• 한국전산유체공학회:학술대회논문집
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• 2007.10a
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• pp.60-66
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• 2007

Separation control has been performed using synthetic jets on airfoil at high angle of attack. Computed results demonstrated that stall characteristics and control surface performance could be substantially improved by resizing separation vortices. It was observed that the actual flow control mechanism and flow structure is fundamentally different depending on the range of synthetic jet frequency. For low frequency range, small vortices due to synthetic jet penetrated to the large leading edge separation vortex, and as a result, the size of the leading edge vortex was remarkably reduced. For high frequency range, however, small vortex did not grow up enough to penetrate into the leading edge separation vortex. Instead, synthetic jet firmly attached the local flow and influenced the circulation of the virtual airfoil shape which is the combined shape of the main airfoil with the separation vortex. Theses results show the characteristic of unsteady flow of single synthetic jet. Beside, we researched on multi-array synthetic jet to obtain applicable synthetic jet velocity. Multi-location synthetic jet is proposed to eliminate small vortex on suction surface of airfoil. With the results, we concluded that the flow around airfoil is stable by high frequency synthetic jet with elimination of small vortex and confirmation of stable flow. Moreover, performance of multi-array/multi-location synthetic jet can be improved by changing phase angle of multi-location synthetic jet.

• Journal of ILASS-Korea
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• v.19 no.4
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• pp.197-203
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• 2014

The liquid jet breakup has been studied in the areas such as aerosols, spray and combustion. The breakup depends on several physical parameters such as the jet velocity, the nozzle inner diameter, and the density ratio of the water to the jet. This paper deals with characteristics of the jet breakup according to the jet velocity and the nozzle diameter. In order to consider only hydrodynamic factors, all the experiments were conducted in non-boiling conditions. The jet behavior in the water pool was observed by high-speed camera and PIV technique. For the condition of the inner diameter of 6.95 mm and the jet velocity of 2.8 m/s, the debris size of 22 mm gave the largest mass fraction, 39%. For higher jet velocity of 3.1 m/s, the debris size of 14 mm gave the largest mass fraction, 36%. For the nozzle with inner diameter of 9.30 mm, the debris size distribution was different. For jet velocity of 2.8 m/s and 3.1 m/s, the debris size with the largest mass fraction was found to be 14 mm. It was identified that the debris size decreased as the diameter or the jet velocity increased.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.3
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• pp.386-398
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• 1998

The present experiment is conducted to investigate heat transfer characteristics on the impinging surface with secondary flows around circular nozzle jets. The changed vortex pattern around jet affects significantly the flow characteristics and heat transfer coefficients on the impinging surface. The effects of the jet vortex control are also considered with jet nozzle-to-plate distances and main jet velocities. The vortex pattern around a jet is changed from a convective instability to an absolute instability with a velocity suction ratio of the main jet and the secondary counterflow. With the absolute instability condition, the jet potential core length increases and the heat transfer on the impinging surface is increased by small scale eddies. The region of high heat transfer coefficients is enlarged with the high Reynolds number due to increasing secondary peak values. The effect of suction flows is influenced largely with collars attached the exit of the jet nozzle because the attached collar guides well the counterflow around the main jet.

• International Journal of Air-Conditioning and Refrigeration
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• v.13 no.4
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• pp.196-205
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• 2005

In this paper, recent research trend on heat transfer in impinging jet is reviewed. We focused on submerged jet that air issued into air or liquid issued into liquid. To control and enhance the heat transfer in single jet, researchers have performed a lot of experiments by considering the nozzle geometry, impinging surface and active method such as jet vibration, secondary injection and suction flow. The studies on multiple jet have been mainly focused on finding out the optimum condition and on investigating many different factors concerned with application condition (crossflow, rotation and geometry etc.) and combined techniques (rib turbulator, pin fin, dimple and effusion hole etc.). All most experiments showed the detailed heat transfer data by using liquid crystal method, infrared camera technique and naphthalene sublimation method. Many numerical calculations have been performed to investigate the flow and heat transfer characteristics in laminar jet region. Various turbulence models such as $k-\varepsilon-\bar{\nu^2}$, modified $k-\varepsilon-f_{\mu}$ were applied to the calculation for turbulent jet and the predicted results showed a good agreement with the experimental data. Although a lot of studies on impinging jet have performed consistently up to recently, further studies are still required to understand the flow and heat transfer characteristics more accurately, and to give a guideline for optimum impinging jet design in various applications.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.46 no.6
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• pp.452-463
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• 2018

Tip-jet gyroplane is a type of compound helicopter that employs the tip-jet system to rotate the rotor by a reaction force from the gas jetted at the rotor tips in hovering. In forward flight, tip-jet gyroplane converts into a form of a gyroplane. Therefore, it is necessary to develop a new conceptual design method to consider three flight modes: tip-jet mode, gyroplane mode, and transient mode. This study developed the numerical code of conceptual design methodology that can consider three flight modes. The developed code was validated against the available experiment data. Based on the developed code, initial sizing of tip-jet gyroplane was performed for two mission profiles including high speed forward flight of 150knots with a mission range of 300km or 400km. Subsequently, the configuration and performance of the 3,000lb tip-jet gyroplane were analyzed.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.323-328
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• 2001

Supersonic coaxial, axisymmetric, jets issuing from various kinds of dual coaxial nozzles were experimentally investigated. Four different kinds of coaxial, dual nozzles were employed to characterize the major features of the supersonic, coaxial, dual jets. Two convergent-divergent supersonic nozzles with an impinging angle in the jet axis of the annular jets were designed to have the Mach number 2.0 and used to compare the coaxial jet flows with those discharging from two sonic nozzles. The primary pressure ratio was changed in the range from 4.0 to 10.0 and the assistant jet ratio from 1.0 to 4.0. The results obtained show that the assistant jets from the annular nozzle affect the coaxial jet flows and an increase of both the primary jet pressure ratio and assistant jet pressure ratio produces longer supersonic length of the dual, coaxial jet.

• Transactions of the Korean Society of Mechanical Engineers
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• v.7 no.4
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• pp.361-371
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• 1983

The turbulent structures of the free plane jet and two dimensional impinging jet are investigated experimentally. In order to get the two dimensional jet, the contour of the cubic equation suggested by Morel is used for a contracting nozzle. A linearized constant-temperature hot-wire anemometer is used for measurement. Mean velocities and turbulent intensities are measured along the centerline of the jet. Jet halp width spatial double velocity correlation coefficients and integral length scales are obtained. It is established that the free plane jet is truly self-preserving about 40 slot widths downstream of the nozzle. The experiments for the impinging jet are carried out at four different impingement wall locations within the self-preserving region of the free plane jet, and comparing the results with that of free plane jet, the mean velocity is changed in the region of 0.25H and turbulent intensities are affected in the region of 0.2H from the wall, respectively, where H means the distance between the nozzle exit and the wall.

• Journal of Astronomy and Space Sciences
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• v.20 no.3
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• pp.163-174
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• 2003

The motion of 3C 120 Jet relative to the core is reasonably uniform and the VLBI scale jet connects outwards to a VLA ~ 100kpc scale. We measured the jet width variation from the center and found some indication of a power law which indicates the jet expands roughly with a constant opening angle and a constant flow velocity, $V_{f}{\cong}c$, from subparsec scales to ~ 100 kpc. With such a constant flow velocity and based on other physical parameters deduced from observed emission characteristics of the jet, we have established an equipartition jet model which might accommodate the basic parameters of the jet on subparsec scales, with which one can fit the radio intensities over all the scale of the jet even to ~100 kpc.

• International Journal of Precision Engineering and Manufacturing
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• v.9 no.2
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• pp.11-17
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• 2008

Critical erosion kinetic energy models for radial/median cracks and lateral cracks in a workpiece are established in this study. We used experimental results to demonstrate that the fracture erosion resistance and erosion machining number could be used to evaluate the brittle fracture resistance and machinability of a workpiece. Erosion kinetic energy models were developed to predict brittle fracture and ductile shear, and a critical erosion kinetic energy model was developed to predict the transition from brittle fracture to ductile shear. These models were verified experimentally.

• Journal of KSNVE
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• v.9 no.5
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• pp.1005-1011
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• 1999

Generation of hole-tones and the instability of circular impinging jets are investigated based on the frequency characteristics and the radiated sound field. The experiment is carried out with varying hole sizes, jet speeds and impinging distances. It is found that hole-tones occur by both the low-speed laminar jet and the high-speed turbulent jet, but not by the transient jet, while plate-tones without holes are produced only in the high-speed turbulent impinging jet. When the diameter ratio of the hole to the nozzle is nearly one, hole-tones occur most easily. At low speed, it is found that hole-tones are due to the symmetrical unstable jet and the impinging distance decreases with jet speed. And the Strouhal number and the sound pressure level increase with jet speed. At high speed, hole-tones show the same characteristics as plate-tones. It is found that the ratio of the convection speed varies with the Strouhal number and the jet speed.