Hunki Lee

Linear Stability Analysis for Combustion Instability in Solid Propellant Rocket

Corresponding author. E-mail: hjmoon@kau.ac.krABSTRACT Linear stability analysis for combustion instability within a cylindrical port of solid rocket motor has been conducted. The analysis of acoustic energy has been perfor med by a commercial COMSOL code to obtain the mode function associated to each acoustic mode pr ior to the calculation of stability alpha. An instability diagnosis based on the linear stability a nalysis of Culick is performed where special interests have been focused on 5 stability factors(alph a) such as pressure coupling, nozzle damping, particle damping and additionally, flow turning effect and viscous damping to take into account the flow and viscosity effect near the fuel surface. The instability decay characteristics depending on the particle size is also analyzed.초 록 본 연구에서는 고체로켓 모터의 연소 불안정성을 예측하고 분석 할 수 있는 해석도구의 개발을 위해 음향에너지의 분석과 선형 안정성 해석을 수행하였다. 음향 해석의 경우 상용 프로그램인 COMSOL을 이용하여 단면적이 일정한 실린더 형상의 연소실 음향 해석 및 모드 해를 도출하였다. Culick에 의해 정립된 고체추진 로켓의 선형 안정성 해석에 기초하여 연소 불안정성을 진단하였으며 압력결합, 노즐감쇠, 입자감쇠의 안정성 요소(stability alpha) 외에 유동방향변환(flow turning) 요소와 점성감쇠(viscous loss) 요소를 추가하여 연료 표면 근처의 유동 및 점성효과를 포함하는 연소 불안정의 경향을 파악하였다. 또한 입자의 크기에 따른 주파수 영역별 연소 불안정 감쇠 특성을 파악하였다.Key Words: Solid Rocket(고체로켓), Acoustic(음향학), Combustion Instability(연소 불안정), Linear Stability Analysis(선형 안정성 해석), Stability Alpha(안정성 요소)Nomenclature : speed of sound

Two-dimensional virtual array for ultrasonic nondestructive evaluation using a time-reversal chaotic cavity

Despite its introduction more than a decade ago, a two-dimensional ultrasonic array remains a luxury in nondestructive evaluation because of the complexity and cost associated with its fabrication and operation. This paper describes the construction and performance of a two-dimensional virtual array that solves these problems. The virtual array consists of only two transducers (one each for transmit and receive) and an aluminum chaotic cavity, augmented by a 10  ×  10 matrix array of rectangular rods. Each rod, serving as an elastic waveguide, is calibrated to emit a collimated pulsed sound beam centered at 2.5 MHz using the reciprocal time reversal. The resulting virtual array is capable of pulse-echo interrogation of a solid sample in direct contact along 10  ×  10 scan lines. Three-dimensional imaging of an aluminum test piece, the nominal thickness of which is in the order of 1 cm, is successfully carried out using the virtual array.

Jet noise-based diagnosis of combustion instability in solid rocket motors

Diagnosis of combustion instability in a solid rocket motor usually involves in-situ measurements of pressure in the combustor, a harsh environment that poses challenges in instrumentation and measurement. This paper explores the possibility of remote diagnosis of combustion instability based on far-field measurements of rocket jet noise. Because of the large pressure oscillations associated with combustion instability, the wave process in the combustor has many characteristic features of nonlinear acoustics such as shocks and limit cycles. Thus the remote detection and characterization of instability can be performed by listening for the tell-tale signs of the combustor nonlinear acoustics, buried in the jet noise. Of particular interest is the choice of nonlinear acoustic measure (e.g., among skewness, bispectra, and Howell-Morfey Q/S) that best brings out the acoustic signature of instability from the jet noise data. Efficacy of each measure is judged against the static test data of two tactical motors...

Acoustically sticky topographic metasurfaces for underwater sound absorption

A class of metasurfaces for underwater sound absorption, based on a design principle that maximizes thermoviscous loss, is presented. When a sound meets a solid surface, it leaves a footprint in the form of thermoviscous boundary layers in which energy loss takes place. Considered to be a nuisance, this acoustic to vorticity/entropy mode conversion and the subsequent loss are often ignored in the existing designs of acoustic metamaterials and metasurfaces. The metasurface created is made of a series of topographic meta-atoms, i.e., intaglios and reliefs engraved directly on the solid object to be concealed. The metasurface is acoustically sticky in that it rather facilitates the conversion of the incident sound to vorticity and entropy modes, hence the thermoviscous loss, leading to the desired anechoic property. A prototype metasurface machined on a brass object is tested for its anechoicity, and shows a multitude of absorption peaks as large as unity in the 2-5 MHz range. Computations also indicate that a topographic metasurface is robust to hydrostatic pressure variation, a quality much sought-after in underwater applications.

Thermoacoustics of a T-burner: How to better estimate the combustion response function from a T-burner experiment

The combustion response function, representing the coupling between the pressure oscillation and the combustion rate of a solid propellant, is a key indicator of combustion instability (large unwanted pressure oscillations in a combustion chamber) of a solid rocket motor. To obtain the response function experimentally, a T-burner is often used, in which the lowest longitudinal acoustic resonance of the cylindrical enclosure is excited by the combustion of propellant samples (acting like speakers) at both ends. The existing formula used to derive the combustion response function from a T-burner experiment is based on a series of rather crude assumptions. For example, acoustically rigid boundaries are assumed at both ends, whereas impedance boundary conditions may be more appropriate to reflect the introduction of combustion products from the propellant. In this talk, we propose a few modifications to the current T-burner analysis regime, which could improve the accuracy of the measured response function. T...

Investigation of nonlinearity parameter B/A of saturated, unconsolidated marine sediments via a statistical approach

In this talk, we present a theory and the supporting experiment for the nonlinearity parameter B/A of saturated, unconsolidated marine sediments. The model is composed of the quadratic nonlinearity of the equivalent suspension of grains and the interstitial fluid and the nonlinearity resulting from the random grain contacts. To capture the random nature of grain contacts, the number of Hertzian contacts on each grain and the distribution of contact forces between grains are treated statistically. Predictions of B/A are compared with measurements performed with the finite amplitude insert-substitution (FAIS) method. Dependence of B/A on external compression and the incident sound pressure level is also discussed. [This work was conducted in the Unmanned Technology Research Center (UTRC) sponsored by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD) in the Republic of Korea.]

Conceptual Design of Cylindrical Hydrophone Arrays for Stabilization of Receiving Characteristics under Ocean Ambient Noise

An underwater sound surveillance system detects and tracks enemy ships in real-time using hydrophone arrays, in which seabed-mounted sensor arrays play a pivotal role. In this paper the conceptual design of seabed-mounted, cylindrical hydrophone arrays for use in shallow coastal waters is performed via finite element calculations. To stabilize the receiving characteristics under the ocean ambient noise, a technique for whitening the ambient noise spectrum using a metal baffle is proposed. Optimization of the array configuration is performed to achieve the directivity in the vertical and azimuthal directions. And the effects of the sonar dome shape and material on the structural vibration and sound scattering properties are studied. It is demonstrated that a robust hydrophone array, having a sensitivity deviation less than 4 dB over the frequency range of interest, can be obtained through the whitening of the ambient noise, the optimization of the array configuration, and the design of acoustically transparent sonar domes.

Derivation of the nonlinearity parameter B/A of saturated, unconsolidated marine sediments via a statistical approach

Marine sediments are known to exhibit the nonlinearity parameter B/A that is much greater than that of homogeneous fluids and solids. The enormous value of B/A is particularly sensitive to the state of grain contacts. Although there exists a large body of literature that deals with the B/A of marine sediments, few studies have addressed how the random nature of grain contacts affects B/A. Here, we develop a model for the B/A of saturated, unconsolidated marine sediments, which accounts for random grain contacts. The number of contacts per grain and the distribution of contact stress between grains are treated statistically. The quadratic nonlinearity of the equivalent suspension of grains and the interstitial fluid is then combined with the nonlinearity arising from the random Hertzian contacts to produce the effective B/A of marine sediments.

Effects of time-varying grain shape on combustion instability of a solid rocket motor

A common approach to modeling combustion instability in a rocket propulsion system is to express the pressure oscillation in a combustor as a superposition of acoustic modes with time-varying amplitudes. Here, the tacit assumption is that the acoustic modes themselves remain more or less the same over the entire burning time. However, in the case of a solid rocket motor, the grain shape changes significantly as combustion progresses. This can gradually alter the shapes and frequencies of the acoustic modes, the influence of which on combustion instability has rarely been discussed in the existing literature. In this study, the effects of time-varying grain shape are modeled by introducing a slow time scale associated with the progressive burning of the grain. The resulting model equation accounts for the evolution of acoustic modes as well as their growth/decay in amplitude. Predictions with and without the use of the slow scale are compared with respect to measurements of a static firing test.

Comparison between the Biot-Stoll model, the grain-shearing model, and the effective density fluid model with respect to sediments in the Yellow Sea

There are some distinctive geophysical properties of sediments in the Yellow Sea, where mudflats are developed extensively. To select a physical model for acoustic wave propagation in the sediments is a critical issue in the analysis and the conceptual design of buried object scanning sonars. We consider three different sediment models to describe physical and geoacoustic properties of the sediments in the Yellow Sea: the Biot-Stoll model, the grain-shearing model, and the effective density fluid model (EDFM). These sediment models are evaluated in terms of their predictions of sediment properties compared with the measurement data. [This work was conducted in the Unmanned Technology Research Center (UTRC) sponsored by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD) in the Republic of Korea.]