• 대한기계학회논문집B
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• 제22권4호
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• pp.450-467
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• 1998

The characteristics of the unstable impinging circular jet is investigated based on the frequency characteristics and the sound field of the impinging-tones. Two symmetric modes S1 and S2, associated with low frequency and high frequency respectively, and one helical mode H have been observed. At low speed the S2 mode is dominant and switched by the S1 mode as the speed increases. When the jet speed is high the S1 mode is very active over the impinging distance from half the nozzle diameter to its ten times, while the S2 mode occurs at shorter distance corresponding to stage 2 and 3. The helical mode H seems unstable, likely to be influenced much by the experimental environment, and occurs at relatively high speed with almost the same frequency characteristics as the S2 mode. By estimating the convection speed of the unstable jet, it is found that the ratio of the convection speed to the jet speed decreases with both Strouhal number and Reynolds number and the speed of S2 mode is faster than the Si mode. When the present experimental results are compared with the previous investigations performed for the hole tone and the impinging tone with a small plate, the S1 mode is found to be associated with the ring vortex of large diameter with low speed, but the S2 mode with the vortex of small diameter with high speed. In addition, the frequency is found to be influenced by the nozzle configuration but the characteristics is almost the same. From the impinging distance and frequency range, it can be deduced that S1 mode is related with the jet column mode and S2 mode with the shear mode.

• 대한조선학회논문집
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• 제28권2호
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• pp.118-131
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• 1991

자유수면 근처에서의 박리유동 현상을 수치 시뮬레이션 방법과 유선 가시화 시험 방법으로 연구하였다. 수치 시뮬레이션은 물체표면에서 유기되고 확산(diffusion) 및 대류(convection)에 의해 유동중으로 박리되어 방출되는 보오티시티(vorticity)를 다수의 보오텍스로 치환하여 유동의 변화와 유체력을 구하는 보오텍스블럽 법을 사용하여 수행하였다. 이 방법으로 대규모의 와류의 생성 및 변화를 추정할 수 있으며 또한 자유수면과 와류유동의 상호간섭현상 그리고 동 유체력 등을 추정할 수 있었다. 유선 가시화 시험은 해사기술연구소의 공동수조에서 수행하였으며, 직류 전원에 의해 얻어진 수소기포를 가시화 입자로 이용하였다. 조명장치는 할로겐 램프를 이용하였고, 연속사진을 가시화의 결과로 사용하였다. 또한 자유수면의 시간적 공간적 변화를 파고계를 이용하여 계측하고 이로부터 와류유동의 변화 주기를 추정하였다.

• Journal of Astronomy and Space Sciences
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• 제21권1호
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• pp.11-28
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• 2004

고위도의 이온권 전기장이 열권 역학에 어떻게 영향을 주는지를 이해하기 위하여, 미 국립대기연구소(NCAR)의 열권-이온권 전기역학적 대순환 모델(TIEGCM)을 이용하여 고위도 하부 열권의 바람을 연구하였다. 1992-1993년 기간의 남반구 여름철 조건에 대해 모델을 가동하였으며, 행성간 자기장(IMF)에 좌우되는 이온권 대류가 바람에 미치는 영향을 살펴보기 위하여 IMF와 열권 바람과의 관련성을 또한 조사하였다. 비록 모델로부터 추정된 바람의 세기가 WINDII관측치에 비해 대체적으로 약하긴 하지만, 바람의 형태는 잘 일치하였다. 고위도 여름철 열권 바람에 대한 이온권 대류의 영향이 105km까지 나타나는 것으로 확인되었다. IMF$\neq$O와 IMF=0인 경우의 바람차이(difference wind)는 IMF$B_y$성분이 양과 음일 때 각각 시계방향과 반시계방향의 강한 소용돌이 형태를 보이며, 이 소용돌이 양상은 고도 105km까지 나타났다. IMF $B_z$가 양인 경우의 바람차이는 극관에 아주 국한되는 반면, IMF $B_z$가 음일 경우에는 아오로라(subauroral) 위도까지 확장되었다. IMF $B_z$에 좌우되는 일주풍(diurnal wind) 성분과 이온권 대류 성분 사이에는 뚜렷한 상관관개를 보이며, 그 관련성은 고도 108km까지 나타나고, 그때 일주풍은 강한 회 전성을 나타냈다 하부 열권의 여름철 동서성분바람의 자기지방시(MLT) 평균에 대한 IMF $B_y$ 영향은 고위도에서 상당히 크며, 최대 풍속은 지자기 위도 $77^{\circ}$부근의 고도 130km에서 약$60ms^-1$로 나타났다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.290-295
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• 2000

An experimental study was carried out in a cavity with upper channel and square heat surface by visualization equipment with Mach-Zehnder interferometer and laser apparatus. The visualization system consists of 2-dimensional sheet light by Argon-Ion Laser with cylindrical lens and flow picture recording system. Instant simultaneous velocity vectors at whole field were measured by 2-D PIV system(CACTUS'2000). Obtained result showed various flow patterns. Severe unsteady flow fluctuation within the cavity are remarkable and sheared mixing layer phenomena are also found at the region where inlet flow is collided with the counter-clockwise rotating main primary vortex. Photographs of Mach-Zehnder are also compared in terms of constant heat flux.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2010년 춘계학술대회논문집
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• pp.534-541
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• 2010

A high-order accurate Euler flow solver based on a discontinuous Galerkin method has been developed for the numerical simulation of unsteady flows on unstructured meshes. A multi-level solution-adaptive mesh refinement/coarsening technique was adopted to enhance the resolution of numerical solutions efficiently by increasing mesh density in the high-gradient region. An acoustic wave scattering problem was investigated to assess the accuracy of the present discontinuous Galerkin solver, and a supersonic flow in a wind tunnel with a forward facing step was simulated by using the adaptive mesh refinement technique. It was shown that the present discontinuous Galerkin flow solver can capture unsteady flows including the propagation and scattering of the acoustic waves as well as the strong shock waves.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2011년도 추계학술대회 초록집
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• pp.40.2-40.2
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• 2011

A Counter-rotating wind turbine is one of the new concepts that are proposed to increase the performance of a wind turbine. It has two rotors rotating in the same axis, and it is known that its power coefficient can reach to 0.64 in the ideal case. While the BEMT is widely used to analyze the aerodynamic performance of wind turbines, the analysis of the counter-rotating wind turbine by using it is limited due to the aerodynamic interaction between the two rotors. In this study, the vortex lattice method is used to consider the effect of the front rotor on the rear rotor of the counter-rotating wind turbine and calculate the aerodynamic performance of it. The power and thrust sharing in the two rotors of the counter-rotating wind turbine are predicted and the total power and thrust are compared with that of a single rotor. Moreover, the wake convection and expansion rate is also compared with that of a single rotor.

• 대한기계학회논문집B
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• 제23권7호
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• pp.918-927
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• 1999

In the fully developed internal flow fields, there are complex transition flows caused by interaction of the cross flow and jet when jet is Injected Into the flow. These interactions are studied by means of the flow visualization methods. An instantaneous laser tomographic method is used to reveal the physical mechanism and the structure of vortices formation in the branch pipe flow. The velocity range of cross flow of the pipe is 0.7m/s and the corresponding Reynolds number $R_{cf}$, based on the duct height is $5.6{\times}10^3$, diameter/height ratios(d/H) 0.14 and velocity ratios 3.0. Oil mist with the size of $10{\mu}m$ diameter is used for the scattering particle. The instantaneous topological features of the vortex ring roll-up of the jet shear layer and characteristics of this flow are studied in detail by performing flow visualization in rectangular duct flow. It is found that the formation and roll-up of ring vortices is a periodic phenomenon. The detailed topology of the vortices in the near field of a cross -flow jet and the mechanism associated with them give enforced hints of vortex breakdown within the vortex system due to the interaction of the jet and the cross-flow.

• 설비공학논문집
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• 제7권4호
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• pp.622-632
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• 1995

The present numerical study is aimed to investigate time-dependent characteristics of a two-dimensional lid-driven square cavity flow of three high Reynolds numbers, $7.5{\times}10^3$, $10^4$ and $3{\times}10^4$. A conservative convection term on irregular grids was adopted by renewing the MAC type difference schemes on regular grids. Relaxation of velocity and pressure is implemented by SOLA algorithm. In case of $Re=7.5{\times}10^3$, flow behavior converges to steady state after a transient period. But for $Re=10^4$, periodic unsteady sinusoidal fluctuation of local velocity and kinetic energy is found and continuous movements of small eddies in the secondary flow regions are also discovered. Random generation of eddies and their active migrating behavior are detected for $Re=3{\times}10^4$, resulting in complete unsteady and non-linear flow characteristics. And, an organized structure similar to a Moffat vortex is also observed from the time-mean flow patterns. Furthermore, a typoon-like vortex(TLV) appears intemittently and rotates along the separation regions and boundary layers.

• 한국음향학회지
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• 제37권5호
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• pp.263-270
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• 2018

외부 유동소음 문제를 다루는 대부분의 산업현장에서 FW-H(Ffowcs Williams and Hawkings) 방정식을 이용한 복합전산공력음향 기법이 수치적인 효율성으로 인하여 널리 사용되고 있다. 그러나 사중극자항을 무시한 면적분 형태의 FW-H 방정식을 사용할 때 경우에 따라 무시할 수 없는 비물리적인 소음이 발생한다고 알려져 있다. 특히, 수중 프로펠러와 같이 날개 끝 와류 공동(tip vortex cavitation)이 하류방향으로 길게 형성되는 유동에 대해서는 적절하게 모델링하지 않으면 소음 예측의 정확도가 떨어지게 된다. 따라서 본 연구에서는 사중극자 보정항을 추가하여 적분면에서 FW-H 방정식으로부터 발생하는 비물리적인 음향을 저감시키고자 하였다. 먼저 FW-H 방정식에 기초하여 개발한 내부 예측코드의 정확성을 확인하기 위하여 에어컨 실외기에 사용되는 축류팬을 대상으로 검증을 수행하였으며, ANSYS Fluent의 결과와 비교하여 잘 일치하는 것을 확인하였다. 사중극자 보정항의 효과를 확인하기 위하여 등엔트로피 와류 전파에 대한 소음 해석을 수행하였으며, 사중극자 보정항에 의한 오차의 저감 효과가 발생하는 것을 확인하였다. 마지막으로 Clark-Y 수중익형에서 발생하는 공동 유동장을 대상으로 소음 해석을 수행하였으며, 공동이적분면을 통과할 때 발생하는 오차를 사중극자 보정항을 이용하여 저감할 수 있다는 것을 확인하였다.

• 대한조선학회논문집
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• 제42권6호
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• pp.619-630
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• 2005

A two-frame PIV (Particle Image Velocimetry) technique is used to investigate the wake characteristics behind a marine propeller with 4 blades at high Reynolds number. For each of 9 different blade phases from $ 0^{\circ} $ to $ 80^{\circ} $, one hundred and fifty instantaneous velocity fields are measured. They are ensemble averaged to study the spatial evolution of the propeller wake in the region ranging from the trailing edge to one propeller diameter (D) downstream location. The phase-averaged mean velocity shows that the trailing vorticity is related to radial velocity jump, and the viscous wake is affected by boundary layers developed on the blade surfaces and centrifugal force. Both Galilean decomposition method and vortex identification method using swirling strength calculation are very useful for the study of vortex behaviors En the propeller wake legion. The slipstream contraction occurs in the near-wake region up to about X/D : 0.53 downstream. Thereafter, unstable oscillation occurs because of the reduction of interaction between the tip vortex and the wake sheet behind the maximum contraction point.