• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.6
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• pp.2160-2170
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• 1991

Three-dimensional turbulent structures in the near field of elliptic jet were experimentally investigated by using a three-color, three-component Laser Doppler Velocimeter. The Reynolds number based on the nozzle exit velocity and nozzle equivalent diameter(De) was about 4*10$^{4}$. The turbulent characteristics of a sharp-edged elliptic nozzle with aspect ratio of 2 were analyzed along major and minor axis at X/De=2,3,5,7 and along the centerline up to X/De=14. Quantities measured at each point with the 3-D LDV system were three orthogonal velocity components, turbulent intensity, skewness, flatness, and Reynolds shear stress. The nondimensional mean velocities coincided well with the Schlichting's empirical curve with going downstream. Elliptic jet of AR=2 had two switching points at about X/De=2 and 16. The turbulent intensity along the minor axis was distributed widely than that along the major axis. In the near field, X/De<5, the Reynolds shear stresses of the inner part of the elliptic jet had negative value, which indicated the enhancement of entrainment toward the inner part.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.6
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• pp.1156-1162
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• 1992

The effect of aspect ratio on the flow characteristics of elliptic jets was experimentally investigated. The flow characteristics of sharp-edged elliptic nozzles with aspect ratio of 1 (round nozzle), 2 and 4 were measured by using a 3-D LDV system along the major and minor axis at X/De = 2, 3, 5, 7 and along the centerline up to X/De = 14. At each measurement point mean velocites, turbulent intensities, skewness of three orthogonal velocity components, and Reynolds shear stress were obtained. The Reynolds number based on the nozzle exit velocity and nozzle equivalent diameter(De) was about 4 * 10$^{4}$. Difference in the spreading rate along the major and minor axis was remarkable. The jet half width along the major axis decreased at first and then increased again with going downstream. But the jet width along the minor axis increased steadly. The elliptic jet of AR = 2 had two switching points within the measurement range, while that of AR = 4 had only one. (AAA) : The elliptic jet of AR = 2 showed larger velocity decay rate than that of AR = 1 and AR = 4. The effect of aspect ratio on the flow characteristics of elliptic jets was dominant in the near jet regions of X/De < 7, and the skewness and Reynolds shear stress had quite different distribution depending on the aspect ratio of the elliptic nozzle.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.221-224
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• 2002

A technique of mixing enhancement in an underexpanded sonic round jet is studied with fully expanded jet Mach number 1.5. Tonal sound, jet screech can be produced at some underexpansion pressure ratio in a sonic jet. Since the jet screech excites the initial Jet shear layer to change the flow, a reflector which focuses the jet screech near the nozzle lip is designed. The reflector has an elliptic shape of which two foci are located near the nozzle lip and the jet screech source region. Jet screech tone near the nozzle lip increases with the elliptic reflector and spreading of the jet largely increases. It is concluded that mixing enhancement of the jet with the elliptic reflector is attributed to large scale structures which are initially excited by the increased jet screech.

• Bulletin of the Korean Space Science Society
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• 2003.10a
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• pp.38-38
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• 2003

전리층은 우주환경의 변화에 매우 중요한 역할을 하고 있다. 그 중 전리층에서 발생하는 오로라는 우주환경의 변화를 가시적으로 보여주는 유일한 현상이다. 본 연구에서는 오로라 타원체를 따라 흐르는 오로라 제트전류의 위도에 따른 분포를 알아보고 그 특징을 조사하였다. 그리고 오로라 발생지역과 이들 전류분포를 비교하였으며 전기장과 전기전도도를 계산하여 오로라 제트전류에 대한 상대적인 기여도를 알아보았다. 먼저 CANOPUS 지자기 관측소의 1998년 데이터를 사용하여 1년간 발생했던 Isolated Substorm을 추려내었다. 그리고 이들 Substorm이 발생하는 동안 위도에 따른 오로라 제트전류 분포의 특징을 구하여 전류의 방향에 따라 이들 분포가 어떠한 형태로 나타나는지 알아보았으며, 이때 오로라 발생지역과의 연관성을 조사하였다. 여기서 전리층에 흐르는 전류를 무한판상으로 가정하여 오로라 제트전류의 분포를 지자기변화 성분으로부터 추론하였다. 또한 DMSP의 하강입자를 이용하여 계산한 전기전도도를 이용해 전기장을 추정함과 동시에 오로라제트전류에 대한 이들의 상대적 기여도를 조사해보았다.

• Proceedings of the Optical Society of Korea Conference
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• 1997.08a
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• pp.69-69
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• 1997

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.8 s.227
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• pp.887-894
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• 2004

A technique of mixing enhancement by using an elliptic jet screech reflector has been examined experimentally in an underexpanded sonic round jet where jet screech tone is generated. Since jet screech is known to enhance jet spreading, a reflector was designed to focus jet screech waves near the nozzle lip at an underexpanded jet. The reflector has an elliptic cross section of which one focus is located near the nozzle lip and the other in the jet screech source region in a plane including the jet axis. In the jet with the elliptic reflector, the mass flow rate showed a significant increase in the jet entrainment when compared to that for the small disk reflector. This was attributed to the increased screech amplitude near the nozzle lip as well as the mode change of the jet. The jet mixing was also increased by the amplified jet screech at two other underexpanded jets, but the jet oscillation mode did not change.

• Journal of the KSME
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• v.34 no.4
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• pp.262-276
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• 1994

본 연구를 통하여 3차원 LDV 시스템의 측정기법을 개발하였으며, 측정결과의 신뢰성을 확인하 였다. 또한 이것을 타원제트 연구에 적용함으로써 그 응용 가능성을 확인하였다. 앞으로 3-D LDV 시스템을 사용하여 신뢰성 있는 측정결과를 얻기 위해서는 아래에 기술한 몇 가지 사항을 고려하여야 한다. 1) 3차원 레이저 유속계는 정교하고 복잡한 광학시스템으로 정확한 배열을 요구한다. 광학계와 실험장치의 좌표축이 일치하지 않으면 축방향보다 측방향(lateral) 속도변동 성분에 큰 영향을 준 다. 2) LDV 측정에서 속도편심을 줄이기 위해서는 적당한 출력의 레이저, 적절한 신호처리(signal conditioning), 실험조건에 알맞는 입자를 선정하여야 한다 3) 입자를 연속적으로 균일하게 공급하여야하며 신호분석기 조작에 익숙하여 도플러신호의 질을 최적화하여야 한다.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.13 no.2
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• pp.109-121
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• 2001

This paper describes results of an experimental study to examine the effect of a submerged horizontal breakwater to sea water exchange. Flow measurements were taken by using a PIV(Particie Image Velocimetry) system, and mean currents and wave ellipses extracted through the harmonic analysis are presented. As results, the rates of circulating flow were closely connected with the volume flux of incident waves and the counter-rotating vortex pair was observed at the onshore side of a plate. The dye study showed that incoming sea water and polluted water body mixed up significantly due to turbulent motions induced by a jet-like flow.