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κ-є 모형의 난류점성상수 C<SUB>μ</SUB> 에 관한 연구
정명균(Myung Kyoon Chung),김동훈(Dong Hun Kim),소홍윤(Hong Yun So) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11
The turbulent eddy viscosity coefficient C<SUB>μ</SUB> that assumes constant value in the standard κ-? model is found to be a function of the time scale ratio Sκ/ε in boundary-free homogeneous shear flows. This functional form is derived by analyzing homogeneous turbulent shear flows. When such a functional coefficient C<SUB>μ</SUB> is applied to near-wall region of wall bounded flows, a universal wall damping function ?(y?) can be observed. And when it is applied to symmetric wake region of channel flows and free shear flows, it is found that there exists a universal wake function W(y/δ). Further when the functional coefficient C<SUB>μ</SUB> is applied to the outer part of free shear flows such as jets, wakes and mixing layers, it seems feasible to define a well-behaving function G(?) that takes account of the intermittency effect on the turbulent eddy viscosity.
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.
오형우,정명균,김상철,양근영,하진수,Oh, Hyoung Woo,Chung, Myung Kyoon,Kim, Sang Chul,Yang, Keun Yung,Ha, Jin Soo 대한기계학회 1999 大韓機械學會論文集B Vol.23 No.2
An optimal design code for centrifugal pumps has been developed to determine geometric and fluid dynamic variables under appropriate design constraints. The optimization problem has been formulated with a nonlinear objective function to minimize one, two or all of the fluid dynamic losses, the net positive suction head required and the product price of a pump stage depending on the weighting factors selected as the design compromise. The optimal solution Is obtained by means of the Hooke and Jeeves direct search method. The performance analysis Is based on the mean streamline analysis using the present state-of-the-art loss correlations. The optimized efficiency and design variables of centrifugal pumps are presented in this paper as a function of non-dimensional specific speed in the range, $0.5{\leq}N$, ${\leq}1.3$. The diagrams presented herein can be used efficiently in the preliminary design phase of centrifugal pumps.
관 유동과 Blasius 유동에서 가장 불안정한 교란에 관하여
최상규,정명균,Choi, Sang-Kyu,Chung, Myung-Kyoon 대한기계학회 2003 大韓機械學會論文集B Vol.27 No.6
The pseudospectral method for stability analysis was used to find the most influential disturbance mode for transition of plane channel flows and Blasius flow at their critical Reynolds numbers. A number of various oblique disturbance waves were investigated for their pseudospectra and resolvent norm contours in each flow, and an exhaustive search method was employed to find the disturbing waves to which the flows become most unstable. In plane Poiseuille flow an oblique disturbance with a wavelength of 3.59h (where h is the half channel width) at an angle $28.7^{\circ}$ was found to be the most influential for the flow transition to turbulence, and in plane Couette flow it is an oblique wave with a wavelength of 3.49h at an angle of $19.4^{\circ}$. But in Blasius flow it was found that the most influential mode is a normal wave with a wavelength of $3.44{\delta}_{999}$. These results imply that the most influential disturbance mode is closely related to the fundamental acoustic wave with a certain shear sheltering in the respective flow geometry.
遠心 임펠러의 相對 渦流 크기 모델에 根據한 이론적인 미끄럼 係數
팽기석,정명균,Paeng, Kee-Seok,Chung, Myung-Kyoon 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.3
By calculating the location and size of the relative eddy formed in the rotating impellers with the logarithmic spiral vanes, a new simple but accurate slip factor is analytically derived. The proposed slip factor depends on only one parameter that is a function of the number of vanes and the vane exit angle. Predicted slip factor for various cases are compared with those estimated by a number of previous slip factors as well as a recent theoretical calculation by Visser et al. ( JFM, Vol. 268, pp. 107-141, 1994). It is found that the present slip factor yields almost similar results to Wiesner's which has been empirically formulated based on the theoretical calculation of Busemann.
오형우,정명균,김재원,Oh, Hyoung Woo,Chung, Myung Kyoon,Kim, Jae Won 대한기계학회 1999 大韓機械學會論文集B Vol.23 No.9
This paper presents a systematic two-zone modeling for reliable performance prediction of centrifugal compressors. In order to improve the predictive capability, a modified jet slip factor is developed and new corrections for the wake flow deviation and mass fraction are suggested based on the comprehensive experimental data of the three Eckardt impellers. The proposed two-zone modeling is tested against nine sets of measured data of centrifugal compressors. The results are also compared with those obtained by the mean streamline analysis. It was found that the predictions by the present two-zone modeling agree fairly well with experimental data for a variety of centrifugal compressors over the wide operating conditions.
이찬,정명균,Lee, Chan,Chung, Myung-Kyoon 대한기계학회 1991 대한기계학회논문집 Vol.15 No.1
본 연구에서는 로켓노즐로 부터 분출되는 고온의 초음속제트가 45˚로 경사진 평판으로 분사되는 경우의 열전달을 측정하였다.고온의 제트가 충돌하는 평판표면 의 고온과 열유속을 측정하는 것이 거의 불가능하므로, 국소적열전달은 국소적 준 1차 원 열전도모형을 평판표면 아래에서 측정한 온도자료에 적용하여 구하였다. An experiment was conducted to determine the local heat transfer from a supersonic hot jet impinging at 45.deg. to a plate surface. A semi-analytic method was used to determine the Nusselt number from experimental data. The results indicates that the location of the peak heat transfer is displaced from the geometric center of the axisymmetric jet and that the radial variation of the local heat transfer is steeper than that in the subsonic impinging jet. In the stagnation region, the heat transfer from the supersonic impinging jet is about 10 times larger than that from the subsonic one, while the heat transer away from the stagnation region is of the same magnitude as that of the in compressible turbulent radial wall jet.
A Finite Volume Method for Computations of Two-Dimensional Laminar Flows
김기섭,정명균,Kim, Ki-Sup,Chung, Myung-Kyoon The Society of Naval Architects of Korea 1992 大韓造船學會 論文集 Vol.29 No.3
2차원 비압축성, 정상, 층류 Navier-Stokes 방정식을 일반 곡선 좌표계에서 계산하기 위한 유한체적법을 개발하였다. 수치해석은 정규 격자구조를 채택하였으며, 이때 압력의 불안정한 거동은 모멘텀 보간법에 의하여 제거하였다. NACA0012 날개 단면 주위의 유동을 개발된 컴퓨터 프로그램으로 계산한 결과들은 실험 및 다른 계산결과와 잘 일치하였다.
평행 Poiseuille, 평행 Couette, Blasius Flow의 준안정 해석
최상규,정명균,Choi, Snag-Kyu,Chung, Myung-Kyoon 대한기계학회 2003 大韓機械學會論文集B Vol.27 No.3
We investigate the spectra and the pseudospectra in plane Poiseuille flow, plane Couette flow and Blasius flow. At subcritical Reynolds number, the spectra are lied strictly inside the stable complex half-plane, but the pseudospectra are lied in the unstable half-plane, reflecting the large linear transient growth that certain perturbations may excite. It means that the smooth flows may become to turbulent even though all the eigenmodes decay monotonically. We found that pseudospectra is one reason that causes subcritical transition in plane Poiseuille flow and plane Couette flow and bypass transition in Blasius flow.