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Hybrid RANS/LES 방법을 이용한 아음속 공동 유동의 수치적 모사
장경식(K. S. Chang),박승오(S. O. Park),최석기(S. K. Choi) 한국전산유체공학회 2004 한국전산유체공학회지 Vol.9 No.2
A numerical simulation of an incompressible cavity flow is conducted using the hybrid turbulence model. The model adopted is a modified type of DES using k-ε two-equation model. Cavity geometry and flow condition are based on Cattafesta's experiment. Computational results are compared with the results of Cattafesta's experiment. The simulation successfully predicts the oscillatory features and the Strouhal number of the oscillation compares very favorably with that of the dominant mode of experimental data. Vorticity contours obtained from the simulation data are consistent with the smoke visualization of the Cattafesta's experiment. The coherent structures of cavity flow are also investigated using Q criterion.
Point-Collocation Non-intrusive Polynomial Chaos 기법을 활용한 평판 주위 난류 유동장 해석
장경식(K. Chang),고아림(A.R. Ko) 한국전산유체공학회 2018 한국전산유체공학회지 Vol.23 No.2
In the present work, uncertainty quantification based on the point-collocation Non-intrusive Polynomial Chaos was studied in turbulent flow around the flat plate using Menter’s SST k-ω model. Two random variables of β* , model coefficient of Menter’s SST k-ω model and U<SUB>∞</SUB>, the freestream velocity are considered. β*, model coefficient was assumed to be uniformly distributed and the freestream velocity U<SUB>∞</SUB>, normally distributed with a certain value of variance. First, the uncertainty quantification with the single random variable of each one was simulated for sensitivity analysis of each parameter on the outputs such as drag coefficient and skin friction one. When the normal distribution of freestream velocity is considered, the drag coefficient showed the same type of distribution with 0.9% standard deviation. The uniform distribution of β* with the internals of 0.0784 and 0.1024 had an effect on 5% variance on drag coefficient. When two random variables are considered simultaneously, the distribution of drag coefficient showed closer type with one of model coefficient β* .The convergence rate of mean value was late than that with single random variable and standard deviation was not converged until the 5th order of gPC.
압축성 기저 유동에 대한 GEKO 모델 계수의 불확실성 정량화 연구
정영기(Y.K. Jung),장경식(K. Chang),배재현(J.H. Bae) 한국전산유체공학회 2021 한국전산유체공학회지 Vol.26 No.1
The present work, supersonic flow over an axisymmetric base is simulated using Generalized k-ω (GEKO) model which is proposed by Menter. GEKO two-equation model based on the k-ω formulation provides free model coefficients which can be adjusted by user depending on the specified flow type. Uncertainty Quantification analysis (UQ) is adopted to quantify the uncertainty of the model coefficients and to calibrate the coefficients for the flow. Latin Hypercube Sampling (LHS) method is used for sampling input parameters which are independent as a uniform distribution. Surrogate model is constructed by using ordinary least-squares (OLS). In order to characterize the posterior via Markov Chain Monte Carlo sampling, Affine Invariant Ensemble Algorithm (AIES) is selected. Through Forward problem, the most effective coefficient among the coefficients of GEKO model is figured out (Sensitivity analysis). Calibrated model coefficients are obtained through Bayesian inference. The results obtained using the calibrated coefficients by UQ to the base flow shows better agreement with available experimental measurements than the results obtained using default model coefficients.
변분다중스케일법을 이용한 $Re_{\tau}=180$ 채널 난류 유동의 대와류모사
장경식,이보현,윤범상,이주성,노명일,Chang, K.,Lee, B.H.,Yoon, B.S.,Lee, J.S.,Roh, M.I. 한국전산유체공학회 2010 한국전산유체공학회지 Vol.15 No.2
In the present work, LES with new variational multiscale method is conducted on the fully developed channel flow with Reynolds number, 180 based on the friction velocity and the channel half width. Incompressible Navier-Stokes equations are integrated using finite element method with the basis function of NURBS. To solve space-time equations, Newton's method with two stage predictor multicorrector algorithm is employed. The code is parallelized using MPI. The computational domain is a rectangular box of size $2{\pi}{\times}2{\times}4/3{\pi}$ in the streamwise, wall normal and spanwise direction. Mean velocity profiles and velocity fluctuations are compared with the data of DNS. The results agree well with those of DNS and other traditional LES.
BWB UCAV 형상 주변의 유동해석에 대한 천이모델의 예측능력 연구
조영희(Y.H. Jo),장경식(K.S. Chang),신동진(D.J. Sheen) 한국전산유체공학회 2014 한국전산유체공학회 학술대회논문집 Vol.2014 No.11
A computational simulation for a nonslender BWB UCAV configuration with rounded leading edge and span of 1.0m was performed to analyze its aerodynamic characteristics at low A.o.A.s. Reynolds number based on the mean chord length is 1.25 million. γ-Re<SUB>θ</SUB> transition model and k-ω SST model was used to assess effect of transition for flow behavior. Also γ transition model that accounts for crossflow instability effects was employed to investigate crossflow instability effects in three-dimensional boundary layer on the UCAV aerodynamic surfaces. After assessing aerodynamic coefficients and lift to drag, it was found that crossflow effects plays an important roles at given flow condition.
변분다중스케일법을 이용한 Re<SUB>τ</SUB>=180 채널 난류 유동의 대와류모사
장경식(K. Chang),이보현(B.H. Lee),윤범상(B.S. Yoon),이주성(J.S. Lee),노명일(M.I. Roh) 한국전산유체공학회 2009 한국전산유체공학회 학술대회논문집 Vol.2009 No.11
In the present work, LES with new variational multiscale method is conducted on the fully developed channel flow with Reynolds number is 180 based on the friction velocity and the channel half width. Incompressible Navier-Stokes equations are integrated using finite element method with the basis function of NURBS. To solve space-time equations, Newton's method with two stage predictor multicorretor algorithm is employed. The code is parallelized using MPI. The computational domain is a rectangular box of size 2π×2×4/3π in the stream wise, wall normal and span wise direction. Mean velocity profiles and velocity fluctuations are compared with the data of DNS. The results agree well with those of DNS and other traditional LES.
장경식(K. Chang),G. Constant inescu 한국전산유체공학회 2012 한국전산유체공학회 학술대회논문집 Vol.2012 No.11
The present large eddy simulation (LES) study focuses on the case of developing flow within wide channels containing at the bottom wall an array of 2D dunes (2a/h=0.25, l/h=3.75, / is the wavelength, 2a is the dune height, h is the mean channel depth) of closer shape to the ones observed in natural streams. For the case of an incoming steady flow, the instantaneous flow fields are characterized by the formation of arrays of highly-organized hairpin vortices whose dimensions scale with the dune height. LES shows that for shallow channels (2a/h>0.25), the large-scale hairpins and the streaks penetrate regularly up to the free surface, thus affecting mass transport and mixing over the whole water column. LES shows that the hairpin eddies play the primary role in the formation of the streaks of high and low momentum over the region containing dunes. The presence of resolved turbulence in the incoming flow reduces the streamwise distance needed for the streaks to develop over the rough bed region, but does not affect qualitatively the transition process toward the fully developed flow regime, nor the spacing of the streaks in the fully-developed flow region.
변분다중스케일법을 이용한 Re<SUB>T</SUB>=180 채널 난류 유동의 대와류모사
장경식(K. Chang),이보현(B.H. Lee),윤범상(B.S. Yoon),이주성(J.S. Lee),노명일(M.I. Roh) 한국전산유체공학회 2010 한국전산유체공학회지 Vol.15 No.2
In the present work, LES with new variational multiscale method is conducted on the fully developed channel flow with Reynolds number, 180 based on the friction velocity and the channel half width. Incompressible Navier-Stokes equations are integrated using finite element method with the basis function of NURBS. To solve space-time equations, Newton's method with two stage predictor multicorrector algorithm is employed. The code is parallelized using MPI. The computational domain is a rectangular box of size 2π×2×4/3π in the streamwise, wall normal and spanwise direction. Mean velocity profiles and velocity fluctuations are compared with the data of DNS. The results agree well with those of DNS and other traditional LES.
DES 방법을 이용한 비압축성 열린 공동 유동의 수치적 모사
장경식(K.S. Chang),박승오(S.O. Park),권오준(O.J. Kwon),G. Constantinescu 한국전산유체공학회 2005 한국전산유체공학회지 Vol.10 No.3
Three-dimensional incompressible flow past an open cavity in a channel is investigated using Detached Eddy Simulation(DES). The length to depth ratio of the cavity is 2 and the Reynolds number defined with the cavity depth is 3,360. The DES methods are based on the Menter's SST model. In the present work, two types of inflow conditions are used; one is RANS profile, the other is LES inflow from another Large Eddy Simulation(LES) of fully developed channel flow. The results are compared with experimental data and LES results in terms of the mean statistics, temporal physics and scalar transport phenomenon of the flow.