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Na K.-S.,Seoul National University | Kim J.-H.,Institute of Advanced Aerospace Technology
Composite Structures | Year: 2010

Step-formed Functionally Graded Materials (FGMs) flat panels are investigated for volume fraction optimization by considering stress and critical temperature. The structure is composed of numerous layers with homogeneous and different isotropic material properties from ceramic to metal. Material properties are assumed to be temperature dependent, and remain constant in each layer. Further, the properties are assumed to be varied in the thickness direction according to a simple power law distribution in terms of the ceramic and metal volume fractions for the layer. The effective material properties of the plate are obtained by applying linear rule of mixtures for the layers. The 3-D finite element model is adopted to analyze more accurately the variation of material properties and temperature field in the thickness direction. For the various FGM volume fraction distributions and geometric parameters, mechanical stress analysis and thermo-mechanical buckling analysis are performed to get the critical conditions. Based on the results, the volume fraction optimization of the flat panels is performed for stress reduction and improving thermo-mechanical buckling behavior and compared with previous results. © 2009 Elsevier Ltd. All rights reserved. Source

Lee I.,Seoul National University | Shin S.,Seoul National University | Shin S.,Institute of Advanced Aerospace Technology | Kim Y.,Agency for Defense Development
Aerospace Science and Technology | Year: 2013

Forced vibration analysis including a vortex lattice prediction given an external aerodynamic force is conducted in this paper based on a standing wave formulation. The starting point of the standing wave formulation is a set of blade disk normal modes that incorporate all forms of the blade, disk, and shroud elastic coupling. The Küssner gust function was used in a few previous investigations of forced vibration based on the standing wave formulation. However, it was found to be valid only for low engine-order excitation. Therefore, a two-dimensional unsteady vortex lattice method is employed in this paper to predict the gust excitation up to higher engine-order excitation. Thus, the present unsteady vortex lattice analytical model is capable of capturing compressibility and higher engine-order excitation. It features advantages in terms of its computational time and level of accuracy. The effects of mistuning a cascaded blade are also examined in the present aeroelastic analysis to determine the possible advantages obtained by doing this. Numerical results for the mistuned bladed disk are presented regarding its forced response characteristics. In a low engine-order excitation condition, it is shown that similar predictions are obtained between the present and earlier analyses. The maximum discrepancy in the blade amplitude is 70% for a single-blade mistuned rotor and 62.6% for an alternately mistuned rotor, in the worst case, compared to a completely tuned rotor. Single-peak frequencies are presented and analyzed in the higher engine-order excitation levels. © 2011 Elsevier Masson SAS. All rights reserved. Source

Park K.H.,Seoul National University | Jun S.O.,Samsung | Jun S.O.,Development Group | Baek S.M.,Hyundai Heavy Industries | And 5 more authors.
Journal of Aircraft | Year: 2013

In this study, an aerostructural analysis using a proper orthogonal decompositionwith a neural network is proposed for accurate and efficient aerostructural wing design optimization using the reduced-order model. Because reducedorder- model basis weighting estimation has a limitation in that its robustness cannot be guaranteed by various design variables and wing deformation due to fluid structure interaction, this study employs the neural network, which is capable of perceiving the relationship between the input variables and reduced variables for the proper orthogonal decomposition to complement the defects. To construct the proper orthogonal decomposition with a neural network, the neural network is learned using pairs of design variables and reduced variables from snapshot data obtained from the aerostructural analysis. Because the proposed aerostructural analysis using a proper orthogonal decomposition with a neural network is applied to validation cases and its results are compared to those of the full-order analysis, it is investigated that the proposed analysis algorithm has the capability to accurately and efficiently predict the aerodynamic and structural performances of wings that are considered aboutwing deformation. Furthermore, because the design optimization problem minimizing the weight of a wing design is performed with the analysis algorithm, it is confirmed that it can be a more efficient design than a conventional design method using a second-order polynomial model, which consists of a greater number of experiment designs than the number of snapshots. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Source

Kim S.-B.,Seoul National University | Na Y.-H.,Seoul National University | Kim J.-H.,Institute of Advanced Aerospace Technology
Journal of Sound and Vibration | Year: 2010

Sensitive devices such as resonant sensors and radio frequency micro-electro-mechanical system (RF-MEMS) filters etc., require high Quality factors (Q-factors) defined as the ratio of total system energy to dissipation that occurs due to various damping mechanisms. Also, thermoelastic damping is considered to be one of the most important factors to elicit energy dissipation due to the irreversible heat flow of oscillating structures in the micro scales. In this study, the Q-factor for thermoelastic damping is investigated in rotating thin rings with in-plane vibration. First, in order to obtain the temperature profile of the model, a heat conduction equation for the thermal flow across the radial direction is solved based on the bending approximation so-called in-extensional approximation of the ring. Using the temperature distribution coupled with a displacement, a governing equation of the ring model can then be derived. Eventually, an eigen-value analysis is performed to obtain the natural frequency of rotating thin rings, and the analytical and numerical values of Q-factors can then be determined by the definition. Furthermore, the effects of rotating speed, dimensions of the ring, mode numbers and ambient temperatures on the Q-factor are discussed in detail. © 2009 Elsevier Ltd. All rights reserved. Source

Lee H.-K.,Seoul National University | Lee H.-K.,Hyundai Motor Company | Viswamurthy S.R.,Seoul National University | Viswamurthy S.R.,National Aerospace Laboratories, Bangalore | And 6 more authors.
Journal of Aircraft | Year: 2010

In this paper, an accurate structural dynamic analysis was developed for a helicopter rotor system including rotor control components, which was coupled to various aerodynamic and wake models in order to predict an aeroelastic response and the loads acting on the rotor. Its blade analysis was based on an intrinsic formulation of moving beams implemented in the time domain. The rotor control system was modeled as a combination of rigid and elastic components. A multicomponent analysis was then developed by coupling the beam finite element model with the rotor control system model to obtain a complete rotor-blade/control-system aeroelastic analysis. The rotor blade analysis was in good agreement and validated by comparing with DYMORE. Numerical results were obtained for a four-bladed, small-scale, articulated rotor rotating in vacuum and in a wind tunnel to simulate forward-flight conditions and its aerodynamic effects. The complete rotor-blade/control-system model was loosely coupled with various inflow and wake models in order to simulate both hover and forward-flight conditions. The resulting rotor blade response and pitch link loads areingood agreement with those predictedby CAMRAD II. The present analysis features both model compactness and robustness inits solution procedure while capturing the sophisticated behavior of individual rotor components. The analysis is expected to be part of a framework useful in the preliminary design phase for helicopters. Copyright © 2010. Source

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