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예조건화된 압축성유동 수치기법에서의 풍상차분법의 수치특성 검토
길재흥(J. H. Gill),이두환(D. H. Lee),최윤호(Y. H. Choi),권장혁(J. H. Kwon),이승수(S. S. Lee) 한국전산유체공학회 2002 한국전산유체공학회 학술대회논문집 Vol.2002 No.-
Studies of the numerical characteristics of implicit upwind. schemes, such as upwind ADI, Line Gauss-Seidel(LGS) and Point Gauss-Seidel(LU) algorithms, for preconditioned Navier-Stokes equations are performed. All the algorithms are expressed in approximate factorization form and Van Neumann stability analysis and convergence studies are made. Preconditioning is applied for efficient convergence at low Mach numbers and low Reynolds numbers. For high aspect ratio computations, the ADI and LGS algorithms show efficient and uniform convergence up to moderate aspect ratio if we adopt viscous preconditioning based on min-CFL/max-VNN time-step definition. The LU algorithm, on the other hand, shows serious deterioration in convergence rate as the grid aspect ratio increases. Computations for practical applications also verify these results.
오픈로터 소음 저감을 위한 비정상 유동 하의 강건최적설계
권형일(H. I. Kwon),최성임(S. Choi),권장혁(J. H. Kwon) 한국전산유체공학회 2014 한국전산유체공학회 학술대회논문집 Vol.2014 No.5
Robust design optimization for open rotor has been conducted. To alleviate computational burden of robust measurement in the design, Kriging surrogate model was employed to model responses. For performance evaluations of open rotor especially, unsteady flow simulation by state-of-the-art CFD technique of harmonic balance method was used, and it coupled with aeroacoustic analysis based on acoustic analogy. Thanks to the combined analysis between aerodynamic and aeroacoustic analyses, unsteady noise characteristics of the open rotor was efficiently investigated. The baseline configuration is 8×8 pusher-type of DLR CROR system, and rated condition is a cruise regime. Two types of aleatory uncertainties were modeled by probability density functions, and reflected in design variables and free-stream Mach number. The result showed that about 3dB of noise reduction was achieved, with it maintained thrust level of the baseline rotor system. Moreover, the validation of robustness for the uncertainties was performed, and showed the robust optimum rotor was more insensitive to the uncertainties than the baseline open rotor.
저속 압축성 유동에서 예조건화 방법을 이용한 수렴성 증진에 대한 연구
이재은(J. E. Lee),박수형(S. H. Park),권장혁(J. H. Kwon) 한국전산유체공학회 2004 한국전산유체공학회 학술대회논문집 Vol.2004 No.-
It is well known that preconditioning methods are efficient for convergence acceleration at compressible low Mach number flows. In this study, the original Euler equations and three preconditioners nondimensionalized differently arc implemented in two dimensional inviscid bump flows using the 3rd order MUSCL and DADI schemes as flux discretization and time integration respectively. The multigrid and local time stepping methods are also used to accelerate the convergence. The test case indicates that a properly modified local preconditioning technique involving concepts of a global preconditioning one produces Mach number independent convergence. Besides, an asymptotic analysis for properties of preconditioning methods is added.
Noisy 한 CFD 결과에 대한 구속조건을 고려한 EGO 최적설계
배효길(H.G. Bae),권장혁(J.H. Kwon) 한국전산유체공학회 2012 한국전산유체공학회 학술대회논문집 Vol.2012 No.5
Efficient Global Optimization (EGO) method is a global optimization technique which can select the next sample point systematically by infill sampling criteria (ISC) and search for the global minimum with less samples than the conventional global optimization. ISC function consists of the predictor and mean square error (MSE) provided from the kriging model which is a stochastic metamodel. Also the constrained EGO method can minimize the objective function dealing with the constraints under EGO concept. In this study the constrained EGO method-applied to the RAE2822 airfoil shape design formulated with the constraint. Even though the optimum shape was obtained efficiently by EGO method, the noisy CFD data caused the kriging model to fail to depict the true function. This study showed that the noisy CFD data could be managed to keep from distorting the true function by changing the correlation function or using the re-interpolation kriging model while retaining the property of global convergence.
Noisy 한 CFD 결과에 대한 구속조건을 고려한 EGO 방법 연구
배효길(H.G. Bae),권장혁(J.H. Kwon) 한국전산유체공학회 2012 한국전산유체공학회지 Vol.17 No.4
Efficient Global Optimization (EGO) method is a global optimization technique which can select the next sample point automatically by infill sampling criteria (ISC) and search for the global minimum with less samples than what the conventional global optimization method needs. ISC function consists of the predictor and mean square error (MSE) provided from the kriging model which is a stochastic metamodel. Also the constrained EGO method can minimize the objective function dealing with the constraints under EGO concept. In this study the constrained EGO method applied to the RAE2822 airfoil shape design formulated with the constraint. But the noisy CFD data caused the kriging model to fail to depict the true function. The distorted kriging model would make the EGO deviate from the correct search. This distortion of kriging model can be handled with the interpolation(p=free) kriging model. With the interpolation(p=free) kriging model, however, the search of EGO solution was stalled in the narrow feasible region without the chance to update the objective and constraint functions. Then the accuracy of EGO solution was not good enough. So the three-step search method was proposed to obtain the accurate global minimum as well as prevent from the distortion of kriging model for the noisy constrained CFD problem.
근사모델을 이용한 RAE2822 운용 불확실성 강건최적설계
배효길(H.G. Bae),권장혁(J.H. Kwon) 한국전산유체공학회 2010 한국전산유체공학회 학술대회논문집 Vol.2010 No.5
In the view of robust design optimization, RAE2822 airfoil was designed to achieve not only the maximum lift-to-drag ratio but also insensitivity of that. While the RAE2822 is flying at the cruise speed, Mach variation is considered as the operational uncertainty. In order to explore the design space, metamodels were introduced instead of consecutively computing the gradient. Also a metamodel was used to represent the sigma space. Using the metamodel, an optimum value was searched in the view of global optimization.
이재훈(J. Lee),권장혁(J.H. Kwon) 한국전산유체공학회 2008 한국전산유체공학회지 Vol.13 No.1
The fluid-structure interaction analysis such as a static aeroelastic analysis requires the result of each analysis as an input to the other analysis. Usually the grids for the fluid analysis and the structural analysis are different. so the results should be transformed properly for each other. The Infinite Plate Spline(IPS) and the Thin Plate Spline(TPS) are used in interpolating the displacement and the pressure. In this study, such interpolation methods are compared with kriging which provides a precise response surface. The static aeroelastic analysis is performed for the supersonic flow field with shock waves and the pressure field is interpolated by the TPS and kriging. The TPS shows tendency to weaken the shock strength, whereas kriging preserves the shock strength.