• Journal of the Korean Society of Propulsion Engineers
• /
• v.12 no.2
• /
• pp.1-7
• /
• 2008

Effect of radial position of half-wave resonator is investigated experimentally for acoustic damping in a combustion chamber by adopting acoustic cold test. Acoustic damping is quantified by damping factor. When resonator with optimal tuning length is installed, damping is enhanced as its radial location is away from the center of the faceplate. And, spatial profile of damping factor is similar to that of the amplitude of the acoustic mode to be damped. As the location is close to the center, acoustic damping is mitigated and independent of the resonator length. On the other hand, the resonator with non-optimal length dose not show any effects of its radial position. Acoustic-damping capacity can be evaluated as functions of resonator length and position.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2007.11a
• /
• pp.17-20
• /
• 2007

Effect of radial position of half-wave resonator is investigated experimentally for acoustic damping of tangential mode in a combustion chamber by adopting acoustic cold test. When resonator with optimal tuning length is installed， damping is enhanced as its radial location is away from the center of the faceplate. And, radial profile of damping factor is similar to that of the amplitude of the acoustic mode to be damped. As the location is close to the center, acoustic damping is mitigated and independent on the resonator length. on the other hand， the resonator with non-optimal length dose not show any effect of its radial position.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.30 no.4
• /
• pp.28-37
• /
• 2002

Acoustic behavior in combustion chamber with acoustoc cavity is numerically investigated by adopting linear acoustic analysis. Helmholtz-type resonator is employed as a cavity model to suppress acoustic instability passively. The tuning frequency of acoustic cavity is adjusted by varying the sound speed in acoustic cavity. Through harmonic analysis, acoustic pressure responses of chamber to acoustic oscillating excitation are shown and the resonant acoustic modes are identified. Acoustic damping effect of acoustic cavity is quantified by damping factor. As the tuning frequency approaches the target frequency of the resonant mode to be suppressed, mode split from the original resonant mode to lower and upper modes appears and thereby damping effect is degraded significantly. Considering mode split and damping effect as a function of tuning frequency, it is desirable to make acoustic cavity tuned to maximum frequency of those of the possible splitted upper modes.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.12 no.3
• /
• pp.34-40
• /
• 2008

Acoustic design parameters of a half-wave resonator are studied experimentally for acoustic stability in a model chamber. According to the standard acoustic-test procedures, acoustic-pressure signals are measured. Quantitative acoustic properties of damping factor and sound absorption coefficient are evaluated and thereby, the acoustic-damping capacity of the resonator is examined. The diameter and the number of a half-wave resonator, its distribution, and the diameter of an enclosure are selected as the design parameters for optimal tuning of the resonator. Aroustic-damping capacity of the resonator increases with its diameter. When the open-area ratio of the resonator exceeds the optimum value, over-damping appears, leading to the decrease in the peak absorption coefficient and the broadening of absorption bandwidth. As the resonator diameter increases, optimum open-area ratio decreases.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.31 no.6
• /
• pp.79-87
• /
• 2003

Application of combustion stabilization devices such as baffle and acoustic cavity to liquid propellant rocket engine is investigated to suppress high-frequency combustion instability, i.e., acoustic instability. First, these damping devices are designed based on linear damping theory. As a principal design parameter， damping factor is considered and calculated numerically in the chambers with/without these devices. Next, the unbaffled chambers with/without acoustic cavities are tested experimentally for several operating conditions. The unbaffled chamber shows the peculiar stability characteristics depending on the operating condition and it is found to have small dynamic stability margin. As a result, the acoustic cavity with the present design has little stabilization effect in this specific chamber. Finally， stability rating tests are conducted with the baffled chamber, where evident combustion stabilization is observed, which indicates sufficient damping effect.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2008.05a
• /
• pp.18-21
• /
• 2008

Acoustic design parameters of a half-wave resonator are studied experimentally for acoustic stability in a model combustor. According to standard acoustic-test procedures, acoustic-pressure signals are measured. Quantitative acoustic properties of damping factor and sound absorption coefficient are evaluated and thereby, the acoustic damping capacity of the resonator is characterized. The diameter and the number of a half-wave resonator, its distribution are selected as design parameters for optimal tuning of the resonator. Acoustic damping capacity increases as the resonators with diameter increases. The optimum number of resonators or the optimum open-area ratio decreases as boundary absorption decreases.

• Proceedings of the Acoustical Society of Korea Conference
• /
• autumn
• /
• pp.163-166
• /
• 2001

초음파를 이용하여 인체 조직의 특성을 알아내는 방법은 매우 광범위하게 응용되어 오고있다. 그 중에서 초음파를 발생시킨 후 반사되어 되돌아오는 신호를 측정하여 그 감쇠 정도로부터 조직의 특성을 추정하는 방법이 많이 사용되고 있다. 이러한 감쇠현상은 발생된 초음파가 조직 내에서 흡수 또는 산란현상을 거치면서 주파수가 변이를 일으키기 때문에 발생한다. 따라서, 조직의 감쇠 특성을 알아내기 위해서, 주파수의 함수로 근사할 수 있는 감쇠 계수(attenuation coefficient)를 이용하여 시간에 따라 달라지는 주파수 변화를 추정한다. 그러나, 기존의 Ah(Auto-Regressive) 모델을 통한 시간영역 및 주파수 영역에서의 추정 방법을 사용하면 잡음이 존재하는 상황에서 시변 신호를 추정하는데 성능이 많이 저하된다. 본 논문에서는 이러한 단점을 보완하기 위해서, 가변 망각 인자와 재귀적인 TLS(Total Least Squares) 방법을 사용하여 시간에 따라 변하는 신호를 정확하게 추정하고 잡음환경에도 강인한 알고리듬을 제안하였다. 또한, 제안된 알고리듬은 추정 성능을 향상시킬 뿐 아니라 감쇠정도의 강약에 관계없이 망각인자의 값을 적응적으로 변화시켜 동작하는 장점을 가진다.

• Journal of KSNVE
• /
• v.3 no.3
• /
• pp.192-198
• /
• 1993

옥외에서 발생하는 소음은 음원과 수음점 사이의 시선을 차단하는 장애물을 설치하는 방법이외에는 달리 방도가 없는 경우가 많다. 빛과 마찬가지로 소리도 시선이 차단되면 소리의 그늘이 지는데 빛의 경우보다는 상당히 강한 음장이 이 그늘에 존재한다. 그늘 영역에서의 음장은 소리의 회절현상에 기인하는 것으로서 회절음장은 곧 방음벽의 차음효과를 좌우한다. 방음벽의 차음효과는 잉여감쇠(excessive attenuation)로 표시되는데 잉여감쇠에 영향을 주는 인자는 방음벽의 기하학적 조건, 음향학적 성질, 설치지면, 주변지형, 풍속 및 온도분포와 같은 기상조건, 음원의 특성 등 다양하지만 가장 기본적인 인자는 기하학적인 조건이다. 본고는 방음벽의 원리에 국한하여 살펴보기 위해 기술된 것이므로 주로 판 또는 쐐기 형태의 물체에 의한 회절현상을 취급하였다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2008.11a
• /
• pp.82-85
• /
• 2008

A Helmholtz resonator is applied to control high frequency combustion instability in liquid rocket engine. Damping characteristics of the Helmholtz resonator are investigated by the flow characteristic and its design. To simulate combustion instability, resonance in a test section(with fixed volume) is made by a pressure pulsator, and then damping characteristics are investigated. Its orifice length and diameter are selected as the design parameters and flow rates are varied to reveal the effect on damping characteristics. The experimental results show that a Helmholtz resonator is also working with flows. When length and diameter of an orifice are small, the tuning frequency increases as the flow velocity increases.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.14 no.3
• /
• pp.52-60
• /
• 2010

Combustion stability tests of 30 $ton_f$-class LRE thrust chamber with double swirl coaxial injector were carried out in domestic ground combustion test facility by means of artificial perturbation method. In these tests, thrust chambers with varying design factors like recess number of injector, baffle length, types of film cooling and chamber diameter were used and test results showed that these design factors are closely related with high frequency combustion stability. By using the oscillation decrement instead of the decay time in the combustion stability analysis of artificially perturbed LRE thrust chamber, it was confirmed that increment of damping factor results in the improvement of high frequency combustion stability of LRE thrust chamber.