• 한국항공우주학회지
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• 제32권8호
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• pp.37-46
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• 2004

지면효과의 여러 현상을 Navier-Stokes 방정식을 이용하여 해석하고 공기역학적인 관점에서 그 결과를 분석하였다. 2차원 지면효과에 대해서는 이미지 와류에 의한 약형 표면의 압력변화, 두께의 영향, 지면효과의 비점성 유동 현상 등을 확인하였으며, 3차원 지면효과로 익단와류 강도의 증가현상과 유효스팬 증가, 익단와류의 바깥 흐름현상 등을 확인하였다. 또한, 재래식 익형인 NACA 6409와 러시아의 WIG기 전용익형인 DHMTU 8-30에 대해서 Irodov의 조건식을 사용하여 새로 정안정성을 해석하였다. 해석결과, DHMTU 8-30 익형의 세로 정안정성이 NACA 6409 익형보다 훨씬 우수한 것으로 나타났다. 그러므로 DHMTU 8-30을 WIG기에 사용할 경우 NACA 6409에 비해 꼬리날개 부피비를 현저히 줄일 수 있음을 확인하였다.

• 대한기계학회논문집B
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• 제40권8호
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• pp.511-517
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• 2016

3차원의 협착 혈관모델을 3D 프린터를 이용하여 제작하였다. 협착부는 관의 중심축에 대하여 대칭인 형태이며, 협착부가 0도인 직관과 10도로 굽어진 관인 두 가지 모델에 대하여 실험을 수행하였다. 협착모델 내부 속도장을 매질에 대한 왜곡 없이 측정하기 위하여 굴절률일치법을 이용하였다. 정량펌프를 사용하여 발생된 맥동유동은 펌프의 회전속도로 세 가지의 속도조건을 조절하였다. 비정상상태의 속도장은 time-resolved PIV 기법을 이용하여 측정되었다. 주기적인 와류의 생성과 이동은 관 내 최대속도 영역과 관련 있으며, 와류의 크기와 위치 및 대칭성은 레이놀즈수와 관의 기하학적 구조에 영향을 받음을 알 수 있었다. 곡선관에서는 협착부 하류에 재순환 영역이 관찰되며, 이는 혈류역학적 관점에서 혈전의 형성과 침착 가능성을 설명해준다.

• 한국항공우주학회지
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• 제36권9호
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• pp.825-834
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• 2008

고 받음각에서의 방향 안정성 향상을 위한 chine 형상 최적화를 수행하였다. Super ellipse equation을 통하여 다양한 형태의 chine 형상을 생성하고, 3차원 Navier-Stokes 방정식을 이용하여 방향안정성 및 고받음각에서의 공력 특성을 분석하였으며, 가장 높은 방향 안정성을 갖는 형상을 기본형상으로 선정하였다. 파리미터를 이용한 기본형상의 곡면 변형을 통하여 높은 방향 안정성 및 양항비를 동시에 만족하는 최적형상 도출을 위하여, 반응면을 구성하고 가중치를 도입하고 양항비를 구속조건으로 하는 방향안정성 최적화 문제를 수행하였다. 본 연구를 통하여 고받음각에서 chine형상의 공력특성을 파악하여 강한 와류를 발생시키는 chine 형상이 방향안정성에 도움이 된다는 것을 확인할 수 있었으며 최적화를 통해 기본형상보다 방향안정성이 약 29% 향상되는 결과를 얻었다. 또한 파라미터 기반 형상 생성기법과 근사최적화 기법의 연동을 이용한 형상최적설계 과정을 초음속, 고받음각 유동의 chine 형상설계에 적용하여 그 효율성을 확인하였다.

• International Journal of Aeronautical and Space Sciences
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• 제18권2호
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• pp.197-205
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• 2017

We investigate the slat noise generation mechanism by using large-eddy simulation (LES) and simple source modeling based on linearized Euler equations. An incompressible LES of an MD 30P30N three-element airfoil in the high-lift configuration is conducted at $Re_c=1.7{\times}10^6$. Using the total derivative of the hydrodynamic pressure (DP/Dt) acquired from the incompressible LES, representative noise sources in the slat cove region are characterized in terms of simple sources such as frequency-specific monopoles and dipoles. Acoustic radiation around the 30P30N multi-element airfoil is effectively computed using the Brinkman penalization method incorporated with the linearized Euler equation. The directivity pattern of $p^{\prime}_{rms}$ at $r=20c_{slat}$ in the multiple sources is closely compared to that obtained by the application of the LES/Ffowcs-Williams and Hawking's methods to the entire flow field. The power spectrum of p' at ${\theta}=290^{\circ}$ is in good agreement with the data reported in BANC-III, especially the broadband part of the spectrum with a decaying slope ${\propto}f^{-3}$.

• Wind and Structures
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• 제24권5호
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• pp.501-528
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• 2017

In this work, wind tunnel tests of pressure measurements are carried out to assess the global aerodynamic interference factors, the local wind pressure interference factors, and the local lift spectra of an square high-rise building interfered by an identical cross-sections but lower height building arranged in various relative positions. The results show that, when the interfering building is located in an area of oblique upstream, the RMS of the along-wind, across-wind, and torsional aerodynamic forces on the test building increase significantly, and when it is located to a side, the mean across-wind and torsional aerodynamic forces increase; In addition, when the interfering building is located upstream or staggered upstream, the mean wind pressures on the sheltered windward side turn form positive to negative and with a maximum absolute value of up to 1.75 times, and the fluctuating wind pressures on the sheltered windward side and leading edge of the side increase significantly with decreasing spacing ratio (up to a maximum of 3.5 times). When it is located to a side, the mean and fluctuating wind pressures on the leading edge of inner side are significantly increased. The three-dimensional flow around a slightly-shorter disturbing building has a great effect on the average and fluctuating wind pressures on the windward or cross-wind faces. When the disturbing building is near to the test building, the vortex shedding peak in the lift spectra decreases and there are no obvious signs of periodicity, however, the energies of the high frequency components undergo an obvious increase.

• Wind and Structures
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• 제25권1호
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• pp.1-24
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• 2017

This paper presents a 1:25 multi-freedom aero-elastic model for a high lighting pole at the Zhoushan stadium. To validate the similarity characteristics of the model, a free vibration test was performed before the formal test. Beat phenomenon was found and eliminated by synthesis of vibration in the X and Y directions, and the damping ratio of the model was identified by the free decay method. The dynamic characteristics of the model were examined and compared with the real structure; the similarity results were favorable. From the test results, the major along-wind dynamic response was the first vibration component. The along-wind wind vibration coefficient was calculated by the China code and Eurocode. When the peak factor equaled 3.5, the coefficient calculated by the China code was close to the experimental result while Eurocode had a slight overestimation of the coefficient. The wind vibration coefficient during typhoon flow was analyzed, and a magnification factor was suggested in typhoon-prone areas. By analyzing the power spectrum of the dynamic cross-wind base shear force, it was found that a second-order vortex-excited resonance existed. The cross-wind response in the test was smaller than Eurocode estimation. The aerodynamic damping ratio was calculated by random decrement technique and the results showed that aerodynamic damping ratios were mostly positive at the design wind speed, which means that the wind-induced galloping phenomenon is predicted not to occur at design wind speeds.

• 한국항공우주학회지
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• 제42권1호
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• pp.29-36
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• 2014

하이브리드 로켓 연소에서는 다양한 종류의 저주파수 연소 압력진동이 나타난다. 10Hz 대역의 저주파수 압력진동은 고체연료와 연소가스의 열 관성 차이 때문에 발생하지만 그외의 저주파수 진동은 고체로켓에서 관찰되는 헬름홀츠 및 $L^*$ 모드에 의해 발생하는 것으로 연소실 부피 변화와 밀접한 관련이 있다. 따라서 유동 특성이 고체로켓과 유사한 하이브리드 로켓 연소에서 연소실 부피 변화는 저주파수 특성에 영향을 미치는 중요한 인자이다. 본 연구에서는 연소실과 후연소실의 형상 변화에 따른 연소 압력의 저주파수 특성 변화를 관찰하였다. 특히 주 연소실과 후연소실의 부피 비가 특정한 값이 되면 연소 도중에 10~30Hz 연소 압력 진동의 진폭이 갑자기 증폭되는 연소불안정 현상이 나타났다. 산화제 유량 조절 및 연료 변경에 의한 O/F 비 변화는 연소 압력의 저주파수 증폭과 무관한 것으로 밝혀졌다. 후연소실로 연소가스가 팽창할 때 발생하는 와류 흘림 현상이 저주파수 불안정 현상과 직접적인 관련 있는 것으로 판단되며 이에 관한 연구가 더 필요하다.

• 한국전산유체공학회지
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• 제21권1호
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• pp.78-85
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• 2016

While most AUV researches have concerned about single hydrofoil, practical AUV's are generally operated with multiple hydrofoils. Double hydrofoil study attempts to evaluate thrust and efficiency with various flapping motions, and carries out design optimization using parametric analysis. Flow patterns such as vortex shedding and wake-body interaction are carefully investigated during design variable sensitivity analysis. The purpose of this design optimization is to find out the optimal motion that yields maximum thrust and efficiency. The design optimization employes several techniques such as table of orthogonal arrays, Kriging method, ANOVA analysis and MGA. Throughout this research, it is possible to find the optimal values of heaving ratio, heaving shift and pitch shift: Heaving ratio 0.950, heaving shift $23.120^{\circ}$ and pitch shift $89.991^{\circ}$ are found to be optimal values in double hydrofoil motions. Thrust and efficiency are 16.7% and 35.1% higher than existing AUV that did not consider nonlinear dependency of motion parameters. This results may offer an effective framework that is applicable to various AUV motion analyses and designs.

• 한국의류산업학회지
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• 제17권6호
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• pp.1020-1029
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• 2015

This paper investigated the wearing performance of knitted fabrics made of air vortex yarns using PTT/tencel/cotton fibres in comparison with ring and compact yarns for emotional garment. Wicking property of knitted fabric made of MVS yarns was worse than those by ring and compact yarns, however, drying property of knitted fabric made of MVS yarns was better than those by ring and compact yarns, which was explained as more water vapor transport due to larger openness between fibres in the MVS yarns than those in the ring and compact yarns. Thermal conductivity of knitted fabric made of MVS was lower than those of ring and compact yarns and maximum heat flow(Q_max) at the transient state of MVS knitted fabric was lower than those of ring and compact yarns, which may be attributed to MVS yarn structure that has parallel fibres in the core part of the yarn and fasciated fibre bundles on the sheath part with roughness on the yarn surface. However, pilling of MVS knitted fabric was better than those by ring and compact yarns, which was caused by less and shorter hairy fibres protruded from MVS yarn surface than those of ring and compact yarns. It was observed that tactile hand of MVS yarn knitted fabrics was stiffer than those of ring and compact yarns knitted fabrics. It was explained by low extensibility and compressibility and high bending and shear rigidities of the MVS yarn knitted fabrics, which resulted in bad wearing performance of MVS knitted fabric.

• Wind and Structures
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• 제23권2호
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• pp.127-142
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• 2016

Aerodynamic effects, such as drag force and flow-induced vibration (FIV), on civil engineering structures can be minimized by optimally modifying the structure shape. This work investigates the turbulent wake of a square prism with its faces modified into a sinusoidal wave along the spanwise direction using three-dimensional large eddy simulation (LES) and particle image velocimetry (PIV) techniques at Reynolds number $Re_{Dm}$ = 16,500-22,000, based on the nominal width ($D_m$) of the prism and free-stream velocity ($U_{\infty}$). Two arrangements are considered: (i) the top and bottom faces of the prism are shaped into the sinusoidal waves (termed as WSP-A), and (ii) the front and rear faces are modified into the sinusoidal waves (WSP-B). The sinusoidal waves have a wavelength of $6D_m$ and an amplitude of $0.15D_m$. It has been found that the wavy faces lead to more three-dimensional free shear layers in the near wake than the flat faces (smooth square prism). As a result, the roll-up of shear layers is postponed. Furthermore, the near-wake vortical structures exhibit dominant periodic variations along the spanwise direction; the minimum (i.e., saddle) and maximum (i.e., node) cross-sections of the modified prisms have narrow and wide wakes, respectively. The wake recirculation bubble of the modified prism is wider and longer, compared with its smooth counterpart, thus resulting in a significant drag reduction and fluctuating lift suppression (up to 8.7% and 78.2%, respectively, for the case of WSP-A). Multiple dominant frequencies of vortex shedding, which are distinct from that of the smooth prism, are detected in the near wake of the wavy prisms. The present study may shed light on the understanding of the underlying physical mechanisms of FIV control, in terms of passive modification of the bluff-body shape.