• 한국해안·해양공학회논문집
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• 제31권4호
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• pp.209-220
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• 2019

해빈 안정화를 위해 거치되는 강성 구조물의 규모는 해빈의 자기 치유 능력이 구현되는 해빈 대순환 과정이 훼손되지 않는 범위에서 결정되어야 하나 최근 지나치게 대형화 되어 광폭 잠제도 빈번하게 차용되고 있는 실정이다. 이러한 시각에 기초하면 Irie et al.(1994)가 제안한 왜도 된 연흔모양 매트는 규모가 크지 않다는 점에서 현재 선호되는 광폭잠제의 대안이 될 수 있을 것으로 판단된다. 전술한 왜도 된 연흔모양 매트의 해빈 안정화 효과는 매트의 유수 단면 축소부에서 강제되는 와류가 run-down 시 외해방향으로 이송되는 표사를 얼마나 효과적으로 포획할 수 있느냐에 따라 결정되는 것으로 추정된다. 본 논문에서는 이러한 가설을 확인하기 위해 수치모의를 수행하였다. 수치모형은 Navier-Stokes 식과 물리기반 지형모형으로 구성하였으며, 모의 결과 왜도 된 연흔모양 매트 정점부에서 강제된 와류에 의해 포획된 표사가 해안 방향으로 이송되는 등 왜도 된 연흔모양 매트의 해빈 안정화 효과를 구성하는 주요 기작과 해빈 안정화 효과를 확인할 수 있었다.

• Wind and Structures
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• 제26권3호
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• pp.179-190
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• 2018

Translation of tornadoes is an important feature in replicating the near-ground tornado flow field which has been simulated in previous studies based on Ward-type tornado simulators using relative motion of the ground plane. In this laboratory investigation, effects of translation on the near-ground tornado flow field were studied using the ISU Tornado Simulator that can physically translate over a ground plane. Two translation speeds, 0.15 m/s and 0.50 m/s, that scale up to those corresponding to slowly-moving tornadoes in the field were selected for this study. Compared with the flow field of a stationary tornado, the simulated tornado with translation had an influence on the spatial distribution and magnitude of the horizontal velocities, early reversal of the radial inflow, and expansion of the core radius. Maximum horizontal velocities were observed to occur behind the center of the translating tornado and on the right side of its mean path. An increase in translation speed, resulted in reduction of maximum horizontal velocities at all heights. Comparison of the results with previous studies that used relative motion of the ground plane for simulating translating tornadoes, showed that translation has similar effects on the flow field at smaller radial distances (~2 core radius), but different effects at larger radial distances (~4 core radius). Further, it showed that the effect of translation on velocity profiles is noticeable at and above an elevation of ~0.6 core radius, unlike those in studies based on the relative motion of the ground plane.

• International Journal of Fluid Machinery and Systems
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• 제2권1호
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• pp.31-39
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• 2009

The flow instability in a low specific speed mixed-flow pump, having a positive slope of head-flow characteristics was investigated. Based on the static pressure measurements, it was found that a rotating stall in the vaned diffuser occurs at about 65% flow rate of best efficiency point (BEP). A dynamic Particle Image Velocimetry (DPIV) measurement and the numerical simulations were conducted in order to investigate the flow fields. As a result, the diffuser rotating stall was simulated even by Computational Fluid Dynamics (CFD) and the calculated periodic flow patterns agree well with the measured ones by DPIV. It is clarified that a periodical large scaled backflow, generated at the leading edge of the suction surface of the diffuser vane, causes the instability. Furthermore, the growth of the strong vortex at the leading edge of the diffuser vane induces the strong backflow from the diffuser outlet to the inlet. The scale of one stall cell is covered over four-passages in total thirteen vane-passages.

• 한국전산유체공학회지
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• 제9권1호
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• pp.1-9
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• 2004

Large eddy simulation is used to investigate the compressible flow over a cavity with high aspect ratio. The sub-grid scale stresses are modeled using the dynamic model. The compressible Navier-Stokes equations are solved with the sixth order accurate compact finite difference scheme in the space and the 4th order Runge-Kutta scheme in the time. The buffer Bone techniques are used for non-reflecting boundary conditions. The results show the shear layer oscillation over the cavity. The votical disturbances, the roll-up of vorticity, and impingement and scattering of vorticity at the downstream cavity edge can be seen in the shear layer. Several peaks for the resonant frequencies are found in the spectra of the vertical velocity at the center-line. The most energetic Peak near the downstream edge is different from that at the center part of the cavity The pressure has its minimum value in the vortex core inside the cavity, and becomes very high at the downstream face of the cavity. The variation of the model coefficient predicted by the dynamic model is quite large between 0 and 0.3. The model coefficient increases in the stream-wise evolution of the shear layer and sharply decreases near the wall due to the wall effect.

• 대한기계학회논문집B
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• 제33권6호
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• pp.454-460
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• 2009

To analyze the mixture formation process of evaporating diesel spray is important for emissions reduction in actual engines. Then the effects of change in density of ambient gas on spray structure in high temperature and pressure field have been investigated in this study. The ambient gas density was changed from ${\rho}_a=5.0kg/m^3$ to ${\rho}_a=12.3kg/m^3$ with CVC(Constant Volume Chamber). Also, simulation study by modified KIVA-II code was conducted and compared with experimental results. The ambient temperature and injection pressure are kept as 700K and 72MPa, respectively. The images of liquid and vapor phase in the evaporating free spray were simultaneously taken by exciplex fluorescence method. As experimental results, with increasing ambient gas density, the tip penetration of the evaporating free spray decreases due to the increase in the drag force from ambient gas. The spatial structure of a diesel spray can be verified as 2-regions consisted of liquid with momentum decrease and vapor with large-scale vortex. The calculated results obtained by modified KIVA-II code show good agreements with experimental results.

• 오토저널
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• 제12권3호
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• pp.53-62
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• 1990

A wind tunnel experiment for the flow around a 1/5 scale passenger vehicle model has been carried out. A 5-hole Pitot tube is used for measuring velocity distributions around the model and a scanivalve with 48 ports is used for measuring surface pressure distribution at various Reynolds numbers. In order to observe the flow on the surface and in the wake region, a flow visualization experiment has been performed using wool tuft with and without paper cones. In addition, a 2-dimensional viscous calculation considering only the mid-plane section of the model has been performed. A complex wake structure in the immediate rear of the model has been confirmed. The distributions of the surface pressure coefficient are not sensitively dependent on the Reynolds Number. In the first half of the model, they do not seem to vary form section to section. However, in the second half, they do vary from section to section, especially at the bottom surface, which indicates that the cross flow vortex is more affected by the bottom surface than the top surface. The qualitative agreement of the measured and calculated velocity field also explains the usefulness of the 2-dimensional calculation in the limited sense.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2005년도 추계학술대회 논문집
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• pp.54-57
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• 2005

Flow characteristics of hull wake behind a container ship model were investigated experimentally with varying loading condition and Reynolds number. Large-scale bilge vortices of nearly the same strength are formed in the near-wake region. They are symmetric and counter-rotating with respect to the wake centerline for all loading conditions tested. With going downstream for both design and ballast loading conditions, the strength of the bilge vortices decreases and the wake region expands due to diffusion and viscous dissipation. Under the design loading condition, the bilge vortices start to appear at St=0.363 transverse plane above the propeller-boss. For the ballast loading condition, however, the bilge vortices start to appear at St=0.591 below the propeller-boss. They move upward as the hull wake goes downstream and Reynolds number increases. These wake characteristics, under the ballast loading condition, may weaken the propulsion and cavitation performances of the propeller, which are usually optimized for the design loading condition.

• Wind and Structures
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• 제8권1호
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• pp.61-78
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• 2005

This paper presents results from an investigation of the structural behaviour of a very slender 90 m high steel chimney erected at V$\ddot{a}$xj$\ddot{o}$ in southern Sweden in 1995. The chimney is equipped with a mechanical friction-type damper at the top. Due to a mistake during erection and installation of the chimney the transport fixings of the damper were not released properly and the chimney developed extensive oscillations in the very first period of service. This caused a great number of fatigue cracks to occur within a few months of service. After the functioning of the damper had been restored and the fatigue cracks were repaired an extensive program was initiated in 1996 to monitor the structural behaviour of the chimney under wind loading. In the investigation data were collected for more than six years of continuous measurements and regular observations of the chimney. The data obtained have some general relevance with respect to wind data, behaviour of a slender structure under wind loading, and the effect of a mechanical damper. Also some theoretical studies were performed as part of the investigation of the chimney.

• 한국추진공학회지
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• 제7권1호
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• pp.40-48
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• 2003

큰에디모사기법을 사용하여 압축성 공동유동을 수치해석하였다. Dynamic 모델을 사용하여 모델상수를 구했으며 공간으로 6차 유한차분기법, 시간에 대하여 4차 Runge-Kutta 수치기법을 사용하였다. 공동 주위의 유동을 보면 중심선을 따른 전단유동의 발달, 에디의 발생과 소멸 현상을 잘 볼 수 있었다. 유동결과로부터 예측된 공진 주파수는 Rossiter의 실험식 결과와 비교적 잘 일치하였다. 평균 유선의 분포는 공동의 중심선을 따라서 평행하였으며 공동 내부 후반부에서 압력이 급격히 감소됨을 알 수 있었다.

• International Journal of Ocean System Engineering
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• 제3권4호
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• pp.188-208
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• 2013

The motion response of floating structures should be adequately low to permit the operation of rigid risers along with dry well heads. Though Spar platforms have low heave responses under lower sea state, could become unacceptable in near resonance region of wave periods. Hence the hydrodynamic response, heave in particular, must be examined to ensure that it is minimized. To reduce heave motions, external damping devices are introduced and one such effective damping device is heave plate. Addition of heave plate can provide additional viscous damping and additional added mass in the heave direction which influence the heave motion. The present study focuses on the influence of heave plate on the hydrodynamic responses of Classic Spar in regular waves. The experimental investigation has been carried out on a 1:100 scale model of Spar with single and double heave plates in regular waves. Numerical investigation has been carried out to derive the hydrodynamic responses using ANSYS AQWA. The experimental results were compared with those obtained from numerical simulation and found to be in good agreement. The influence of disk diameter ratio, wave steepness, pretension in the mooring line and relative spacing between the plates on the hydrodynamic responses of Spar are evaluated and presented.