• Advances in aircraft and spacecraft science
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• v.1 no.4
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• pp.409-425
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• 2014

Converging flows of a gas and a liquid at a microchannel cross junction, under proper conditions, can result in the formation of periodic, dispersed microslugs. This microslug formation phenomenon has been proposed as the basis for a fuel injection system in a novel, 'discrete' monopropellant microthruster designed for use in next-generation miniaturized satellites. Previous experimental studies demonstrated the ability to generate fuel slugs with characteristics commensurate with the intended application during steady-state operation. In this work, numerical and experimental techniques are used to study the effect of valve actuation on slug characteristics, and the results are used to compare with equivalent steady-state slugs. Computational simulations of a valve with a 1 ms valve-actuation cycle show that as the ratio of the response time of the valve to the fully open time is increased, transient effects can increase slug length by up to 17%. The simulations also demonstrate that the effect of the valve is largely independent of surface tension coefficient, which is the thermophysical parameter most responsible for slug formation characteristics. Flow visualization experiments performed using a miniature valve with a 20 ms response time showed less than a 1% change in the length of slugs formed during the actuation cycle. The results of this study indicate that impulse bit and thrust calculations can discount transient effects for slower valves, but as valve technology improves transient effects may become more significant.

• Computers and Concrete
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• v.18 no.1
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• pp.39-51
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• 2016

This paper considers the tensile strength of concrete samples in direct, CTT, modified tension, splitting and ring tests using both of the experimental tests and numerical simulation (particle flow code 2D). It determined that which one of indirect tensile strength is close to direct tensile strength. Initially calibration of PFC was undertaken with respect to the data obtained from Brazilian laboratory tests to ensure the conformity of the simulated numerical models response. Furthermore, validation of the simulated models in four introduced tests was also cross checked with the results from experimental tests. By using numerical testing, the failure process was visually observed and failure patterns were watched to be reasonable in accordance with experimental results. Discrete element simulations demonstrated that the macro fractures in models are caused by microscopic tensile breakages on large numbers of bonded discs. Tensile strength of concrete in direct test was less than other tests results. Tensile strength resulted from modified tension test was close to direct test results. So modified tension test can be a proper test for determination of tensile strength of concrete in absence of direct test. Other advantages shown by modified tension tests are: (1) sample preparation is easy and (2) the use of a simple conventional compression press controlled by displacement compared with complicate device in other tests.

• Structural Engineering and Mechanics
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• v.75 no.5
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• pp.621-631
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• 2020

A laboratory experiment is conducted is to investigate the behaviour of a low-mass-ratio and high aspect ratio flexible cylinder under vortex-induced vibration (VIV). A flexible cylinder with aspect ratio of 100 and mass ratio of 1.17 is towed horizontally to generate uniform flow profile. The range of Reynolds number is from 1380 to 13800. Vibration amplitude, in-line and cross-flow frequency response, amplitude trajectory, mean tension variation and hydrodynamic force coefficients are analyzed based on the measurement from strain gauges, load cell and CCD camera. Experimental results indicate that broad-banded lock-in region is found for the cylinder with a small Strouhal number. The frequency switches in the present study indicates the change of the VIV phenomenon. The hydrodynamic force responses provide more understanding on the VIV of a low mass ratio cylinder.

• Wind and Structures
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• v.20 no.3
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• pp.405-422
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• 2015

In previous model- and full-scale studies, high-amplitude vertical vibrations of mast-arm traffic signal structures have been shown to be due to vortex shedding, a phenomenon in which alternatingly shed, low-pressure vortices induce oscillating forces onto the mast-arm causing a cross-wind response. When the frequency of vortices being shed from the mast-arm corresponds to the natural frequency of the structure, a resonant condition is created causing long-lasting, high-amplitude vibrations which may lead to the fatigue failure of these structures. Turbulence in the approach flow is known to affect the cohesiveness of vortex shedding. Results from this full-scale investigation indicate that the surrounding terrain conditions, which affect the turbulence intensity of the wind, greatly influence the likelihood of occurrence of long-lasting, high-amplitude vibrations and also impact whether reduced service life due to fatigue is likely to be of concern.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.16 no.3 s.108
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• pp.233-239
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• 2006

The effect of the joint condition of automotive muffler shell and baffle on radiated noise from muffler shell Is studied. Shell vibration characteristics, especially shift In natural frequency and change in of response amplitude, was identified from finite element analysis and modal tests for various joint conditions between shell and baffle. When there is heat expansion and inner pressure from the exhaust 9as in the muffler with elliptical cross section, the shell expands out, and construct gap with baffles. This gap causes more degrees of freedom of shell vibration and natural frequency shift. By welding the muffler shell to the baffles or using a dual shell muffler, the radiated noise problem could be mitigated supplying more a rigid joint and damping, respectively.

• Journal of the Korean Institute of Gas
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• v.21 no.1
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• pp.80-86
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• 2017

In this paper, it is proposed to solve the problem of the harmful exhaust gas reduction device DPF. The exhaust gas is effectively guided through the control of the exhaust gas flow path which is separated in both directions. Through cross control of two DPF units, it helps to improve the purification and regeneration combustion characteristics of harmful emissions. It is possible to prevent an increase in internal pressure caused by deposition of particulate matter in the DPF. It is also an object of the present invention to provide an automobile harmful exhaust gas reduction device capable of solving the problem of lowering the output and lowering the fuel consumption.

• Transactions of the Korean Society of Automotive Engineers
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• v.18 no.5
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• pp.108-114
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• 2010

The injection nozzle of an electro-hydraulic injector is being opened and closed by movement of a injector's needle which is balanced by pressure at the nozzle seat and at the needle control chamber, at the opposite end of the needle. In this study, the effects of needle movement in a piezo-driven injector on unsteady cavitating flows behavior inside nozzle were investigated by cavitation numerical model based on the Eulerian-Lagrangian approach. Aimed at simulating the 3-D two-phase flow behavior, the three dimensional geometry model along the central cross-section regarding of one injection hole with real design data of a piezo-driven diesel injector has been used to simulate the cavitating flows for injection time by at fully transient simulation with cavitation model. The cavitation model incorporates many of the fundamental physical processes assumed to take place in cavitating flows. The simulations performed were both fully transient and 'pseudo' steady state, even if under steady state boundary conditions. As this research results, we found that it could analyze the effect the pressure drop to the sudden acceleration of fuel, which is due to the fastest response of needle, on the degree of cavitation existed in piezo-driven injector nozzle.

• Wind and Structures
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• v.23 no.2
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• pp.157-169
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• 2016

The structural integrity of tube bundles represents a major concern when dealing with high risk industries, such as nuclear steam generators, where the rupture of a tube or tubes will lead to the undesired mixing of the primary and secondary fluids. Flow-induced vibration is one of the major concerns that could compromise the structural integrity. The vibration is caused by fluid flow excitation. While there are several excitation mechanisms that could contribute to these vibrations, fluidelastic instability is generally regarded as the most severe. When this mechanism prevails, it could cause serious damage to tube arrays in a very short period of time. The tubes are therefore stiffened by means of supports to avoid these vibrations. To accommodate the thermal expansion of the tube, as well as to facilitate the installation of these tube bundles, clearances are allowed between the tubes and their supports. Progressive tube wear and chemical cleaning gradually increases the clearances between the tubes and their supports, which can lead to more frequent and severe tube/support impact and rubbing. These increased impacts can lead to tube damage due to fatigue and/or wear at the support locations. This paper presents simulations of a loosely supported multi-span U-bend tube subjected to turbulence and fluidelastic instability forces. The mathematical model for the loosely-supported tubes and the fluidelastic instability model is presented. The model is then utilized to simulate the nonlinear response of a U-bend tube with flat bar supports subjected to cross-flow. The effect of the support clearance as well as the support offset are investigated. Special attention is given to the tube/support interaction parameters that affect wear, such as impact and normal work rate.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2022.05a
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• pp.317-321
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• 2022

In response to Incheon tap water accident in 2019, the Ministry of Environment has created the "Comprehensive Measures for Water Safety Management" to improve water operation management, provide systematic technical support, and respond to accidents. Accordingly, K-water is making a smart water supply management system for the entire process of tap water. In order to advance the response to water accidents, it is essential to secure the reliability of real-time water operation data such as flow rate, pressure, and water level, and to develop and apply a warning algorithm in advance using big data analysis techniques. In this paper, various statistical techniques are applied using water supply operation data (flow, pressure, water level, etc) to prepare the foundation for the selection of the optimal operating range and advancement of the monitoring and alarm system. In addition, the arrival time is analyzed through cross-correlation analysis of changes in raw water turbidity between the water intake and water treatment plants. The purpose of this paper is to study the model that predicts the raw water turbidity of a water treatment plant by applying raw water turbidity data considering the time delay according to the flow rate change.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.1
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• pp.200-214
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• 1993

This paper presents the results of fluidelastic instability analysis performed for the U-tube bundle of a Westinghouse model 51 steam generator, one of the recirculating types designed at an early stage, in which the principal region of external cross-flow is associated with the U-bend portion of tube. The prerequisites for this analysis are detailed informations of the secondary side flow conditions in the steam generator and the free vibration behaviours of the U-tubes. In this study, the three-dimensional two-phase flow field in the steam generator has been calculated employing the ATHOS3 steam generator two-phase flow code and the ANSYS engineering analysis code has been used to calculate the free vibration responses of specific U tubes under consideration. The assessment of the potential instability for the suspect U-tubes, which is the final analysis process of the present work, has been accomplished by combining the secondary side velocity and density distributions obtained from the ATHOS3 prediction with the relative modal displacement and natural frequency data calculated using the ANSYS code. The damping of tubes in two-phase flow has been deduced from the existing experimental data by taking into account the secondary side void fraction effect. In operation of the steam generator, the tube support conditions at the tube-to-tube support plate intersections due to either tube denting degradation or deposition of tube support plate corrosion products or ingression of dregs. Thus, various hypothetical cases regarding the tube support conditions at the tube-to-tube support plate intersections have been considered to investigate the clamped support effects on the forced vibration response of the tube. Also, the effect of anti-vibration bars support in the curved portion of tube has been examined.