• Journal of Ocean Engineering and Technology
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• v.30 no.1
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• pp.10-17
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• 2016

This paper reports a simulation-based design method for the optimized arrangement design of buoyancy modules in a marine riser system. A buoyancy module is used for the safe operation and structural stability of the riser. Engineers design buoyancy modules based on experience and experimental data. However, they are difficult to design because of the difficulty of conducting real sea experiments and quantifying the data. Therefore, a simulation-based design method is needed to tackle this problem. In this study, we developed a simulation-based design algorithm using a multi-body dynamic simulation and genetic algorithm to perform optimization arrangement design of a buoyancy module. The design results are discussed in this paper.

• Journal of the Korean Society of Combustion
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• v.17 no.1
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• pp.37-47
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• 2012

Experimental study on comparison of diffusive-thermal self-excitation with buoyancy-driven one due to accumulation of partially premixed, preheated mixture in front of edge flame was conducted in horizontally and vertically injected laminar free-jet flames with an applied DC electric field of -10 kV. The application of horizontal injection method with the DC electric field to jet flames was experimentally designed to suppress heat-loss-induced self-excitation and thereby to highlight the definite difference between both diffusive-thermal and buoyancy-driven self-excitations with the same order of O(1.0 Hz), in that diffusive-thermal self-excitation has not been so far found experimentally in laminar jet flames. Flame stability maps in vertically and horizontally injected jet flames are presented. The distinct modes of individual self-excitation are shown to be well described by their own phase diagrams. The results show that buoyancy-driven self-excitation due to the accumulation of partially premixed, preheated mixtures in front of edge flame is branched from the buoyancy-induced self-excitation with O(10 Hz) due to a flame flicker. Once the buoyancy-driven self-excitation appears, it suppresses buoyancy-induced as well as diffusive-thermal self-excitation. The key characteristics for individual self-excitation are discussed and their functional dependencies of Strouhal number upon related physical parameters are also presented.

• 한국연소학회:학술대회논문집
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• 2014.11a
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• pp.367-369
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• 2014

A study on laminar coflow jet flames diluted with helium and nitrogen has been conducted to investigate self-excitations. The stability map was provided with a function of nozzle exit velocity and fuel mole fractions of propane or methane. The results show that there exist three types of self-excitations; (1) buoyancy-driven self-excitation (BDSE), (2) Lewis number induced self-excitation coupled with buoyancy (LCB) and (3) Lewis number induced self-excitation (LISE).

• Journal of the Korea Institute of Military Science and Technology
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• v.14 no.5
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• pp.871-879
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• 2011

The internal deck structure of turtle ship has been investigated with the following approach manner. Firstly the 5 species of turtle ship are identified by examining the characteristics of 5 turtle ships revealed by the librarian or paintings. Secondly the centers of gravity and buoyancy for each type are calculated to evaluate the turtle ship's buoyancy stability. Thirdly the optimal deck structures for 5 turtle ships are additionally assessed from the viewpoint of the interoperability of soldier's missions to get more meaningful conclusion. From the results obtained by the well refined analyses above, one can identify that Changjeguyseon and Lee Chung Mu Gong descendent family's turtle ship possess one layer deck, and the tongjeyoung and jwasuyoung turtle ship consist of 1 and half story deck, and the one disclosed by Yoon has 2 story structure.

• 한국연소학회:학술대회논문집
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• 2014.11a
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• pp.129-132
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• 2014

A study on laminar coflow jet flames diluted with helium and nitrogen has been conducted to investigate self-excitations. The stability map was provided with a function of nozzle exit velocity and fuel mole fractions of propane or methane. The results show that there exist three types of self-excitations; (1) buoyancy-driven self-excitation (BDSE), (2) Lewis number induced self-excitation coupled with buoyancy (LCB) and (3) Lewis number induced self-excitation (LISE).

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.8
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• pp.2069-2078
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• 1993

The buoyancy-driven turbulent thermal convection is predicted using an anisotropic hybrid turbulence model, which is incorporated with a low Reynolds k-.epsilon. turbulence model and an anisotropic buoyant part of algebraic stress model(ASM). The numerical predictions are compared with the Davidson's model,(1) the full ASM and the experimental results of Cheesewright et al.(2) All the models are shown to predict good agreements with the experiments for the averaged turbulence quantities. It is found that the effect of an anisotropic part on the Reynolds stress and the turbulent heat fluxes is substantial. In this study, the present hybrid model gives a fairly reasonable prediction in terms of the computational accuracy, convergence and stability. The contribution of an anisotropic buoyant part to turbulent heat fluxes are also scrutinized over the range of Rayleigh numbers $(4.79{\times}10^{10}{\le}Ra{\le}7.46{\times}10^{10}).$

• Nuclear Engineering and Technology
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• v.15 no.1
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• pp.50-56
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• 1983

The effect of flow stratification is of particular concern during transient after scram in the outlet plenum of LMFBR. In this case, buoyancy effects on turbulent mixing are of importance to designers. An investigation has been made to identify the appropriate change in the available turbulence models which are necessary to include the effects of buoyancy on turbulence transport equations. The developed physical model of the buoyant turbulent flow are solved through SMAC method. Testing of the developed numerical model was undertaken and compared with experimental results. The results show that the buoyant turbulent effects account for a significant increase in the stability of the stratification, with a strong suppression of turbulence in the outlet plenum.

• Journal of Mechanical Science and Technology
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• v.15 no.10
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• pp.1451-1460
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• 2001

The critical condition of the onset of buoyancy-driven convective motion of uniformly heated horizontal fluid layer was analysed by the propagation theory which transforms the disturbance quantities similarly. The dimensionless critical time, $\tau$$\sub$c/, is obtained as a function of the Rayleigh number and the Prandtl number. Based on the stability criteria and the boundary-layer instability model, a new heat transfer correlation which can cover whole range of Rayleigh number was derived. Our theoretical results predict the experimental results quite reasonably.

• Journal of Energy Engineering
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• v.10 no.3
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• pp.266-271
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• 2001

Buoyancy-driven natural convection is analysed by employing a linear stability theory in hori-zontal porous media with net through flow. Darcy's law is used to model the flow characteristics in porous media. Bated on the results of linear stability analysis, a heat transfer correlation was obtained by employing weakly nonlinear analysis. As the net through flow increases, the system becomes more stable and the effect of the Darcy-Rayleigh number on the Nusselt number decreases.

• Korean Chemical Engineering Research
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• v.55 no.6
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• pp.868-873
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• 2017

Natural convection is driven by the compositional buoyancy in solidification of a binary melt. The stabilities of convection in a growing mushy layer were analyzed here in the time-dependent solidification system of a near-eutectic melt cooled impulsively from below. The linear stability equations were transformed to self-similar forms by using the depth of the mushy layer as a length scale. In the liquid layer the stability equations are based on the propagation theory and the thermal buoyancy is neglected. The critical Rayleigh number for the mushy layer increases with decreasing the Stefan number and the Prandtl number. The critical conditions for solidification of aqueous ammonium chloride solution are discussed and compared with the results of the previous model for the liquid layer.