평균 벽 전단 응력 경계 조건을 이용한 복잡한 난류유동의 벽모델 큰에디모사

조민정(Minjeong Cho),이정일(Jungil Lee),최해천(Haecheon Choi) 대한기계학회 2012 대한기계학회 춘추학술대회 Vol.2012 No.11

The mean wall shear stress boundary condition was successfully applied to turbulent channel and boundary layer flows using large eddy simulation without resolving near-wall region. In the present study, we apply this boundary condition to more complex flows where flow separation and redeveloping flow exist. As a test problem, we consider flow over a backward-facing step at Reynolds number of 22860 based on the step height. Turbulent boundary layer flow at the inlet is obtained using inflow generation technique by Lund et al. (1998) but with wall shear stress boundary condition. First, we prescribe the mean wall shear stress distribution obtained from DNS (Kim, 2011, Ph.D. Thesis, Stanford U.) as the wall boundary condition of present simulation, and we give no-slip boundary condition at flow-reversal region. The results from present wall-modeled large eddy simulation are in good agreements with the flow statistics by DNS. Currently, a dynamic approach of obtaining mean wall shear stress based on the log-law is being applied.

뇌동맥류 모델에 대한 혈류역학 해석

서태원(Tae won Seo),변준수(Jun SooByun) 대한기계학회 2013 大韓機械學會論文集B Vol.37 No.10

뇌동맥류모델은 CT 영상을 기반으로 추출하여, ANSYS-FLUENT 를 사용해 전산 유체유동해석을 수행하였다. 본 연구를 통해 뇌동맥류에서 최소 벽전단응력은 동맥류가 발생한 영역에서 일어나는 것을 알 수 있다. 또한 뇌동맥류 모델에서 우측중뇌동맥 안쪽벽면에 작용하는 벽전단응력의 크기는 동맥류 전부와 후부의 벽면에 작용하는 벽전단응력의 크기에 비해 20 배 더 크게 발생하는 것을 알 수 있다. 그러나 동맥류 영역에서의 전단응력의 크기는 매우 작게 나타났다. 혈관 수축이 일어나는 동안 동맥류의 영역에서 매우 복잡한 이차유동이 발생하는 것을 볼 수 있다. 동맥류 내부에서의 혈류유동은 나선형 유동형태를 보이며, 본 연구의 혈류역학적 특성 분석을 통해 뇌동맥류의 파열을 예견할 수 있을 것으로 판단한다. The intracranial aneurysm model is extracted based on the Computed Tomography (CT) scan images. Computational fluid dynamics simulations were conducted under both steady and realistic flow conditions in ANSYSFLUENT. The minimum wall shear stress in the intracranial aneurysm tended to occur in the aneurysmal region. The magnitude of wall shear stress along inner wall of the curvature in the right M1 segment of middle cerebral artery is approximately 20 times higher than that along both the proximal and distal walls. However, the magnitudes of the wall shear stress at the aneurysm region were considerably low. The blood flow has the complex distribution in the aneurysmal region during the systolic period. Complex helical flow patterns are observed inside the aneurysm. Through an analysis of the hemodynamic characteristics, one may predict the rupture of the cerebral aneurysms.

총 전단 응력을 RANS 상단 경계 조건으로 이용한 벽모델 큰 에디 모사

성창우(Changwoo Sung),이정일(Jungil Lee),최해천(Haecheon Choi) 대한기계학회 2012 대한기계학회 춘추학술대회 Vol.2012 No.11

In wall-modeled large eddy simulation, how to exchange the flow field information between the solutions from RANS and LES through the boundary condition is one of the important issues. In general, the wall boundary condition of LES is given as a form of the instantaneous wall shear stresses from the solution of RANS, whereas the upper boundary condition of RANS is provided as the instantaneous velocity from the solution of LES. However, in this approach, the total shear stress at the upper boundary is not continuous and thus momentum transfer from LES to RANS is not strictly conserved. In our study, we provide the instantaneous total shear stresses at the upper boundary of RANS with mixing-length model and conduct simulations of turbulent channel flow at high Reynolds numbers. The results show excellent predictions of turbulence statistics.

구조설계실무 현황을 고려한 전단벽 해석모형에 관한 고찰

정성진(Jung, Sung-Jin) 대한건축학회 2018 大韓建築學會論文集 : 構造系 Vol.34 No.9

While computer environments have been dramatically developed in recent years, as the building structures become larger, the structural analysis models are also becoming more complex. So there is still a need to model one shear wall with one finite element. From the viewpoint of the concept of FEA, if one shear wall is modeled by one finite element, the result of analysis is not likely accurate. Shear wall may be modelled with various finite elements. Among them, considering the displacement compatibility condition with the beam element connected to the shear wall, plane stress element with in-plane rotational stiffness is preferred. Therefore, in order to analyze one shear wall with one finite element accurately, it is necessary to evaluate finite elements developed for the shear wall analysis and to develop various plane stress elements with rotational stiffness continuously. According to the above mentioned need, in this study, the theory about a plane stress element using hierarchical interpolation equation is reviewed and stiffness matrix is derived. And then, a computer program using this theory is developed. Developed computer program is used for numerical experiments to evaluate the analysis results using commercial programs such as SAP2000, ETABS, PERFORM-3D and MIDAS. Finally, the deflection equation of a cantilever beam with narrow rectangular section and bent by an end load P is derived according to the elasticity theory, and it is used to for comparison with theoretical solution.

인공신경망 기반 벽모델을 이용한 큰에디모사

이영모(Young Mo Lee),이정일(Jungil Lee),이재화(Jae Hwa Lee) 대한기계학회 2019 대한기계학회 춘추학술대회 Vol.2019 No.11

Wall-modeled LES (WMLES) is promising to simulate a turbulent flow at high Reynolds number with a reasonable cost because the required computational cost of large-eddy simulation in the near-wall region is proportional to approximately square of the friction Reynolds number (Reτ). An equilibrium stress model is the most widely used method due to the high efficiency. However, this method is still required to enhance the accuracy and applicability because of the limitations of the equilibrium assumption. In the present study, an artificial neural network (ANN) is used to obtain the wall shear stress for WMLES. The proposed method shows good prediction on the profiles of mean velocity and Reynolds stresses compared to equilibrium model when direct numerical simulation dataset of turbulent channel flows is trained at Reτ=395, 540, 930 and 2000. In addition, the present method predicts well the turbulent statistics at Re<SUB>τ</SUB>=640 and 5200, which are untrained by the ANN.

벽모델 큰 에디모사에서의 RANS 상단 경계 조건 비교

성창우(Changwoo Sung),이정일(Jungil Lee),최해천(Haecheon Choi) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12

In wall-modeled large eddy simulation based on a zonal approach, how to exchange the flow field information between the solutions from RANS and LES through the boundary condition is one of the important issues. In general, the wall boundary condition of LES is given as a form of the instantaneous wall shear stresses from the solution of RANS, whereas the upper boundary condition of RANS is provided as the instantaneous velocity from the solution of LES. However, in this approach, the total shear stress at the upper boundary is not continuous and thus momentum transfer from LES to RANS is not strictly conserved. In our study, we provide the instantaneous total shear stresses at the upper boundary of RANS with mixing-length model and conduct simulations of turbulent channel and boundary layer flows at high Reynolds numbers. The results from proposed boundary condition show excellent predictions of turbulence statistics.

사각공동내 자연대류에서 측면 단열벽에 의한 영향

허정환(Jeong-Hwan Heo),정범진(Bum-Jin Chung) 대한기계학회 2010 大韓機械學會論文集B Vol.34 No.9

사각공동내 자연대류 열전달 실험에서, 단열벽에 의해 열전달이 저하가 관찰되는 영역을 실험적으로 그리고 수치해석적으로 평가하였다. GrH 수 1.53×10?부터 1.01×10¹?까지 변화시키며, 단열벽이 존재할 때와 그렇지 않을 때를 구분하여 열전달률을 측정하였다. FLUENT 실험의 결과를 예측할 수 있는지 확인하고 실험으로 수행하기 어려운 매우 좁은 영역에 대해 FLUENT를 수행하였다. 실험과 FLUENT의 결과를 다른 연구와 비교한 바 일치함을 보였다. 단열벽이 전체 열전달에 미치는 영향은 예상과 같이 단열벽 근처의 매우 좁은 영역에 국한하여 나타남을 확인하였다. 본 연구는 유사성(Analogy) 원리를 이용하여 열전달계를 전기도금계의 물질전달계로 모사하는 방법론을 개발하는 과정에서 실험을 효율화하고자 하는 방안을 강구하기 위하여 추진되었다. 본 연구를 통하여 단열벽간 거리(전극의 폭)를 매우 줄일 수 있는 이론적 근거를 확보하였다. In this study, we investigated the effects of adiabatic walls on natural convection in various rectangular cavities experimentally and numerically. Heat transfer rates were measured for cavities with and without adiabatic sidewalls by varying Grashof number from 1.53 × 10? to 1.01 × 10¹?. Some typical test results were successfully simulated using FLUENT. In the case of very narrow cavities, where the adiabatic walls were very close to each other, it was difficult to perform experiments; therefore, FLUENT simulations were performed. The existing heat transfer correlations for rectangular cavities were well predicted by the experimental and numerical results. As expected, the effects of adiabatic walls were restricted to the very narrow region near the walls. This study was carried out during the development of an analogy experimental method in which heat-transfer systems are replaced with mass-transfer systems using copper sulfate electroplating systems. The results of this study provide theoretical background of handling adiabatic walls during the design of test facilities.

NUMERICAL ANALYSIS OF BLOOD FLOW DYNAMICS AND WALL MECHANICS IN A COMPLIANT CAROTID BIFURCATION MODEL

투안(T. M. Nguyen),이상욱(S.-W. Lee) 한국전산유체공학회 2011 한국전산유체공학회 학술대회논문집 Vol.2011 No.5

Blood flow simulations in an idealized carotid bifurcation model with considering wall compliance were carried out to investigate the effect of wall elasticity on the wall shear stress and wall solid stress. Canonical waveforms of flowrates and pressure in the carotid arteries were imposed for the boundary conditions. Comparing to rigid wall model, generally, we could find an increased recirculation region at the carotid bulb and an overall reduced wall shear stress. Also, there was appreciable change of flowrate and pressure waveform in longitudinal direction. Solid and wall shear stress concentration occurs at the bifurcation apex.

스텐트 공극률의 뇌동맥류 모델 내부 유동장 영향 수치해석

판다이탄(Dai Thanh Phan),이상욱(Sang-Wook Lee) 대한기계학회 2014 大韓機械學會論文集B Vol.38 No.1

본 연구에서는 상대적으로 얇은 두께의 스트럿을 가진 스텐트를 적용하여 스텐트의 공극률(80%, 74% 및 64%)에 따른 뇌동맥류 내부 유동 특성 변화를 이해하고자 CFD 해석을 수행하고, 기존의 발표된 실험 결과와 비교하였다. 수치해석 방법으로는 이차의 사면체 요소(quadratic tetrahedral element) 기반의 유한요소해석(FEM) 코드를 이용하였다. 뇌동맥류 내부 평균유속 감소비의 정량적인 면에서는 실험결과와 약간의 차이를 보였으나, 스텐트 공극률에 따른 뇌동맥류 내부 유동장 패턴 및 평균유속 감소의 상대적 특성 등은 비교적 잘 일치하는 것을 확인할 수 있었다. 또한 본 연구에서 고려한 가장 높은 80% 공극률을 가지는 스텐트의 경우에도 비교적 우수한 뇌동맥류 유입 유속 감소 효과를 가짐을 확인할 수 있었으며, 이보다 더 낮은 공극률을 가진 스텐트의 경우에는 약간의 추가적인 뇌동맥류 유입 유속 감소 효과를 가지나, 유속 및 벽전단응력 등의 혈류역학적 특성은 큰 변화가 없음을 알 수 있었다. In the present study, CFD simulations were conducted for investigating intra-aneurysmal flow characteristics with different stent porosities (Cα = 80%, 74%, and 64%), and the simulation results were compared with experimental data. Using a quadratic tetrahedral element.based finite element scheme, we estimated velocity fields and wall shear stress. The intra-aneurysmal velocity reduction ratios obtained via simulation agree well with published experimental data. It was found that a stent with a porosity of 80%, which is highest in the present study, is able to effectively reduce flow into the aneurysm, which causes intra-aneurysmal stasis, and that stents with lower porosities afford only incremental benefits in reducing inflow to an aneurysm.

관상동맥의 기하학적 형상변화에 따른 동맥경화 위험도

노형운(Hyung-Woon Roh),서상호(Sang-Ho Suh),박준길(JoonGil Park),이병권(Byung-Kwon Lee),권혁문(Hyuck-Moon Kwon) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5

From the previous study, it found that hemodynamics were correlated with stenosis in coronary artery. The flow characteristics and distributions of the hemodynamic wall parameters in the coronary artery are investigated under physiological flow condition. The present study is also aimed to establish the mechanism of the generation of atherosclerosis by analyzing the hemodynamic variables in the coronary artery where atherosclerosis frequently occured. The stenosis phenomena due to atherosclerosis are related to not only biochemical reaction between blood and blood vessel but also the hemodynamic factors like flow separation and oscillatory wall shear stress.