• Aerospace Engineering and Technology
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• v.12 no.1
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• pp.207-212
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• 2013

To investigate the possibility of using artificial means for the control of shear layers in swirl jets, a shear excitation device is introduced. The device consists of co-centric dual nozzles with internal lobes (i.e., convex surfaces) of small height to induce disturbance. The number of convexities can be varied to produce the various modes in azimuthal direction. The acquired velocity data of the swirling jets, with and without the lobes, are numerically simulated in 2-D contour plots and 3-D particle trace plots. The results are compared with the baseline (i.e., no excitation case) at various excitation modes. Conclusively it is observed that the artificial excitation method is effective in the control of the vortical structure in swirling jets.

• Transactions of the Korean Society of Automotive Engineers
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• v.12 no.1
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• pp.39-47
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• 2004

The objective of this study is to analyse the spray characteristics according to the injection duration under ambient pressure condition and to investigate the relationship between vorticity and entropy for controlling diffusion process that is the most important thing during the intake stroke injection process. Therefore, the spray velocity was obtained by using the PIV method that has been an useful optical diagnostics technology, and vorticity calculated from spray velocity component with vorticity algorithm. In addition, the homogeneous diffusion rate of spray was quantified by using the entropy analysis based on the Boltzmann's statistical thermodynamics. From these method, we found that as injection duration increases, spray velocity increases and the location of vortex is moved to the downstream of spray. In the same condition, as the entropy decrease, mean vorticity increases. This means that the concentration of spray droplets caused by the increase of injection duration is more effective than the increase of momentum dissipation.

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

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1996.10a
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• pp.443-449
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• 1996

헬리콥터, 팬, 프로펠러, 터이빈같이 회전익에서 유체역학적 소음이 발생하는 장치의 설계에 있어서는 공기 역학적 성능 분석과 함께 소음에 대한 해석이 절대적으로 필요하다. 근래에 들어와서 소음에 대한 관심이 급격히 증가하고 공항 주변에서의 국제적인 규약들은 낮은 소음 수준(low noise level)을 규정하고 있으며, 이에 따라서 소음을 감소시키려는 연구가 매우 활발히 진행되고 있는 실정이다. 더욱이 컴퓨터의 냉각 팬을 비롯한 공조기기 및 산업기기에 사용되는 회전기계에서 발생되는 소음의 저감은 보다 더 쾌적한 환경을 요구하는 사회적 요구에 부합하면서 공력소음의 연구 분야가 더 넓어지고 있다. 본 논문에서는 소음예측 방법중의 하나인 음향상사(acoustic analogy)를 주파수 영역 방법(frequency domain method)을 이용하여 헬리콥터 블레이드의 고속 충격소음(High Speed Impulsive Noise)을 해석한다. 고속 충격소음은 블레이드-와류 상호작용 소음과 더불어 헬리콥터의 지배적인 소음원으로서 깃끝 속도가 큰 전진 수평비행(forward level flight)또는 제자리 비행(hovering flight)시 발생하는 소음으로 블레이드의 깃끝 마하수(critical Mach number)보다 크거나 비슷할 경우 충격파의 교란에 의해서 일어나는 충격적인 소음을 말한다. 고속 충격소음은 고주파수 스펙트럼 성분과 큰 소음강도를 가지고 있기 때문에 날카로운 금속성의 소리를 내며 먼 거리까지 전파되는 특징을 가지고 있다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.11a
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• pp.305-310
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• 2007

For the understanding of the complex flow characteristics in the counter-rotating axial flow fan, it is necessary to investigate the three-dimensional unsteady flow fields in the counter-rotating axial flow fan. This information is also essential for the prediction of the aerodynamic and acoustical characteristics of the counter-rotating axial flow fan. Experimental study on the three-dimensional unsteady flow in the counter-rotating axial flow fan is carried out at the design point(operating condition). Three-dimensional unsteady flow fields in the counter rotating axial flow fan are measured at the cross-sectional planes of the upstream and downstream of each rotor using the $45^{\circ}$ inclined hot-wire. Three-dimensional unsteady flow fields in the counter-rotating axial flow such as the wake, the tip vortex and the tip leakage flow are shown the form of the velocity vectors and the velocity contours.

• Journal of Advanced Marine Engineering and Technology
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• v.39 no.4
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• pp.437-442
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• 2015

The swirl ratio in a SI engine is investigated in a steady flow bench according to the measurement methods: an impulse swirl meter and particle image velocimetry (PIV). When measuring the swirl ratio using the PIV, the torque is evaluated based on the cylinder center and swirl center, respectively. The position of the measurement plane is considered. As a result, in the upstream, the swirl ratio measured by the impulse swirl meter is estimated to be larger than that from the PIV measurements due to the unstable vortex motions. Regarding the PIV measurements, the swirl ratio based on the cylinder center has been found to be lower than that based on the swirl center. On the other hand, the difference in swirl ratio has decreased smaller as the measurement plane moved downstream due to the stabilization of the vortex motion.

• Journal of the Korean Society of Propulsion Engineers
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• v.20 no.2
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• pp.94-101
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• 2016

In this paper, aerodynamic effects on the flow field under the wing with varying aspect ratio were investigated by measuring pressures on the lower surface of wing and analysing velocity components using Particle Image Velocimetry at Reynolds numbers of $1.384{\times}10^5$ and $2.306{\times}10^5$. In case of aspect ratio 4.8 which keeps the wing tip at a distance of 80% chord length from the pylon, the vortex from the wing tip influenced the flow field under the wing by reducing static pressures on the lower surface and increasing the velocity in proximity of the wing tip. Throughout the results, it is observed that aerodynamic effects of wing tip on the flow field around pylon under wing become insignificant as the aspect ratio increases.

• Journal of the Korean Society of Marine Environment & Safety
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• v.27 no.1
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• pp.153-160
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• 2021

Nitrous oxide is a global warming substance and is known as the main cause of the destruction of the ozone layer because its global warming effect is 310 times stronger than carbon dioxide, and it takes 120 years to decompose. Therefore, in this study, we investigated the characteristics of NOx emission from N2O reduction by thermal decomposition of N2O. Bunsen premixed flames were adopted as a heat source to form a high-temperature flow field, and the experimental variables were nozzle exit velocity, co-axial velocity, and N2O dilution rate. NO production rates increased with increasing N2O dilution rates, regardless of nozzle exit velocities and co-axial flow rates. For N2O, large quantities were emitted from a stable premixed flame with suppressed combustion instability (Kelvin Helmholtz instability) because the thermal decomposition time is not sufficient with the relatively short residence time of N2O near the flame surface. Thus, to improve the reduction efficiency of N2O, it is considered effective to increase the residence time of N2O by selecting the nozzle exit velocities, where K-H instability is generated and formed a flow structure of toroidal vortex near the flame surface.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.50 no.1
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• pp.1-12
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• 2022

Wind tunnel test for a small scale electric ducted fan with a 104mm diameter was conducted to analyze the aerodynamic characteristics when it was used as a propulsion system of tilt-propeller UAV. Experimental conditions were derived from flight conditions of a sub-scaled OPPAV. Forces and moments of the ducted fan model were measured by a 6-axis balance and 3-dimensional wake vectors which could induce an aerodynamic influence in the vehicle were measured by 5-hole probes. Thrust and torque on hover and cruise conditions were measured and analyzed to drive out the operating conditions when it was applied in the sub-scaled OPPAV. On transition conditions, thrust keep its value with tilt angle variation below 40° and increase after that. But, sideforce increase constantly until 75°. The maximum axial velocity in the wake on hover and cruise conditions was around 60m/s and tangential velocity was around 12m/s. The position of the maximum axial velocity and vortex center move off the fan rotation center line as the tilt angle increases.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.6
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• pp.390-395
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• 2019

Hydrogymnastics so that sufficient exercise effect can be obtained using the resistance of water has a positive effect on patients who have to receive arthritis or rehabilitation treatment. However, the studies on the effect are insufficient, and the main cause of their effects has not been unclear yet. In this study, in order to identify the main cause of the effects of Hydrogymnastics, conducted Unsteady fluid flow simulation under the same conditions as the actual environment. The analysis model based on real hands, and the pressure fluctuation applied to the knuckle was analyzed by the computational fluid method. During the underwater movement of the hands, Various sizes of vortices were generated between fingers due to skin surface velocity and flow resistance. Pressure of about -500 Pa to +500 Pa is applied by the vortex flow. Also It was confirmed that the positive pressure and the negative pressure were continuously repeated up to maximum + 2000 Pa at the minimum of -2000 Pa at the portion where the direction was changed. Pressure fluctuations with a frequency of 20 Hz to 70 Hz were added continuously for each knuckle. These continuous pressure fluctuations provide a direct massage effect on the knuckles, an It is judged that the blood circulation at the relevant part is positively affected.