압력 구배가 없는 평판 천이 경계층 유동을 예측하기 위한 k-$\varepsilon$모형의 개발

백성구,임효재,정명균,Baek, Seong-Gu,Im, Hyo-Jae,Jeong, Myeong-Gyun 대한기계학회 2001 大韓機械學會論文集B Vol.25 No.3

A modified model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a universal model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity (1%∼6%) under zero-pressure gradient. It was found that the profiles of mean velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily predicted throughout the flow regions.

Explicit Algebraic Stress/Heat-Flux 모형을 이용한 벽면가열이 높은 수직관 내의 열전달 감소에 대한 수치적 해석

백성구,박승오,Baek, Seong-Gu,Park, Seung-O 대한기계학회 2003 大韓機械學會論文集B Vol.27 No.12

This paper assesses the prediction performance of explicit algebraic stress and heat-flux models for reduction of heat transfer coefficient in a strongly-heated vertical tube. Two explicit algebraic stress models and four explicit algebraic heat-flux models are selected for assessment. Eight combinations of explicit algebraic stress and heat-flux models are used in predicting the turbulent gas flows with intense heating, which yields the significant property-variation. The results showed that the two combinations of GS-AKN and WJ-mAKN predicted the Nusselt number and the axial wall temperature variations well and that the predictions of Nusselt number with WJ-combinations spread in a wider range than those with Gs-combinations. WJ is the explicit algebraic stress model of Wallin and Johansson and GS is the model of Gatski and Speziale and that AKN is the explicit heat-flux model of Abe, Kondoh and Nagano and mAKN is the modified AKN.

압력구배가 변하는 표면 위의 Bypass 천이 유동의 예측

백성구,정명균,임효재,Baek-Seong-Gu,Chung, Myung-Kyoon,Lim, Hyo-Jae 대한기계학회 2002 大韓機械學會論文集B Vol.26 No.6

A modified $textsc{k}$-$\varepsilon$model is proposed for calculation of transitional boundary-layer flows with changing pressure gradient. In order to develop the model for this problem, the flow is divided into three regions; pre-transition region, transition region and fully turbulent region. The effect of pressure gradient is taken into account in stream-wise intermittency factor, which bridges the eddy-viscosity models in the pre-transition region and the fully turbulent region. From intermittency data in various flows, Narashima's intermittency function, F(${\gamma}$), has been found to be proportional to $\chi$$^{n}$ according to the extent of pressure gradient. Three empirical correlations of intermittency factor being analyzed, the best one was chosen to calculate three benchmark cases of bypass transition flows with different free-stream turbulence intensity under arbitrary pressure gradient. It was found that the variations of skin friction and shape factor as well as the profiles of mean velocity in the transition region were very satisfactorily predicted.

고난류강도 자유유동에서 평판 경계층 천이의 예측을 위한 난류 모형 개발

백성구(Seong Gu Baek),임효재(Hyo Jae Lim),정명균(Myung Kyoon Chung) 한국유체기계학회 2000 유체기계 연구개발 발표회 논문집 Vol.- No.-

A modified k-ε model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a universal model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity (1%~6%) under zero-pressure gradient. It was found that the profiles of mean velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily predicted throughout the flow regions.

천이영역에서 난류에너지의 이동에 관한 실험적 연구

임효재,백성구,이원근 한국에너지학회 2003 에너지공학 Vol.12 No.2

본 연구에서는 평판 경계층의 천이 영역에서 평균속도, 표면마찰계수, 간헐도 분포, 에너지 스펙트럼 등의 신뢰성 있는 실험 자료를 획득하였다. 실험 결과 천이가 진행됨에 따라 난류반점이 지배하는 영역이 벽면 근처에서 전 경계층으로 확산되었으며 이러한 현상의 결과로 속도 섭동량에서 비등방성이 크게 나타난다. 천이 이전영역에서는 에너지가 주로 저주파에 집중되어 있다가 천이를 거치면서 에너지가 고주파 성분으로 이동하였다 이는 두 가지 이상의 에너지 발생과 소멸 메커니즘이 천이영역에서 공존하고 있음을 의미한다. 따라서 천이영역에서의 유동장을 예측하기 위한 난류 계산 모형에서 비등방성과 서로 다른 메커니즘을 표현할 수 있기 위해서는 반드시 두 가지 이상의 척도를 표현할 수 있어야 한다. This paper considered the structural mechanism of transitional boundary layer by the experimental approach. In order to measure the turbulence quantity in the boundary layer, we made a wind tunnel with 400${\times}$190${\times}$2500 mm test section and a flat plate with well fabricated leading edge. Hot wire anemometer was used for acquiring the continuous turbulence signal which is processed by special software. The results of experiment show that the region where turbulence spot is dominant moves from near wall to overall layer and thus the anisotropy of velocity fluctuation shows so large value. Also the turbulence energy originally contained in low frequency band comes up to the high frequency band. Finally the turbulence model needs minimum two length scales to consider the pre-transition region.

압력 구배가 없는 평판 천이 경계층 유동을 예측하기 위한 k-ε 모형의 개발

임효재,백성구,정명균 대한기계학회 2001 大韓機械學會論文集B Vol.25 No.3

천이경계층에서의 간헐도 측정에 관한 실험적 연구

임효재,안재용,백성구,정명균 대한설비공학회 2003 설비공학 논문집 Vol.15 No.1

An experimental study was performed to investigate the turbulence intermittency measuring methods across the boundary layer in the transition region. A single type hot-wire probe was used to measure instantaneous streamwise velocities in laminar, transitional and turbulent boundary layer To estimate wall shear stresses on the flat plate, near wall mean velocities are applied to the principle of CPM. Distribution of intermittency factor is obtained by dual-slope method and compared to the results of four methods,$\'{u},\;\{U}$, TERA and M-TERA method. In these methods, M-TERA shows a good agreement in the near wall region. However, the result of M-TERA method shows that intermittency factor is underestimated in the outer part and outside of the boundary layer and the dimensional constant of M-TERA method should be changed appropriately depending on measuring point.

열유속이 있는 난류 원관 유동에의 FLUENT의 2방정식 난류모델의 적용성 판단

문치명(C. M. Moon),백성구(S.G. Baek),박승오(S.O. Park) 한국전산유체공학회 2003 한국전산유체공학회 학술대회논문집 Vol.2003 No.-

This paper assesses the two-equation turbulence models available in a commercial code, FLUENT, for heat transfer in a turbulent heated pipe flow. In case of flow under RED = 10, 000, Standard k-ε and Realizable k-ε models overpredict the Nusselt number about 20% compared with the experimental correlation, and RNG k-ε model overpredicts about 30% when the two-layer zonal method is employed. When wall function method is adopted, all k-ε models show better predictions. Standard k-ω and SST k-ω models have the dependency on the first grid point (0.3<y₁+<3). As Reynolds number becomes high, the predictions of all k-ε and k-ω models are in a good agreement with the experimental correlation.

Aerosol을 이용한 Direct-Write 시스템에서 침착된 입자의 형상예측 모델에 관한 연구

박준정(Jun Jung Park),백성구(Seong Gu Baek),리광훈(Gwang Hoon Rhee) 한국전산유체공학회 2008 한국전산유체공학회지 Vol.13 No.1

Direct Write Technologies are being utilized in various industrial fields such as antennas, engineered structures, sensors and tissue engineering. With Direct Write Technologies, producing features have the mesoscale range, from 1 to 100 microns. One form of the Direct Write Technologies is based on aerosol dynamics. The shape of deposited aerosols determine the form of products in the Direct Write Technology based on aerosol dynamics. To predict shape of deposited aerosol, a prediction model is created. In this study, we estimated Line-Width and Line-Thickness from the prediction model. Results of prediction model is valid from comparison with experimental results.