• 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 for Aeronautical & Space Sciences
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• v.32 no.4
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• pp.61-66
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• 2004

The objective of the present study is to develop methodology for the assessment of combustion stability of liquid rocket injectors. To simulate actual combustion occurring inside of a thrust chamber, a fullscale injector has been employed in the study, which bums gaseous oxygen and mixture of methane and propane. The main idea of the experiment is that the mixing mechanism is considered as a dominant factor significantly affecting combustion instability in a fullscale thrust chamber. A single split triplet injector has been used with an open-end cylindrical combustion chamber. The characteristics revealed by excited dynamic pressures in gaseous combustion show degrees of relative acoustic damping depending on operating conditions. Upon test results, the direct comparison between various types of injectors can be realized for the selection of the best design among prospective injectors.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.2
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• pp.160-168
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• 2010

An experimental study has been conducted to investigate the effects of forcing amplitude on the tone-excited non-premixed jet flame of the resonance frequency. Visualization techniques are employed using the laser optic systems, which are RMS tomography, PLIF and PIV system. There are three lift-off histories according to the fuel flow rates and forcing amplitudes; the regime I always has the flame base feature like turbulent flame when the flame lift-off, while the flame easily lift-off in the regime II even if a slight forcing amplitude applied. The other is a transient regime and occurs between the regime I and regime II, which has the flame base like the bunsen flame of partial premixed flame. In the regime I and II, the characteristics of the mixing and velocity profile according to the forcing phase were investigated by the acetone PLIF, PIV system. Particular understanding is focused on the distinction of lift-off history in the regime I and II.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.3
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• pp.238-243
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• 2008

Several experimental techniques for identifying the noise sources distributed over a moving vehicle have been developed recently and are used to design a low noise vehicle. The beamforming method, which uses phase information between several microphones to localize the source position, is proved to be one of the promising techniques applicable even under complicated test environments. In this study a beamforming algorithm is developed and applied to measure the dominant noise sources on a passenger car passing by. Unlike the acoustic signals from a stationary noise source, the sound generated from a moving source is distorted due to the Doppler effects. The information about the speed and relative position of the vehicle are used to eliminate the Doppler effects from the measured acoustic signal by using a de-Dopplerization algorithm. The noise generated from a moving vehicle can be grouped in many ways, however, tire noise and the noise generated from the engine are distinguishable at the speeds being tested.

• International Journal of Aeronautical and Space Sciences
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• v.18 no.4
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• pp.797-806
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• 2017

In this study, the reflection coefficient (RC) and the flame transfer function (FTF) were measured by applying acoustic excitation to a duct-type model combustor and were used to predict the frequency of the combustion instability (CI). The RC is a value that varies with the excitation frequency and the geometry of the combustor as well as other factors. Therefore, in this study, an experimentally measured RC was used to improve the accuracy of prediction in the cases of 25% and 75% hydrogen in a mixture of hydrogen and methane as a fuel. When the measured RCs were used, an unstable condition was correctly predicted, which had not been predicted when the RCs had been assumed to be a certain value. The reason why the CI occurred at a specific frequency was also examined by comparing the peak of the FTF with the resonance frequency, which was calculated using Helmholtz's resonator analysis and a resonance frequency equation. As the CI occurred owing to the interaction between the perturbation in the rate of heat release and that in the pressure, the CI was frequent when the peak of the FTF was close to the resonance frequency such that constructive interference could occur.

• The Journal of the Acoustical Society of Korea
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• v.23 no.8
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• pp.576-582
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• 2004

A computer code that is based on the Pseudo-spectral finite difference algorithm using staggered grid is developed for the wave propagation modeling in the time domain. The advantage of a finite difference approximation is that any geometrically complicated media can be modeled. Staggered grids are advantageous as it provides much more accuracy than using a regular grid. Pseudo-spectral methods are those that evaluate spatial derivatives by multiplying a wavenumber by the Fourier transform of a pressure wave-field and performing the inverse Fourier transform. This method is very stable and reduces memory and the number of computations. The synthetic results by this algorithm agree with the analytic solution in the infinite and half space. The time domain modeling was implemented in various models. such as half-space. Pekeris waveguide, and range dependent environment. The snapshots showing the total wave-field reveals the Propagation characteristic or the acoustic waves through the complex ocean environment.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.12
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• pp.1004-1012
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• 2014

In this study, blowoff was investigated in a ducted combustor with the bluff body when acoustic excitation was forced. To observe the flame structure, OH radical chemiluminescence was used and the image was analyzed by using POD (Proper Orthogonal Decomposition) algorithm. Natural gas mainly composed of methane was used as fuel. Blowoff occurred when the equivalence ratio was reduced. Equivalence ratio causing blowoff was measured by changing air flow rate, excitation frequency and sound pressure. Blowoff equivalence ratio was varied depending on the experimental conditions. Vortex frequency behind the bluff body and resonance effect in combustor are the main factors that affect the blowoff equivalence ratios with the excitation.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.46 no.5
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• pp.376-384
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• 2018

The aerodynamic far-field noise was computed by an acoustic analogy code using the permeable surface for the UH-1H rotor blade in hover. The permeable surface surrounding the blade was constructed to include the thickness noise, the loading noise, and the flow noise generated from the shock waves and the tip vortices. The computation was performed with compressible three-dimensional Euler's equations and Navier-Stokes equations. The high speed impulsive noise was predicted and validated according to the permeable surface locations. It is confirmed that the noise source caused by shock waves generated on the blade surface is a dominant factor in the far-field noise prediction.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.7
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• pp.79-88
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• 2006

In the present paper, the swirl flow structure and flame characteristics of turbulent premixed combustion in a model gas turbine combustor are investigated using large eddy simulation(LES). A G-equation flamelet model is employed to simulate the unsteady flame behavior. When inlet swirl number is increased, the distinct flow structures, such as the shapes of corner recirculation and center toroidal recirculation zone, are observed and the flame length is shorted gradually. Also, the phenomena of flashback are identified at strong swirl intensity. In order to get the accurate description of unsteady flame behavior, the predictive ability of the acoustic wave in a combustor is primarily evaluated. It is found that the vortex generated near the edge of step plays an important role in the flame fluctuation. Finally it is examined systematically that the flame and heat release fluctuation are coupled strongly to the vortex shedding generated by swirl flow and acoustic wave propagation from the analysis of flame-vortex interaction.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.6
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• pp.516-525
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• 2015

In this paper, the acoustic pressure of a helicopter rotor in hovering and low speed descent flight is predicted and compared with experimental data. Ffowcs Williams-Hawkings equation is used to predict the acoustic pressure. Two different wind tunnel test data are used to validate the predicted results. Boeing 360 model rotor test results are used for the low-frequency noise in hover, and HART II test results are employed for the mid-frequency noise, especially BVI noise, in low speed descent flight. A simple free-wake model as well as the state-of-the-art CFD/CSD coupling method are adopted to perform the analysis. Numerical results show good agreement against the measured data for both low-frequency and mid-frequency harmonic noise signal. The noise carpet results predicted using the FFT(Fast Fourier Transform) shows also reasonable correlation with the measured data.