직접 수치 모사법을 이용한 섬유 강화 복합재료의 열팽창계수 예측

남윤식(Youn-Sic Nam),오민환(Min Hwan Oh),김광식(Kwang Sik Kim),조진연(Jin Yeon Cho) 한국항공우주학회 2007 韓國航空宇宙學會誌 Vol.35 No.9

본 논문에서는 직접 수치 모사 기법을 이용해 섬유 강화 복합재료의 열팽창계수를 예측하였다. 직접 수치 모사 기법을 통해 구한 열팽창계수 예측치와 실험치의 비교를 통해 본 논문에서 제안한 직접 수치 모사 기법을 이용하면 인위적인 가정을 최소화하면서 기존의 방법과 유사하게 복합재료 열팽창계수를 예측할 수 있음을 확인하였다. 또한 섬유 체적비 변화에 따른 열팽창계수의 변화를 예측하고 그 경향성을 고찰하였다. In this paper, thermal expansion coefficients of fiber-reinforced composite materials are predicted by direct numerical simulation. From comparing the predicted results with experimental results, it is confirmed that direct numerical simulation gives similar results to the previously proposed methods while minimizing artificial assumptions. Additionally trend of variation in thermal expansion coefficients is investigated according to the fiber volume fraction.

직접 수치 모사를 통한 캐비테이션 소음 예측 및 모델링

서정희(Jung-Hee Seo),문영준(Young J. Moon) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5

Prediction methods for cavitation noise are presented. At first, direct numerical simulation of cavitating flow noise has been performed, and acoustic analogy equation based on the cavitation noise modeling is derived. For the direct numerical simulation, a density based homogenous equilibrium model is employed to simulate cavitating two-phase flow and the governing equations are solved with high-order numerical schemes to resolve cavitation noise. The compressible Navier-Stokes equations for mixture fluids are discretized with a sixth-order central compact scheme, and the steep gradient of flow variables and supersonic regions are treated with the selective spatial filtering technique. The direct simulation of cavitating flow noise is performed for a 2D circular cylinder at cavitation number 0.7 and 1. The far-field noise is also predicted with the derived analogy equation. Noise spectrum predicted with the equation is well compared with the result of direct numerical simulation and also agree well with the theory.

막대형 표면조도가 있는 난류경계층 직접수치모사: 표면조도 높이에 따른 효과

최윤경(Yun Kyung Choi),황현규(Hyeon Gyu Hwang),이영모(Young Mo Lee),이재화(Jae Hwa Lee) 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.12

Direct numerical simulations (DNSs) of spatially developing turbulent boundary layers (TBLs) over two-dimensional (2D) rod-roughened walls are performed to investigate the effects of the roughness heights on the flow characteristics. The roughness height (k) is varied in the range of k/θ<SUB>in</SUB>=0.1~1.5 (θ<SUB>in</SUB> is the inlet momentum thickness) and the Reynolds number based on the momentum thickness is varied in the range Reθ=300~1400. Based on the mean velocity profiles at equilibrium states, the roughness function (△U<SUB>+</SUB>) is decreased with a decrease of k/θ<SUB>in</SUB> and magnitude of the Reynolds stresses in the outer layer is also reduced continuously. The present observation indicates that the wall-similarity between the rough- and smooth-walls in the outer layer is satisfied when the roughness height is less than δ/k =300, where δ is the boundary layer thickness.

단순화된 도시지형을 지나는 경계층유동의 직접수치모사

문기영(Kiyoung Moon),이창훈(Changhoon Lee) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11

Boundary-layer flows over urban-like regularly distributed obstacles were investigated by direct numerical simulation. To enable interpretation of the flow within the arrays as a canopy flow, results are analysed in terms of a spatial averaging procedure and of the flow above as a roughwall boundary-layer. Spatial and time averages of velocity and turbulent stresses are computed. The statistics are compared with data from wind-tunnel experiments and other direct numerical simulations. Below about half of canopy layer, dispersive stress is more significant than the Reynolds stress. In the lower canopy layer, unsteady effects are important where turbulent fluctuations are dominant over the spatial and time averaged flow.

3 차원 입방형 표면조도가 난류경계층에 미치는 영향

이재화(Jae Hwa Lee),성형진(Hyung Jin Sung) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.4

3 차원 표변조도의 영향을 조사하기 위하여 규칙적으로 배열된 3 차원의 입방형 표면조도를 갖는 난류경계층을 직접수치모사하였다. 표면조도는 주 유동방향과 횡 방향으로 각각 8k과 2k의 주기를 갖도록 배열되었으며 표면조도의 크기 (k)는 입구 모멘텀 두께(θ<SUB>in</SUB>)의 1.5 배이다. 주 유동 방향을 따라 공간 발달하는 3 차원 표면조도 위의 난류특성을 2 차원의 막대형표면조도에서의 결과와 비교하였다. 2 차원 표면조도와 마찬가지로 3 차원 표면조도의 경우에도 조도저층 뿐만 아니라 바깥영역에서 표면조도의 영향이 존재하였으며 이러한 결과는 주 유동 방향의 표면조도의 주기와 사각형의 면에 의한 막음현상이 2 차원의 표면조도와 마찬가지로 크게 나타나기 때문인 것으로 판단된다. Direct numerical simulation of a spatially developing turbulent boundary layer with regularly arrayed cubical roughness was performed to investigate the effects of three-dimensional surface elements. The staggered cubes along the downstream were periodically arranged in the streamwise and span wise directions with pitches of p<SUB>x</SUB>/k=8 and p<SUB>z</SUB>/k=2, where p<SUB>x</SUB>, and p<SUB>z</SUB> are streamwise and spanwise spacing of the cubes and the roughness height (k) was k=1.5θ<SUB>in</SUB>, where θ<SUB>in</SUB> is the momentum thickness at the inlet. Spatially developing characteristics over the 3-D cubical roughness were compared with the data from the DNS over the two-dimensional rod rough and smooth walls. Introduction of the cubical roughness on the TBL also affected the turbulent Reynolds stresses not only in the roughness sub layer but also in the outer layer, consistent with those over the 2-D rough wall.

저속충격을 받는 적층복합재료 평판의 직접 수치모사

지국현 ( Kuk Hyun Ji ),김승조 ( Seung Jo Kim ) 한국복합재료학회 2006 Composites research Vol.19 No.1

역압력 구배 난류 경계층 유동 내 대형 구조가 표면 저항에 미치는 진폭 변조 영향

윤민(Min Yoon),황진율(Jinyul Hwang),성형진(Hyung Jin Sung) 대한기계학회 2017 대한기계학회 춘추학술대회 Vol.2017 No.11

Direct numerical simulation of a turbulent boundary layer subjected to an adverse pressure gradient (β = 1.45) at Re<SUB>τ</SUB> = 834 is performed to explore amplitude modulation influences of large scales on the skin friction. The correlations 〈vω<SUB>z</SUB>〉 and , 〈-wω<SUB>y</SUB>〉 representing the advective vorticity transport and the vortex stretching, respectively, are directly related to the skin friction coefficient (C<SUB>f</SUB>), and they affect the decrease and increase in C<SUB>f</SUB>, respectively. Large-scale streamwise velocity fluctuations (uL) are extracted by employing a long-wavelength-pass filter with a cut-off wavelength of , λ⁺<SUB>z,c</SUB>=400. The correlations 〈vω<SUB>z</SUB>〉 and 〈-wω<SUB>y</SUB>〉 are conditionally averaged with respect to u<SUB>L</SUB>. The enhanced large scales strongly modulate the vortical motions related to the skin friction. In particular, the negative-u<SUB>L</SUB> events augment the contribution of 〈vω<SUB>z</SUB>〉to the skin friction, and the contribution of -wω<SUB>y</SUB>〉 is enhanced by the positive-u<SUB>L</SUB> events.

비정상 압력 구배에 의한 난류 경계층 유동 박리 현상의 동적 모드 분할 분석

박준신(Junshin Park),유동현(Donghyun You) 대한기계학회 2020 大韓機械學會論文集B Vol.44 No.2

시간에 따라 변하는 비정상 압력 구배에 의해 발생한 난류 경계층 유동 박리 현상에 대해 직접수치 모사와 동적 모드 분할 분석을 수행하였다. 압력 구배가 시간에 따라 진동하며, 각기 다른 전환 주파수 k=wLb /2U를 가지는 두 경우에 대해 계산을 수행하였다. 터빈 유동의 작동 범위내로 k=0.75인 경우, 대조군으로 이보다 빠른 영역의 k=3.75인 경우에 대하여 수치 해석을 진행하였다. k=3.75인 경우 박리 기포는 시간에 따라 높이 변화가 없는 데 반해 k=0.75인 경우 박리 기포의 높이, 재점착된 전단층의 거동도 시간에 따라 큰 변화를 보였다. 이 현상은 동적 모드 분할 기법을 적용한 결과 유의한 모드들로 분할되었으며, 이는 전단층의 거동을 잘 표현할 뿐만 아니라 주요 물리현상의 위치 및 주파수를 특정할 수 있음을 확인하였다. Direct numerical simulation and dynamic mode decomposition are conducted for a separated turbulent boundary layer under unsteady adverse pressure gradients. Two simulation cases are chosen based on the reduced frequency k=wLb /2U . First case is when k=0.75 which lies inside the operating condition of turbomachinery. The second case is when k=3.75 for comparison. When k=3.75, the height of the shear layer above the separation bubble does not change with time. However, when k=0.75, the height of the shear layer and the response of the reattached shear layer changes with time. Flow fields are decomposed into significant modes using dynamic mode decomposition. It is shown that significant modes not only describe the response of the shear layer effectively, but also specify locations and frequencies of significant features of the separated turbulent boundary layer.

Direct numerical simulation of the turbulent boundary layer with rod-roughened wall

Seung-Hyun Lee(이승현),Hyung Jin Sung(성형진) 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-

The effects of surface roughness on a spatially-developing turbulent boundary layer (TBL) were investigated by performing direct numerical simulations of TBLs over rough and smooth walls. The Reynolds number based on the momentum thickness was varied in the range Reθ=300~1400. The roughness elements used were periodically arranged two-dimensional spanwise rods, and the roughness height was k=1.5θin, which corresponds to k/δ=0.045~0.125. To avoid generating a rough wall inflow, which is prohibitively difficult, a step change from smooth to rough was placed 80θin downstream from the inlet. The spatially-developing characteristics of the rough-wall TBL were examined. Along the streamwise direction, the friction velocity approached a constant value and a self-preserving form of the turbulent stress was obtained. Introduction of the roughness elements affected the turbulent stress not only in the roughness sublayer but also in the outer layer. Despite the roughness-induced increase of the turbulent stress in the outer layer, the roughness had only a relatively small effect on the anisotropic Reynolds stress tensor in the outer layer. Inspection of the triple products of the velocity fluctuations revealed that introducing the roughness elements onto the smooth wall had a marked effect on vertical turbulent transport across the whole TBL. By contrast, good surface similarity in the outer layer was obtained for the third-order moments of the velocity fluctuations.

3 차원 입방형 표면조도가 난류경계층에 미치는 영향

이재화(Jae Hwa Lee),성형진(Hyung Jin Sung) 대한기계학회 2010 大韓機械學會論文集B Vol.34 No.10

3 차원 표면조도의 영향을 조사하기 위하여 규칙적으로 배열된 3 차원의 입방형 표면조도를 갖는 난류경계층을 직접수치모사하였다. 표면조도는 주 유동방향과 횡 방향으로 각각 8k 과 2k 의 주기를 갖도록 배열되었으며 표면조도의 크기 (k)는 입구 운동량 두께(θin)의 1.5 배이다. 주 유동 방향을 따라 공간 발달하는 3 차원 표면조도 위의 난류특성을 2 차원의 막대형표면조도에서의 결과와 비교하였다. 2 차원 표면조도와 마찬가지로 3 차원 표면조도의 경우에도 조도저층 뿐만 아니라 바깥영역에서 표면조도의 영향이 존재하였으며 이러한 결과는 주 유동 방향의 표면조도의 주기와 사각형의 면에 의한 막음현상이 2 차원의 표면조도와 마찬가지로 크게 나타나기 때문인 것으로 판단된다. Direct numerical simulation (DNS) of a spatially developing turbulent boundary layer (TBL) with regularly arrayed cubical roughness elements was performed to investigate the effects of three-dimensional (3D) surface elements. The staggered cubes downstream were periodically arranged in the streamwise and spanwise directions with pitches of px/k=8 and pz/k=2, where px and pz are the streamwise and spanwise spacings of the cubes; the roughness height (k) was k=1.5θin, where θin is the momentum thickness at the inlet. Spatially developing characteristics over the 3D cubical roughness were compared with the data obtained from the DNS over the two-dimensional (2D) rod roughened wall and smooth wall. Introduction of the cubical roughness on the TBL affected the turbulent Reynolds stresses not only in the roughness sublayer but also in the outer layer; and these effects are consistent with those observed over the 2D rough wall.