J. A. Bucaro

Broadband acoustic scattering from a ribbed shell

Monostatic acoustic scattering measurements have been made on a large aspect ratio (L/D=6.2) ribbed cylindrical shell over the frequency range 0.8<ω/ωr<12 (where ωr is the ring frequency for the shell). The ribs are arranged in a near periodic array with small random deviations in the rib locations of magnitude da/a=0.05. The physical processes that give rise to peaks in the scattering cross section are discussed and the ability of various theoretical models to predict the locations of the highlights is examined. The measurements provide further confirmation that the principle sources of scattering for typical ribbed shells in this frequency range are specular reflection, phase matching to fast membrane waves, phase matching to flexural Bloch waves, and Bragg scattering from the ribs. They also reveal that the scattered spectrum of the flexural Bloch waves can be more complicated than originally thought.

Resonant response of complex shell structures

Resonances of shell structures play a prominent role in determining their acoustic characteristics. Among the most important of such characteristics are radiated noise, scattering, and self-noise levels. The nature of the structural resonances, for example the number of modes within a particular frequency band or the overall spatial dependence of the response, determines the importance of these phenomena in various contexts and what steps may be taken to modify their acoustic impact. In the past, knowledge of the resonant response of highly idealized systems has been used to guide the development of quiet platforms. The results of a scale model investigation of the effects of increased complexity on the nature of the resonances of submerged shell structures are reported. Effects presented here are flexural Bloch mode resonances, localized resonances resulting from structural irregularity, and the effects of non-axisymmetry on these phenomena. The implications of these results on acoustic design and the development of systems is discussed.

The effect of internal oscillators on the acoustic response of a submerged shell

The effect of a large number (≈1000) of internal mechanical oscillators on the acoustic backscattering cross section of a submerged ribbed shell has been experimentally investigated. A comparison of the backscattering cross section of the shell with oscillators to that of a simple ribbed shell with no oscillators shows a number of significant effects arising from the internal structure and overall a significantly increased average scattering strength. These results are presented and several physical mechanisms which can account for the salient aspects of the observations are discussed.

Active control of payload fairing interior noise using physics-based control laws

This paper presents results of an overview study that draws on active control technologies developed at the Naval Research Laboratory for submarine structural acoustics and aircraft interior noise problems and explores their use in reducing the interior acoustic levels inside rocket payload fairings. Research in controls at NRL includes wavenumber domain control, structural impedance control, and acoustic boundary control (ABC). ABC employs active blankets comprised of a collocated actuator and pressure-velocity sensor layers. A feedback controller is used to impose a spatially averaged local acoustic impedance at the structure-fluid boundary over the region that the blanket is attached. In the study reported here a high fidelity finite-elementinfinite-element model is used to conduct control simulations using active blankets. In this model, the main rocket body is modeled as a finite cylinder with an attached rib stiffened faking. The interior fluid is modeled with finite acoustic elements while the entire exterior fluid is modeled with finite and infinite acoustic elements. In order to better understand the physics of the problem and identify optimum physical control laws, we first conducted a study to uncover the relevant structural acoustics. Using this understanding, we use our numerical model and examine the performance of ABC active blankets as well as evaluate some simple anti-sound configurations.

Bistatic, above-critical angle scattering measurements of fully buried unexploded ordnance (UXO) and clutter

Laboratory grade bistatic scattering measurements are conducted in order to examine the acoustic response of realistic fully buried unexploded ordnance (UXO) from above-critical angle insonification, between 2 and 40 kHz. A 127 mm diameter rocket UXO, a 155 mm diameter artillery shell, a natural rock of approximately the same size, and a cinder block are fully buried in water-saturated medium grained sand (mean grain diameter, 240 μm) at depths of 10 cm below the water-sediment interface. A two-dimensional array of bistatic scattering measurements is generated synthetically by scanning a single hydrophone in steps of 3 cm over a 1 m × 1 m patch directly above the targets at a height of 20 cm above the water-sediment interface. Three-dimensional volumetric acoustic images generated from the return waveforms reveal scattering components attributed to geometric and elastic scattering, as well as multiple-scattering interactions of returns between the sediment-water interface and the buried objects. The far-field target strength of the objects is estimated through extrapolation of the angular spectrum. Agreement is found between experimental data and simulated data generated from a finite-element-based, three-dimensional time-harmonic model (2-25 kHz). Separation of the measured UXO from the clutter objects is demonstrated through exploitation of structural-acoustics-based features.

Bistatic scattering from submerged unexploded ordnance lying on a sediment.

The broadband bistatic target strengths (TSs) of two submerged unexploded ordnance (UXO) targets have been measured in the NRL sediment pool facility. The targets-a 5 in. rocket and a 155 mm projectile-were among the targets whose monostatic TSs were measured and reported previously by the authors. Bistatic TS measurements were made for 0 degrees (target front) and 90 degrees (target side) incident source directions, and include both backscattered and forward scattered echo angles over a complete 360 degrees with the targets placed proud of the sediment surface. For the two source angles used, each target exhibits two strong highlights: a backscattered specular-like echo and a forward scattered response. The TS levels of the former are shown to agree reasonably well with predictions, based on scattering from rigid disks and cylinders, while the levels of the latter with predictions from radar cross section models, based on simple geometric optics appropriately modified. The bistatic TS levels observed for the proud case provide comparable or higher levels of broadband TS relative to free-field monostatic measurements. It is concluded that access to bistatic echo information in operations aimed at detecting submerged UXO targets could provide an important capability.

Local admittance model for acoustic scattering from a cylindrical shell with many internal oscillators

This paper considers whether local admittance fluctuations can explain the acoustic scattering from a submerged shell with many internal oscillators observed by Bucaro et al. [Proc. ASME, Noise Control and Acoustics Div., NCA 22, 87–92 (1996)] and Photiadis et al. [J. Acoust. Soc. Am. 101, 895–899 (1997)]. The scattering is computed using a Helmholtz-based approach for a locally reacting surface whose admittance is affected by the random placement of internal oscillators. The predicted scattering patterns versus frequency and monostatic scattering angle are shown to agree with what is observed in the measured results.

The structural acoustics and active control of interior noise in a ribbed cylindrical shell

Numerical studies are carried out regarding the structural acoustics of a ribbed aluminum cylindrical shell structure intended to represent the essential structural features of a small aircraft fuselage. Calculations are made to determine both the wall normal displacements and the interior acoustic pressures for the case in which the shell wall is forced dynamically at a point. The structural responses are further decomposed into their frequency‐wave‐number components. Through a series of comparisons between those responses for an infinite shell, the ribbed vacuum‐filled shell, and the ribbed air‐filled shell, the relevant structural acoustic mechanisms are interpreted. The frequencies at which interior acoustic ‘‘resonances’’ are observed are connected to specific mechanisms, including cavity responses driven by structural modes, cavity modes forcing the structure, and mixed structure/air‐cavity modes. Numerically based active control experiments are carried out using end actuation, and the relative performance of this control ‘‘system’’ is compared when operating on the various mode types.

Structural-acoustic modeling for three-dimensional freefield and littoral environments with verification and validation

This paper describes a high-order, finite-element-based, three-dimensional time-harmonic model for large-scale exterior structural-acoustics problems. It is applicable to both freefield and littoral environments. For the freefield case, the infinite exterior is treated as a homogeneous linear acoustic medium. For littoral applications, the water or air and the sediment domains are each treated as linear homogeneous, semi-infinite half-spaces with piecewise-constant properties. Both domains admit complex-valued wave speeds to enable the inclusion of damping. The finite element formulation uses a variational statement which naturally incorporates the transmission-condition at the water or air-sediment interface. The truncation of the infinite exterior is realized using an infinite-element for the freefield case, and the perfectly-matched-layer approximation for littoral applications. Computation of the farfield quantities is done based on an integral representation which, for the littoral cases, uses efficient approximations for the appropriate Greens function. Numerical computations are presented for a series of progressively more complex problems, and are used to verify the model against analytic and other numerical solutions and validate it based on the experimental data for scattering from elastic scatterers as measured in freefield and sediment pool laboratory facilities.

Exploiting forward scattering for detecting submerged proud/half-buried unexploded ordnance.

Laboratory underwater bistatic scattering measurements are reported for free, proud, and half-buried unexploded ordnances for 0 degrees and 90 degrees source angles. Forward echoes are larger than backscattered returns, and half burial significantly decreases the latter but not the former. Results agree with analytic predictions borrowed from radar. The forward echo and source signal are separated by measurements made with and without the target, a method not possible in a target search. For this, a method is described that uses knowledge of the source location and the hyperbolic character in time-cross range of the signals received at points along a line.