• 대한기계학회논문집
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• 제15권2호
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• pp.630-643
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• 1991

The hydrodynamic stability equations are formulated for buoyancy-induced flows adjacent to a vertical, planar, isothermal surface in cold pure water. The resulting stability equations, when reduced to ordinary differential equation by a similarity transformation, constitute a two-point boundary-value(eigenvalue) problem, which was numerically solved for various values of the density extremum parameter R=( $T_{m}$ - $T_.inf./) / ( $T_{o}$ - $T_.inf./). These stability equations have been solved using a computer code designed to accurately solve two-point boundary-value problems. The present numerical study includes neutral stability results for the region of the flows corresponding to 0.0.leq. R. leq.0.15, where the outside buoyancy force reversals arise. The results show that a small amount of outside buoyancy force reversal causes the critical Grashof number $G^*/ to increase significantly. A further increase of the outside buoyancy force reversal causes the critical Grashof number to decrease. But the dimensionless frequency parameter $B^*/ at $G^*/ is systematically decreased. When the stability results of the present work are compared to the experimental data, the numerical results agree in a qualitative way with the experimental data.erimental data.

• 로봇학회논문지
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• 제12권2호
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• pp.228-238
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• 2017

The CALEB10 is a multi-legged biomimetic underwater robot. In the last research, we developed a swimming pattern named ESPG (Extended Swimming Pattern Generator) by observing diving beetle's swimming actions and experimented with a positive buoyancy state in which CALEB10 floats on the water. In this paper, however, we have experimented with CALEB10 in a neutral buoyancy state where it is completely immersed in water for pitch motion control experiment. And we found that CALEB10 was unstably swimming in the pitch direction in the neutral buoyancy state and analyzed that the reason was due to the weight proportion of the legs. In this paper, we propose a pitch motion control method to mimic the pitch motion of diving beetles and to solve the problem of CALEB10 unstably swimming in the pitch direction. To control the pitch motion, we use the method of controlling additional joints while swimming with the ESPG. The method of obtaining propulsive force by the motion of the leg has a problem of giving propulsive force in the reverse direction when swimming in the surge direction, but this new control method has an advantage that a propulsive moment generated by a swimming action only on a target pitch value. To demonstrate validity this new control method, we designed a dynamics-based simulator environment. And the control performance to the target pitch value was verified through simulation and underwater experiments.

• Journal of Pharmaceutical Investigation
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• 제26권2호
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• pp.137-144
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• 1996

The study was carried out for the preparation and evaluation of a buoyant hydrogel matrix (BHM), which is buoyant in a neutral or in pH 2.0 buffer solution, by the aspects of buoyancy, swelling, and drug release. Physical mixtures of HPC and CP in various molar ratio were employed as a mucoadhesive polymer which swells and controls the rate of drug release. Anhydrous citric acid and sodium bicarbonate in the molar ratio of 1:3 were employed as effervescing agents which provide a buoyancy for the mucoadhesive polymeric matrix. The buoyancy in vitro was expressed as both floating time$(T_{fl})$ and surfing time$(T_{sf})$, which are the time required for floating from the bottom to the surface of the medium and the time to keep the floated state at the surface of medium during release studies, respectively. A close relationship was observed between the buoyancy and the amount of effervescing agent added. $T_{fl}$ of the buoyant hydrogel matrices were decreased to about 10 seconds linearly with increasing the amount of effervescing agent in the range of 5 to 15%. $T_{sf}$ of the buoyant hydrogel matrices were varied from 1 to 3 hr depending on the amount of effervescing agent. The swelling was observed by changes in diameter of the buoyant hydrogel matrices in distilled water or acidic buffer solution, resulted in dependences on pH and the amount of effervescing agents. The release of hydrochlorothiazide from the buoyant hydrogel matrices were followed by apparent zero-order kinetics, while the buoyant hydrogel matrices were floated at the surface and maintaining their swollen shapes.

• 대한기계학회논문집A
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• 제36권1호
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• pp.1-7
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• 2012

이 논문에서는 최소의 배터리를 소비하여 물고기 로봇을 구동하고, 물고기와 같은 유연한 운동을 할 수 있는 생체 모방(biomimetic) 물고기 로봇의 설계, 제작, 제어에 관하여 제안 하였다. 두 개 모터를 적용하여 물고기와 같이 유연하게 움직일 수 있는 방법을 제시 하였다. 중성 부력을 유지하는 방법과 빠르게 잠영하고, 방향을 전환 하기 위한 방법을 제시 하였다. 로봇 물고기의 꼬리는 유연한 움직임을 만들기 위하여 폴리머 재질을 사용하여 만들었다. 꼬리 내부는 관절과 강선으로 구성된다. 로봇 물고기에 척추에 해당하는 우레탄 골격과 관절을 이루는 핀에 연결된 강선을 당겨 꼬리에 정현파 명령을 주어 물고기와 비슷한 유영을 할 수 있도록 하였으며, 부력 조절 장치를 설치하였으며, 이 부력 조절 장치를 이용하여 물고기 로봇이 상승, 하강을 할 수 있도록 하였다.

• 한국환경과학회지
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• 제27권6호
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• pp.359-369
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• 2018

A model has been developed to predict natural ventilation in a single zone building with large openings. This study first presents pressure-based equations on natural ventilation, that include the combined effect of wind and thermal buoyancy. Moreover, the concept of neutral pressure level(NPL) is introduced to consider the two-way flow through a large opening. The total pressure differences across the opening and the NPL are calculated, and nonlinear equations are solved to find the zonal pressure to satisfy mass conservation. For this analysis, an iterative technique of successively approximating the zonal pressure is used. The results of applying this study model to several simple cases are as follows. When there is no wind and only the stack effect is caused, a one-way flow occurs in both the top and bottom openings in the case of two openings of equal-area, and a one-way flow occurs in the top opening; however, a two-way flow occurs in the bottom opening in the case of two openings of unequal-area. When there is a wind effect, regardless of whether the outside air temperature is lower or higher than the indoor air temperature, air flows into the room through the bottom opening and out of the room through the top opening. As the wind velocity increases, the wind effect appears to be more influential than the stack effect owing to the temperature difference.

• 수산해양기술연구
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• 제49권4호
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• pp.483-491
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• 2013

The sound-speed and density contrasts are important factors in estimating the target strength (TS) of moon jellyfish (Aurelia aurita). In this study, the sound-speed and density contrasts were measured using time-of-flight and neutral buoyancy methods, respectively. The sound-speed contrast of A. aurita was from 0.9966 to 1.0031 (mean${\pm}$SD, $0.9999{\pm}0.0017$) and no distinct differences in temperature or pulsation activity and weak were found. The density contrast was from 0.9994 to 1.0004 (mean${\pm}$SD, $1.0000{\pm}0.0002$). The density of A. aurita was substantially different but the density contrast of A. aurita was shown to be similar to that in the sampling location. The results can be used to estimate of TS of A. aurita by acoustic model.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2001년도 ICCAS
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• pp.148.2-148
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• 2001

The most important difference of an airship from conventional vehicle is that it has the apparent mass and inertia provided from the existence of Helium gas inside the airship. To acquire To acquire the exact response of the airship, the longitudinal responses of airship with respect to the vertical gust, which is the non-linear system, have been studied. An Airship has neutral buoyancy in equilibrium state. When it moves, its motion shows much difference comparing with conventional aircraft. Here, we compare two cases, the one has the apparent mass and the other hasn´t. With the apparent mass, the magnitude of the former response is smaller than the latter, while the frequency is higher. However, the apparent mass delay ...

• 한국해양공학회지
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• 제19권1호
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• pp.95-99
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• 2005

This paper presents the optimum design of cylindrical shell under external pressure loading. Two kinds of material, Al7075-T6, Ti-6Al-4V, are considered. For each material, the design variable is a thickness of the unstiffened parallel middle body shell, and the state variable, constraint, is hoop stress and the object .function is total weight of the cylindrical shell. Optimization is performed by conventional FE Program, ANSYS. In addition, buckling analysis is performed for the middle body of the cylindrical shell. Finally, we calculates the payload of the cylindrical shell to keep neutral buoyancy with optimized thickness in deep-sea applications.

• 문화기술의 융합
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• 제8권3호
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• pp.493-499
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• 2022

본 논문은 제주도의 화산이 폭발 시에 얼마만한 속도로 폭발했으며, 폭발 후에 생기는 튜물러스와 한라산 중턱에서 폭발된 현무암이 바다까지 밀려온 결과를 보고한다. 현무암 지하 마그마가 약 절대온도 1000K로 지상의 중성대를 뚫고 1950m 상공에서 절대온도 1200K로 폭발하면서 나올 때의 속도에 대해 분석했다. 부력(浮力)에 의한 화염 기둥의 열기류인 Plume 화산 폭발로 인한 주변의 공기보다 연소가스의 밀도가 작아져 부력이 발생하고 상승 기류가 형성된다. 화염 기둥은 연속, 간헐, 부력 화염 영역으로 분류된다. 한라산(1950m) 화염 기둥의 속도는 상승한 최고점에서 낙하함으로 위치에너지가 운동에너지로 변환되어 유체흐름에 의해 이 두 식을 같게 놓아 풀면 화산 폭발 속도87.5m/s를 얻었다. 이때 마그마의 밀도는 온도에 반비례한다. 거문오름(456m)은 폭발 속도는 42.6m/s로 계산했다.

• 한국군사과학기술학회지
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• 제19권5호
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• pp.559-566
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• 2016

Cables for Towed Array Sonar System(TASS) were developed. In order to verify the performance of cables, environmental and operational conditions as well as functional requirements were investigated during design stage. Double armored high and low voltage integrated cable for towed body and two kinds of cables, armored and light weight power and optic hybrid cables for towed array sensor system were developed by modeling and simulation. Customized manufacturing process and test method, such as foam extrusion and dynamic fatigue test were applied to this development. In conclusion, underwater towed hybrid cable with high tensile strength and compact structure were developed.