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Cylindrical containers, such as liquid tanks and pressure vessels, are ubiquitous in storage applications. Traditional lines of non-destructive evaluation (NDE) are mostly focused on the integrity of containers, but studies on solid contents within using external sensors are lacking. In previous work, metrics were developed to estimate the structural integrity of mock-up fuel assemblies inside a lab-scale nuclear dry storage cask. Linear acoustic resonance techniques were shown to be sensitive down to single assembly level. In this work, this problem is further examined by studying contact nonlinearity in a simplified system using Nonlinear Ultrasound Resonance Spectroscopy (NRUS). This system consists of a single layer of identical spheres with varying composition and size evenly distributed at the bottom of a cylindrical aluminum container. The resonance frequency shifts due to varying amplitudes were mostly affected by the total mass of spheres inside, while diameter and composition of spheres played minor roles. A phenomenological model was developed based on the resulting shifts and was studied numerically using finite element simulations. The agreement between simulations and experiments suggests that the contact nonlinearity is predominated by a contact loss mechanism. This NRUS technique may complement linear acoustic techniques for solid cargo NDE inside sealed vessels.

In many countries, permanent repositories for high burnup spent nuclear fuel assemblies have yet to be established. As a result, much of the spent fuel from nuclear reactors is contained within dry storage casks, with many being beyond their designed service life. The assessment of the internal structural integrity of these casks and the fuel assemblies contained therein is of critical importance for both extended storage and transport to permanent repositories. The large size, structural complexity, and inaccessibility of the interior of the casks make this task challenging. To address these difficulties, a 1:6 scaled model based closely on the design of a Transnuclear (TN-32) dry storage cask was fabricated to facilitate controlled studies of these structures in the laboratory. Vibrational spectroscopy was used to evaluate the state of the cargo and internal structures within the cask utilizing only measurements on the outer surface. Using modes identified through Finite Element modeling corresponding to those previously measured on a full-scale TN-32 cask, we report on the development of amplitude- and phase-based metrics that are sensitive to internal conditions in the lab cask. Steel rod bundles and steel shot were used as surrogates for intact and damaged fuel assemblies, and various internal configurations of these materials were investigated. The metrics were based on acquired spectra involving the (1, 2) global bending mode and the (2, 1) radial-with-shearing mode. The results show that the metrics are sensitive to the condition of a single assembly and have some ability to determine the locations of damaged and empty slots.

We observe that our experimentally measured emission power enhancement of a speaker inside a previously proposed metacavity agrees with our numerically calculated enhancement of the density of states (DOS) of the source-cavity system. We interpret the agreement by formulating a relation between the emitted sound power and the acoustic DOS. The formulation is an analog to Fermi's golden rule in quantum emission. The formulation complements the radiation impedance theory in traditional acoustics for describing sound emission. Our study bridges the gap between acoustic DOS and the acoustic Purcell effect for sound emission enhancement.

The assessment of the internal structural integrity of dry storage casks for used high burnup nuclear fuel assemblies is of critical importance before transporting these to permanent repositories. The large size and structural complexity of the Transnuclear-32 (TN-32) cask as well as the inability to access its interior make this a challenging task. To address these difficulties, we use an active acoustics approach to develop metrics that are sensitive to the internal configuration of these casks. A 6:1 scaled model of the TN-32 cask was constructed in order to study the internal configuration of the fuel assemblies including various damage scenarios. Each mock-up fuel assembly consists of bundled steel rods, and their structural failure is mimicked by steel shot of equal weight. This talk will report the amplitude- and phase-based active acoustics metrics that we developed to characterize different levels of internal damage. Our studies indicate that vibrometric signatures of various internal conditions can be measured using sources and sensors mounted on the exterior shell. Our current methodology is sensitive enough to detect structural failures at the single fuel assembly level. [Work supported by DOE NEUP Award No. DENE0008400.]

The assessment of the internal structural integrity of dry storage casks with high burnup nuclear fuel assemblies is of critical importance for their transport to permanent repositories and during their extended storage service lives. The large size, structural complexity, and inability to access the interiors of the casks make this task challenging. The aim of this work is to evaluate the ability of vibrational characterization to assess the state of the internal cargo (i.e., fuel assemblies) in a cask strictly using measurements on the outer surface. In this study, we report on the identification of five modes of an unloaded full-scale Transnuclear-32 (TN-32) cask and examine the potential of using these modes to assess the internal configuration in a loaded condition. Vibrational spectra were acquired using impulse and continuous-wave techniques on both storage and transportation configurations from which the modal frequencies and the associated quality factors were determined. A finite element model of the TN-32 was constructed and used to identify the observed modes. The model was extended to include loaded configurations to assess how the presence of assemblies impacts the modal structure of the cask and its surface vibrations.

Acoustic based Hostile Fire Detection Systems (HFDS) depend on multi-element arrays of microphones which are simultaneously sampled at sample rates often in the 10–50 ksps range. A driving component of the electronic hardware complexity of the system is the signal buffering, filtering, and analog to digital conversion required for each microphone channel. Digital MEMS microphones consolidate these functions to within the microphone element giving a digital output representative of the measured acoustic signal. Digital MEMS microphones allow for a completely digital hardware solution which reduces complexity and allows more flexibility in utilizing higher element number arrays if desired. The performance parameters relevant to HFDSs of current best-in-class digital MEMS microphones were measured and compared to current analog microphones commonly utilized in these systems. The purpose of these tests was to determine the viability of digital MEMS microphones as an alternate acoustic sensing element for a HFDS system being implemented on a multi-rotor drone.

The assessment of the internal structural integrity of dry storage casks for used high burnup nuclear fuel assemblies is of critical importance before these are transported to permanent repositories. The size of the casks (5.2 m in height and 2.4 m in diameter), structural complexity and the inability to access the interior make this a challenging task. This project addresses these difficulties through a multi-modal approach involving nuclear, charged particle and acoustic methods. This paper reports on measurements of the vibrational spectra and mode shapes using a 6:1 scaled TN-32 model cask constructed in house. 2-D vibrometric scans were performed with a laser Doppler vibrometer to measure the vibrational mode shapes exhibited by the intact cask. Good agreement is observed between the experimentally measured mode shapes on both full-scale and scaled cask and those determined numerically using a detailed finite element model. Several modes identified will be important in assessing the internal characteristics from external measurements. Furthermore, compared to linear techniques, nonlinear responses acquired suggest higher sensitivity to changes in internal structure.

The assessment of the internal structural integrity of dry storage casks for used high burnup nuclear fuel assemblies is of critical importance before these are transported to permanent repositories. The size of the casks (5.2 m in height and 2.4 m in diameter), structural complexity and the inability to access the interior make this a challenging task. This project addresses these difficulties through a multi-modal approach involving nuclear, charged particle and acoustic methods. This paper reports on linear and nonlinear vibrational spectra of an unloaded full-scale TN-32 cask. These studies use both impulsive and swept continuous-wave excitations with a variety of sensor placement and cask configurations. From the resulting spectra, resonant frequencies, quality factors and harmonic responses of various vibrational modes were determined. A detailed finite element model of the TN-32 was constructed and the experimental results are compared to the modal structure determined numerically.

The assessment of the internal structural integrity of dry storage casks for used high burnup nuclear fuel assemblies is of critical importance before these are transported to permanent repositories. The size of the casks (5.2 m in height and 2.4 m in diameter), structural complexity and the inability to access the interior make this a challenging task. This project addresses these difficulties through a multi-modal approach involving nuclear, charged particle and acoustic methods. In this work, we report on measurements of the vibrational spectra and mode shapes using a 6:1 scaled TN-32 model cask constructed in our lab. 2-D vibrometric scans were performed with a laser Doppler velocimeter to measure the vibrational mode shapes exhibited by the cask. Good agreement is observed between the experimentally measured mode shapes and those determined numerically using a detailed finite element model. Several modes are identified that will be important in assessing the internal characteristics from external mea...

The assessment of the internal structural integrity of dry storage casks for used high burnup nuclear fuel assemblies is of critical importance before these are transported to permanent repositories. The size of the casks (5.2 m in height and 2.4 m in diameter), structural complexity, and the inability to access the interior make this a challenging task. This project addresses these difficulties through a multi-modal approach involving nuclear, charged particle, and acoustic methods. In this talk, we report on linear and nonlinear vibrational spectra of intact TN-32 casks. These studies use both impulsive and swept continuous-wave excitations with a variety of sensor placement configurations. From the resulting spectra, resonant frequencies, quality factors, and harmonic responses of various vibrational modes were determined. A detailed finite element model of the TN-32 was constructed and the experimental results are compared to the modal structure determined numerically. [This work was supported by DOE ...

Due to background noise issues, effective implementation of an acoustic array as part of a Hostile Fire Detection System on moving vehicles has been a significant challenge to date. A new method utilizing both software and hardware design parameters has been developed and successfully demonstrated to be very effective at high speeds on various vehicles. This method has been shown to be effective against both flow based noise sources and acoustic wave based noise sources in low signal to noise ratio conditions. The technology will be discussed and results from measurements utilizing conformally mounted acoustic sensors on moving ground vehicles will be shown.

This paper investigates the wind noise pressure spectra measured by aerodynamically designed devices in turbulent flow. Such measurement probes are often used in acoustic measurements in wind tunnels to reduce the pressure fluctuations generated by the interaction of the devices with the incident flow. When placed in an outdoor turbulent environment however, their performance declines noticeably. It is hypothesized that these devices are measuring the stagnation pressures generated by the cross flow components of the turbulence. Predictions for the cross flow contribution to the stagnation pressure spectra based on measured velocity spectra are developed, and are then compared to the measured pressure spectra in four different probe type devices in windy conditions outdoors. The predictions agree well with the measurements and show that the cross flow contamination coefficient is on the order of 0.5 in outdoor turbulent flows in contrast to the published value of 0.15 for measurements in a turbulent jet indoors.

An aerostat based acoustic array data collection system was deployed at the NATO TG-53 "Acoustic Detection of Weapon Firing" Joint Field Experiment conducted in Bourges, France during the final two weeks of June 2008. A variety of impulsive sources including mortar, artillery, gunfire, RPG, and explosive devices were fired during the test. Results from the aerostat acoustic array will be presented against the entire range of sources.

Miltec Research & Technology along with the National Center for Physical Acoustics, Raspet Flight Research Laboratory, and Mississippi State University Aerospace Department have performed a detailed study of the benefits and issues associated with the implementation of an array of acoustic sensors on airborne platforms both moving and stationary. In order to facilitate this a platform has been developed for the testing of airborne acoustic arrays used in detection, tracking, and identification of objects of interest. The test bed has been selected such that it is optimized for this effort within reasonable limitations on complexity and costs. This approach allows for the determination of best case performance parameters for acoustic arrays on aerial platforms. Issues related to the design of the platform as well as measured results will be presented.

We report on the measurements of the pressure fields set up inside a water‐filled steel shell over a spectral range (10–50 kHz) that includes 75 modes of the composite system. The 9.5‐in.‐diameter shell has a wall thickness to radius ratio of h/R = 0.158, which is beyond the thin shell limit. Hydrophone scans of the acoustic pressure fields in the fluid were performed, from which 46 modes of the composite resonator were identified. The modal frequencies are in good agreement with those predicted by Mehl for a fluid‐filled shell of arbitrary wall thickness. The radial pressure profiles of the modes are shown to fit the expected spherical Bessel profiles for solutions of the Helmholtz equation under spherical symmetry. The results are interpreted in terms of the idealized rigid shell and empty shell limits.

As part of an overall effort to place acoustic sensors on airborne platforms for surveillance applications, analysis studies were performed of acoustic probe windscreens that could minimize wind noise and be made relatively insensitive to small angle of attack and sideslip excursions. Through a collaborative effort between Miltec Research & Technology, Mississippi State University Department of Aerospace Engineering, Raspet Flight Research Laboratory, and the Engineering Research Center several probe designs were developed and analyzed using inviscid and viscous simulations with the objective of characterizing their flow fields and identifying potential modifications that could improve suitability for use as acoustic measurement probes. Several of the recommended probe shapes were fabricated and tested under real world conditions on an acoustically optimized 1/3 scale Schleicher ASH26 glider test bed.

One means of driving an oscillatory mode of a system is to modulate a parameter upon which the natural frequency of the mode depends. This ‘‘parametric excitation’’ occurs most readily if the modulation frequency is twice the frequency of the mode. In addition, the modulation amplitude must exceed a threshold that depends upon the dissipation of the mode. The response amplitude then grows exponentially until it is saturated by a nonlinearity of the system, so large amplitudes are possible. Previous efforts to parametrically excite an acoustic mode will be briefly described, and the theory for a length‐modulated tube will be discussed. An analysis of this case indicates that it is feasible to parametrically excite the fundamental mode. Results of an experiment that is currently in progress will be presented. The tube has ambient length 2.0 m and diameter 30 cm, and the length is modulated by low‐frequency loudspeakers at the ends. Sulfur hexafluoride gas is employed in order to decrease the required drive ...

Probe microphone measurements of air‐borne sound penetrating into air‐filled sands and soils indicate two absorption coefficients for the frequency range 40–4000 Hz. The probe microphone signal attenuates rapidly with depth in a region near the surface. Below that region the microphone signal attenuates significantly slower with depth. Rigid‐capillary‐tube porous models, which allow for pore‐fluid motion only, correctly describe the rapid attenuation of probe pressure. Using these models, pore properties (tortuosity and air permeability) are typically deduced from the measured complex absorption coefficient. The Biot poro‐elastic‐capillary tube model describes both attenuation regimes. The two absorption coefficients are associated with the Biot type I and II waves. The large attenuation in the region near the surface is associated with the Biot type II wave. The much smaller attenuation coefficient is a consequence of the elasticity of the matrix and is associated with the Biot type I wave. Biot’s model ...