Wave Reverberation Research Articles

On the pressure buildup behind an array of perforated plates impinged by a normal shock wave

The pressure buildup behind an array of perforated plates following a shock wave impingement was experimentally studied. The experiments were performed in a shock tube facility and the arrays were varied in the number of their perforated plates and the porosities of the plates. It was found that the pressure buildup behind all of the configurations that were examined displayed similar characteristics. Using simplified assumptions about the nature of the flow through the perforated plates array, we could account for the influence of the volume confined between the plates, and the porosity of the plates. Once accounted for, experiments were performed with argon and SF6, in addition to air, in order to examine the effect of the gas. It was found that the pressure buildup depended on a constant parameter, which was gas-dependent. The constant parameter for each gas was found in this study experimentally. To the best of our knowledge this result has never been reported before. The large spread of different experiments performed throughout the present study enabled the separation of the effects facilitated by the perforated plates array into two distinct processes that affected the pressure buildup at the end-wall. The first was the diffraction of the incident shock wave and subsequent shock reverberations that were found to depend on the number of perforated plates in the array and their porosity. The second was the inhibition of the flow through the perforated plate that was found to depend on the confined volume, type of gas and the porosity of the plates.

Role of wave reverberations in exit vibrations of projectiles and shells

Carlucci et al. (2013) proposed a theoretical model describing exit vibrations of a shell launched by a gun. The model treats the shell as a system of a spring, damper and a rigid mass. In this paper, similar models describing vibrations of shells and monolithic projectiles are compared with some 1D models taking into account wave reverberations. The comparison proves that the models taking into account the wave motion, forecast similar amplitudes and frequencies of vibrations as discrete models. However, details of acceleration time courses differ considerably. The problem of the influence of the friction force between the shell wall and the filling is also discussed. The results of modelling serve as well for assessing if some projectile velocity oscillations detected by making use of the Doppler radar can be solely attributed to the exit vibrations.

In situ impulse response method of oblique incidence sound absorption coefficient with microphone array

The paper reports the development of the methods for measuring oblique incidence sound absorption coefficient in situ. The standing wave tube method and reverberation room method are not suitable for the test in situ. And the transfer function method presented by Allard fails to consider scattering effect of unsmooth surfaces. Based on the use of microphone array, the paper presents an impulse response method to study oblique incidence sound absorption coefficient. The impulse signal generator is designed to simulate pseudo-random noise including many different frequencies. The microphone array can eliminate the effect of reflection and scattering. The paper compares the impulse response methods of in situ measurement in reverberation room and free field, with another method which used in standing wave tube. The specimen is made of polyurethane foam. The results show that the absorption coefficient obtained by the standing wave tube is similar to reverberation room, but smaller than free field. Although the results obtained by three methods are not the same, the three curves reflect the same varying tendency. This in situ method can satisfy the technical requirement of measuring.

Characterization of a local scatterer in a plate from reverberation of flexural waves

In media where acoustic waves are subject to multiple propagation paths (scattering or reverberation), recorded acoustic signals present a random aspect. Still, such signals carry some information about the medium, and ensemble averaging can give access to the estimation of a number of useful parameters. A well-known example, in room acoustics, is the estimation of absorption coefficients of walls from average decrease of the reverberation envelopes. In the work reported here, we have proposed a statistical model allowing to relate the scattering properties of a local heterogeneity (defect) to the average properties of reverberated acoustic signals in a solid plate. A theoretical expression of the averaged envelope of signals produced by scattered and reverberated flexural waves has been derived and both numerically and experimentally validated. A simple curve-fitting procedure applied to signals recorded on a few receivers then allows an experimental estimation of the scattering cross-section of the heterogeneity.

DEM/FEM Simulations of Dynamic Response of Projectile Penetrating into Granular Medium

Dynamic elastic Finite Element Method (FEM) and Discrete element method (DEM) simulations are carried out to investigate dynamic penetration of a projectile into a target of granular medium. It was found that the highly densified region of granular medium was generated just ahead of the projectile and began to propagate spherically with much higher velocity than that of projectile which leaves relatively rarefied medium region. This propagation phenomenon was probably the result of a collision and momentum transfer between particles in target granular medium. The propagation velocity of the densified region decreased during penetration as depending not only on the packing ratio of target medium but also on the projectile velocity. The resistance force of projectile was also investigated in the case of penetration of projectiles with various body lengths. The resistance force increased rapidly and reached to the peak. The peak value was expressed in terms of momentum change of target particles. The resistance force decreased periodically after the peak value. The period clearly depended on the length of projectile. It is obvious that this was caused by the stress wave reverberations in the projectiles with various body lengths.

Experimental Dosimetry in a Mode-Stirred Reverberation Chamber in the 60-GHz Band

This paper aims at introducing a calibration procedure for experimental dosimetry in a millimeter wave reverberation chamber. The proposed procedure is based on surface temperature measurements performed on a parallelepiped phantom using an infrared camera. The temperature rise measurement is compared to the value predicted by an analytical model. The latter is based on heat transfer formulation using the statistically homogeneous and isotropic electromagnetic field generated in the reverberation chamber. Temperature measurements are performed, at 59.3 GHz, on a dielectric phantom constituted of distilled water with a concentration of 4% of agar. Good agreement is observed between experimental and theoretical models. The difference between the peak specific absorption rate values estimated by two different approaches is about 16%.

Shatter cones at the Keurusselkä impact structure and their relation to local jointing

AbstractShatter cones are the only distinct meso‐ to macroscopic recognition criterion for impact structures, yet not all is known about their formation. The Keurusselkä impact structure, Finland, is interesting in that it presents a multitude of well‐exposed shatter cones in medium‐ to coarse‐grained granitoids. The allegedly 27 km wide Keurusselkä impact structure was formed about 1150 Ma ago in rocks of the Central Finland Granitoid Complex. Special attention was paid in this work to possible relationships between shatter cones and local, as well as regionally occurring, fracture or joint systems. A possible shatter cone find outside the previously suggested edge of the structure could mean that the Keurusselkä impact structure is larger than previously thought. The spacing between joints/fractures from regional joint systems was influenced by the impact, but impact‐induced fractures strongly follow the regional joint orientation trends. There is a distinct relationship between shatter cones and joints: shatter cones occur on and against joint surfaces of varied orientations and belonging to the regional orientation trends. Planar fractures (PF) and planar deformation features (PDF) were found in three shatter cone samples from the central‐most part of the impact structure, whereas other country rock samples from the same level of exposure but further from the assumed center lack shock deformation features. PDF occurrence is enhanced within 5 mm of shatter cone surfaces, which is interpreted to suggest that shock wave reverberation at preimpact joints could be responsible for this local enhancement of shock deformation. Some shatter cone surfaces are coated with a quasi‐opaque material which is also found in conspicuous veinlets that branch off from shatter cone surfaces and resemble pseudotachylitic breccia veins. The vein‐filling is composed of two mineral phases, one of which could be identified as a montmorillonitic phyllosilicate. The second phase could not be identified yet. The original composition of the fill could not be determined. Further work is required on this material. Observed joints and fractures were discussed against findings from Barringer impact crater. They show that impact‐induced joints in the basement rock do not follow impact‐specific orientations (such as radial, conical, or concentric).

Reverberation of flexural waves scattered by a local heterogeneity in a plate.

A statistical model is proposed to relate the scattering properties of a local heterogeneity in a plate to the statistical properties of scattered and reverberated flexural waves. The contribution of the heterogeneity is isolated through the computation of differential signals consisting of a subtraction of the signals recorded after and before introduction of the heterogeneity. The theoretical expression of the average reverberation envelope of these differential signals is obtained as a function of the scattering cross-section of the heterogeneity. Successful numerical and experimental validations in various cases of canonical heterogeneities with known scattering cross-sections are shown. These satisfying results offer a way to estimate the scattering cross-section of an unknown scatterer from the reverberated differential signals.

Delamination detection in foam core composite structures using transient flexural wavefields

This paper investigates a health monitoring technique for foam sandwich structures based on flexural wavefields imaged by Laser Doppler vibrometry. A study of calibrated artificial defect responses in harmonic regime demonstrates that the use of a low frequency regime (below 30kHz) makes it possible to excite defects in their first flexural resonance modes. The analysis performed in harmonic regime is used to interpret signature patterns obtained with accumulated energy maps from transient wavefield recordings. The potential of the technique is demonstrated on a real impact-induced defect. The robustness of the method relatively to the excitation center frequency selection and to the presence of wave reverberation is demonstrated.

The Hugoniot and Strength of Ultem 1000 Polyetherimide

Parallel-plate impact studies using a single stage powder gun have been performed to investigate the shock and subsequent release behavior of the commercial polyetherimide polymer Ultem™ up to 13 GPa. Two different experimental configurations were used to observe both the shock and unloading behavior. In one configuration, the unloading was continuously tracked by observing transmitted wave profiles in the Ultem samples through a transparent interferometer window. In the other configuration, the unloading was inferred from observing stress wave reverberations in an elastic target plate resulting from impact by the Ultem sample. The loading behavior indicated by the two methods agreed very well and the resulting Hugoniot was represented by US = 2.42 + 1.61*uP. This study also demonstrated that the plate reverberation method of following the unloading response, though not observing the continuous unloading of the sample, agrees extremely well with the unloading response recorded using continuous data obtained using interferometry windows. The results are used to build a case that the strength τ of Ultem increases with shock stress from 0.03 to 0.08 GPa over the range investigated. Furthermore, an investigation of the ratio of the release wave velocity to the shock wave velocity indicates that a transition to bulk liquid (no strength) behavior is not achieved until Hugoniot strains exceed 0.35 for amorphous polymers such as Ultem.

Electrical resistance of high-pressure phases of tin under shock compression

The normalized electrical resistance R/R0 of tin for different pressures of shock compression p is measured. The resultant dependence R/R0(p) differs significantly from the known dependences for static and quasi-isentropic compression and demonstrates the growth of the electrical resistance with increasing pressure. The dependence has an inflection testifying to a phase transition. The inflection corresponds to pressures of 4.7–5.3 GPa in the dielectric embracing a thin sample and to pressures of 8.4–9.6 GPa in the first shock wave. The latter parameters qualitatively agree with the characteristics of the phase transition β-Sn → γ-Sn. The first shock wave in tin determines the final electrical resistance of the sample after wave reverberation. The experimental data obtained in this study are indicative of the kinetic behavior of the electrical resistance in the transition β-Sn → γ-Sn, which is accompanied by generation of crystalline structure defects with a characteristic time greater than 1 μs. A drastic increase in the electrical resistance of the sample in the expansion wave is observed. This increase is attributed to tin melting.

Elastic features visible on canonical targets with high frequency imaging during the 2014 St. Andrews Bay experiments

During the 2014 St. Andrews Bay experiments some canonical metallic targets (a hollow sphere and some circular cylinders) were viewed with a synthetic aperture sonar (SAS) capable of acquiring data using a 110–190 kHz chirped source. The targets rested on mud-covered sand and were typically at a range of 20 m. Fast reversible SAS processing using an extension of line-scan quasi-holography [K. Baik, C. Dudley, and P. L. Marston, J. Acoust. Soc. Am. 130, 3838–3851 (2011)] was used to extract relevant signal content from images. The significance of target elastic responses in extracted signals was evident from the frequency response and/or the time-domain response. For example, the negative group velocity guided wave enhancement of the backscattering by the sphere was clearly visible near 180 kHz. [For a ray model of this type of enhancement see: G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994).] In another example, the timing of a sequence of near broadside echoes from a solid aluminum cylinder was consistent with reflection and internal reverberation of elastic waves. These observations support the value of combining reversible imaging with models interpreted using rays. [Work supported by ONR and SERDP.]

A modified split Hopkinson torsional bar system for correlated study of τ – γ relations, shear localization and microstructural evolution

The conventional split Hopkinson torsional bar (SHTB) system consists of two bars, which can successfully produce the data for the construction of dynamic torsional shear stress and strain relationships. However, the system cannot provide reliable information on the progression of the deformed micro-structure during the test. The reverberation of waves in the bars and the tested specimen can spoil the microstructural pattern formed during the effective loading. This paper briefly reviews a modified version of the SHTB system consisting of four bars that has been developed. This modified system can eliminate the reverberation of waves in the specimen and provide only a single rectangular torsional stress pulse, thus it can properly freeze the microstructure formed during the effective period of loading in the specimen. By using the advantage of the modified SHTB system, together with a new design of specimen, it is possible to perform a correlated study of the dynamic stress-strain response, shear localization and the evolution of the microstructure at a fixed view-field (position) on a given specimen during the sequence of the loading time. The principles, experimental set-up and procedure, calibration and some preliminary results of the correlated study are reported in this paper.

Scattering of teleseismic P-waves by the Japan Trench: A significant effect of reverberation in the seawater column

We detected a scattered wave train in data from the high-sensitivity seismograph network in Japan (Hi-net) following the arrival of the near-vertically incident P-wave generated by the 2009 earthquake (Mw 7.8) off the South Island of New Zealand. The scattered wave train represented predominantly vertical ground motion at a period of 20 to 50 s and with an apparent velocity of 3.5 km/s; it propagated cylindrically westward through the Kanto area of central Japan. Array analysis showed that the scattered wave train developed beneath the Pacific Ocean near the Boso triple junction, southeast of the Kanto area. A 3D finite-difference simulation of seismic wave propagation using a high-resolution model incorporating subsurface structure, topography, and bathymetry revealed that the strong scattered waves that were generated along the Japan Trench and propagated normal to the trench axis represented multiple reverberations of seismic waves between the seafloor and the Pacific plate boundary. In addition, strong reverberation of acoustic waves in the seawater column above the Boso triple junction causes elongated scattered waves, which reasonably explains our observations.

Equation of state of polytetrafluoroethylene for calculating shock compression parameters at megabar pressures

Semi-empirical equations of state (thermal and caloric) are obtained to calculate not only the kinematic parameters (shock wave velocity, particle velocity, and reverberation of waves) but also the thermodynamic parameters (temperature, pressure, and compression) of monolithic and porous polytetrafluoroethylene at high shock pressures. The equations of state are used to model wave interaction in shock-wave experiments using the developed hydrocode. The equations are verified by comparison simulation results with published results of experiments and the data of our shock compression tests of solid and porous samples of PTFE in the range of 10–170 GPa.

Ultrasonic metal sheet thickness measurement without prior wave speed calibration

Conventional ultrasonic mensuration of sample thickness from one side only requires the bulk wave reverberation time and a calibration speed. This speed changes with temperature, stress, and microstructure, limiting thickness measurement accuracy. Often, only one side of a sample is accessible, making in situ calibration impossible. Non-contact ultrasound can generate multiple shear horizontal guided wave modes on one side of a metal plate. Measuring propagation times of each mode at different transducer separations, allows sheet thickness to be calculated to better than 1% accuracy for sheets of at least 1.5 mm thickness, without any calibration.

Numerical Simulation of the Rock SHPB Test with a Special Shape Striker Based on the Discrete Element Method

A split Hopkinson pressure bar (SHPB) system with a special shape striker has been suggested as the test method by the International Society for Rock Mechanics (ISRM) to determine the dynamic characteristics of rock materials. In order to further verify this testing technique and microscopically reveal the dynamic responses of specimens in SHPB tests, a numerical SHPB test system was established based on particle flow code (PFC). Numerical dynamic tests under different impact velocities were conducted. Investigation of the stresses at the ends of a specimen showed that the specimen could reach stress equilibrium after several wave reverberations, and this balance could be maintained well for a certain time period after the peak stress. In addition, analyses of the reflected waves showed that there was a clear relationship between the variation of the reflected wave and the stress equilibrium state in the specimen, and the turning point of the reflected wave corresponded well with the peak stress in the specimen. Furthermore, the reflected waves can be classified into three types according to their patterns. Under certain impact velocities, the specimen deforms at a constant strain rate during the whole loading process. Finally, the influence of the micro-strength ratio ( $${{\tau_{\text{c}} } \mathord{\left/ {\vphantom {{\tau_{\text{c}} } {\sigma_{\text{c}} }}} \right. \kern-0pt} {\sigma_{\text{c}} }}$$ ) and distribution pattern on the dynamic increase factor (DIF) of the strength DIF were studied, and the lateral inertia confinement and heterogeneity were found to be two important factors causing the strain rate effect for rock materials.

A New Method to Measure the Absorption Coefficient Based on the Sound Pressure Delaying

Absorption coefficient is an important parameter of the absorption function of the absorption material. Traditional measurement methods of absorption coefficient are standing wave tube and reverberation which have some shortcomings. In this paper, phase of the sound pressure measured by two equal distance microphones placed in the front of the absorption material is delayed in order to attain the absorption coefficient. At the last, an experiment for one absorption material is carried out, the experiment results compare with the results of the other methods above mentioned that denotes that the theory is correct and practicable. Key words: absorption material standing wave reverberation time delaying

A Binomial Model for Radiated Immunity Measurements

We propose a statistical analysis of immunity testing in EMC based on binomial distributions. This approach aims at extracting the immunity properties of a device from its probability of failure during a test. We show that under certain conditions, this approach can be applied to plane wave testing environments and reverberation chambers. This approach allows one to control the uncertainty of the immunity level estimation and to reduce the duration of a test by both reducing significantly the number of observations needed to reach a given uncertainty budget and giving an optimal number of power level tested. We show the benefits of such an approach for immunity testing and we present some experimental results.

Spatial cross-correlation modeling for propagation channels in indoor distributed antenna systems

Abstract In this contribution, the spatial cross-correlation of composite channels in a distributed antenna system (DAS) is studied for the case that a nine-antenna DAS is deployed in an indoor environment. As few measurement data of DAS propagation channels are available, the propagation graph modeling based on the electromagnetic wave reverberation theory is used to generate the synthetic impulse responses (IRs) for the composite channels induced by the different groups of antennas in the DAS. The simulations take into account the geographic parameters of the environment, the cluttering scatterers, the realistic visibility conditions, as well as the flexible locations for the antennas, and the user equipment. The uncorrelated scattering assumption is proved to be valid by using the real measurement data from the indoor environments. Based on this assumption, narrowband channel cross-correlation is computed by integrating the small-scale fading cross-correlation within the same delay bins in two channel IRs. The characteristics of the cross-correlation in four cases with different antenna grouping are investigated for the nine-antenna DAS. Part of the modeling results are shown to be consistent with the empirical counterparts for specific DAS constellations calculated using the real measurement data.