http://chineseinput.net/에서 pinyin(병음)방식으로 중국어를 변환할 수 있습니다.
변환된 중국어를 복사하여 사용하시면 됩니다.
Development of a viscoelastic turbulent flow solver without any artificial diffusivity
Kyoungyoun Kim(김경연) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11
We developed a finite difference code for the simulation of viscoelastic turbulent flow using the FENE-P model. The conformation tensor in the FENE-P model is time-marched using a second-order Runge-Kutta scheme. The advection term in the polymer constitutive equation is discretized based on the slope-limiting approach of Vaithianatha et al. (2006, J. Non-Newton. Fluid), which guarantees the positive-definiteness of the conformation tensor without introducing any artificial diffusivity. The present code has been verified through a DNS of a viscoelastic drag-reduced turbulent channel flow. The accuracy and stability of the code were evaluated for various FENE-P model parameters.
김경연(Kyoungyoun Kim) 대한기계학회 2012 大韓機械學會論文集B Vol.36 No.8
벽면 난류의 항력과 밀접한 관련이 있는 유동구조를 조사하기 위해 Reτ = 180, 395, 590 의 난류채널유동에 대한 직접수치모사를 수행하였다. 확률밀도함수를 조사하여 레이놀즈 전단응력에 가장 큰 기여를 하는 Q2 이벤트를 파악하였으며 Q2 이벤트의 각도의 변화가 y+ < 50 에서는 벽 단위로 스케일링되며, y/h > 0.5 에서는 채널의 높이로 스케일링 됨을 확인하였다. Q2 이벤트를 조건으로 하는 조건부 평균 유동장을 조사하여 레이놀즈 전단응력의 발생과 관련이 있는 유동구조는 주 유동방향의 보텍스 및 헤어핀 형상의 보텍스임을 보였다. 또한, 순간 유동장을 관찰하여 높은 레이놀즈 전단 응력의 분포가 이러한 보텍스 구조와 관련이 있으며 1.5 ~ 3h 의 크기를 갖는 대형유동구조를 구성함을 확인하였다. Direct numerical simulations were carried out for turbulent channel flows with Reτ = 180, 395 and 590 to investigate the turbulent flow structure related to the Reynolds shear stress. By examining the probability density function, the second quadrant (Q2) events with the largest contribution to the mean Reynolds shear stress were identified. The change in the inclination angle of Q2 events varies with wall units in y+ < 50 and with the channel half height in y/h > 0.5. Conditionally averaged flow fields for the Q2 event show that the flow structures associated with Reynolds shear stress are a quasi-streamwise vortex in the buffer layer and a hairpin-shaped vortex in the outer layer. Three-dimensional visualization of the distribution of high Reynolds shear stress reveals that the organization of hairpin vortices in the outer layer having a size of 1.5~3 h is associated with large-scale motions with high Reynolds shear stress in the outer layer.
비압축성 Navier-Stokes 방정식에 대한 내재적 속도 분리 방법
김경연(Kyoungyoun Kim),백승진(Seung-Jin Baek),성형진(Hyung Jin Sung) 한국전산유체공학회 2000 한국전산유체공학회 학술대회논문집 Vol.2000 No.10
An efficient numerical method to solve the unsteady incompressible Navier-Stokes equations is developed. A fully implicit time advancement is employed to avoid the CFL(Courant-Friedrichs-Lewy) restriction, where the Crank-Nicholson discretization is used for both the diffusion and convection terms. Based on a block LU decomposition, velocity-pressure decoupling is achieved in conjunction with the approximate factorization. Main emphasis is placed on the additional decoupling of the intermediate velocity components with only n th time step velocity. The temporal second-order accuracy is preserved with the approximate factorization without any modification of boundary conditions. Since the decoupled momentum equations are solved without iteration, the computational time is reduced significantly. The present decoupling method is validated by solving the turbulent minimal channel flow unit.
기체확산층의 기공률이 고분자 전해질 연료전지 성능에 미치는 영향에 관한 전산해석 연구
김경연(Kyoungyoun Kim),손영준(Young-Jun Sohn),김민진(Minjin Kim),이원용(Won-Yong Lee) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.12
Numerical analysis was carried out to investigate the effect of GDL (Gas diffusion layer) porosity on the performance of PEMFC (proton exchange membrane fuel cell). A complete three-dimensional model was chosen for single straight channel geometry including cooling channel. Main emphasis is placed on the heat and mass transfer through the GDL with different porosity. The present numerical results show that at high current densities, the cell voltage is influenced by the GDL porosity while the cell performance is nearly the same at low current densities. At high current densities, low value of GDL porosity results in decrease of the fuel cell performance since the diffusion of reactant gas through GDL becomes slow with decreasing porosity. On the other hand, for high GDL porosity, the effective thermal conductivity becomes low and the heat generated in the cell is not removed rapidly. This causes the temperature of fuel cell to increase and gives rise to dehydration of the membrane, and ultimately increase of the ohmic loss.