• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.12
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• pp.35-40
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• 2006

The combustion instability in a model dump combustor with an exhaust nozzle and the possibility of combustion control using a loudspeaker to these instabilities were studied. By changing inlet velocity, combustor length and equivalence ratio, dynamic pressure signals and flame structures were simultaneously taken. Because inlet velocity and combustor length affect the life time of vortex in the dump combustor, the results showed that as the combustor length increased and the inlet velocity decreased, the instability frequency decreased and the maximum power spectral density of the dynamic pressure generally decreased. Also, instability frequency and maximum power spectral density of the dynamic pressure increased with the increment of equivalence ratio. From the data of close-loop control, the optimum time-delay control using a loudspeaker was confirmed to be able to reduce the vortex shedding induced from the mixed acoustic-convective mode combustion instability.

• Journal of the Korean Society of Propulsion Engineers
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• v.19 no.4
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• pp.15-23
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• 2015

This study predicts the basic characteristics of combustion instabilities in a gas turbine lean premixed combustor using ASCI3D code which is a FEM(Finite Element Method)-based Helmholtz solver. The prediction results show the good agreement with the measured data in modeling the overall combustion instability features, however, the code is found to overpredict the unstable conditions. As one of the efforts to improve the model accuracy, the effects of acoustic boundary conditions on the instability growth rate are analyzed. As a result, it is shown that the acoustic reflection coefficient has a great impact on the instability and the prediction accuracy can be enhanced by defining the precise acoustic conditions.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.46 no.12
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• pp.1021-1027
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• 2018

Experimental studies have been conducted to verify that the positive coupling between pressure oscillation (p') and combustion oscillation (q') of high frequency range is a prerequisite for the initiation of low frequency instability in hybrid rocket combustion. The post-chamber length and combustion equivalence ratio were selected as critical parameters to control the phase difference between p' and q', and p' amplitude in relation to the suppression of LFI. In the results, even if the post-chamber length increases, the phase difference between p' and q' maintains below pi/2, which is a necessary condition for the LFI development, but the amplification of RI (Rayleigh index) was substantially decreased leading to a stable combustion. In addition, results confirmed that combustion stability is achieved by changing the momentary equivalence ratio and/or by suppressing the positive coupling status of p' and q'. Thus, the periodic amplification of RI was identified as the middle path of the mechanism of occurrence of LFI.

• Journal of Advanced Navigation Technology
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• v.25 no.1
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• pp.52-59
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• 2021

According to the International Air Transport Association (IATA), 61% of the accidents occurred during the approach and landing phase of the flight, with 16% of the accidents caused by unstable access of the commercial aircraft. It was identified that the pilot's unstable approach and poor manipulation of correction led to accidents by continuing the excessive approach without go-around manuever. The causes of unstable access may vary, including airport approach procedures, pilot error, misplanning, workload, ATC (Air Traffic Contol) congestion, etc. In this study, we use the flight data analysis system to select domestic case airports and aircraft type where unstable approach events occur repeatedly. Through flight data analysis, including main events, airport approach procedures, pilot operations, as well as various environmental factors such as weather and geographical conditions at the airport. It aims to identify and eliminate the tendency of unstable approach events and the causes and risks of them to derive implications for mitigating unstable approach events and for developing navigation safety measures.

• Aerospace Engineering and Technology
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• v.4 no.1
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• pp.171-178
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• 2005

An acoustic analysis of a fuel-rich gas generator for the drive of a turbopump in a liquid rocket engine has been performed and the length of a simulating duct has been determined by comparing the resonant frequency of unstable acoustic modes to simulate an actual flight model gas generator. To simulate more realistically, a realistic short-length simulating duct has been determined by considering 1 or 2 wavelength of the unstable modes. Duct-length adjustment to turbopump can be a method to suppress a combustion instability problem by decoupling of acoustic mode and combustion characteristics. This method has been set up and validated with acoustic analysis and hot firing tests.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.31 no.2A
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• pp.71-77
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• 2011

In this paper elasto-plastic dynamic behavior of solid is analyzed by using smoothed particle hydrodynamics (SPH) without tensile instability which caused by a clustering of SPH particles. In solid body computations, the instability may corrupt physical behavior by numerical fragmentation which, in some cases of elastic or brittle solids, is so severe that the dynamics of the system is completely wrong. The instability removed by using an artificial stress which introduces negligible errors in long-wavelength modes. Applications to several test problems show that the artificial stress works effectively. These problems include the collision of rubber cylinders, fracture and crack of plate.

• Korean Business Review
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• v.21 no.2
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• pp.17-41
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• 2008

Although job insecurity has attracted many scholars for long time, the research of job insecurity has not been accumulated because of nonexistence of and comprehensive and systematic model on job insecurity. This study, therefore, has provided integrated model of job insecurity and has also generated important propositions of job insecurity. This study aims to design the antecedents and consequences(outcomes) of job insecurity of employees. First, we have postulated important hypothesis based on the existing literatures. Second, we have tested hypothesis by the comprehensive review of existing local empirical studies. Third we have provided underlying logics of designing integrated model of job insecurity. And we have also generated Important propositions underlying the model focusing on job insecurity. We propose that employees' job insecurity increase with their expectation about further organizational change, role ambiguity. We also propose that employees' job insecurity raise their psychological strain while reducing their organizational commitment, job satisfaction and work effort. Theoretical implications are discussed, as are the implications of study and suggestions for future research.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.05a
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• pp.128-132
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• 2009

Digital control system for active combustion control of lab-scale combustor equipped with secondary fuel injection is designed. Controllability with adaptive law is revealed with the Cambridge Combustor model and the requirement for control system is derived. The input and output requirements of frequency estimator and fuel supply actuator for the adaptive control law is verified with cold tests. The system can be used as digital based active combustion controller having 150Hz combustion instability.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.05a
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• pp.111-115
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• 2009

The method of test for observing the combustion instability and controling the instability actively by using secondary injection of fuel through flame stabilizer was studied by constructing model combustor of supersonic engine. The frequency of combustion instability was detected by measuring the pressure of combustor using pressure sensor and by optical sensing of flame intensity. Electro-magnetic valve was adopted as actuator for active control and the characteristics of modulated fuel was studied by measured pressure of valve outlet and scattering signal of spray at secondary fuel injection.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.7
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• pp.695-702
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• 2008

Flow instability in the rocket turbo pump systems can be caused by various elements such as valve, orifice and venturi and etc. The formation of cavitation specially in the propellant feeding system can trigger the mass flow and pressure oscillation due to cyclic formation and depletion of cavitations. If the cryogenic propellant are used, which is very sensitive to temperature variation, the change of propellant properties due to thermodynamic effect should be accounted for in the flow analysis. This study focuses on the formation of cryogenic cavitation adopting MUSHY IDM model suggested by Shyy and coworkers. Also, the flow instability is investigated with developed numerical code in the downstream of orifice flow. To this end, three different orifices are selected and investigated by the numerical calculation.