• Journal of the Korean Society of Safety
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• v.31 no.2
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• pp.76-82
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• 2016

A 2-D hydrodynamic model for predicting the movement of debris flow was developed. The developed model was validated against a dam break flow problem conducted in EU CADAM project, and the performance of the model was shown to be satisfactory. In order to suggest structural alternative for hazardous zone vulnerable to debris flow disaster, two types of initial mass distribution and two shapes of defensive structure were considered. It was found that 1) the collapse of debris mass initiated with square pyramid shape induced more damage compared with that of cubic shape; and 2) a defensive structure with semi-circular shape was vulnerable to debris flow disaster in terms of debris control or primary defense compared with that of rectangular-shaped structure.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.681-687
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• 2001

Two-phase cross-flow exists in many shell-tube heat exchangers such as steam generators, condensers and reboilers. An understanding of flow-induced vibration excitation mechanism is necessary to avoid problems due to excessive tube vibration. This paper presents the results of a series of experiments done on tube bundles of different geometries subjected to two-phase cross-flow simulated by air-water mixtures. Normal(30$^{\circ}$) and rotated (60$^{\circ}$)triangular, and normal(90$^{\circ}$) and rotated (45$^{\circ}$) square tube bundle configurations of pitch-to-diameter ratio of 1.2 to 1.5 were tested over a range of mass fluxes from 0 to 1,000kg/$m^2$ㆍ s and void fraction from 0 to 100%. The effects of tube bundle geometry on vibration excitation mechanism such as fluidelastic instability and random turbulence, and on dynamic parameters such as damping and hydrodynamic mass are discussed. A lower pitch-to-diameter results in a higher hydrodynamic mass. The effect of tube bundle configurations on damping and random turbulence excitation is minor. The effect of pitch-to-diameter on the fluidelastic instability, however, is significant.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.77.1-77.1
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• 2012

We have performed cosmological hydrodynamic simulations that include the effects of radiative heating/cooling, star formation, feedback by supernova explosions, and metallicity evolution. Our simulations cover a cubic box of a side length 4 Mpc/h with 130 million particles. The mass of each particle is $3.4{\times}10^4M_{\odot}$, thus sub-galactic mini-halos can be resolved with more than hundred particles. Our simulation follows the whole formation process of the mini-halos (M< $10^7M_{\odot}$) around dwarf galaxies. We discuss various properties of the mini halos such as mass function, specific frequency, baryon-to-dark matter ratio, metallicity, spatial distribution, and orbit eccentricity distribution as functions of redshift and host galaxy mass. We also discuss how the formation and evolution of the mini halos are affected by the epoch of the reionization.

• Bulletin of the Society of Naval Architects of Korea
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• v.14 no.3
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• pp.13-22
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• 1977

The hydrodynamic forces acting on a forced oscillating 2-dimensional cylinder on a free surface of a fluid of a finite depth are calculated by distributing singularities on the immersed body surface. And the Haskind-Newman relation in a fluid of a finite depth is derived. The wave exciting force of the cylinder to an oscillation is also calculated by using the above relation. The method is applied to a circular cylinder swaying in a water of finite depth, and then, to a rectangular cylinder heaving, swaying, and rolling. The results of above cases give a good agreement with those by earlier investigators such as Bai, Keil, and Yeung. Also, this method is applied to a Lewis form cylinder with a half beam-to-draft ratio of 1.0 and a sectional area coefficient of 0.941, and to a bulbous section cylinder which is hard to represent by a mapping function. The results reveal that the hydrodynamic forces in heave increase as the depth of a water decrease, but in sway or roll, the tendency of the hydrodynamic forces is difficult to say in a few words. The exciting force to heave for a bulbous section cylinder becomes zero at two frequencies. The added mass moment of inertia for roll is seemed to mainly depend on the sectional shape than the water depth.

• Journal of Ocean Engineering and Technology
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• v.28 no.2
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• pp.119-125
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• 2014

A vertical planar motion mechanism(VPMM) test was used to increase the prediction accuracy for the maneuverability of an underwater glider model. To improve the accuracy of the linear hydrodynamic coefficients, the analysis techniques of a pure heave test and pure pitch test were developed and confirmed. In this study, the added mass and damping coefficient were measured using a VPMM test. The VPMM equipment provided pure heaving and pitching motions to the underwater glider model and acquired the forces and moments using load cells. As a result, the hydrodynamic coefficients of the underwater glider could be acquired after a Fourier analysis of the forces and moments. Finally, a motion control simulation was performed for the glider control system, and the results are presented.

• Journal of Ocean Engineering and Technology
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• v.30 no.5
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• pp.374-380
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• 2016

The large drift and angle of attack motion of an ROV (Remotely operated vehicle) cannot be modeled using the typical hydrodynamic coefficients of conventional straight running AUVs and specific slender bodies. In this paper, the ROV hull is divided into several simple-shaped components to model the hydrodynamic force and moment. The hydrodynamic force and moment acting on each component are modeled as the components of added mass force and drag using the known values for simple shapes such as a cylinder and flat plate. Since an ROV is operated under the water, the only environmental force considered is the current effect. The target ROV dealt with in this paper has six thrusters, and it is assumed that its maneuvering motion is determined using a thrust allocation algorithm. Tracking simulations are carried out on the ship’s surface near the stern, bow, and midship sections based on the modeling of the hydrodynamic force and current effect.

• Structural Engineering and Mechanics
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• v.82 no.3
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• pp.355-367
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• 2022

The effect of hydrodynamic damping on intake tower is twofold: one is fluid damping and another is structural damping. Fluid damping can be derived analytically from the governing equation of the fluid-structure-interaction (FSI) problem which yields a very complicated solution. To avoid the complexity of the FSI problem water-tower system can be simplified by considering water as added mass. However, in such a system a reconsideration of structural damping is required. This study investigates the effects of this damping on the dynamic response of the intake tower, where, apart from the "no water (NW)" condition, six other cases have been adopted depending on water height. Two different cross-sections of the tower are considered and also two different damping properties have been used for each case as well. Dynamic analysis has been carried out using horizontal ground motion as input. Finally, the result shows how hydrodynamic damping affects the dynamic behavior of an intake tower with the change of water height and cross-section. This research will help a designer to consider more conservative damping properties of intake tower which might vary depending on the shape of the tower and height of water.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.04a
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• pp.693-699
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• 2011

The outermost SG tubes have more structural problems than inside tubes. Many studies have used a uniform added mass coefficient for all of the SG tubes during the FIV analysis. The purpose of this study is to find out the added mass coefficients for each tube in a cylindrical shell. When a number of tubes are increased, added mass coefficients are also increased. And added mass coefficients at outermost tubes are less than those of inside tubes. According to gap changes between outermost tube and cylindrical shell, added mass coefficients are decreased with increasing the gap. When the gap has very large value, it shows that the added mass coefficient is asymptotically converged to the value of the tube array in a free fluid field.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.491-500
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• 2020

The present study proposes a time domain model for the Vortex-induced Vibration (VIV) simulation of a catenary riser under the combination of the current and oscillatory flow induced by vessel motion. In this model, the hydrodynamic force of VIV comprises excitation force, hydrodynamic damping and added mass, which are taken as functions of the non-dimensional frequency and amplitude ratio. The non-dimensional frequency is related with the response frequency, natural frequency, lock-in range and the fluid velocity. The relatively oscillatory flow induced by vessel motion is taken into account in the fluid velocity. Considering that the added mass coefficient and the non-dimensional frequency can affect each other, an iterative analysis is conducted at each time step to update the added mass coefficient and the natural frequency. This model is in detail validated against the published test models. The results show that the model can reasonably reflect the effect of the added mass coefficient on the VIV, and can well predict the riser's VIV under stationary and oscillatory flow induced by vessel motion. Based on the model, this study carries out the VIV simulation of a catenary riser with harmonic vessel motion. By analyzing the bending moment near the touchdown point, it is found that under the combination of the ocean current and oscillatory flow the vessel motion may decrease the VIV response, while increase the excited frequencies. In addition, the decreasing rate of the VIV under vessel surge is larger than that under vessel heave at small vessel motion velocity, while the situation becomes opposite at large vessel motion velocity.

• Journal of applied mathematics & informatics
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• v.23 no.1_2
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• pp.127-140
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• 2007

We present the boundary value problem (BVP) for the heave motion due to a vertical circular cylinder in water of finite depth. The BVP is presented in terms of velocity potential function. The velocity potential is obtained by considering two regions, namely, interior region and exterior region. The solutions for these two regions are obtained by the method of separation of variables. The analytical expressions for the hydrodynamic coefficients are derived. Computational results are presented for various depth to radius and draft to radius ratios.