• Nuclear Engineering and Technology
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• v.17 no.4
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• pp.279-289
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• 1985

The purpose of this paper is to investigate the density-wave oscillation type instability in the recirculating loop of U-tube steam generator (UTSG). The perturbed and nodalized conservations equations based on the drift-flux model have been derived to obtain the single-and two-phase pressure drop perturbations, by taking into account the slip between phases, nonuniform heat flux and heated wall dynamics. To assess the stability, the frequency domain technique with the Nyquist criterion has been used under the constant pressure drop boundary condition through the loop. The computer implementation of this model, SASG, was used for the parametric study of the steam generator in Kori-Unit 1. The results of the parametric study revealed important factors influencing UTSG stability margin.

• Journal of Mechanical Science and Technology
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• v.17 no.6
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• pp.906-913
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• 2003

Lean premixed combustion has been considered as one of the promising solutions for the reduction of NOx emissions from gas turbines. However, unstable combustion of lean premixed flow becomes a real challenge on the way to design a reliable, highly efficient dry low NOx gas turbine combustor. Contrary to a conventional diffusion type combustion system, characteristics of premixed combustion significantly depend on a premixing degree of combusting flow. Combustion behavior in terms of stability has been studied in a model gas turbine combustor burning natural gas and air. Incompleteness of premixing is identified as significant perturbation source for inducing unstable combustion. Application of a simple convection time lag theory can only predict instability modes but cannot determine whether instability occurs or not. Low frequency perturbations are observed at the onset of instability and believed to initiate the coupling between heat release rate and pressure fluctuations.

• Proceedings of the KSME Conference
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• 2001.06c
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• pp.246-250
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• 2001

In resin transfer molding (RTM), resin is forced to flow through the fiber perform of inhomogeneous permeability. This inhomogeneity is responsible for the mismatch of resin velocity within and between the fiber tows. The capillary pressure of the fiber tows exacerbates the spatial variation of the resin velocity. The resulting microscopic perturbations of resin velocity at the flow front allow numerous air voids to form. In this study, a mathematical model was developed to predict the formation and migration of micro-voids during resin transfer molding. A transport equation was employed to account for the migration of voids between fiber tows. Incorporating the proposed model into a resin flow simulator, the volumetric content of micro-voids in the preform could be obtained during the simulation of resin impregnation.

• Journal of Advanced Marine Engineering and Technology
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• v.10 no.4
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• pp.31-40
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• 1986

With the tendency toward high speed and high pressure in centrifugal pumps, the problem of sub-synchronous vibration has arisen, caused by the hydraulic forces of the working fluid, such as wearring, balance piston, impeller, etc.. These forces can drastically alter the rotor critical speeds and stability characteristics, and can be acted significant destabilizing forces. For preventing such self-excited vibration, the desing of the rotor system needs, which would secure the stability of the machine. In this paper, a procedure is presented for dynamic modeling of rotor-bearing-seal-impeller systems which consist of rigid disks, distributed parameter finite rotor elements and discrete bearings, seals and impellers. A finite element model including the effects of rotatory inertia and gyroscopic moments is developed using the consistent matrix approach. The technique of dynamic matrix reduction is applied to the shaft matrices to reduce them to a set of matrices of dynamic of significantly fewer degrees of freedom. The representation of bearing, seal and impeller elements is in term of linearized stiffness and damping matrices by reasonably small perturbations from equilibrium. The stability behavior of a typical double suction centrifugal pump is presented. Results show the influence of clearance and flow conditions on running speeds and stability characteristics.

• Journal of the Korean Society of Combustion
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• v.23 no.1
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• pp.21-27
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• 2018

The effect of swirl flow structures on combustion dynamics of two interacting, lean-premixed flames was experimentally investigated, with a particular emphasis on swirl numbers and swirl rotational directions. Our results show that the amplitude of limit cycle oscillations is very sensitive to the combination of swirl numbers and rotational directions, while the instability frequency remains nearly unchanged. The counter-rotating cases show significantly lower pressure perturbations, and this behavior appears to be related to the formation of compact interacting zone with higher heat release rate, indicating the presence of increased flame surface wrinkling caused by intense turbulence.

• Tribology and Lubricants
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• v.13 no.4
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• pp.10-17
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• 1997

The present study describes an experimental investigation of temperature fluctuations associated with thermal instability. Surface temperatures of brake disk and pad were monitered at various locations in a caliper type brake system during drag braking conditions. It was found that the thermal instability appeared in pad more seriously than in disk. The temperatures at various circumferential positions fluctuate synchronously, whereas the center temperature fluctuates with 180$^{\circ}$ phase difference from the outer and inner radius temperatures. The temperature and amplitude of the temperature perturbations are increased due to the increase of contact area in the center location. It was also found that the thermal instability was dominantly determined by the increase of rotation speed and pressure. And the modification of ventilation path could retard the onset of thermal instability.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.62.2-62.2
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• 2018

From the gaussian, near scale-invariant density perturbations observed in the CMB to the late time clustering of galaxies, CDM provides a minimal theoretical explanation for a variety of cosmological data. However accepting this explanation, requires that we include within our cosmic ontology a vacuum energy that is ~122 orders of magnitude lower than QM predictions, or alternatively a new scalar field (dark energy) that has negative pressure. Alternatively, modifications to Einstein's General Relativity have been proposed as a model for cosmic acceleration. Recently there have been many works attempting to test for modified gravity using the large scale clustering of galaxies, ISW, cluster abundance, RSD, 21cm observations, and weak lensing. In this work, we compare various modified gravity models using cosmic shear data from the Deep Lens Survey as well as data from CMB, SNe Ia, and BAO. We use the Bayesian Evidence to quantify the comparison robustly, which naturally penalizes complex models with weak data support. In this poster we present our methodology and preliminary constraints on f(R) gravity.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.10
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• pp.1436-1444
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• 2001

Stall inception in a high-speed centrifugal compressor has been examined. The main objective was to find stall precursor and to develop a reliable stall warning method. Eight equally spaced fast-response pressure transducers in the inducer detected the spatial structure of small amplitude perturbations, via spatial Fourier transform, as stall is approached. Near the stall inception point, the phase of spatial courier coefficients increased linearly with the speed of impeller relation for several impeller revolutions at all test speeds, and the spectrum at impeller frequency increased as stall is approached. These are the clear evidences that the impeller frequency participates in the stalling process.

• Geophysics and Geophysical Exploration
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• v.18 no.3
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• pp.133-142
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• 2015

Displacement of the Earth's surface due to atmospheric loading has been recognized since a century years ago, and its accurate estimation is required in present day geodesy and surveying, particularly in space geodesy. Atmospheric load deformation in continental region can readily be calculated with the given atmospheric pressure field and the load Green's function, and, in near coastal area, approximate model is used for the calculation. The changes in the Earth's atmospheric circulation and the seasonal variation of atmospheric pressure on two hemispheres of the Earth are the each main causes of variation of the Earth's spin angular velocity and polar motion respectively. Wind and atmospheric pressure do the major role in other periodic and non-periodic perturbations of the positions in the Earth's reference frame and variations in the Earth's spin rotational state. In this reviewing study, the developments of related theories and models are summarized along with brief description of phenomena, and the geodetic perturbing effects of a hypothetical typhoon passing Korea are shown as an example. Finally related existing problems and further necessary studies are discussed in general.

• Journal of Advanced Navigation Technology
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• v.22 no.6
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• pp.585-590
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• 2018

GPS satellite orbits could be estimated precisely using GPS dynamics as well as GPS observations. Most of the perturbations are available to be generated using satellite position, velocity, well known model equations and coefficients. However, who wants to calculate solar radiation pressure (SRP) should choose a SRP model and estimate the coefficients of a SRP model. The accuracy of SRP model depends on the model characteristics. Therefore this paper has estimated coefficients of SRP models using CODE precise orbit product and compared the accuracy of orbit propagator depending on SRP model. The results show that the extended CODE orbit model (ECOM) and the reduced ECOM achieved cm level fitting orbit for the CODE 1-day orbit. Also orbit propagation model without SRP model consideration could get tens of meter orbits compared to CODE orbits.