Then, the remaining data were used to check the predictability of the neural network. Given measurable performance parameters as inputs to the neural network, characteristic parameters of each component were predicted and compared with original data. The neural network produced sufficiently accurate prediction. Using a smaller number of input parameters decreased prediction accuracy. However, an acceptable accuracy was observed even without information on several input parameters. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Mr. J. E. Yoon received his MS degree from Dept. of Mechanical Engineering, Inha University in 2008. His thesis topic was test and simulation of micro gas turbines. He has been working at LG Digital Appliance Company. Mr. J.J. Lee received his MS degree from Dept. of Mechanical Engineering, Inha University in 2006, and is now Doctoral student at the same department. His research topics include simulation and diagnosis of gas turbines. Prof. T.S. Kim received his PhD degree from Dept. of Mechanical Engineering, Seoul Na-tional University in 1995. He has been with Dept of Mechanical Engineering, Inha Univeristy since 2000, and is Associate Professor as of Oct. 2008. His research area is aero-thermodynamc simulation and test of gas turbine systems including microturbine and their components. His recent research concern also includes analysis on fuel cells and fuel cell/gas turbine hybrid systems. Prof. J.L. Shon received his PhD degree from Dept. of Mechanical Engineering, The University of Alabama in Huntsville in 1986. He has been with School of Mechanical & Aerospace Engineering, Seoul National University since 2000, and is BK Associate Professor as of Oct. 2008. His research area is design, simulation and test of gas turbine system and components. He is also interested in researches on fuel cells and fuel cell/gas turbine hybrid systems.     

A numerical study on oil retention and migration characteristics in the heat pump system

In HVAC system, the oil circulation is inevitable because the compressor requires the oil for lubrication and sealing. A small portion of the oil circulates with the refrigerant flow through the system components while most of the oil stays or goes back to the compressor. Because oil retention in refrigeration systems can affect system performance and compressor reliability, proper oil management is necessary in order to improve the compressor reliability and increase the overall efficiency of the system. This paper describes a numerical analysis of oil distribution in each component of the commercial air conditioning system including the suction line, discharge line and heat exchanger. In this study, system modeling was conducted for a compressor, discharge line, condenser, expansion valve, evaporator and suction line. Oil separation characteristics of the compressor were taken from the information provided by manufacturer. The working fluid in the system was a mixture of a R-410A refrigerant and PVE oil. When the oil mass fraction (OMF) was assumed, oil mass distribution in each component was obtained under various conditions. The total oil hold-up was also investigated, and the suction line contained the largest oil hold-up per unit length of all components. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Min Soo Kim received his B.S., M.S., and Ph.D. degree at Seoul National University, Korea in 1985, 1987, and 1991, respectively. After Ph.D. degree, Prof. Kim worked at National Institute of Standards and Technology (NIST) in U.S.A. for about three years. He is currently a professor at the School of Mechanical and Aerospace Engineering of Seoul National University, Korea. Jong Won Choi received B.S. degree in Mechanical Engineering from Korea University in Seoul, Korea, in 2004, and then received M.S. degrees from Seoul National University in 2006. He is currently a student in Ph.D. course at the School of Mechanical and Aerospace Engineering of Seoul National University in Seoul, Korea. His research interests include refrigeration system, micro-fluidic devices, and PEM fuel cell as an alternative energy for next generation. Mo Se Kim received B.S. degree in Mechanical and Aerospace Engineering from Seoul National University in Seoul, Korea, in 2007. He is currently a student in M.S. course at the School of Mechanical and Aerospace Engineering of Seoul National University in Seoul, Korea. He had studied on the oil migration in the heat pump system, and now he studies on the refrigeration system using an ejector. Baik-Young Chung received his B.S., M.S., and Ph.D. degrees in Mechanical Engineering from Inha University, Korea in 1984, 1986, and 2001, respectively. He is currently a research fellow of HAC Research Center at LG Electronics. He is responsible for the commercial air conditioner group. Sai-Kee Oh received B.S. degree in Mechanical Engineering from Seoul National University, Korea in 1989, and then received M.S. and Ph.D. degrees from KAIST, Korea in 1991 and 1997, respectively. He is currently a principal research engineer of HAC Research Center at LG Electronics. He is responsible for the residential air conditioner group. Jeong-Seob Shin received B.S. degree in Machine Design and Production Engineering from Hanyang University, Korea in 1988, M.S. degree in Mechanical Engineering from KAIST, Korea in 1991, and Ph.D. degree in Mechanical Engineering from POSTECH, Korea in 2004. He has joined HAC Research Center at LG Electronics since 2006 as a principal research engineer.     

Numerical solutions for unsteady flow past a semi-infinite inclined plate with temperature oscillations

A study of the velocity and thermal boundary layers on a semi-infinite inclined plate with temperature oscillations is presented in this work. The non-linear, coupled parabolic integro-partial differential equations governing flow and heat transfer have been solved numerically using an implicit finite difference scheme of Crank-Nicolson type. The numerical values for the flow field, temperature, shearing stress, and heat transfer coefficients are presented in a graphical form. It is observed that the velocity and temperature profiles decrease as the frequency parameter increases. This paper was recommended for publication in revised form by Associate Editor Yang Na G. Palani received his B.Sc. and M.Sc. degrees from Madras University, India, in 1991 and 1993, respectively, and his Ph.D. degree from Anna University, India in 2001. Dr. G. Palani is currently a Post Doctoral Research Fellow at the School of Mechanical Engineering of Inha University in Incheon, Korea. Kwang-Yong Kim received his B.S. degree from Seoul National University in 1978, and his M.S. and Ph.D. degrees from the Korea Advanced Institute of Science and Technology (KAIST), Korea, in 1981 and 1987, respectively. He is currently a professor and the chairman of the School of Mechanical Engineering of Inha University, Incheon, Korea. Professor Kim is also the current editor-inchief of the Transactions of Korean Society of Mechanical Engineers (KSME), the editor-in-chief of the International Journal of Fluid Machinery and Systems (IJFMS), and the chief vice president of the Korean Fluid Machinery Association (KFMA). He is likewise a fellow of the American Society of Mechanical Engineers (ASME).     

Influence of steam injection through exhaust heat recovery on the design performance of solid oxide fuel cell — gas turbine hybrid systems

This study analyzed the influence of steam injection on the performance of hybrid systems combining a solid oxide fuel cell and a gas turbine. Two different configurations (pressurized system and ambient pressure system) were examined and the effects of injecting steam, generated by recovering heat from the exhaust gas, on system performances were compared. Performance variations according to the design of different turbine inlet temperatures were examined. Two representative gas turbine pressure ratios were used. Without steam injection, the pressurized system generally exhibits higher system efficiency than the ambient pressure system. The steam injection augments gas turbine power, thus increasing the power capacity of the hybrid system. The power boost effect due to the steam injection is generally greater in the relatively higher pressure ratio design in both the pressurized and ambient pressure systems. The effect of the steam injection on system efficiency varies depending on system configurations and design conditions. The pressurized system hardly takes advantage of the steam injection in terms of system efficiency. On the other hand, the steam injection contributes to the efficiency improvement of the ambient pressure system in some design conditions. In particular, a higher pressure ratio provides a better chance of efficiency increase due to the steam injection. This paper was recommended for publication in revised form by Associate Editor Kyoung Doug Min Mr. S. K. Park received his MS degree from Dept of Mechanical Engineering, Inha University in 2007, and is now Doctoral student at the same department. His research topics include performance analysis of fuel cell and fuel cell/gas turbine hybrid sys-tem and advanced energy systems. Prof. T. S. Kim received his PhD degree from Dept of Mechanical Engineering, Seoul National University in 1995. He has been with Dept of Mehanical Engineering, Inha University since 2000, and is Associate Professor as of Nov. 2008. His research area includes simulation and test of gas turbines and aerodynamic performance of their components. He is also interested in researches on fuel cells and fuel cell/gas turbine hybrid systems. Prof. J. L. Sohn received his PhD degree from Dept of Mechanical Engineering, The University of Alabama in Huntsville in 1986. He has been with School of Mechanical & Aerospace Engineering, Seoul National University since 2000, and is BK Associate Proessor as of Nov. 2008. His research area is design, simulation and test of gas turbine system and components. He is also interested in researches on fuel cells and fuel cell/gas turbine hybrid systems.     

Free vibration of multi-layered circular cylindrical shell with cross-ply walls,including shear deformation by using spline function method

The spline function technique is used to analyze the vibration of multi-layered circular cylindrical shells with cross-ply walls including first-order shear deformation theory. Both antisymmetric and symmetric cross-ply laminations are considered in this analysis. The governing equilibrium equations are obtained in terms of displacement and rotational functions. A system of coupled ordinary differential equations in terms of displacement and rotational functions are obtained by assuming the solution in a separable form. These functions are approximated by using Bickley-type splines of suitable order to obtain the generalized eigenvalue problem by applying point collocation techniques with appropriate boundary conditions. Parametric studies are performed to analyze the frequency response of the shell with reference to the material properties, number of layers, fiber orientation, thickness to radius ratio, length to radius ratio and circumferential node number. Reasonable agreement is found with existing results obtained by FEM and other methods. Valuable results are presented as graphs and discussed. This paper was recommended for publication in revised form by Associate Editor Maenghyo Cho Dr. K. K. Viswanathan was born in 1962 in Vellore District, India. He received his B.Sc. in Mathematics from University of Madras and M.Sc. in 1992 and Ph.D. in 1999 from Anna University, India. Later he was a Project Associate in Indian Institute of Science, Bangalore. He served as lecturer in Crescent Engg. College and as Asst. Professor in SRM University, India. He did his post doctoral research in Korea for three years. At present he serves as Professor in the Dept. of Naval Architecture, Inha University, Incheon, Korea. His research areas of interest includes vibration of plates, shells and the application of numerical techniques in Engineering problems. Dr. Kyung Su Kim was born in Korea in 1954. He is a professor in Naval Architecture and Ocean Engineering at Inha University, Korea. He obtained his B.Sc. degree in Naval Architecture and Ocean Engineering from Seoul National University, Korea, in 1981. He worked for KR (Korean Register of Shipping) from 1981 to 1983. He obtained M.Sc. degree in Naval Architecture and Ocean Engineering in 1986, and Ph.D. degree in Structural Mechanics in 1991 from Rheinisch — Westfaelische Technische Hoch-schule Aachen, Germany. From 1986 to 1992, he was a Post Doctoral Research Engineer of Engineering Research Institute at Rheinisch — Westfaelische Technische Hochschule Aachen. He was appointed as a professor of Inha University, Korea, in 1994. His major area of study is Impact and Fatigue Fracture. Dr. Jang Hyun Lee was born in Korea in 1969. Currently, he is an Assistant professor of the Department of Naval Architecture and Ocean Engineering at Inha University, Korea. He obtained his B.Sc., M.Sc. and Ph.D. degrees in Naval Architecture and Ocean Engineering from Seoul National University, Korea, in 1993, 1995 and 1999 respectively. From 1999 to 2002, he was a Post Doctoral Research Engineer of Engineering Research Institute at Seoul National University. He joined the Inha University in 2005 after holding the Chief Technology Officer at Xinnos for four years. His research interests include press forming of thick plates and shells, computational welding mechanics and Product Lifecycle Management.     

The development of a GUI program for automated generating rotor blade airfoil performance table

A user-friendly, Windows-based Graphic User Interface (GUI) program that automates the generation of the aerodynamic performance tables for comprehensive helicopter load analysis codes was developed. With this program, computational grids are automatically created for the given airfoil surface coordinates, and users can verify the grids immediately. In addition, the program’s post-processing feature can enable the real-time inspection of the interim results of the computation. The aerodynamic performance tables can be generated by performing automated aerodynamic analysis of the airfoil in various angles of attack and Mach numbers by using validated CFD code. The Mixed-Language technique was employed to combine the FORTRAN-based CFD codes to the C++-based GUI program without needing any transformation between these two languages. The MS Access was used to construct the data base of the aerodynamic tables. The real-time inputs of the analysis results of the ODBC Access data base make it easy to retrieve the aerodynamic tables. Of the entire range of the angle of attack (−180° to 180°), the CFD analysis is only performed in the significant angles depending on the Mach number. The aerodynamic data for the other angles were obtained by using an interpolation method; therefore, the tables could be generated quickly. Various aerodynamic information such as drag divergence Mach number and stall angle can be extracted from the computed results stored in the database. This paper was recommended for publication in revised form by Associate Editor Yang Na Taewoo Kim received a B.S. degree in Aerospace Engineering from Pusan National University in 2007. He is currently a M.S. candidate at the graduate school of Aerospace Engineering at Pusan National University in Pusan, Korea. His research interests are in the area of aerodynamics, optimization design, and helicopter aerodynamics. Kwanjung Yee received a B.S. degree in Aerospace Engineering from Seoul National University in 1992. He then went on to receive his M.S. and Ph.D. degrees from Seoul National University in 1992 and 1998, respectively. Dr. Yee is currently an Assistant Professor at the Department of Aerospace Engineering in Pusan National University, Korea. His major research area covers unsteady aerodynamics, rotorcraft flight dynamics and multidisciplinary design optimization. Sejong Oh received his B.S. and M.S. degrees in Aerospace Engineering from Seoul National University in 1979 and 1982 respectively. He finished his Ph.D. degrees from Stanford University in 1988. Prof. Oh is currently a Professor at Department of Aerospace at Pusan National University in Busan, Korea. He is currently a Board Member of Korean Society of Aeronautical and Space Sciences. Prof. Oh’s research interests are in the area of Vortex flow and Rotorcraft aerodynamics. Hee Jung Kang received a B.S. degree in Aerospace Engineering from KAIST in 1994. He then went on to receive his M.S. and Ph.D. degrees from KAIST in 1996 and 2001, respectively. Dr. Kang is currently a senior research engineer at Rotor Department in KARI (Korea Aerospace Research Institute). Dr. Kang’s research interests are in the area of helicopter rotor aerodynamics, CFD (Computational Fluid Dynamics) and simulation of fluid-structure interaction.     

Tolerance effects on natural frequencies of multibody systems undergoing constant rotational motion

A general multi-body formulation to analyze the tolerance effects on the statistical property variations of natural frequencies of multi-body systems undergoing constant rotational motion is proposed in this paper. To obtain the tolerance effects, Monte-Carlo simulation method is conventionally employed. However, the Monte-Carlo simulation has serious drawbacks; spending too much computation time for the simulation and achieving very slow convergence around some dynamically unstable regions. To resolve such problems, a method employing analytical sensitivity information is suggested in this paper. To obtain the sensitivities of natural frequencies the eigenvalue problem should be differentiated with respect to a design variable. The sensitivities of mass and stiffness matrices should be calculated at the dynamic equilibrium. By employing the sensitivities of natural frequencies along with the tolerance of the design variable, the statistical property variations of the natural frequencies can be calculated. This paper was recommended for publication in revised form by Associate Editor Seockhyun Kim Seung Man Eom graduated from the Department of Mechanical Engineering at Incheon University in 2005 and received his master degree from the Department of Mechanical Engineering at Hanyang University in 2007. He is currently working as a Researcher of Aircraft Development Team in KIAT(Korea Institute of Aerospace Technology, Koreanair), DaejeonDeajeon, Korea. Bum Suk Kim graduated from the School of Mechanical Engineering at Hanyang University in 2006 and received his master degree from the same department in 2008. He is currently working as a Ph.D. student in the School of Mechanical Engineering in Hanyang University, Seoul, Korea. Hong Hee Yoo graduated from the Department of Mechanical Design and Production Engineering at Seoul National University in 1980 and received his master degree from the same department in 1982. He received his Ph.D. degree in 1989 from the Department of Mechanical Engineering and Applied Mechanics in the University of Michigan at Ann Arbor, U.S.A. He is currently working as a professor in the School of Mechanical Engineering in Hanyang University, Seoul, Korea.     

Optimal operating points of PEM fuel cell model with RSM

The output power efficiency of the fuel cell system mainly depends on the required current, stack temperature, air excess ratio, hydrogen excess ratio, and inlet air humidity. Therefore, the operating conditions should be optimized to get maximum output power efficiency. In this paper, a dynamic model for the fuel cell stack was developed, which is comprised of a mass flow model, a gas diffusion layer model, a membrane hydration, and a stack voltage model. Experiments have been performed to calibrate the dynamic Polymer Electrolyte Membrane Fuel Cell (PEMFC) stack model. To achieve the maximum output power and the minimum use of hydrogen in a certain power condition, optimization was carried out using Response Surface Methodology (RSM) based on the proposed PEMFC stack model. Using the developed method, optimal operating conditions can be effectively selected in order to obtain minimum hydrogen consumption. This paper was recommended for publication in revised form by Associate Editor Tong Seop Kim Dong-Ji Xuan received his B.S. degree in Mechanical Engineering from Harbin Engineering University, China in 2000. He then received his M.S. degree in Mechanical Engineering from Chonnam National University, South Korea in 2006. Currently, he is a Ph.D. candidate of the Department of Mechanical Engineering, Chonnam National University, South Korea. His research interests include control and optimization of PEM fuel cell system, dynamics and control, and mechatronics. Zhen-Zhe Li received his B.S. degree in Mechanical Engineering from Yanbian University, China in 2002. He then received his M.S. degree in Aerospace Engineering from Konkuk University, South Korea in 2005 and his Ph.D. degree in Mechanical Engineering from Chonnam National University, South Korea in 2009. Dr. Li is currently a Researcher of the Department of Mechanical Engineering in Chonnam National University, South Korea. Dr. Li’s research interests include applied heat transfer, fluid mechanics, and optimal design of thermal and fluid systems. Jin-Wan Kim received his B.S. degree in Aerospace Engineering from Chosun University, South Korea in 1990. He then received his M.S. degree in Aerospace and Mechanical Engineering from Korea Aerospace University, South Korea in 2003 and his Ph.D degree in Mechanical Engineering from Chonnam National University, South Korea in 2008. He is currently a Post Doctor of the Department of Mechanical Engineering in Chonnam National University, South Korea. His research interests include control of hydraulic systems, dynamics and control, and mechatronics. Young-Bae Kim received his B.S. degree in Mechanical Design from Seoul National University, South Korea in 1980. He then received his M.S. degree in Mechanical Engineering from the Korean Advanced Institute of Science and Technology (KAIST), South Korea in 1982 and his Ph.D. degree in Mechanical Engineering from Texas A&M University, USA in 1990. Dr. Kim is currently a Professor of the School of Mechanical and Systems Engineering in Chonnam National University, South Korea. Dr. Kim’s research interests include mechatronics, dynamics and control, and fuel cell hybrid electric vehicle (FCHEV) systems.     

Finite element analysis of piezoelectric underwater transducers for acoustic characteristics

This paper presents a simulation technique for analyzing acoustic characteristics of piezoelectric underwater transducers. A finite element method is adopted for modeling piezoelectric coupled problems including material damping and fluid-structure interaction problems by taking system matrices in complex form. For the finite element modeling of unbounded acoustic fluid, infinite wave envelope element (IWEE) is adopted to take into account the infinite domain. An in-house finite element program is developed and technical issues for implementing the program are explained. Using the simulation program, acoustic characteristics of tonpilz transducer are analyzed in terms of modal analysis, radiated pressure distribution, pressure spectrum, transmitting-voltage response and impedance analysis along with experimental comparison. The developed simulation technique can be used for designing ultrasonic transducers in the areas of nondestructive evaluation, underwater acoustics and bioengineering. This paper was recommended for publication in revised form by Associate Editor Maenghyo Cho Jaehwan Kim received his B.S. degree in Mechanical Engineering from Inha University, in 1985. He received his M.S. degree from KAIST in 1987 and his Ph.D. degree from The Pennsylvania State University in 1995. Dr. Kim is currently a Professor of the Dept. of Mechanical Engineering at Inha University, Inchoen, Korea. He serves as an Associate Editor of Smart Materials and Structures. He is the director of Creative Research Center for EAPap Actuator supported by KOSEF. Dr. Kim’s research interests are smart materials such as piezoelectric materials, electroactive polymers and their applications including sensors, actuators, motors and MEMS devices. Heung Soo Kim received his B.S. and M.S. degrees in the Department of Aerospace Engineering from Inha University, Korea in 1997 and 1999, respectively. He obtained his Ph. D degree in the Department of Mechanical and Aerospace Engineering from Arizona State University in 2003. He is now working as an assistant professor in the School of Mechanical and Automotive Engineering, Catholic University of Daegu. His main research interests are in biomimetic actuators and sensors, structural health monitoring, smart materials and structures as applied to aerospace structures and vehicles.     

Improvement of flame stability and NOx reduction in hydrogen-added ultra lean premixed combustion

Lean premixed combustion is a well known method in gas turbine combustors that can reduce fuel consumption and decrease flame temperature. In lean premixed flames, flame instabilities can occur because the combustion takes place near the lean flammable limit. For the purpose of increasing flame stability, a small amount of hydrogen was added into a fuel, which has ultra low lean flammable limit. The extinction stretch rate increased and total equivalence ratio at extinction decreased with hydrogen addition; consequently, ultra lean premixed combustion was possible and flame stability could be achieved at low temperature conditions. The NO_x emission increased with hydrogen addition for the same stretch rate and equivalence ratio, but the extinction stretch rate and lean flammability limit was enlarged. Consequently, NO_x emission decreased with hydrogen addition in the near extinction conditions. Hydrogen addition could improve flame stability and reduce NO_x emission in ultra lean premixed combustion. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Dr. Eun-Seong Cho received his B.S. and M.S. degrees in Mechanical Engineering from Hanyang University, Korea, in 1996 and 1998, respectively. He then received his Ph.D. degree from Seoul National University, Korea, in 2005. He was a principal engineer of KD Navien research center and currently a research associate at Delft University of Technology, The Netherlands. His research interests include eco-friendly clean combustion technology, new and renewable energy systems. Prof. Suk Ho Chung received his B.S. degree from Seoul National University, Korea, in 1976 and Ph.D. degree in Mechanical Engineering from Northwestern University, USA, in 1983. He is a Professor since 1984 in the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests cover combustion fundamentals, pollutant formation, laser diagnostics, and plasma-assisted combustion.     

Topology optimization with displacement-based nonconforming finite elements for incompressible materials

This investigation is concerned with the topology optimization using displacement-based nonconforming finite elements for problems involving incompressible materials. Although the topology optimization with mixed displacement-pressure elements was performed, a displacement-based approach can be an efficient alternative because it interpolates displacement only. After demonstrating the Poisson locking-free characteristics of the employed nonconforming finite elements by a simple patch test, the developed method is applied to solve the design problems of mounts involving incompressible solid or fluid. The numerical performance of the nonconforming elements in topology optimization was examined also with existing incompressible problems. This paper was recommended for publication in revised form by Associate Editor Tae Hee Lee Gang-Won Jang received his M.S. degree in 2000, and Ph.D. degree in 2004, both from the School of Mechanical and Aerospace Engineering, Seoul National University, Seoul, Korea. He is currently an Assistant Professor at the School of Mechanical and Automotive Engineering, Kunsan National University, Jeonbuk, Korea. His current interest concerns topology optimization of multiphysics problems and thin-walled beam analysis. Yoon Young Kim received his B.S. and M.S degrees from Seoul National University, Seoul, Korea, and the Ph.D. degree from Stanford University, Palo Alto, CA, in 1989. He has been on the faculty of the School of Mechanical and Aerospace Engineering, Seoul National University, since 1991. He is also the Director of the National Creative Research Initiatives Center for Multiscale Design. His main research field is the optimal design of multiphysics systems, mechanisms, and transducers. He has served as an editor of several Korean and international journals, and as an organizing committee member of several international conferences.     

Aerodynamic analysis of flapping foils using volume grid deformation code

Nature-inspired flapping foils have attracted interest for their high thrust efficiency, but the large motions of their boundaries need to be considered. It is challenging to develop robust, efficient grid deformation algorithms appropriate for the large motions in three dimensions. In this paper, a volume grid deformation code is developed based on finite macro-element and transfinite interpolation, which successfully interfaces to a structured multi-block Navier-Stokes code. A suitable condition that generates the macro-elements with efficiency and improves the robustness of grid regularity is presented as well. As demonstrated by an airfoil with various motions related to flapping, the numerical results of aerodynamic forces by the developed method are shown to be in good agreement with those of an experimental data or a previous numerical solution. This paper was recommended for publication in revised form by Associate Editor Do Hyung Lee Jin Hwan Ko received his B.S. degree in Mechanical Engineering from KAIST, Korea, in 1995. He then received his M.S. and Ph.D. degrees from KAIST in 1997 and 2004, respectively. Dr. Ko is currently a research professor at the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests include fluid-structure interaction analysis, structural dynamics of a micro-scale resonator, and model order reduction. Soo Hyung Park received his B.S. degree in Aerospace Engineering from KAIST, Korea, in 1996. He then received his M.S. and Ph.D. degrees from KAIST in 1999 and 2003, respectively. Prof. Park is currently an assistant professor at the Dept. of Aerospace Information Engineering at Konkuk University in Seoul, Korea. His research interests include computational fluid dynamics, fluid-structure interaction analysis, rotorcraft aerodynamics, and turbulence modeling.     

Flow dynamics at the minimum starting condition of a supersonic diffuser to simulate a rocket’s high altitude performance on the ground

A numerical analysis was conducted to investigate and characterize the unsteadiness of the flow structure and oscillatory vacuum pressure inside of a supersonic diffuser equipped to simulate high-altitude rocket performance on the ground. A physical model including a rocket motor, vacuum chamber, and diffuser, which have axisymmetric configurations was employed. Emphasis was placed on investigating the physical phenomena of very complex and oscillatory flow evolutions in the diffuser operating very close to the starting condition, i.e. at a minimum starting condition, which is one of the major important parameters from a diffuser design point of view. This paper was recommended for publication in revised form by Associate Editor Jun Sang Park Hyo-Won Yeom received a B.S. degree in the department of Aerospace & Mechanical Engineering from Korea Aerospace University in 2007. He is currently a master candidate at the school of Aerospace & Mechanical Engi-neering at Korea Aerospace Uni-versity in Goyang-city, Korea. His research interests are in the area of numerical analysis for High-speed propulsion system. Sangkyu Yoon received a B.S. degree in the department of Aerospace & Mechanical Engineering from Korea Aerospace University in 2006 and M.S. degrees in the school of Aerospace & Mecha-nical Engineering from Korea Aerospace University in 2008. He currently works in Hanwha Corporation R&D Center. Hong-Gye Sung received a B.S. degree in the department of Aerospace Engineering from Inha University in 1984 and Ph.D. degree in Nuclear and Mechanical Engineering from The Pennsylvania State University in 1999. Dr. Sung has various research experiences in the fields of high-speed propulsion and rocket propulsion in Agency for Defense Development for 22 years (1984–2006). He is currently a professor at the School of Aerospace and Mechanical Engineering of Korea Aerospace University in Goyang, Korea. Dr. Sung’s research interests are in the area of propulsion, combustion, and its control.     

Friction-induced ignition modeling of energetic materials

The heat released during the external frictional motion is a factor responsible for initiating energetic materials under all types of mechanical stimuli including impact, drop, or penetration. We model the friction-induced ignition of cyclotrimethylenetrinitramine (RDX), cyclotetramethylene-tetranitramine (HMX), and ammonium-perchlorate/ hydroxylterminated-polybutadiene (AP/HTPB) propellant using the BAM friction apparatus and one-dimensional time to explosion (ODTX) apparatus whose results are used to validate the friction ignition mechanism and the deflagration kinetics of energetic materials, respectively. A procedure to obtain the time-to-ignition for each energetic sample due to friction is outlined. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Min-cheol Gwak received his B.S. degree in Mechanical Engineering from Korea Aerospace University, Korea, in 2007. Now he is a graduate student of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests are ignition of high energy material and combustion phenomena. Tae-yong Jung received his B.S. degree in Mechanical and Aerospace Engineering from Seoul National University, Korea, in 2007. Now he is a graduate student of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests are solid propellant combustion and phase transformation. Professor J. Yoh received his BSME from UC Berkeley in 1992 and MSME from UCLA in 1995. His doctoral degree is in Theoretical & Applied Mechanics from the University of Illinois at Urbana-Champaign, 2001. His research interest is in high energy system design using high power lasers and condensed energetic materials.     

Numerical simulation on soot deposition process in laminar ethylene diffusion flames under a microgravity condition

A numerical study on soot deposition in ethylene diffusion flames has been conducted to elucidate the effect of thermophoresis on soot particles under a microgravity environment. Time-dependent reactive-flow Navier-Stokes equations coupled with the modeling of soot formation have been solved. The model was validated by comparing the simulation results with the previous experimental data for a laminar diffusion flame of ethylene (C₂H₄) with enriched oxygen (35% O₂ + 65% N₂) along a solid wall. In particular, the effect of surrounding air velocity as a major calculation parameter has been investigated. Especially, the soot deposition length defined as the transverse travel distance to the wall in the streamwise direction is introduced as a parameter to evaluate the soot deposition tendency on the wall. The calculation result exhibits that there existed an optimal air velocity for the early deposition of soot on the surface, which was in good agreement with the previous experimental results. The reason has been attributed to the balance between the effects of the thermophoretic force and convective motion. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Jae Hyuk Choi received his B.S. and M.S. degrees in Marine System Engineering from Korea Maritime University in 1996 and 2000, respectively. He then went on to receive a Ph.D. degrees from Hokkaido university in 2005. Dr. Choi is currently a BK21 Assistant Professor at the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. Dr. Choi’s research interests are in the area of reduction of pollutant emission (Soot and NOx), high temperature combustion, laser diagnostics, alternative fuel and hydrogen production with high temperature electrolysis steam (HTES). Junhong Kim received his B.S., M.S., and Ph. D degrees in Mechanical Engineering from Seoul National University in 1998, 2000, and 2004, respectively. His research interests include lifted flames, edge flames, and numerical simulation. Sang Kyu Choi received his B.S. degree in Mechanical Engineering from Seoul National University in 2004. He is a Ph. D student in the School of Mechanical Engineering, Seoul National University. His research interests include edge flames, oxy-fuel combustion, and numerical simulation. Byoung ho Jeon received his B.S degrees in Mechanical Engineering from kangwon University in 1998, and M.S., Ph. D. degrees in Mechanical Engineering from Hokkaido University in 2002, 2008, respectively. Dr Jeon is working at Korea Aerospace Research Institute from 2007. June. as Gasturbine engine developer. Jeon’s research interests are in the area of reduction of pollutant emission (Soot and Nox), High temperature combustion, combustion system (Furnace, Combine Generation system, IGCC, CTL), and Fire safety in building. Osamu Fujita received his B.S., M.S., and Ph. D. degrees in Mechanical Engineering from Hokkaido University in 1982, 1984, and 1987, respectively. Prof. Fujita is currently a Professor at the division of Mechanical and space Engineering at Hokkaido University in sapporo, Japan. Prof. Fujita’s research interests are in the area of reduction of pollutant emission (Soot and Nox), solid combustion, catalytic combustion, high temperature combustion, alternative fuel and fire safety in space. Suk Ho Chung received his B.S. degree in Mechanical Engineering in 1976 from Seoul National University, and his M.S. and Ph. D. degree in Mechanical Engineering in 1980 and 1983, respectively from Northwestern University. He is a professor since 1984 in the School of Mechanical and Aerospace Engineering, Seoul National University. His research interests cover combustion fundamentals, pollutant formation, and laser diagnostics.