Aldo A. Ferri

Scattering of plane waves from submerged objects with partially coated surfaces

This paper addresses the scattering of acoustic plane waves from submerged objects that are partially covered with a compliant coating. Beam incidence of a planar wave on cylinders of finite and infinite length are considered. The infinite cylinder is analyzed using a two‐dimensional collocation technique, while a three‐dimensional boundary element technique is used to study the finite cylinder. The coated and uncoated surfaces are modeled by assuming that the specific local impedance is constant along arcs of the cylindrical surfaces. It is shown that discontinuities in the impedance factor between adjacent regions leads to large pressure and normal velocity gradients on the surface in the vicinity of the discontinuity, but such effects have little influence on the scattered far‐field pressure.

Sensitivity analysis of coupled structural-acoustic problems using perturbation techniques

The analysis of acoustic radiation and scattering from submerged elastic structures is an important and challenging problem. Often, numerical solutions are hampered by the fact that the acoustic pressure field can be very sensitive to structural detail. In this paper, a procedure for obtaining the sensitivity of a structural-acoustic system to structural perturbations is presented. The theoretical development assumes a finite element description of the structure and a boundary element description of the fluid. The analysis includes full coupling between the fluid and the structure, and can accommodate both internal and/or external fluid regions. The types of perturbations are limited to those that do not alter the geometry of the wetted surface, but are otherwise completely general in nature. The technique is then applied to acoustic scattering from a cylindrical shell. Two perturbations are considered: A change in the shell thickness and a change in the elastic modulus. The effect of these structural per...

Effect of Far-Field Structure on Joint Properties

Classical structural analysis techniques have proven time and time again to be remarkably accurate for systems consisting of a single, continuous piece of material. Unfortunately, nearly all real engineering structures are assembled from multiple parts, joined by bolts, rivets, or other fasteners, and these joints introduce nonlinearities and uncertainties into systems’ structural stiffness and damping. Nonlinear damping due to jointed connections in particular is critical to limiting the resonant response of a structure, yet it remains poorly understood. This work seeks to understand the degree to which joint properties are dependent on the rest of the structure. The testable hypothesis is that the boundary conditions and the far-field structure itself (i.e. distribution of the stiffness and mass) change the way in which the interface is loaded, thus altering the perceived or deduced nonlinear properties of the mechanical joint. This hypothesis is investigated using experimental impact hammer testing methods in order to understand the extent to which alteration in the boundary conditions and far-field structure change the interface properties as well as the underlying mechanics during loading. Numerical tools are also employed to investigate and complement the experimental results, focusing on two fronts: replicating the experimental results with discrete joint models, and investigating joint loading for different modes using numerical modal analysis.

Comparison of time‐integration schemes for fluid‐structure interaction.

Computational structural‐dynamics codes invariably use finite element (FE) methods and relatively course meshes, whereas finite‐difference methods with fine meshes are popular for modeling the fluid domain. Fluid‐structure interaction (FSI) problems in which there is a mean flow feature can be addressed by simultaneously employing both techniques. Because of the dissimilarities of the two formulations, a time‐domain solution is most readily obtained by allowing each code to march forward in time in a loosely coupled manner. A proper FSI implementation must address the physical and computational issues associated with compatibility of displacements and surface tractions at the boundary of the two domains. It is also necessary that one identifies a suitable numerical scheme to perform unsteady time‐marching simulations of the coupled system. It is the latter issue that concerns this paper. Various techniques for accurate and stable time integration of loosely coupled systems are compared. A two‐dimensional ...

Identification of very closely spaced modes using an iterative algorithm

The algorithm of mode isolation is a frequency domain method for processing measured response data in order to identify the modal properties of a system. It has been shown [M. V. Drexel and J. H. Ginsberg, Proceedings of the 19th IMAC, Orlando, FL, 5–8 February 2001] to accurately evaluate a pair of damped modes whose natural frequencies differ by an amount that is commensurate with the bandwidth of either mode. Here, SIMO measurement of a two‐degree‐of‐freedom system is simulated analytically by adding white noise to the computed response. This system is selected because the natural frequency difference can be made as small as desired by adjusting a parameter that has little effect on the modal damping ratios in the range of interest. Analytically, the two normal modes are unique and mutually orthogonal if the frequency difference is nonzero, while repeated frequencies lead to two arbitrary, and not necessarily mutually orthogonal, modes. The present work explores the degree to which AMI tracks the analy...

The effect of large structural perturbations on the response of structural‐acoustic systems

Due to high sensitivity, first‐order perturbation analysis of structural‐acoustic systems can be inaccurate for large perturbations. Using high‐order derivatives to create a Taylor series approximation for the perturbed solution can result in slow convergence or even divergence. A rational polynomial or Pade approximation may overcome the poor convergence of the Taylor series by canceling out the pole causing the poor convergence. In this study, a finite element framework is used to describe the structural‐acoustic system. External radiation and scattering problems are accommodated by truncating the infinite fluid region using exponential decay infinite elements. Changes to a nominal model are introduced through a structural perturbation that modifies the nominal structural stiffness matrix. An efficient method for calculating the solution derivatives with respect to the structural perturbation is presented. A Taylor series expansion is constructed using the derivative information and the convergence crit...

Structural sensitivity analysis in structure‐fluid interaction problems

Using a finite‐element formulation with infinite elements to approximate the external fluid, well‐developed structural sensitivity analysis techniques can be applied to acoustic radiation and scattering problems. Changes to a nominal structure are introduced through a structural perturbation matrix, eΔS, which modifies the system matrix, S, of the nominal system. The perturbed surface pressures and normal velocities are expressed as a binomial series in ΔS. The convergence criteria for the series is examined for structural‐acoustic systems. A perturbation consisting of a change in the Young’s modulus is examined for two numerical examples. First, a one‐dimensional problem of plane‐wave scattering from a viscoelastic layer is examined. The results of the 1‐D problem are then extended to the multidimensional problem of plane‐wave scattering from a cylindrical shell. The perturbations consist of varying the Young’s modulus of the layer and the shell. It is shown that the convergence of the binomial series is...

Multiple scattering from submerged bodies with dissimilar acoustical properties. Part II. Experimental results

In Part I [Turek et al.] a numerical solution for a two‐body acoustical multiple scattering problem based on the analytical infinite series solution of the wave equation was presented. Surface and far‐field solutions involv‐ ing permutations of the two ‘‘degenerate’’ boundary conditions (pressure release and rigid) were compared to solutions obtained with the combined Helmholtz integral equation formulation problem (CHIEF) program. In this paper the case in which one of the spheres is fully elastic and the other pressure release is modeled and the results verified experimentally.

Multiple scattering from submerged bodies with dissimilar acoustical properties. Part I. Local impedance models

A numerical solution for a two‐body acoustical multiple scattering problem based on the analytical infinite series solution of the wave equation was developed. FORTRAN 77 codes implementing this solution were written which are capable of simulating the case of two spheres of arbitrary radius and distinct material properties subject to an acoustic plane wave of arbitrary incidence. Far‐field solutions involving permutations of the two ‘‘degenerate’’ boundary conditions (pressure release and rigid) were compared to solutions obtained with the combined Helmholtz integral equation formulation problem (CHIEF) program. Surface pressures and velocities were also calculated. [Work supported by ONR.]

Radiation sensitivity of baffled plates to structural perturbation

The analysis of acoustic radiation and scattering from submerged elastic structures is an important and challenging problem. Often, numerical solutions are hampered by the fact that the acoustic pressure field can be very sensitive to structural detail. In this presentation, a baffled plate submerged in a semi‐infinite acoustic medium is used study this sensitive. A finite element description of the structure and a boundary element description of the fluid are used to model the plate‐fluid‐baffle arrangement. The sensitivity information is obtained by applying perturbation techniques to the matrix equations that arise, yielding exact derivatives of the chosen field variable. Several types of local structural perturbations (point masses, springs, dampers, and ribs) as well as global perturbations (Young’s modulus, density, damping) are considered. The nature of the structural sensitivity is examined and some investigation is made into what types of structural features have the greatest impact on radiation and scattering. Finally, the use of the sensitivity information for engineering and design is discussed.