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Park K.H.,Seoul National University | Jun S.O.,Samsung | Baek S.M.,Hyundai Heavy Industries | Baek S.M.,Construction Equipment Research Institute | And 4 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.

Kim J.-H.,Seoul National University | Kim C.,Seoul National University | Kim C.,Institute of Advanced Aerospace Technology
AIAA Journal | Year: 2011

This paper investigates the unsteady flowfield characteristics around three-dimensional insect flapping motion under forward flight conditions. A realistic wing trajectory, called the "figure-of-eight" motion, is extracted from a blowfly's (phormia regina) tethered flight experiment under a freestream velocity of 2:75 m=s. In the authors' preliminary research [Lee, J., Kim, J., and Kim, C., "Numerical Study on the Unsteady-Force-Generation Mechanism of Insect Flapping Motion," AIAA Journal, Vol. 46, No. 7, 2008, pp. 1835-1848. doi:10.2514/1.35646], the two-dimensional blowfly's wing motion was computationally investigated, and the results revealed very interesting and distinctive vortical flowfields, which provide a decisive clue in understanding the rapid maneuverability of insects' flight. On the line of continuous efforts, the present work investigates the role of three-dimensional vortical structure in unsteady aerodynamic force generation. Detailed numerical simulation and analysis on threedimensional flapping motion are conducted, and interesting flow features of insects' flapping flight are observed, such as the existence of a spanwise flow component, leading-edge vortex, wing tip vortex, trailing-edge vortex, and various forms of vortex tubes and vortex rings. It turns out that vortical structures play a crucial role in determining unsteady characteristics of lift and thrust generation. In particular, the vortex pairing and vortex staying phenomena, which have been observed in two-dimensional flapping motion, are also observed but with a more complicated pattern. On top of that, distinctive lift and thrust generation is observed owing to three-dimensional wing shape, trajectory, and vortex structure. Consequently, the results of the present work provide an important clue in understanding generation of aerodynamic force for rapid maneuverability in insects' flight. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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.

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.

Kim W.J.,Institute of Advanced Aerospace Technology | Jang E.Y.,Institute of Advanced Aerospace Technology | Seo D.K.,Institute of Advanced Aerospace Technology | Kang T.J.,Institute of Advanced Aerospace Technology | And 3 more authors.
Langmuir | Year: 2010

A fabrication technique is developed for the preparation of metal oxide/CNT composites. An essential feature of the technique lies in the use of nonaqueous electrolyte in place of the usual aqueous electrolyte, which ensures well-dispersed CNTs without surfactants. After a "seed" is formed by electroplating on the anode, the seed is simply pulled up at a certain speed to grow a 1D CNT composite structure. The technique leads to a uniform distribution of metal oxide and a high weight fraction of CNT in the composite structure. Moreover, the conductivity of the composite is much higher than that of the CNT fibers fabricated with polymer. © 2010 American Chemical Society.

Lee I.,Seoul National University | Kim T.,Seoul National University | Kim T.,Institute of Advanced Machinery Design | Shin S.,Seoul National University | Shin S.,Institute of Advanced Aerospace Technology
AIAA Journal | Year: 2012

A brand new reduced-order aeroelastic analysis is proposed for a turbine cascade in both time and frequency domains based on the z and p transformation techniques, exhibiting no limitation upon the interblade phase angle considered in the analysis. Based on the proposed model, flutter prediction results are obtained for various interblade phase angles and are compared with the other existing results. Transient aeroelastic responses due to an initial condition are also presented, and they are compared with results from the original full-order aeroelastic model. The two analyses show a good match with a difference less than 3.65%. Such a discrepancy could be attributed to the difference in the aerodynamic coefficients between the present and former approaches. The computational time consumed by the present model is decreased by 68.52% relative to that for the full-order model.

Na Y.-H.,Seoul National University | Shin S.,Seoul National University | Shin S.,Institute of Advanced Aerospace Technology
Journal of Aircraft | Year: 2013

An efficient method to analyze a low-aspect-ratio composite wing with a control surface is developed in this paper. To reduce the problem size and the computational cost, two special schemes are applied. The first is an equivalentplate methodology and the second is an expanded type of component-mode synthesis. An equivalent-plate analysis was adopted for a semimonocoque main wing, which consists of skins, spars, and ribs, and a thin solid control surface. The main wing and control surface were connected by torsional springs, and the expanded component-mode synthesis was used to combine these two separate components with torsional springs. The first-order shear-deformation theory of plates was the basis of the present equivalent-plate analysis, and a finite element method was applied to solve it. To validate this development, a free-vibration analysis for a cantilevered plate with torsional springs was conducted. Various semimonocoque wings were analyzed, and the results were compared with those using MSC Nastran. A further complex three-dimensional analysis for a composite wing with a control surface was then conducted. The results were compared with those obtained from MSC Nastran to demonstrate the levels of accuracy and the reduction of the problem size. © 2012 by Ernesto Benini.

Moon Y.,Seoul National University | Moon Y.,Korea Aerospace Research Institute | Kim D.,Seoul National University | Yoon Y.,Seoul National University | Yoon Y.,Institute of Advanced Aerospace Technology
Journal of Propulsion and Power | Year: 2010

A modified sheet breakup model was applied to a thin, viscous liquid film generated by a swirl injector similar to that installed in a liquid propellant rocket engine combustor. The sheet breakup model consists of three steps: determination of the swirl injector characteristics for the prediction of initial sheet conditions at the injector exit as input, linear stability analysis for primary sheet breakup, and the Taylor analogy breakup model for final drop formation. Under atmospheric pressure, the liquid sheet breakup occurs under a long-wave regime, sometimes according to simple theoretical analysis. But in high ambient pressure conditions, like a liquid propellant rocket engine combustor, the sheet breakup regime changes from a long wave to a short-wave regime due to a high gas Weber number (We2 > 27=16), although the same injector was used. The sheet breakup model was, therefore, modified to be applicable to both long- and short-wave regimes and validated by the comparison of breakup length and Sauter mean diameter to experimental results. In both experimental and computational results, the spray cone angle and breakup length decreased as the ambient pressure increased, even though the pressure difference of the injector was constant. Local Sauter mean diameters, predicted by computation, were smaller at high ambient pressures. The comparative results show that the computational model is able to accurately predict sheet breakup length, spray cone angle, local Sauter mean diameter, and overall spray shape. Therefore, the model can be used as a design tool, ahead of analyzing spray characteristics of an injector in both atmospheric and high ambient pressure conditions. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

Lee K.R.,Seoul National University | Park J.H.,Seoul National University | Kim K.H.,Seoul National University | Kim K.H.,Institute of Advanced Aerospace Technology
AIAA Journal | Year: 2011

To enhance accuracy in a high-order flow solver with an overset grid method, a high-order interpolation method was developed based on finite volume method in the Euler equations. To improve stability when calculating a nonlinear discontinuity with the high-order interpolation, the interpolation was combined with a multidimensional limiting process to remove oscillatory phenomena. Thus, the proposed method can be robustly applied to a discontinuous region as well as a continuous one. It was compared with conventional methods using test cases, thereby verifying its accuracy and efficiency. It can be extendable to the Navier-Stokes equations without any modification, even though it was tested in the Euler equations in the present paper. Copyright©2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Won S.-H.,Seoul National University | Jeung I.-S.,Seoul National University | Jeung I.-S.,Institute of Advanced Aerospace Technology | Parent B.,Pusan National University | Choi J.-Y.,Pusan National University
AIAA Journal | Year: 2010

A three-dimensional unsteady reacting flowfield that is generated by transverse hydrogen injection into a supersonic mainstream is numerically investigated using detached-eddy simulation and a finite-rate chemistry model. Grid refinement with the grid-convergence-index concept is applied to the instantaneous flowfield for assessing the grid resolution and solution convergence. Validation is performed for the jet penetration height, and the predicted result is in good agreement with experimental trends. The results indicate that jet vortical structures are generated as the interacting counter-rotating vortices become alternately detached in the upstream recirculation region. Although the numerical OH distribution reproduces the experimental OH-planar-laser-induced fluorescence well, there are some disparities in the ignition delay times due to the restricted availability of experimental and numerical data. The effects of the turbulence model on combustion are identified by a comparative analysis of the Reynolds-averaged Navier-Stokes and detached-eddy simulation approaches. Their effects are quantified by the production of H2O, which is the primary species of hydrogen combustion.

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