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Haecheon CHOI,Jungil LEE 한국산업응용수학회 2011 한국산업응용수학회 학술대회 논문집 Vol.6 No.2
In Choi & Moin (2011), it was shown that the number of grid points (N) required for solving turbulent boundary layer flow using wall-modeled large eddy simulation (WMLES) is proportional to ReLx, but a wall-resolving LES requires N ∼ Re<SUP>13/7</SUP>Lx , where Lx is the flat-plate length in the streamwise direction. This grid-point requirement indicates the importance of WMLES for high Reynolds number flow. In this study, we provide the mean wall shear stress as a boundary condition for WMLES without any further modeling near the wall. The motivation of using this wall boundary condition for WMLES is such that in the framework of finite volume method, an accurate information of mean wall shear stress is the most important in the momentum transport near the wall, even if the first grid used in WMLES locates far away from the wall. For turbulent channel flow, the mean wall shear stress is balanced with the mean pressure gradient and thus is provided a priori during the simulation. DNS and LES with this boundary condition show that the results agree very well with those with no-slip boundary condition. WMLES up to Reτ = 2 × 10? (based on the wall shear velocity and boundary layer thickness) with current boundary condition predicts the log law very well.
A Dynamic Globalization Model for Large Eddy Simulation of Complex Turbulent Flow
Haecheon Choi(최해천),Noma Park(박노마),Jinseok Kim(김진석) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
A dynamic subgrid-scale model is proposed for large eddy simulation of turbulent flows in complex geometry. The eddy viscosity model by Vreman [Phys. Fluids, 16, 3670 (2004)] is considered as a base model. A priori tests with the original Vreman model show that it predicts the correct profile of subgrid-scale dissipation in turbulent channel flow but the optimal model coefficient is far from universal. Dynamic procedures of determining the model coefficient are proposed based on the 'global equilibrium' between the subgrid-scale dissipation and viscous dissipation. An important feature of the proposed procedures is that the model coefficient determined is globally constant in space but varies only in time. Large eddy simulations with the present dynamic model are conducted for forced isotropic turbulence, turbulent channel flow and flow over a sphere, showing excellent agreements with previous results.
이해천(Haecheon Lee),윤웅섭(Woongsup Yoon) 한국추진공학회 2010 한국추진공학회 학술대회논문집 Vol.2010 No.11
감압비등에 의한 미립화는 더 미세한 직경의 액적을 얻을 수 있고, 분무각이 증가하며, 더 좋은 혼합특성을 갖는 다는 점에서 단순 압력식 미립화와 차이가 난다. 감압비등이란 과열된 액체를 포화 압력 이하의 대기 중으로 분사하여 급격한 비등에 의해 이루어지는 미립화이다. 본 연구는 금속 연료를 이용한 추진기관의 산화제인 물 공급기술에 감압비등과정을 이용한 와류분무를 적용시키기 위해 수행됐다. 분열길이와 같은 거시적 특성은 Charge-Couple Device(CCD) 카메라를 이용해 측정하였고, Sauter Mean Diameter(SMD)와 SMD분포와 같은 미시적 특성은 Global Sizing Velocimetry(GSV) 시스템을 이용해 측정했다. 실험은 압력과 온도, 대류 속도를 변화시키며 진행했다.
Aerodynamics of Heavy Vehicles
Choi, Haecheon,Lee, Jungil,Park, Hyungmin Annual Reviews 2014 Annual review of fluid mechanics Vol.46 No.-
<P>We present an overview of the aerodynamics of heavy vehicles, such as tractor-trailers, high-speed trains, and buses. We introduce three-dimensional flow structures around simplified model vehicles and heavy vehicles and discuss the flow-control devices used for drag reduction. Finally, we suggest important unsteady flow structures to investigate for the enhancement of aerodynamic performance and future directions for experimental and numerical approaches.</P>