• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.1
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• pp.1-10
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• 2005

An aerodynamic shape optimization technique has been developed for helicopter rotor blades in hover based on a continuous adjoint method on unstructured meshes. The Euler flow solver and the continuous adjoint sensitivity analysis were formulated on the rotating frame of reference for hovering rotor blades. In order to handle the repeated evaluation of the design cycle efficiently, the flow and adjoint solvers were parallelized using a domain decomposition strategy. A solution-adaptive mesh refinement technique was adopted for the accurate capturing of the tip vortex. Applications were made for the aerodynamic shape optimization of Caradonna-Tung rotor blades and UH60 rotor blades in hover. The results showed that the present method is an effective tool to determine optimum aerodynamic shapes of rotor blades requiring less torque while maintaining the desired thrust level.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.7
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• pp.1-8
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• 2005

For study on the unsteady wall interference effect, flows around a circular cylinder in closed test-section wind tunnels have been numerically investigated by solving compressible Navier-Stokes equations. The numerical scheme is based on a node-based finite-volume method with the Roe's flux-difference splitting and an implicit time-integration method coupled with dual time-step sub-iteration. The computed results showed that the unsteady pressure gradient over the cylinder is enhanced by the wall interference, and as a result the fluctuations of lift and drag are augmented. The drag is further increased because of the lower base pressure. The vortex shedding frequency is also increased by the wall interference. The pressure on the test section wall shows the harmonics having the shedding frequency contained in the wall effect.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.4
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• pp.298-304
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• 2014

The lifting-line method based on Weissinger's method is extended to be able to analyze the ground effect. The method is applied to predict the variation of aerodynamic performance due to ground effect for the elliptic wing with aspect ratio of 10 and the wing of human powered aircraft. While the vortex strength of the wing increases slightly, the downwash decreases significantly as the wing approaches to the ground. For the wing of human powered aircraft, the increment of lift at the height of 2m is 5% than the lift outside the influence of ground effect. The decrease of induced drag at the height of wing span is 10% and at the height of 2m is 55% than that out of ground effect.

• Atmosphere
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• v.26 no.2
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• pp.337-345
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• 2016

This paper is to assess the state of climate over East Asia and Korea during 2015/16 winter. There was a distinct intraseasonal climate variation during the period: the record-breaking warmth in December 2015 vs. strong cold surge outbreaks in January 2016. It is suggested that the anomalous warming in December 2015 was contributed by an intensification of Kuroshio anticyclone associated with 2015/16 El $Ni{\tilde{n}}o$ and polar vortex intensification. In January 2016, a strong cold surge outbroke over East Asia bringing severe cold more than two weeks. The cold surge was a blocking-type one which followed extremely negative AO developed from early January. It was suggested that the intensification of cold surge might be contributed indirectly by a strong Arctic warming and MJO activity during the period.

• Proceedings of the Korean Magnestics Society Conference
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• 2013.12a
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• pp.145-146
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• 2013

• Journal of the Korean Society of Propulsion Engineers
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• v.18 no.4
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• pp.18-25
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• 2014

In this paper, the mechanism of the combustion instability which may occur in a hybrid rocket motor with a diaphragm was studied. And the new design for a hybrid motor grain was suggested. It could increase a regression rate of solid fuel, and reduce a large pressure oscillation in a hybrid rocket motor with a diaphragm. It was confirmed that the main mechanism of a large pressure oscillation was hole-tone, and it was caused by a collision between a diaphragm and a vortex which was generated in a pre-chamber. And 'Stepped Grain' design which had the mechanism for high regression rate in a motor with a diaphragm and could reduce a combustion instability was suggested.

• Journal of the Korean Society of Marine Environment & Safety
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• v.22 no.7
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• pp.892-899
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• 2016

Gravity current flow past a subsea pipe above a scour based on computational fluid dynamics. For comparison, gravity current flow over pipe above a smooth bed also calculated, this configuration conventionally employed to consider the scour effect from an ideal approach. Interestingly, there different flow features and hydrodynamic forces between the scour and smooth bed cases. These results indicate that realistic conditionvery important investigatthe scour effect on gravity current flow around subsea pipe.

• Journal of the Society of Naval Architects of Korea
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• v.49 no.6
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• pp.528-533
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• 2012

Flexible composite propellers are subject to large deformation under heavy loading, and hence the hydrodynamic performance of deformed propeller might deviate from that of the metallic propeller under negligible deformation. To design the flexible propeller, it is therefore necessary to be able to evaluate the structural response of the blades to the hydrodynamic loadings, and then the influence of the blade deformation upon the hydrodynamic loadings. We use the lifting-surface-theory-based propeller analysis and design codes in solving the hydrodynamic problem, and the finite-element-method program formulated with 20-node iso-parametric solid elements for the analysis of the structural response. The two different hydrodynamic and structural programs are arranged to communicate through the carefully-designed interface scheme which leads to the derivation of the geometric parameters such as the pitch, the rake and the skew distributions common to both programs. The design of flexible propellers, suitable for manufacturing, is shown to perform the required thrust performance when deformed in operation. Sample design shows the fast iteration scheme and the robustness of the design procedure of the flexible propellers.

• Journal of the Society of Naval Architects of Korea
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• v.44 no.3 s.153
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• pp.267-278
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• 2007

This experimental study investigated on the eddy making effect on the roll motion of a rectangular barge in a two-dimensional wave tank. The structure was used to simulate a simplified rectangular barge in the beam sea condition. The structure with a draft one half of its height was hinged at the center of gravity and free to roll by waves. The rectangular barge was tested with regular waves with a range of wave periods that are shorter, equal to, and longer than its roll natural period. Particle image velocimetry (PIV) was employed to obtain the velocity field in the vicinity of the structure. The coupled interactions between the incident wave and the barge were demonstrated by examining the vortical flow fields to elucidate the eddy making effect during the roll motion. For incoming wave with a wave period same as the roll natural period, the barge roll motion was reduced by the eddy making damping effect. At the wave period shorter than the roll natural period, the structure roll motion was slightly reduced by the vertical flow around the barge. However, at the wave period longer than the roll natural period, the eddy making effect due to flow separation at structure corners indeed amplifies the roll motion. This indicates that not only can the eddy making effect damp out the roll motion, it can also increase the roll motion.

• International Journal of Naval Architecture and Ocean Engineering
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• v.5 no.3
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• pp.414-430
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• 2013

This paper shows the study of preswirl duct as an effective energy saving devices that have been devised and reviewed to support the propeller performance, especially for the ship of VLCC with large block coefficients. From the bare hull wake measurements, typical upper/lower asymmetry of hull wake at the propeller disk was found. The 2 kinds of pre-swirl duct, Unconventional half circular duct and Conventional circular pre-swirl duct have been designed and reviewed to recover the loss of propeller running in that condition. The general function of the pre-swirl duct was set to work against this asymmetry of wake and generate pre-swirled flow into the propeller against the propeller rotating direction. The optimum self propulsion tests with various angle configurations were carried out and the best configuration was decided. Accordingly, cavitation test was carried out with best configuration of unconventional half circular duct. The blade surface and tip vortex cavitation behaved smoother when the duct was mounted. The hull pressure amplitudes reflected this difference, so the hull pressure amplitude with duct was smaller than that of without duct.