• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.11a
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• pp.313-317
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• 2009

The orifice in the propellent feeding pipe line of a Liquid Rocket Engine(LRE) is used to balance the pressure of the pipe line. When a LRE starts up, pressure at the upstream of the orifice rapidly increases. In this case, pressure waves occuring by resistance of the orifice may induce low frequency instability in the pipe line. For this reason the study of dynamic characteristics of orifices is needed to prevent the instability. A pump is used to build up the pressure, and the pressure is measured upstream and downstream of the orifice when the orifice diameter is changed. With the increase of orifice diameter, water hammer decreases, but the effect of resistance downstream is increases.

• Journal of the Korean Society of Marine Environment & Safety
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• v.23 no.3
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• pp.320-329
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• 2017

In this research, we have calculated characteristics of wave-piercing high-speed planing hull, by using a RANS solver and overset grid method, for comparing with experimental measurements of that and simulating with several appendages, since the computed results of commercial CFD code look reasonable for the prediction of the performances of planing hulls on calm water in planing conditions. As a result, it is confirmed that the dynamic instability phenomena in pitch and heave motions (porpoising) occurred after a certain $Fn_V$, and effectively suppressed using some of appendages, especially the 0.5L spray rail is suppressed to 24-55 % in the pitch motion and 33-55 % in the heave motion. In spray phenomenon, 1L hard chine suppress spray effectively and it is effective to set the angle of appendages to be less than $0^{\circ}$ in order to suppress wave.

• Nuclear Engineering and Technology
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• v.38 no.4
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• pp.343-352
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• 2006

Convenient analytical tools for evaluation of the aperiodic and the fluctuating instabilities of the passive residual heat removal system (PRHRS) of an integral reactor are developed and results are discussed from the viewpoint of the system design. First, a static model for the aperiodic instability using the system hydraulic loss relation and the downcomer feedwater heating equations is developed. The calculated hydraulic relation between the pressure drop and the feedwater flow rate shows that several static states can exist with various numbers of water-mode feedwater module pipes. It is shown that the most probable state can exist by basic physical reasoning, that there is no flow rate through the steam-mode feedwater module pipes. Second, a dynamic model for the fluctuating instability due to steam generation retardation in the steam generator and the dynamic interaction of two compressible volumes, that is, the steam volume of the main steam pipe lines and the gas volume of the compensating tank is formulated and the D-decomposition method is applied after linearization of the governing equations. The results show that the PRHRS becomes stabilized with a smaller volume compensating tank, a larger volume steam space and higher hydraulic resistance of the path $a_{ct}$. Increasing the operating steam pressure has a stabilizing effect. The analytical model and the results obtained from this study will be utilized for PRHRS performance improvement.

• The korean journal of orthodontics
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• v.45 no.5
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• pp.268-272
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• 2015

Clear thermoplastic retainers have been widely used in daily orthodontics; however, they have inherent limitations associated with thermoplastic polymer materials such as dimensional instability, low strength, and poor wear resistance. To solve these problems, we developed a new type of clear orthodontic retainer that incorporates multi-layer hybrid materials. It consists of three layers; an outer polyethylenterephthalate glycol modified (PETG) hard-type polymer, a middle thermoplastic polyurethane (TPU) soft-type polymer, and an inner reinforced resin core. The resin core improves wear resistance and mechanical strength, which prevent unwanted distortion of the bucco-palatal wall of the retainer. The TPU layer absorbs impact and the PETG layer has good formability, optical qualities, fatigue resistance, and dimensional stability, which contributes to increased support from the mandibular dentition, and helps maintain the archform. This new type of vacuum-formed retainer showed improved mechanical strength and rate of water absorption.

• Journal of Ocean Engineering and Technology
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• v.25 no.5
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• pp.27-32
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• 2011

Recently, a ship-shaped barge has been developed to improve the resistance performance, as well as course-keeping capability. However, the stern of the barge is still similar to a box shape, and the vortex generated at the side of the barge creates drag and yaw instability. In order to solve this problem, stern skegs are normally used. The present paper deals with the correlation between the size of the stern skegs and the barge dimensions and hull coefficient. A stern skeg was designed to prevent yaw instability and minimize any additional resistance. The resistance test and course keeping test were performed in the towing tank at Pusan National University. To determine the correlation parameters between the designed stern skeg size and barge dimensions, a parametric study was also performed. Based on the experimental data from five barges, the optimum skeg dimensions were successfully derived. It is expected that the validation of the present study will be carried out by further experiments and computational comparisons in the near future.

• Wind and Structures
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• v.23 no.6
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• pp.485-503
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• 2016

To investigate the nonlinear aerostatic stability of the Hutong cable-stayed rail-cum-road bridge with ultra-kilometer main span, a FEM bridge model is established. The tri-component wind loads and geometric nonlinearity are taken into consideration and discussed for the influence of nonlinear parameters and factors on bridge resistant capacity of aerostatic instability. The results show that the effect of initial wind attack-angle is significant for the aerostatic stability analysis of the bridge. The geometric nonlinearities of the bridge are of considerable importance in the analysis, especially the effect of cable sag. The instable mechanism of the Hutong Bridge with a steel truss girder is the spatial combination of vertical bending and torsion with large lateral bending displacement. The design wind velocity is much lower than the static instability wind velocity, and the structural aerostatic resistance capacity can meet the requirement.

• Structural Engineering and Mechanics
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• v.9 no.1
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• pp.51-68
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• 2000

The paper presents theoretical and experimental investigations on three varying thickness circular cylinders, which were tested to destruction under external hydrostatic pressure. The five buckling theories that were presented were based on inelastic shell instability. Three of these inelastic buckling theories adopted the finite element method and the other two theories were based on a modified version of the much simpler von Mises theory. Comparison between experiment and theory showed that one of the inelastic buckling theories that was based on the von Mises buckling pressure gave very good results while the two finite element solutions, obtained by dividing the theoretical elastic instability pressures by experimentally determined plastic knockdown factors gave poor results. The third finite element solution which was based on material and geometrical non-linearity gave excellent results. Electrical resistance strain gauges were used to monitor the collapse mechanisms and these revealed that collapse occurred in the regions of the highest values of hoop stress, where considerable deformation took place.

• Steel and Composite Structures
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• v.18 no.2
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• pp.325-344
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• 2015

Experimental tests on cellular beams with sinusoidal openings showed two main failure modes around the openings. They concern the formation of four plastic hinges and the local instability of the sinusoidal part of the opening. In parallel, numerical analysis of the sinusoidal part of the opening revealed the existence of an elastic rotational restraint between the intermediate web-post and the adjacent opening panel. The aim of the present study is to present an approach to quantify this rotational restraint. Through the response surface method, a mathematical model is proposed. It shows a great ability to predict the rotational restraint value as a function of the geometrical parameters of the opening. This model can be used to perform an extensive study with various geometrical configurations of beams with the aim to develop a reliable and realistic analytical model predicting the resistance of the sinusoidal openings.

• KIEE International Transactions on Power Engineering
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• v.12A no.1
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• pp.6-14
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• 2002

In this paper, an EMM(Equivalent Mechanical Model) Is developed to explain the voltage collapse mechanism by reflecting the effects of reactive powers. The proposed EMM exactly represents the voltage instability mechanism described by the system equations. By the use of the EMM model, the voltage collapse mechanism has been illustrated by showing the exactness of the results. The stable region has been investigated with a reactive-power-controlled two-bus system, which shows that special alerts are required when the system operates with leading power factor. It is also discussed a system transform technique to eliminate the resistance component of the Thevenin equivalent impedance for practical applications. Finally, the results adopting the proposed method fur sample systems which were transformed are listed

• Steel and Composite Structures
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• v.5 no.2_3
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• pp.103-126
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• 2005

This paper describes the theoretical background and underlying principles behind the American Institute of Steel Construction Load and Resistance Factor Design (AISC LRFD) Specification for the analysis and stability design of steel frames. Various analysis procedures that can take into consideration the effects of member instability, frame instability, member-frame interaction, geometric imperfections, and inelasticity are reviewed. Design approaches by which these factors can be incorporated in the design of steel moment frames are addressed. Current specification guidelines for member and frame design in the U.S. are summarized. Examples are given to illustrate the validity of the design equations. Some future directions for the analysis and stability design of steel frames are discussed.