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CPM을 이용한 평판위 천이경계층에서 벽 마찰응력의 계측
전우평,강신형 대한기계학회 1995 대한기계학회논문집 Vol.19 No.1
A CPM (computational preston tube method) was developed to measure wall shear stress in a transitional boundary layer on a flat plate using Preston tubes. Correlation for the displacement factor of Preston tubes was improved for a CPM to be used in the transitional boundary layer. The distribution of skin-friction coefficient was reasonably predicted in the uniform free stream of 3.1% turbulence intensity. Reasonable and accurate estimation of displacement factor of Preston tubes was found to be of crucial importance for the CPM, especially in the laminar boundary layer. The mean velocity profiles of the boundary layer on the plate were also measured and presented.
전우평(Woo-Pyung Jeon),김훈도(Hoondo Kim),최해천(Haecheon Choi) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
The aerodynamic role of cactus grooves is experimentally investigated in a wind tunnel by measuring drag, surface pressure and near-wall streamwise velocity. The cactus is modeled by mounting spanwise triangular grooves on a circular cylinder. The height of the span wise groove is 7% of outer diameter of the model, which is the biological exterior feature of Saguaro cactus. The Reynolds numbers based on the free-stream velocity and model diameter are 126000~410000. The spanwise grooves reduce the drag by 13% compared to a smooth circular cylinder, and the drag remains almost constant as the Reynolds number increases. The separation point of the flow over the grooved cylinder is fixed at θ=90° regardless of the Reynolds number investigated. The shear-layer instability induced by the local separation bubble increases turbulent production over the grooved surface, which generates strong near-wall momentum and leads to delay of main separation point.
최진,전우평,최해천,Choi, Jin,Jeon, Woo-Pyung,Choi, Hae-Cheon 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.8
The objective of this study is to increase lift and decrease drag of an airfoil at high angles of attack by delaying flow separation with piezo-ceramic actuators. The airfoil used is NACA 0012 and its chord length is 0.3m. An experiment is performed at the freestream velocity of 15m/s at which the Reynolds number based on the chord length is $2{\times}10^5$. Seven rectangular actuators are attached to the airfoil surface and move up and down based on the electric signal. Drag and lift are measured using an in-house two-dimensional force-balance and the surface pressures are also measured. At the attack angle of $16^{\circ}$, the separation point is delayed downstream due to momentum addition induced by the movement of the actuators. Lift is increased by 10%, drag is reduced by 37%, and the efficiency is increased up to 170%. The flow fields with and without control are visualized using the smoke-wire and tuft techniques.
자유유동 난류강도 변화에 따른 평판위 천이 경계층의 유동특성에 관한 실험적 연구
신성호,전우평,강신형,Shin, Sung-Ho,Jeon, Woo-Pyung,Kang, Shin-Hyoung 대한기계학회 1998 大韓機械學會論文集B Vol.22 No.9
Flow characteristics in transitional boundary layers on a flat plate were experimentally investigated under three different freestream conditions i. e. uniform flow with 0.1 % and 3.7% freestream turbulent intensity and cylinder-wake with 3.7% maximum turbulent intensity. Instantaneous streamwise velocities in laminar, transitional and turbulent boundary layers were measured by I-type hot-wire probe. For estimation of wall shear stresses on the flat plate, measured mean velocities near the wall were applied to the principle of Computational Preston Tube Method (CPM). Distributions of skin friction coefficients were reasonably predicted in all developed boundary layers. Intermittency profiles, which were estimated using Conditional Sampling Technique in transitional boundary layers, were also consistent with previously published data. It was predicted that the incoming turbulent intensity had more influence on transition onset point and transition process than freestream turbulent intensity existed just over the transition region. It was also confirmed that non-turbulent and turbulent profiles in transitional boundary layers could not be simply treated as Blasius and fully turbulent profiles.
균일유동에 수직인 2차원 스크린 후류의 점성유동에 관한 수치적 연구
강신형,전우평 대한기계학회 1988 대한기계학회논문집 Vol.12 No.3
본 연구에서는 난류유동이론을 적용하여 2차원 스크린의 후류에 관한 유동특 성을 연구하였다. 이를 위해 적합한 난류모델 및 스크린의 존재를 고려한 수치해석 방법을 연구하였다. 스크린의 저항이 커서 스크린 하류유동이 역류가 되는 경우도 포함하였다. Viscous flows through a screen normal to an uniform flow are numerically simulated. A .kappa.-.epsilon. model is adopted for evaluation of the Reynolds stresses. The existence of a screen is regarded as extra sources in the momentum equations. The amount of extra sources is related to the resistance coefficient and the refraction coefficient of the screen. Flows are numerically simulated for various resistance coefficients and heights of the screen and Reynolds numbers. The present method has been verified to reasonably simulate viscous wakes and shear layers of the screen, for which the inviscid theory is quite limitted. As the fluids approach the screen, the velocity is reduced and the pressure is raised to satisfy the Bernoulli equation. After passing the screen, the velocity shows its minimum value at the down-stream, but static pressure is slowly recovered. A detached separation-bubble from the screen appears as the resistance coefficient is increased to a certain level. Such results are qualitatively in agreement with limitted experimental data available. The turbulent kinetic energy shows its maximum value at further down stream and decrease thereafter.
김철규,전우평,박진일,김동주,최해천,Kim, Chul-Kyu,Jeon, Woo-Pyung,Park, Jin-Il,Kim, Dong-Joo,Choi, Haecheon 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.9
In this study, the modification of turbulent boundary layer flow by local wall vibration is investigated. The wall is locally vibrated using a wall deformation actuator, which moves up and down at the frequencies of 100Hz and 50Hz. Simultaneous measurements of the streamwise velocities in the spanwise direction are performed at several wall-normal and streamwise locations using an in-house multi-channel hot wire anemometer and a spanwise hot-wire-probe rake. The mean velocity is reduced in most places due to the wall vibration and its reduced amount becomes small as flow goes downstream. Interestingly, the mean velocity is found to increase very near the wall and near the actuator. This is due to the motion induced by the streamwise vortices which are generated by the downward motion of the actuator. In case of the streamwise velocity fluctuations, their magnitude increases as compared to the unperturbed turbulent boundary layer, and the increased amount becomes small as the flow moves downstream. The modified flow field at the forcing frequency of 50Hz is not much different from that of 100Hz, except the reduced amount of modification.