• IEMEK Journal of Embedded Systems and Applications
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• v.12 no.5
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• pp.311-318
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• 2017

A reduced order observer is developed for depth control of a hybrid underwater glider which combines the good aspects of a conventional autonomous underwater vehicle and a underwater glider. State variables include the center of gravity of the robot and the weight of the buoyancy bag, which can not be directly measured. By using the mathematical model and available information such as directional velocities, accelerations, and attitudes, we developed a Luenberger's reduced order observer to estimate the center of gravity and the buoyancy weight. By simulations using Matlab/Simulink, the efficiency of the proposed observer is shown, where a LQR controller using full state variables is adopted as a depth controller.

• 한국연소학회:학술대회논문집
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• 2003.12a
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• pp.177-184
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• 2003

Stabilization mechanism of lifted flame in the near field of coflow jets has been investigated experimentally and numerically for methane fuel diluted with nitrogen. Lifted flames were observed only in the near field of coflow jets until blowout occurred in the normal gravity condition. To elucidate the stabilization mechanism for the stationary lifted flames in the near field of coflow jets for the diluted methane having the Schmidt number smaller than unity, the behaviors of the stationary lifted flame in microgravity and unsteady propagation phenomena were investigated numerically at various conditions of jet velocity. It has been founded that the buoyancy plays an important role for flame stabilization of lifted flame in normal gravity and the stabilization mechanism is due to the significant variation of the propagation speed of lifted flame edge compared to the local flow velocity at the edge.

• Proceedings of the Korea Institute of Fire Science and Engineering Conference
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• 2008.04a
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• pp.234-237
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• 2008

A direct numerical simulation (DNS) code suitable for the prediction of buoyant jet diffusion flames was developed in this study. The thermodynamic and transport properties were evaluated using CHEMKIN package to enhance the prediction performance of the developed DNS code. A two dimensional simulations were performed for the jet diffusion flames in normal and zero-gravity conditions where the Froude numbers are 5 and infinity, respectively. The simulated buoyant jet diffusion flame in normal gravity showed that the unsteady and dynamic motion although the reynolds number is low (400). It was identified that the flame in normal gravity flickered periodically. The periodic motion of the flame disappeared in zero-gravity condition. The dynamic motion of the buoyant jet diffusion flame could be well understood by comparing the flame structures obtained by the simulations of normal and zero-gravity conditions.

• Journal of Science Education
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• v.36 no.2
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• pp.369-380
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• 2012

The purpose of this study was to investigate middle students' conceptions of buoyancy. The questionnaires composed of 9 items which asked the reasons for situations with buoyancy; recognized of buoyancy, the facts affecting buoyancy(bottom of the Area of the object, the shape of the object, the depth of the object in the water, amount of water, the degree of the object submerged in the water, volume of the object, weigh of the object), the relationship between the amount of replaced fluid and buoyancy. The subjects were 140 students from the Busan Area who answered questionnaires composed of 9 items which asked the reasons for situations with buoyancy. Among the 140, 132 students were selected who answered all of question in the test. The students' answer types were follow: 1. Related to the shape of the submerged object, students answered that buoynacy depended on the bottom dimensions or the surface area of the object. 2. Students confused press, water press and buoyancy, who answered buoyancy was great when the water press was great. 3. Students answered that the weight and the size of the buoyancy of the object was same in the water. 4. Students answered, gravity was smaller in the water. Students recognized the existence of buoyancy but the reason of the answer depend on their experience. Also students answered using the words that water pressure, pressure, force instead of buoyancy. Students didn't understand the factors affecting the buoyancy. Many students responded that buoyancy was changed depend on weight of object, sinking depth, shape and others. It was considered that these responses didn't understand the cause of buoyancy.

• Journal of Ocean Engineering and Technology
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• v.15 no.2
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• pp.141-147
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• 2001

The launching process is one of the most critical operations for large structure in offshore installation. Since as the size increases it limits the availability of offshore crane facilities, the large jacket structures are often installed by launching. As the structure approaches to tilt beam, it reaches critical load, and there are parameters to affect on launching procedure. The major influential parameters are trim, draft of barge, center of gravity, center of buoyancy and reserved buoyancy of jacket. As increasing of trim and draft, structural loads tend to decrease. The trim is found to be more contributing than draft on structural loads. Therefore the trim should be increased so as to decrease structural loads and to avoid stalling of structure and submergence of stern. During the launching process, the distance between jacket and seabed should be investigated which differs from the amount of reserved buoyancy and launching condition of barge. In this paper the effects of parameters on launching process are numerically investigated.

• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.2
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• pp.865-874
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• 2019

A controller for an underwater glider is presented. Considered underwater glider is a torpedo-shaped autonomous underwater vehicle installing adjustable buoyancy bag and movable battery in it. The controller is composed of an LQR controller to maintain zigzag vertical movement for gliding and two PD controllers to control elevator/rudder angles. The LQR controller controls the pumping speed into the buoyancy bag and the moving speed to locate the battery. One of the PD controller controls the elevator angle to assist the LQR controller, and the other controls the rudder angle to adjust the direction of the underwater glider. A reduced order Luenberger observer is adopted to estimates the center of gravity of the glider and the buoyancy mass that are essential but cannot be measured. Mathematical simulation using Matlab proved the validity of the proposed controller to obtain better performance than conventional LQR only controller under the influence of sea current.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.62 no.1
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• pp.43-48
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• 2013

In this paper, magnetic buoyancy force on nonmagnetic solid object submerged in magnetic liquid was simulated and measured. For the evaluation of the force, a multi-physics approach of hydrostatic equilibrium considering magnetic body force as well as gravity is presented. The magnetic body force should be regarded as an additional forcing term in the momentum equation of hydrodynamics. It is also shown that the virtual air-gap based Kelvin's force formula is a useful method for the calculation of force distribution in the magnetic liquid. The experimental result which was performed by a load-cell measurement system agreed quantitatively well with the numerical one.

• Journal of Advanced Marine Engineering and Technology
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• v.37 no.8
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• pp.807-815
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• 2013

A numerical study on soot formation in a laminar ethylene diffusion flame at atmospheric pressure was conducted to obtain a better understanding of the effects of buoyancy on sooting flames under 0g and 1g using a gas-phase reaction mechanism and thermal and transport properties. A simple model was employed to predict soot formation, growth and oxidation with interactions between the gas phase chemistry and the soot chemistry taken into account. Results showed that the flames in 0g are much wider than that of 1g because of the thicker diffusion layer and reduction in axial velocity. The reduction in the axial velocity in 0g results in longer residence times, and resulting in greatly enhanced soot volume fraction. And, under zero-gravity, due to the lack of a buoyancy-induced instability, flame instability disappears.

• Journal of the Korean Society of Safety
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• v.17 no.4
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• pp.66-70
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• 2002

The transient soot distributions within the region bounded by the droplet surface and the flame were measured using a full-filed light extinction technique and subsequent tomographic inversion using Abel transforms. The soot volume fraction results for n-heptane droplets represent the first quantitative assessment of the degree of sooting for isolated droplets burning under microgravity condition. The absence of buoyancy(which produces longer residence times) and the effects of thermophoresis produce a situation in which a significant concentration of soot is produced and accumulated into a soot-cloud. Results indicate that indeed the soot concentration within the microgravity droplet flames(with maximum soot volume fractions as high as ~60ppm) are significantly higher than corresponding values that are reports for normal-gravity flames. This increase in likely due to longer residence times and thermophoretic effects that manifested under microgravity conditions.

• Journal of the Korean Society of Safety
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• v.17 no.3
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• pp.107-111
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• 2002

A near extinction nonpremixed counterflow flame of 19% methane diluted by 81% nitrogen by volume and undiluted air at a low global strain rate, 20 s-1, was computed. Investigations were focused on effects of the duct thickness and velocity boundary conditions on the flame structure in normal and zero gravity conditions. The results showed that, under normal gravity conditions, the effects of the duct thickness and velocity boundary conditions were significant by shifting the flame position, but negligible in zero gravity. The differences in flame structure were caused by buoyancy, and hence should be considered in the measurements in normal gravity.