Seismoelectric Fluid/Porous-Medium Interface Response Model and Measurements

Coupled seismic and electromagnetic (EM) wave effects in fluid-saturated porous media are measured since decades. However, direct comparisons between theoretical seismoelectric wavefields and measurements are scarce. A seismoelectric full-waveform numerical model is developed, which predicts both the fluid pressure and the electric wavefields in a fluid in which a porous disc is embedded. An experimental setup, in which pressure and electric signals in the fluid are simultaneously measured, is presented. The setup allows the detection of the EM field that is generated when an acoustic wave crosses the interface between the fluid and the thin porous disc, without interference of electrical fields that are present within seismic body waves. The predicted pressure wavefield agrees well with the measurements in terms of acoustic wave travel times, waveforms, and amplitudes. The electric wavefield predictions agree with the recordings in terms of travel times, waveforms, and spatial amplitude decay. A discrepancy in amplitude of the converted EM signal is observed. Theoretical amplitudes that are smaller than the measurements were also reported in previous literature. These results seem to validate seismoelectric theory.     

LES Investigation of the Flow Through an Effusion-Cooled Aeronautical Combustor Model

The present study is devoted to the analysis of the behaviour of the flow through an effusion-cooled aeronautical combustor model. High-fidelity calculations are performed on an experimental model of a combustion chamber multi-perforated wall and compared to experimental measurements. The effect of combustion instability on the effusion-cooling system is investigated by studying the interaction of an acoustic wave with the jets-in-crossflow issued from the cooling plate. It is shown that the mass-flow rate through the plate can be drastically reduced by the acoustic wave, which demonstrates the destructive effect that such instability may have on the cooling of an aeronautical combustion chamber.     

Shock tube study of n-decane ignition at low pressures

Ignition delay times for n-decane/O 2 /Ar mixtures were measured behind reflected shock waves using endwall pressure and CH* emission measurements in a heated shock tube. The initial postshock conditions cover pressures of 0.09-0.26 MPa, temperatures of 1 227-1 536 K, and oxygen mole fractions of 3.9%-20.7% with an equivalence ratio of 1.0. The correlation formula of ignition delay dependence on pressure, temperature, and oxygen mole fraction was obtained. The current data are in good agreement with available low-pressure experimental data, and they are then compared with the prediction of a kinetic mechanism. The current measurements extend the kinetic modeling targets for the n-decane combustion at low pressures.     

Scattered noise prediction using acoustic velocity formulations V1A and KV1A

Aeroacoustic scattering prediction generally relies on boundary integral methods which require evaluation of the impermeability condition on the scattering surface. The boundary condition implies zero normal velocity relative to the scattering surface. This condition has been expressed by relating the acoustic velocity to the acoustic pressure gradient, allowing indirect evaluation of the boundary condition by existent acoustic pressure gradient formulations. In the present paper, a direct evaluation of the hardwall boundary condition in scattering problems is demonstrated by time-domain analytic acoustic velocity formulae. Acoustic velocity formulations V1A and KV1A are implemented for acoustic scattering prediction, by hybrid approaches based on the FW–H equation and the Kirchhoff method These formulations can be coupled to any scattering solver, allowing time-domain prediction of the incident acoustic field when broadband noise generation is concerned. Formulation V1A offers mathematical simplicity and computational efficiency, which can be advantageous for realistic scattering applications. Implementation of formula KV1A enables acoustic scattering prediction by existing solvers based on the Kirchhoff method. The validity of the suggested methodology is assessed through the analytical test case of harmonic sound scattered by a rigid sphere. Sound propagation and scattering effects are analyzed by examination of the acoustic velocity field characteristics.     

Moving shocks through metallic grids: their interaction and potential for blast wave mitigation

Numerical simulations and laboratory measurements have been used to illuminate the interaction of a moving shock wave impacting on metallic grids at various shock strengths and grid solidities. The experimental work was carried out in a large scale shock tube facility while computational work simulated the flow field with a time-dependent inviscid and a time-dependent viscous model. The pressure drop measured across the grids is a result of two phenomena which are associated with the impact of the shock on the metallic grids. First are the reflection and refraction of the incoming shock on the grid itself. This appears to be the main inviscid mechanism associated with the reduction of the strength of the transmitted shock. Second, viscous phenomena are present during the reflection and refraction of the wave as well as during the passage of the induced flow of the air through the grid. The experimental data of pressure drop across the grid obtained in the present investigation are compared with those obtained from computations. The numerical results slightly overpredict the experimental data of relative pressure drop which increases substantially with grid solidity at fixed flow Mach numbers. The processes of shock reflection and refraction are continuous and they can be extended in duration by using thicker grids that will result in lower compression rates of the structural loading and increase the viscous losses associated with these phenomena which will further attenuate the impacting shock. Preliminary theoretical analysis suggests that the use of a graded porosity/solidity material will result in higher pressure drop than a constant porosity/solidity material and thus provide effective blast mitigation.      

Diagnostic modelling of an expansion tube operating condition

Computational simulations of an expansion tube were conducted to estimate flow parameters and verify experimental uncertainties. Two types of simulations of the complete facility were undertaken: a one-dimensional simulation, and a hybrid simulation where a one-dimensional simulation of the shock tube section was coupled with a two-dimensional simulation of the acceleration tube. Good agreement between the one-dimensional simulations and experiments were obtained in the shock tube portion of the facility. In the acceleration section, initial two-dimensional simulations did not match the experimentally measured pitot pressure and showed a discrepancy in the shock speed. Further studies examined how the accelerator gas composition affected shock speed, static pressure and pitot pressure levels in expansion tube operation. Subsequent two-dimensional simulations, using an 8% level of air contamination in helium, showed reasonable agreement with experimental data. This prediction of air contamination was later confirmed with experimental measurements of the air partial pressure before operation.      

密闭空间可燃气体爆炸超压预测

为避免密闭空间内可燃预混气体爆炸事故造成的伤害，对其进行较为准确的爆炸超压预测是抗爆设计和日常安全管理的关键。结合已有文献实验数据，利用光滑层流火焰传播理论模型建立了爆炸超压模型；对比发现，当体积较大时，光滑层流火焰传播理论模型存在较大的误差。较大体积密闭空间爆炸火焰传播过程中的不稳定性造成火焰前锋面褶皱并引起湍流燃烧，导致火焰前锋面表面积大幅增加，且在火焰传播过程中表现出自相似分形特征。依据褶皱及湍流火焰传播过程中的自相似分形特征，基于分形燃烧理论和相关经验数据，进一步建立了考虑可燃预混气体爆炸火焰褶皱及湍流火焰传播的爆炸超压预测模型，并与实验所得结果进行了对比。结果表明:当密闭空间体积较大时，利用褶皱及湍流火焰传播理论建立的爆炸超压模型进行峰值压力估算时，两种工况下实验所得和理论计算所得相对误差分别为10.4%和11.1%，较光滑层流火焰传播理论爆炸超压模型相比，误差分别减少了72.3%和50.6%。本文所建立理论模型与实验所得结果具有较好的一致性，在一定程度上可满足结构抗爆设计或日常安全管理的需要。     

Acoustic, thermal and flow processes in a water filled nanoporous glass by time-resolved optical spectroscopy

We present heterodyne detected transient grating measurements on water filled Vycor 7930 in the range of temperature 20-90 °C. This experimental investigation enables to measure the acoustic propagation, the average density variation due to the liquid flow and the thermal diffusion in this water filled nano-porous material. The data have been analyzed with the model of Pecker and Deresiewicz which is an extension of Biot model to account for the thermal effects. In the whole temperature range the data are qualitatively described by this hydrodynamic model that enables a meaningful insight of the different dynamic phenomena. The data analysis proves that the signal in the intermediate and long time-scale can be mainly addressed to the water dynamics inside the pores. We proved the existence of a peculiar interplay between the mass and the heat transport that produces a flow and back-flow process inside the nano-pores. During this process the solid and liquid dynamics have opposite phase as predicted by the Biot theory for the slow diffusive wave. Nevertheless, our experimental results confirm that transport of elastic energy (i.e. acoustic propagation), heat (i.e. thermal diffusion) and mass (i.e. liquid flow) in a liquid filled porous glass can be described according to hydrodynamic laws in spite of nanometric dimension of the pores. The data fitting, based on the hydrodynamic model, enables the extraction of several parameters of the water-Vycor system, even if some discrepancies appear when they are compared with values reported in the literature.     

Shock tube studies of thermal radiation of diesel-spray combustion under a range of spray conditions

A tailored interface shock tube and an over-tailored interface shock tube were used to measure the thermal energy radiated during diesel-spray combustion of light oil, α-methylnaphthalene and cetane by changing the injection pressure. The ignition delay of methanol and the thermal radiation were also measured. Experiments were performed in a steel shock tube with a 7 m low-pressure section filled with air and a 6 m high-pressure section. Pre-compressed fuel was injected through a throttle nozzle into air behind a reflected shock wave. Monochromatic emissive power and the power emitted across all infrared wavelengths were measured with IR-detectors set along the central axis of the tube. Time-dependent radii where soot particles radiated were also determined, and the results were as follows. For diesel spray combustion with high injection pressures (from 10 to 80 MPa), the thermal radiation energy of light oil per injection increased with injection pressure from 10 to 30 MPa. The energy was about 2% of the heat of combustion of light oil at P _inj = about 30 MPa. At injection pressure above 30 MPa the thermal radiation decreased with increasing injection pressure. This profile agreed well with the combustion duration, the flame length, the maximum amount of soot in the flame, the time-integrated soot volume and the time-integrated flame volume. The ignition delay of light oil was observed to decrease monotonically with increasing fuel injection pressure. For diesel spray combustion of methanol, the thermal radiation including that due to the gas phase was 1% of the combustion heat at maximum, and usually lower than 1%. The thermal radiation due to soot was lower than 0.05% of the combustion heat. The ignition delays were larger (about 50%) than those of light oil. However, these differences were within experimental error.

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An abridged version of this paper was presented at the 18th Int. Symposium on Shock Waves at Sendai, Japan during July 21 to 26, 1991 and at the 19th Int. Symposium on Shock Waves at Marseille, France during July 26 to 30, 1993.     

Acoustic damping rate measurements in binary mixtures of atomic species via transient-grating spectroscopy

The objective of this work is to investigate the ability of transient-grating spectroscopy (TGS) to measure accurately the acoustic damping rate by analyzing the temporal behavior of laser-induced gratings. Experiments are performed in a binary gaseous mixture, with a trace amount of NO₂, as a function of both composition and pressure. Measured and theoretically calculated acoustic damping rates are compared using both a classical model and a more comprehensive model that included additional diffusive mechanisms. The TGS technique demonstrated here provides a nearly instantaneous measurement with reasonably high spatial resolution. The experimental data agree well with theoretical predictions.     

探索发展激波风洞爆轰驱动技术

发现了燃烧驱动激波管中入射激波马赫数异常升高的起因. 实验显示爆轰驱动能力强于燃烧驱动, 从而推动爆轰驱动技术的发展. 采用卸爆管消除爆轰波反射高压以及双爆轰驱动段全部消除爆轰波后的Taylor稀疏波, 使反向和前向爆轰驱动模式具有实用价值. 反向爆轰驱动技术还成功用来延长激波风洞试验时间.      

Experimental investigation of dynamic pressure loads during dam break

The objective of this research work has been to conduct experimental measurements on a dam break flow over a horizontal dry bed in order to provide a detailed insight, with emphasis on the pressure loads, into the dynamics of the dam break wave impacting a vertical wall downstream the dam. The experimental setup is described in detail, comprising state of the art miniaturized pressure sensors, high sampling rate data acquisition systems and high frame-rate video camera. It is a 1:2 scale of the highly cited (Lee et al., 2002, Journal of Fluids Engineering, 124) article experimental apparatus. Kinematics has been analyzed focusing on the free surface and wave front evolution. Experimental observations regarding liquid height and wave front speed have found to be in agreement with existing literature. This agreement enables the authors, assuming a similar framework, to discuss the measured pressure loads as a consequence of the dam break wave front impacting on the downstream wall. These loads show a substantial variability which has been statistically characterized. The measured quantities have been compared with the scarce available data in the literature, whose consistency is discussed. Measurements have been conducted with two filling heights. Scaling effects for such heights are also analyzed. As a direct result of the present initiative, an extensive set of data for computational tools validation is provided as Supplementary Materials, including pressure signals, wave height measurements and experimental videos.     

铝粉湍流火焰诱导激波现象的实验研究

粉尘湍流火焰诱导激波问题是工业灾害研究中的重要课题．本文在自行设计的大型卧式燃烧管内，对铝粉火焰诱导激波现象进行了实验研究，测试了湍流火焰阵面前压缩波到激波的转捩过程，并将实验结果与数值模拟结果进行了比较．     

Topology optimization of a plate coupled with acoustic cavity

Optimization of the topology of a plate coupled with an acoustic cavity is presented in an attempt to minimize the fluid–structure interactions at different structural frequencies. A mathematical model is developed to simulate such fluid–structure interactions based on the theory of finite elements. The model is integrated with a topology optimization approach which utilizes the moving asymptotes method. The obtained results demonstrate the effectiveness of the proposed approach in simultaneously attenuating the structural vibration and the sound pressure inside the acoustic domain at several structural frequencies by proper redistribution of the plate material.Experimental verification is carried out by manufacturing topology optimized plates and monitoring their vibration and sound radiation into a rigid acoustic cavity. The measured sound pressure and plate vibration are found to be in good agreement with the predictions of the mathematical model.The presented theoretical and experimental techniques present valuable tools in the design of a wide variety of critical structures which must operate quietly when subjected to fluid loading.     

快速热点火熔奥梯铝炸药燃烧转爆轰实验研究

以熔铸型含铝混合炸药熔奥梯铝为对象，研究铸装含铝混合炸药快速热点火后的燃烧转爆轰特性。建立了快速热点火燃烧转爆轰实验平台，由实验装置（加热装置、约束钢管、炸药）、压力测试系统、光纤测速系统组成；加热装置加热15 mm厚45钢钢板，峰值温度大于1 100 ℃，温升速率为85～95 ℃/s。开展了快速热点火带壳熔奥梯铝炸药燃烧转爆轰实验，由加热装置加热约束钢管内熔奥梯铝炸药，炸药化学反应阵面压力和传播速度分别由压电性高压压力传感器和光纤探针测定；实测阵面压力约1 GPa，传播速度最大约2 600 m/s。由光纤数据获得炸药化学反应阵面传播轨迹，通过特征线方法获得冲击形成点，半定量给出冲击形成距离大于850 mm；并比较了管体破片质量实测值与炸药完全爆轰时破片平均质量计算值，实测值远小于计算值。综合实测化学反应阵面传播速度和压力、冲击形成距离分析、破片质量比较，可确定熔奥梯铝炸药没有发生完全爆轰，其化学反应状态为爆燃。另外，采用Adams和Pack模型、CJ燃烧模型，都能够半定量的预估冲击形成距离和燃烧波后压力，为实验设计提供依据，但CJ燃烧模型的计算结果更接近于实测值。     

Effect of diameter on two-phase pressure drop in narrow tubes

The effect of tube diameter on two-phase frictional pressure drop was investigated in circular tubes with inner diameters of 0.6, 1.2, 1.7, 2.6 and 3.4 mm using air and water. The gas and liquid superficial velocity ranges were 0.01-50 m/s and 0.01-3 m/s, respectively. The gas and liquid flow rates were measured and the two-phase flow pattern images were recorded using high-speed CMOS camera. Unique flow patterns were observed for smaller tube diameters. Pressure drop was measured and compared with various existing models such as homogeneous model and Lockhart-Martinelli model. It appears that the dominant effect of surface tension shrinking the flow stratification in the annular regime is important. It was found that existing models are inadequate in predicting the pressure drop for all the flow regimes visualized. Based on the analysis of present experimental frictional pressure drop data a correlation is proposed for predicting Chisholm parameter “C” in slug annular flow pattern. For all other flow regimes Chisholm’s original correlation appears to be adequate except the bubbly flow regime where homogeneous model works well. The modification results in overall mean deviation of pressure drop within 25% for all tube diameters considered. This approach of flow regime based modification of liquid gas interaction parameter appears to be the key to pressure drop prediction in narrow tubes.     

Measurement of acoustic velocity in the stack of a thermoacoustic refrigerator using particle image velocimetry

Thermoacoustic refrigeration systems generate cooling power from a high-amplitude acoustic standing wave. There has recently been a growing interest in this technology because of its simple and robust architecture and its use of environmentally safe gases. With the prospect of commercialization, it is necessary to enhance the efficiency of thermoacoustic cooling systems and more particularly of some of their components such as the heat exchangers. The characterization of the flow field at the end of the stack plates is a crucial step for the understanding and optimization of heat transfer between the stack and the heat exchangers. In this study, a specific particle image velocimetry measurement is performed inside a thermoacoustic refrigerator. Acoustic velocity is measured using synchronization and phase-averaging. The measurement method is validated inside a void resonator by successfully comparing experimental data with an acoustic plane wave model. Velocity is measured inside the oscillating boundary layers, between the plates of the stack, and compared to a linear model. The flow behind the stack is characterized, and it shows the generation of symmetric pairs of counter-rotating vortices at the end of the stack plates at low acoustic pressure level. As the acoustic pressure level increases, detachment of the vortices and symmetry breaking are observed.     

An experimental study on the interactions between surface waves and floating viscoelastic covers

A possible mathematical ice model for the wave interactions in polar seas was developed based on the assumption that an ice cover behaved as a Voigt viscoelastic material. The dispersion relation was found to depend on the rheological properties of the cover. In the present study, an experimental approach was developed that can enable the verification of the theoretical predictions in the laboratory. The approach utilized the blended mixture of white oil and Polydimethylsiloxane (PDMS) material with various mass percentages of a curing agent, to create a floating layer with a range of targeted viscoelastic properties. Due to the large coverage required for wave flume experiments, special curing procedures were also established for the preparation of PDMS material. The rheological results showed that the mechanical behavior of the floating cover was close to a Voigt material. Experiments were conducted to analyze the wave interactions with the floating viscoelastic cover. The measured data showed an obvious change of wavelength when waves propagated along the cover region. It is observed that the change in wavelength can be linked quantitatively to the viscoelastic properties based on the numerical predictions by Wang and Shen (2010). Some differences were however noted for less viscous covers under longer wave periods. A direct comparison of the PDMS covers with a polypropylene (PP) cover was also performed for verification. Only wave lengthening was observed under the PP cover. With a shear modulus more than three orders of magnitude greater than that of PDMS, the theoretical wavelength for the PP cover from Wang and Shen (2010) is very close to that of the thin elastic plate theory from Fox and Squire (1990). Comparison between these two theoretical results and the measured data again deviated with longer wave periods. In both PDMS and PP cases, edge effects and pitching motion of the covers were present at various degrees. In addition, the materials were not strictly a Voigt type. The small deviation from the idealized rheological behavior could also contribute to differences between theoretical and experimental results.     

Two-dimensional unstructured elastic model for acoustic pulse scattering at solid-liquid interfaces

Abstract. A numerical model to simulate elastic waves and acoustic scattering in two spatial dimensions has been developed and thoroughly tested. The model universally includes elastic solids and liquids. The equations of motion are written in terms of stresses, displacements and displacement velocities for control volumes constructed about the nodes of a triangular unstructured grid. The latter conveniently supports various geometries with complex external and internal boundaries separating sub-domains of different elastic properties. Theoretical dispersion for zero mode symmetric () and antisymmetric () waves in a plate has been reproduced numerically with high accuracy, thus verifying the method and code. Comparison of simulated acoustic pulse scattering at water-immersed steel plate with the respective experiments reveals a very good agreement in such delicate features as excitation of the surface (A) wave. The numerical results explain the peculiar location of the surface wave relative to the other ones in experimental registrations. Examples of acoustic pulse interactions with curvilinear metallic shells in water demonstrate flexibility of the method with respect to complex geometries. Potential applications as well as some directions for further improvement to the technique are briefly discussed. Received 5 September 2002 / Accepted 25 November 2002 Published online 4 February 2003 RID="*" ID="*"Permanent address: Ioffe Physical-Technical Institute, 26 Polytekhnicheskaya, 194021 St. Petersburg, Russia Correspondence to: P. Voinovich (e-mail: vpeter@scc.ioffe.ru)     

Propagation of a pressure wave in two-phase flow with very high void fraction

An experimental and theoretical study of a finite amplitude pressure wave propagating through a two-phase media of about 0.9999–0.99999 void fraction is performed. This two-phase media consists of many parallel liquid films in a gas. The films are perpendicular to the wave propagation direction and result in a two-phase fluid of extremely high void fraction. Experiments are done in a vertical shock tube and show that the shock wave is broken down into an initial sharply rising wave and a second gradually rising wave. The velocity of the first wave agrees well with the theoretical prediction assuming an adiabatic thermal equilibrium change, which approaches the gas sonic velocity in the two-phase flow in the low void fraction region. The second wave is caused by the complex reflection and destruction of the waves.