• 한국가시화정보학회:학술대회논문집
• /
• 2004.11a
• /
• pp.14-17
• /
• 2004

A purpose of this visual experiment is to investigate the effect of reduced frequency qualitatively by examining wake pattern change for insect flying motion. Insect is composed of two pair wing with forewing and hindwing, flying motion of insect is performed pitching and plunging so it makes a separation over the wings. The separation affects at the wake pattern and changed wake pattern has an influence on lift, drag and propulsion. This experiment is conducted by using a smoke wire technique and a camera is fixed at hindwing to take a photograph of wake. An electronic device is mounted below test section to find exact the mean positional angle of wing. The reduced frequency in experiment is 0.15, 0.3 and 0.45. We obtained the result which that reduced frequency is closely related to wake pattern that determines flight efficiency.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.31 no.1 s.256
• /
• pp.8-15
• /
• 2007

The jet impingement cooling characteristics are investigated experimentally. The study is motivated by the potential application of local hot spot cooling by means of the vortex tube. The purposes of this research are to examine the effect of the nozzle-block spacing and flow rate. The results of jet through vortex tube is compared with ones of circular Jet. Flow visualization by the smoke-wire technique is also performed to investigate the flow structure. As the nozzle-block spacing is increased and flow rate decreased, the cooling effect of the Jet through the vortex tube decreases mere remarkably than that of the circular jet. So the cooling effect for the jet through the vortex tube is higher than that for the circular jet at $H/D{\leq}3$, $Q{\geq}10m^3/h$.

• Journal of the Korean Society of Visualization
• /
• v.2 no.2
• /
• pp.58-65
• /
• 2004

The purpose of this visualization study is to investigate the effect of reduced frequency qualitatively by examining wake patterns for dragonfly flight motion. Dragonflies have two pairs of wing (a forewing and hindwing) and flight is achieved by a pitching and plunging, so it makes a separation over the wings. The separation affects the wake pattern and changed wake pattern has an influence on lift, drag, and propulsion. This experiment was conducted by using a smoke wire technique and a camera fixed above the test section used to take a photograph of the wake. An electronic device is mounted below the test section to find the exact mean positional angle of the wing. The reduced frequency in the experiment is 0.15, 0.3 and 0.45. Results show that reduced frequency is closely related to the wake pattern that determines flight efficiency.

• Proceedings of the KSME Conference
• /
• 2000.04b
• /
• pp.508-513
• /
• 2000

An experimental investigation has been carried out for two circular cylinders having different groove configurations(U and V-shape). The results were compared with those for the smooth circular cylinder. The drag force, mean velocity and turbulent intensity profiles of wake behind the cylinders were measured with varying the Reynolds number $Re=8000{\sim}14,000$ based on the cylinder diameter. As a results, the U-groove circular cylinder was found to be most effective riblet shape with reducing the drag up to 21%. As the Reynolds number increases, the vortex shedding frequency of the grooved cylinders becomes a little larger, compared to the smooth cylinder. The flow visualization using the smoke-wire technique was also carried out to see the flow structure qualitatively.

• Journal of Advanced Marine Engineering and Technology
• /
• v.25 no.4
• /
• pp.846-856
• /
• 2001

An experimental study has been made to investigate the effect on wake flow and vortex shedding frequency by vortex stabilizer in Karman vortex type air flow sensor. The conditions investigated include 3 types of shapes and 3 types of separation distances of the vortex stabilizer. The phase averaged technique and smoke-wire flow visualization method are used to understand the detail information. The rolling up position of shear layer is fixed by the influence of the vortex stabilizer. Especially, the convex type vortex stabilizer has shown the more stable repeatability and linearity regarding the vortex shedding frequency compared to the other types.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.25 no.5
• /
• pp.722-730
• /
• 2001

The local heat transfer of an axisymmetric submerged air jet impinging on a heated flat plate is investigated experimentally with the variation of mesh-screen solidity. The screen installed in front of the nozzle exit modifies the flow structure and local heat transfer characteristics. The mean velocity and turbulence intensity profiles of streamwise velocity component are measured using a hot-wire anemometry. The temperature distribution on the heated flat surface is measured with thermocouples. The smoke-wire flow visualization technique was employed to understand the near-field flow structure qualitatively for different mesh screens. Large-scale toroidal vortices and high turbulence intensity enhance the heat transfer rate in the stagnation region. For a higher solidity, turbulence intensity become higher which increases the local heat transfer at small nozzle-to-plate spacings such as L/D<6. The local and average Nusselt numbers of impinging jet from the $\sigma$(sub)s=0.83 screen at L/D=2 are about 5.6∼7.5% and 7.1% larger than those for the case of no screen, respectively. For the nozzle-to-plate spacings larger than 6, however, the turbulence intensities for all tested screens approach to an asymptotic curve and the mean velocity along the jet centerline decreases monotonically. As the nozzle-to-plat spacing increases for high solidity screens, the heat transfer rate decreases due to the reduction in turbulence intensity and jet momentum.

• Proceedings of the KSME Conference
• /
• 2004.11a
• /
• pp.1586-1591
• /
• 2004

Flow visualization experiments have been performed to investigate the effects of phase lag, reduced frequency qualitatively by examining wake pattern on a dragonfly type wing. The model was built with a scaled-up, flapping wings, composed of paired wings with fore- and hindwing in tandem, that mimicked the wing form of a dragonfly. The present study was conducted by using the smoke-wire technique, and an electronic device was mounted to find the exact positional angle of wing below the tandem wings, which amplitude is ranged from $-16.5^{\circ}$ to $+22.8^{\circ}$. Phase lag applied on the wings is $0^{\circ}$, $90^{\circ}$, $180^{\circ}$ and $270^{\circ}$. The reduced frequency is 0.15, 0.3 and 0.45 to investigate the effect of reduced frequency. It is inferred through observed wake pattern that the phase lag clearly plays an important role in the wake structures and in the flight efficiency as changing the interaction of wings. The reduced frequency also is closely related to wake pattern and determines flight efficiency.

• Proceedings of the KSME Conference
• /
• 2003.04a
• /
• pp.2089-2094
• /
• 2003

The flow around a circular cylinder was controlled by attaching O-rings to reduce drag force acting on the cylinder. Four experimental models were tested in this study; one smooth cylinder of diameter D (D=60mm) and three cylinders fitted with O-rings of diameters d=0.0167D, 0.05D and 0.067D with pitches of PPD=1D, 0.5D and 0.25D. The drag force, mean velocity and turbulent intensity profiles in the near wake behind the cylinders were measured for Reynolds numbers based on the cylinder diameter in the range of $Re_D=7.8{\times}10^3{\sim}1.2{\times}10^5$. At $Re_D=1.2{\times}10^5$, the cylinder fitted with O-rings of d=0.0167D in a pitch interval of 0.25D shows the maximum drag reduction of about 5.4%, compared with the smooth cylinder. The drag reduction effect of O-rings of d=0.067D is not so high. For O-ring circulars, as the Reynolds number increases, the peak location of turbulence intensity shifts downstream and the peak magnitude is decreased. Flow field around the cylinders was visualized using a smoke-wire technique to see the flow structure qualitatively. The size of vortices and vortex formation region formed behind the O-ring cylinders are smaller, compared with the smooth cylinder.

• Journal of the Korean Society of Visualization
• /
• v.4 no.2
• /
• pp.44-50
• /
• 2006

A visualization study on the boundary layer and near-wake of an NACA 0012 airfoil was con-ducted in order to investigate the influence of boundary layer behavior on the near-wake at low Reynolds numbers. The present study is investigated at static angles of attack ${\alpha}=0^{\circ},\;3^{\circ},\;6^{\circ}$ and $Re=2.3{\times}10^4,\;3.3{\times}10^4,\;4.8{\times}10^4$ by using a smoke-wire technique. The results of this study show that the laminar boundary layer on the airfoil surface is attached to the surface at ${\alpha}=0^{\circ}$, and that laminar separation of boundary layer on the airfoil surface occurs at ${\alpha}=3^{\circ}$. Furthermore, reattachment of the boundary layer occurs in the case of ${\alpha}=6^{\circ}$. In the current study, the location of the laminar separation point moves upstream as the Reynolds number and the angle of attack increase.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.13 no.5
• /
• pp.1023-1031
• /
• 1989

An experimental study is conducted to investigate the turbulent wake flow around tubes in vertical single row tube banks. All measurements are performed at Reynolds number(Re$_{max}$) 4.2*10$_{3}$ - 2.5*10$_{4}$ with varying tube spacings from the wide pitch ratio(H/D=3.07) to the very narrow one(H/D=1.23). Flow patterns are visualized using the smoke-wire method. Mean static pressures, velocity components, and various statistical quantities of turbulence are obtained by the computer on-line technique. In the case of wide tube spacings, the near wakes of tube show similar trends to those of a single tube, and their flow indicats an anisotropic turbulence. However, as the pitch ratio decreases, wide and narrow wakes appear alternately behind adjacent tubes due to the deflected flow. Also, in the case of H/D .leq. 1.54, Karman vortex is not formed at the side of relatively wide wake.e.