거친 지표면 조도구분의 패턴에 따른 난류경계층의 유동 및왜도에 미치는 영향에 대한 연구

박권호,이시열,임희창 한국풍공학회 2018 한국풍공학회지 Vol.22 No.3

In this study, the characteristics of flow field on the rough surface were examined. The fully developed thick turbulent boundary layer has been made in the boundary layer wind tunnel with the cube arrays patch on the floor of boundary layer wind tunnel, which has patterns of staggered and aligned roughness. The boundary layer properties such as mean flow and turbulent statistics such as stresses and skewness were measured using thermal anemometry. In order to compare with wind tunnel measurement, the turbulent boundary layer was also simulated using the numerical simulation. Rough wall surface was basically used generate the turbulent boundary layer and periodic boundary condition was coaxially applied for both inlet and outlet sections. Results show that turbulent boundary layer have successfully been generated on the roughness surface, and the boundary layer properties vary depending on the pattern of surface roughness. 본 연구는 거친 표면에서의 유동장 특성에 대해 실험 및 수치해석적 연구를 수행하였다. 완전 발달한 두꺼운 난류경계층은풍동 내 바닥에 깔린 거친 표면을 이용하여 생성하였다. 평균유동과 난류강도 및 왜도와 같은 난류경계층의 특성을 열선유속계를 이용해 측정하였다. 풍동실험결과와의 비교를 위해 난류경계층은 수치해석을 이용하여 모사하였다. 거친 벽표면은 기본적으로 난류경계층 생성에 사용되었으며 입출구면에는 주기경계조건이 적용되었다. 본 연구결과로서 난류경계층이 거친 표면 위에서 성공적으로 생성되었으며 평균유동과 난류강도 및 왜도와 같은 난류경계층의 특성은 지표면조도에 따라 달라진다는 것을 확인하였다.

합성난류경계층이 벽면에서의 변동압력에 미치는 영향

이영우(Y.W. Yi),이동섭(D.S. Lee),신구균(K.K. Shin),홍진숙(C.S. Hong),임희창(H.C. Lim) 한국가시화정보학회 2021 한국가시화정보학회지 Vol.19 No.3

Large Eddy Simulation (LES) has been popularly applied and used in the last several decades to simulate turbulent boundary layer in the numerical domain. A fully developed turbulent boundary layer has also been applied to predict the complicated wake flow behind bluff bodies. In this study we aimed to generate an artificial turbulent boundary layer, which is based on an exponential correlation function, and generates a series of realistic three-dimensional velocity data in two-dimensional inlet section which are correlated both in space and in time. The results suggest its excellent capability for high Reynolds number flows. To make an effective generation, a hexahedral mesh has been used and Cholesky decomposition was applied to possess suitable turbulent statistics such as the randomness and correlation of turbulent flow. As a result, the flow characteristics in the domain and fluctuating pressure near the wall are very close to those of fully developed turbulent boundary layers.

표면조도가 있는 난류경계층 내 난류구조

이재화(Jae Hwa Lee),이승현(Seung-Hyun Lee),김경연(Kyoungyoun Kim),성현진(Hyung Jin Sung) 대한기계학회 2008 大韓機械學會論文集B Vol.32 No.6

Turbulent coherent structures near rod-roughened wall are investigated by analyzing the database of directnumerical simulation of turbulent boundary layer. The surface roughness rods with the height k/δ =0.05 are arranged periodically in Reδ = 9000 . The roughness sublayer is defined as two-point correlations are not independent of streamwise locations around roughness. The roughness sublayer based on the two-point spatial correlation is different from that given by one-point statistics. Quadrant analysis and probability-weighted Reynolds shear stress indicate that turbulent structures are not affected by surface roughness above the roughness sublayer defined by the spatial correlations. The conditionally-averaged flow fields associated with Reynolds shear stress producing Q2/Q4 events show that though turbulent vortices are affected in the roughness sublayer, these are very similar at different streamwise locations above the roughness sublayer. The Reynolds stress producing turbulent vortices in the log layer ( y /δ = 0.15) have almost the same geometrical shape as those in the smooth wall-bounded turbulent flows. This suggests that the mechanism by which the Reynolds stress is produced in the log layer has not been significantly affected by the present surface roughness.

난류흐름의 화학적 동역학 모델 (TKM) 개발: 대류경계층 내의 NO_x-O₃ 단순 광화학 기작에 대한 난류혼합의 영향 연구

김미숙 ( Mi-sug Kim ) 한국환경기술학회 2017 한국환경기술학회지 Vol.18 No.2

많은 환경모델에서는 물질의 이송 또는 확산현상에서의 난류의 영향을 잘 묘사하면서도 보통 2 개의 반응물질에 대한 화학반응속도에서는 난류의 영향이 무시된 평균값만을 표기한다. 본 연구에서는 난류의 영향이 고려된 비선형의 2차 화학반응속도로 표현된 1차원 수직 난류흐름의 화학적 동역학 모델(1-D TKM)을 개발하였으며 그 첫 번째 적용사례를 제시하였다. 연구목적은 한 낮에 형성되는 대기의 대류혼합층(CBL) 내에서 상호작용하는 두 물질(지표에서 배출되는 물질 A와 대류혼합층의 상부로부터 유입되는 물질 B)간의 불완전 혼합과정을 정량적으로 설명하는 것으로 비균질의 CBL내에서 두 물질간의 하향식-상향식 확산에 대한 시뮬레이션을 수행하고 기존의 다른 모델형식과 비교하였다. 완전혼합을 가정하는 CBL에서 난류의 영향은 거의 무시될 수 있었으나 하층부와 상층부에서는 분리의 영향과 유입농도는 화학적 특성에 매우 민감하였으며 물리적 특성과 수치해석에는 덜 민감하였다. 반응속도 상수에 가장 민감한 인자로는 분리영향이며 상부의 유입농도의 영향은 매우 작았고 지표의 배출인자의 민감도는 그 특성에 따라 다양하게 나타났다. Numerous environmental models well describe turbulent effects on transport and dispersion phenomena. However, a chemical reaction rate between two species uses an only mean value ignored the turbulent effect. This study aims at developing a simple one-dimensional model to quantitatively account for turbulence-induced incomplete mixing of two interacting chemical species. Top-down and bottom-up(TDBU) diffusion of two reactive species in an inhomogeneous convective boundary layer(CBL) are simulated by means of 1-D vertical turbulent chemical kinetics model(TKM). The model incorporates an asymmetric convective model(ACM) scheme and a concentration field splitting method(CSM) using a phenomenal extent of reaction. The derived governing equations are integrated using forward finite differences. The TKM is applied to both irreversible and reversible reactions between ozone and nitric oxide in the summer daytime convective atmospheric boundary layer. Results, in the form of vertical profiles of reactants concentrations and intensities of segregation in the convective boundary layer show that for both the irreversible and reversible reactions, segregation is significant near the boundaries where the reactants sources are located. Segregation for the irreversible reaction is everywhere larger than for the reversible reaction because the photolysis of NO₂ tends to increase the NO concentrations while the covariances of the reacting species remain relatively constant. Comparison of the reactant concentrations from the TKM to a conventional kinetic model(CKM) that ignores effects of segregation reveals consistency in that everywhere in the CBL the TKM results lie between a no-reaction case and the CKM results.

합성난류경계층이 벽면에서의 변동압력에 미치는 영향

이영우,이동섭,신구균,홍진숙,임희창 한국가시화정보학회 2021 한국가시화정보학회지 Vol.19 No.3

Large Eddy Simulation (LES) has been popularly applied and used in the last several decades to simulate turbulent boundary layer in the numerical domain. A fully developed turbulent boundary layer has also been applied to predict the complicated wake flow behind bluff bodies. In this study we aimed to generate an artificial turbulent boundary layer, which is based on an exponential correlation function, and generates a series of realistic three-dimensional velocity data in two-dimensional inlet section which are correlated both in space and in time. The results suggest its excellent capability for high Reynolds number flows. To make an effective generation, a hexahedral mesh has been used and Cholesky decomposition was applied to possess suitable turbulent statistics such as the randomness and correlation of turbulent flow. As a result, the flow characteristics in the domain and fluctuating pressure near the wall are very close to those of fully developed turbulent boundary layers.

난류경계층에 놓인 2차원 및 3차원 트렌치 공동 주위의 표면압력변동 특성에 관한 연구

이영태,임희창 한국풍공학회 2011 한국풍공학회지 Vol.15 No.4

The paper aims to study on the surface pressure variation around open cavities under turbulent boundary flow. RANS (Reynolds-Averaged Navier-Stokes) modeling was applied for making a numerical simulation of turbulent flow. 2-equation models used in this paper include standard k-ε and k-ω SST model. In order to understand the flow and surface pressure variation around the cavities, the calculations are made on a series of 2 and 3-dimensional open trench cavities placed in a turbulent boundary layer at a Reynolds number of 1.6×10^4 based on the velocity at free stream velocity U_∞ and length of the cavity L. For getting an appropriate solution in the RANS equation for a cavity flow, the computational mesh is densely attracted to the cavity surface and coarsely far-field, as this also aids saving the computation cost and rapid convergence. The results include the flow and surface pressure variations inside cavities with the two different turbulence models. 본 논문은 난류경계층에 놓인 2차원 및 3차원 트렌치 공동 주위에서 나타나는 유동 및 표면압력 변동특성에 관한 연구이다. RANS (Reynolds-Averaged Navier-Stokes) 기법을 적용하여 공동 주위 난류 유동을 2차원 및 3차원 격자를 구성하여 수치적으로 모사하였다. 여기서 사용된 난류모델은 RANS 2-방정식 난류모델로 표준 k-ε모델과 k-ω SST 모델이 사용되었으며, 적용된 모델링의 차이에 따른 트렌치 공동 주위의 유동 및 표면의 압력특성을 알아보았다. 적용된 난류모델이 타당한 해를 가지기 위해서 공동의 벽면에 격자를 밀집시켜 격자의존성에 의한 영향을 최소화 하였다. 트렌치 공동유동의 특성을 일반화하기위해 사용된 레이놀즈수는 특성길이와 유동장의 자유흐름속도를 기준으로 하였으며, 그 크기는 1.6×10^4이다. 본 연구의 최종 목적은 2가지의 난류모델에 의한 영향과 2차원 및 3차원 트렌치 공동내부의 표면압력 결과를 통해 공동 주위 유동 특징을 파악하고자 하였다.

직접수치모사를 이용한 난류경계층 내의 거대난류구조 연구

이재화(Jae Hwa Lee),성형진(Hyung Jin Sung) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.12

Direct numerical simulation (DNS) of a turbulent boundary layer with moderate Reynolds number was performed to scrutinize streamwise-coherence of hairpin packet motions. The Reynolds number based on the momentum thickness (θin) and free-stream velocity (U∞) was varied in the range Reθ=1410~2540 which was higher than the previous numerical simulations in the turbulent boundary layer. In order to include the groups of hairpin packets existing in the outer layer, large computational domain was used (more than 50δo, where δo is the boundary layer thickness at the inlet in the streamwise domain). Characteristics of packet motions were investigated by using instantaneous flow fields, two-point correlation and conditional average flow fields in xy-plane. The present results showed that a train of hairpin packet motions was propagating coherently along the downstream and these structures induced the very large-scale motions in the turbulent boundary layer.

주기변동하는 합성입구유동이 난류경계층에 미치는 영향

이영우,임희창 한국풍력에너지학회 2020 풍력에너지저널 Vol.11 No.4

Large eddy simulation (LES) has been popularly applied and used in the last several decades to implement the atmospheric environment of a turbulent boundary layer in the numerical domain. One of its representative applications would be the development of wind turbines and farms under the turbulent boundary layer. A fully developed turbulent boundary layer is also applied to predict the flow around the wake behind bluff bodies such as wind turbines and aerodynamic structures. In this study, we aimed to generate an artificial turbulent boundary layer in the numerical domain, which can be made by a synthetic method of inflow generation. In order to get an appropriate, well-developed boundary layer in a three-dimensional domain, this generation method has been coupled with oscillating flow, which was expected to make faster convergence in the calculation. To make an effective flow analysis, a hexahedral mesh was been and Cholesky decomposition was applied to possess turbulent statistics such as the randomness and correlation of turbulent flow. As a result, the flow characteristics in the domain are very close to the calculation without the oscillating flow.

유동입구 적분길이 스케일의 변화에 따른 난류경계층생성 특성

이영태,임희창 한국풍공학회 2017 한국풍공학회지 Vol.21 No.4

Large Eddy Simulation (LES), which has recently been developed and used for the climate environment and turbulent flow analysis, can be applied for a variety of area. In particular, in order to achieve a faster performance, an artificial generation of inflow turbulent flow would be able to make the faster calculation as well as obtain the real turbulent flow in the domain. The study aims to build a synthetic inflow generator, which generates the real turbulent boundary flow in the domain. The results are compared with the existing data which was similarly obtained in the calculated and measured domain. In addition, the variation in inlet integral length scale on the inlet section shows that the larger length scale in spanwise direction has the faster recovery of wall shear stress, which means that the development of the turbulent boundary layer is fast on high length scales in spatial direction. 최근 대기환경 및 난류해석분야에 관심을 받고 있는 대와류모사(LES)는 그 적용분야도 다양하다. 특히 LES를 이용한 난류 유동장 해석 시, 실제와 유사한 난류의 특성을 지니고 있는 입구조건을 부여하면 해석 도메인 내에서 실제 난류를 보다 빠르게 생성시 킬 수 있다. 본 연구에서는 LES를 이용한 해석으로 난류경계층을 수치해석 도메인 내에 빠르게 생성시킬 수 있도록 하기 위해 입구 조건으로 기존에 많이 적용하는 방법 중 하나인 합성법(synthesis method)이 이용되었고, 수치해석 도메인 내에 만들어진 난류특성들을 기존의 연구결과들과 비교하였다. 입구에 유입되는 몇몇 유동특성들은 기존의 연구결과와 동일한 값을 입력할 수 있으나 유동방향의 길이크기는 쉽지 않다. 그로인해 기존의 연구결과들과 비교하였을 때, 채널 내에서 발달한 난류특성들은 약간의 차이를 나타내지만 유입데이터와 큰 차이는 나타나지 않았다. 또한, 본 합성법을 통한 난류 입구유동의 길이크기에 따른 응답특성을 확인하기 위하여, 폭방향으로 다양한 길이스케일 변화를 주어 그 특성을 확인하였다. 그 결과, 폭방향의 길이스케일이 커질수록 벽면의 전단응력의 회복이 빨라졌으며, 이는 난류경계층의 발달이 빨라지는 것을 의미한다.

역압력 구배가 있는 난류 경계층 내 횡 방향 와류의 분포 경향

이진(Jin Lee),이재화(Jae Hwa Lee),이중호(Joung-Ho Lee),성형진(Hyung Jin Sung) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.4

직접수치모사를 이용하여 역압력 구배가 있는 난류 경계층 유동의 난류 구조를 통계적으로 조사하였다. 평형 역압력 구배 난류 유동장을 고찰하여 횡 방향 와류의 분포 경향을 구하였다. 벽 수직 방향 거리에 따른 와류 구조의 분포를 역압력 구배의 세기에 따라 조사하였고, 역압력 구배가 있을 때 강한 소용돌이가 압력 구배가 없을 때 보다 빈번하게 나타남을 확인하였다. 역방향으로 회전하는 횡 방향 와류가 압력 구배에 상관없이 비슷한 밀도를 나타낸 것에 반해, 정방향으로 회전하는 횡 방향 와류의 밀도는 역압력 구배에 의해 후류역에서 크게 증가하였다. 이들 와 구조는 역압력 구배가 있는 난류 경계층의 후류역에서 Reynolds 전단 응력의 최대값을 형성하는 큰 난류 구조를 유발한다. The coherent structures in turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) were investigated by analyzing a database of direct numerical simulations. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution. The distributions of vortical structures were obtained by analyzing the population trends of the spanwise vortices. Strong swirling motions are more frequently observed in the APG TBLs than in the ZPG TBL. Few retrograde span wise vortices are present near the wall due to the large viscous mean shear. In the wake region, the densities of the span wise vortices increase significantly. These vortical structures induce low-momentum regions in the middle of the boundary layers, which result in an outer peak in the Reynolds shear stress in an APG boundary layer.