선인장 표면 그루브의 공기역학적 역할

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.

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.

국소 벽면 진동에 의한 난류경계층 유동 변화

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.

압전세라믹 액추에이터를 이용한 익형 후류 제어

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.

구에 설치한 딤플과 표면 거칠기에 의한 항력 감소 메커니즘

최진(Jin Choi),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi) 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-

In this paper, we present a detailed mechanism of drag reduction by dimples and roughness on a sphere by measuring the streamwise velocity above the dimpled and roughened surfaces, respectively. Dimples cause local flow separation and trigger the shear layer instability along the separating shear layer, resulting in generation of large turbulence intensity. With this increased turbulence, the flow reattaches to the sphere surface with high momentum near the wall and overcomes strong adverse pressure gradient formed in the rear sphere surface. As a result, dimples delay main separation and reduce drag significantly. The present study suggests that generation of a separation bubble, i.e. a closed-loop streamline consisting of separation and reattachment, on a body surface is an important flow-control strategy for drag reduction on a bluff body such as the sphere and cylinder. In the case of roughened sphere, the boundary layer flow is directly triggered by roughness and changes to a turbulent flow. Due to this change, the drag significantly decreases. As the Reynolds number further increases, transition to turbulence occurs earlier on the sphere surface. Because of faster growth of turbulent boundary layer by roughness, earlier transition thickens the boundary layer, resulting in earlier separation and drag increase with increasing Reynolds number.

돛새치와 황새치의 유체역학적 특성

사공웅(Woong Sagong),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11

The sailfish and swordfish are known as the fastest sea animals. They have a long bill, which has been regarded as a drag-reduction device by the separation delay through turbulence generation. In the present study, we investigate the hydrodynamic characteristics of sailfish and swordfish through the wind-tunnel experiment. From the direct measurement of drag forces on the fishes, we find that the drag coefficients of sailfish and swordfish are comparable to or smaller than those of other fishes such as the dogfish, tuna, dolphin and trout. The role of bill is also investigated in this study. The drag with the artificial short bill is smaller than that with the original bill, indicating that the bill does not make any role in reducing drag. From the velocity measurement near the body surface, we find that the flow separation does not occur even with the short or smooth bill, and thus there is no need to generate turbulence using the bill for the form-drag reduction.

횡 방향 공동을 이용한 마찰 저항 감소

김철규(Chulkyu Kim),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi) 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-

In this study, we experimentally investigate the possibility of skin-friction drag reduction by series of transverse cavities in a turbulent boundary layer flow. The effects of cavity depth (d), cavity length (l) and cavity spacing (s) on the skin friction drag are examined in the range of Reθ=4030 ~ 7360, d/θ? = 0.13 ~ 1.03, l/d = 1 ~ 4 and s/d = 5 ~ 20. We perform experiments for twenty different cavity geometries and directly measure total drag force using in-house force measurement system. In most cases, the skin friction drag is increased. At several cases, however, small drag reduction is obtained. The variation of the skin friction drag is more sensitive to the cavity length than to the cavity depth or cavity spacing, and drag is reduced at s/l≥10 and l/θ?≤ l irrespective of the cavity depth. At l/θ?=0.13 and s/l = 10, maximum 2 % drag reduction is achieved. When the skin friction drag is reduced, there is little interaction between the flows inside and outside cavity, and the flow changed by the cavity is rapidly recovered at the following crest. A stable vortex is formed inside a cavity in the case of drag reduction. This vortex generates negative skin friction drag at the cavity bottom wall. Although there is form drag due to the cavity itself, total drag is reduced due to the negative skin friction drag.

골프공에서의 딤플의 역할

최진(Jin Choi),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11

It is an open question why the drag coefficient on golf ball remains nearly constant with increasing Reynolds number after its sharp decrease. In order to investigate this interesting phenomenon, we measure the drag, separation angle, wall pressure and streamwise velocity inside/outside dimples before main separation. When drag reduction occurs, the separation angle measured is nearly constant and the wall pressure distributions outside dimples are nearly the same even with increasing Reynolds number. From the streamwise velocity measurement, it is found that dimples located at the angles of 65˚~90˚ make an important role in changing flow characteristics. Inside one or two rows of dimples located at those angles, a small separation bubble exists and flow becomes quickly transitional and turbulent with reattachment. The main separation occurs further downstream at 110˚ even with increasing Reynolds number, because downstream dimples do not make an important role in changing flow characteristics any more.

수직 탭을 이용한 후향 계단 유동 제어

박형민(Hyungmin Park),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi),유정열(Jung Yul Yoo) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11

In this study, mixing enhancement behind a backward-facing step is experimentally investigated using thin rectangular tabs attached on the trailing edge at the Reynolds number of 24,000 based on the free-stream velocity and step-height (h). The control parameters considered are the height and width of the tab and the spanwise spacing (λ) between the adjacent tabs. The reattachment length (x<SUB>R</SUB>) is about 5.8h for uncontrolled flow. For each tab configuration, we measure the distributions of wall static pressure and reattachment length along the spanwise direction. With the tab, the reattachment length and wall static pressure show significant variations in the spanwise direction. With single tab, x<SUB>R</SUB> slightly increases at the spanwise location of the tab, but significantly decreases at other spanwise locations. With multiple tabs, we also find that there exists optimal λ minimizing reattachment length. It is shown that the tabs attached on the trailing edge drastically increase mixing behind the backward-facing step.

글라이딩하는 제비나비의 꼬리가 공력성능에 미치는 영향

박형민(Hyungmin Park),배기수(Kisoo Bae),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi) 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-

Among the various nature’s flyers, the butterfly is known as one of the excellent aerial species that are very good at both gliding and flapping flight. Considering the aerodynamically poorly-shaped wings of the butterfly (cross section of flat-plate and low aspect ratio), the high flight capability of the butterfly wing is worthwhile to be noted. Several researchers have studied the effects of the wing morphology on the aerodynamic performances of the butterfly, based on the idea that the wing should have additional devices to overcome the disadvantages and enhance the aerodynamic performance. The hind-wing tails of swallowtail butterfly has been also reported to affect the gliding flight of the butterfly, but the detailed evidences supporting the conjectured aerodynamic function of the tails have not been given. Therefore, in the present study, the effect of hind-wing tails on the aerodynamic performance of a swallowtail butterfly is experimentally investigated in a wind tunnel. The butterfly model is made of a thin brass sheet and its shape is precisely duplicated from a specimen of real swallowtail butterfly (Papilio Ulysses). With the tails, the lift and drag increase by about 10~20% and 5%, respectively, at the angles of attack larger than 15°, which results in the enhancement of lift-to-drag ratio in a wide range of attack angles. Also due to the tails, the slope of pitching moment curve with respect to the attack angle is more negative (steeper), which indicates the enhanced longitudinal static stability. From the flow visualization, the wake width is increased at the spanwise location near the tails and thus modifying the vortical structure behind the wing as a V-shaped one. This change in vortical structure induces the vortex core-region with low-pressure near the upper-surface of the wing, thereby increases the lift force.