Aeroacoustic Sensitivity Analysis and Its Application for Noise Barrier Design

An automated design optimization method is demonstrated by computationally modeling the wheel-on-rail noise. The noise propagation is simulated by solving the wave equation using the boundary element method and compared with field test data. Then, the computational model was coupled with an optimization methodology through finite-differenced sensitivities. Using this method, a number of new designs was obtained and evaluated for their relative effectiveness. Three new designs were found to improve the insertion loss within the design space delineated by the given geometrical constraints. Finally, for improved computational efficiency, a continuous adjoint sensitivity methodology was applied to the wave equation to obtain the shape sensitivity derivatives of the acoustic pressure field. Its accuracy and validity are demonstrated for objectives defined on the barrier surface and out in the acoustic field.

Parallel Iterative Methods for the Helmholtz Equation With Exact Nonreflecting Boundaries

Parallel iterative methods for fast solution of large-scale acoustic radiation and scattering problems are developed using exact Dirichlet-to-Neumann (DtN), nonreflecting boundaries. A separable elliptic nonreflecting boundary is used to efficiently model unbounded regions surrounding elongated structures. We exploit the special structure of the non-local DtN map as a low-rank update of the system matrix to efficiently compute the matrix-by-vector products found in Krylov subspace based iterative methods. For the complex non-hermitian matrices resulting from the Helmholtz equation, we use a distributed-memory parallel BICG-STAB iterative method in conjunction with a parallel Jacobi preconditioner. Domain decomposition with interface minimization was performed to ensure optimal interprocessor communication. For the architectures tested, and using the MPICH version of MPI, we show that when implemented as a low-rank update, the non-local character of the DtN map does not signicantly decrease the scale up and parallel eciency versus a purely approximate local boundary condition.

An Active Flow Distortion Control System for Serpentine Inlets

An active flow control system for reducing distortion in serpentine inlets was developed using non-intrusive microphones as feedback sensors. While the serpentine inlet can provide large benefits to an air vehicle by reducing its overall size and therefore weight, it unfortunately delivers a non-uniform (spatially-distorted) flow to the compressor due to the formation of a secondary flow created by separation of the turbulent boundary layer in the aggressive turn. An active means of controlling distortion has been developed using an array of micro air jet vortex generators. It was hypothesized that microphones in the vicinity of the distorted flow would record higher amplitudes pressure fluctuations compared to those microphones in the vicinity of the undistorted flow. Experiments showed that the difference between the microphone readings in these two flow regimes was correlated to the distortion level. This difference in microphone signals was then used as feedback in a PID control system that regulated spatial distortion levels during steady flight conditions, as well as sudden ramps in aircraft speed.

Experimental Study of Sound Transmission Loss in Electrorheological Liquids

Electrorheological (ER) liquids possess the ability to change their physical properties like the apparent viscosity and modulus of elasticity under the influence of an external electric field. They serve successfully in the field of active vibration control—as well as in many other areas. In the Acoustic Laboratory at the College of Engineering, Southern Illinois University in Carbondale, research on the possibility of applying ER liquids to the control of a sound transmission loss (STL) was conducted. The STL was investigated for various kinds of ER suspensions in the frequency range 100 Hz to 2 kHz. An influence of the electric field density on the STL was different for normal and shear stress developed by DC voltage. In both cases the STL decreased with the increasing electric field density. These properties could be potentially useful in sound propagation control applications.

Recovering Acoustic and Elastic Parameters From Travel Times

The relation between travel times and waves in anisotropic media is explained using the geometrical optics method in a phase space setting. This approach also covers caustics and multiple arrivals. We than consider the question of whether one can determine an anisotropic index of refraction by measuring travel times. We show that this is indeed the case if the index of refraction satisfies some additional assumptions.

Statistical Energy Analysis of Fluid-Filled Piping Vibrations and Acoustics

The flow of vibratory energy in turbo-machinery piping systems can contribute significantly to the noise emission. Fluctuating pressures and mechanical vibrations of pumps and valves generate coupled vibration and acoustic waves that propagate throughout the system and radiate noise to the surrounding acoustic space. Statistical energy analysis provides a method to analyze the energy transmitted by these waves and to develop noise and vibration mitigation designs. The development of SEA models requires that special consideration be given to piping elbows and tees, where the coupling between structural vibrations and fluid acoustic waves may be high. This paper reviews the development of piping system prediction models and their limitations. A mobility-based approach is described to improve predictions at mid-frequencies where both statistical energy and finite element procedures often fail to provide accurate predictions.

Noise Transmission Control Studies on a Chamber Core Composite Cylinder

The vibroacoustic behavior and sound transmission properties of a mock-scale chamber core composite cylinder were studied, and the feasibility of the active structural acoustic control and passive control was also investigated. A box-beam model of the chamber core cylindrical shell was used for calculating the critical frequency and the ring frequency. The coupling problems between structural and acoustic modes were investigated, and the structural and acoustic modal parameters were identified from measured data. The sound transmission into the chamber core cylindrical structure was measured with and without fill materials in its wall chambers. The structural stiffness-, cavity resonance-, coincidence-, and mass-controlled zones were identified and verified.

Attenuation of High Amplitude Vibrations With Particle Dampers

Fluid transport systems are rarely at steady state. Transient phenomena, such as water hammer, can inflict severe physical damage. Repair costs can soar into the millions of dollars (Myers, 1997), and can reduce or even halt operation. Such high amplitude vibrations may be attenuated with particle dampers, which are beds of small particles placed in an attached enclosure or contained void. Vibration of the enclosure causes the particles to collide with each other and with the enclosure walls, resulting in energy dissipation through inelastic impacts and friction. Particle dampers have many advantages over conventional viscoelastic treatments including lower cost, increased robustness, greater effectiveness at high amplitudes and the ability to operate in extreme-temperature environments. Previous papers focus on exploration of sensitivity to design parameters, modeling techniques, and effective applications. However, there still remains much that is unknown about the phenomena and design of particle dampers. In this paper, experiments were performed to explore the effects of friction, excitation amplitude, and particle gap size. The formation of an oily residue on the colliding surfaces when certain materials were used increased friction and reduced damper effectiveness. This agrees with the theoretical prediction made by Mansour and Filho (1974). Damping was found to peak at an optimum gap size. Increasing the excitation amplitude resulted in higher damping and reduced sensitivity to the optimum gap size. Overall, the particle damper was deemed to be successful, increasing the loss factor of a clamped beam by over 10 times with a damper/structure mass ratio of only 0.016.

Enhancement of Reconstruction Accuracy of the HELS Method

The validity of the HELS method (Wu, 2000) for reconstructing the acoustic pressure field inside the minimum circle that encloses an arbitrary object is examined. Results show that the HELS solutions are approximate and the corresponding matrix equation is ill conditioned in general for back propagation of the acoustic field. Accordingly, the further the reconstruction point moves inside the minimum circle, the worse the reconstruction accuracy becomes. To overcome this difficulty new strategy for sensor placement is proposed. This strategy together with a constrained minimization are shown to yield satisfactory reconstruction inside the minimum circle. The same procedures can be extended to three-dimensional problems.

An Adaptive Feedback Noise Control Approach Using Q-Parameterization

An algorithm is presented which uses adaptive Q-parameterized compensators for control of sound. All stabilizing feedback compensators can be described in terms of plant coprime factors and a free parameter, Q, which can be any stable function. By generating a feedback signal containing only disturbance information, the parameterized compensator allows Q to be designed in an open-loop fashion. The problem of designing Q to yield desired noise reduction is formulated as an on-line gradient descent-based adaptation process. Coefficient update equations are derived for different forms of Q, including digital finite impulse response (FIR) and lattice infinite impulse response (IIR) filters. Simulations predict good performance for both tonal and broadband disturbances, and a duct feedback noise control experiment results in a 37 dB tonal reduction.