Browsing by Author "Ruckman, Christopher E."

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    Numerical simulation of active structural-acoustic control for a fluid-loaded, spherical shell
    Ruckman, Christopher E.; Fuller, Chris R. (Acoustical Society of America, 1994-11-01)
    Numerical methods are used to investigate active structural-acoustic control, a noise control technique in which oscillating force inputs are applied directly on a flexible structure to control its acoustic behavior. The goal is to control acoustic radiation from a thin-walled shell submerged in a dense fluid and subjected to a persistent, pure-tone disturbance. For generality the fully coupled responses are found numerically, in this case using the computer program that combines finite-element and boundary-element techniques. A feedforward control approach uses linear quadratic optimal control theory to minimize the total radiated power. Results are given for a thin-walled spherical shell, and are compared to analytical results. The numerical solution is shown to be suitably accurate in predicting the radiated power, the control forces, and the residual responses as compared to the analytical solution. A relatively small number of control forces can achieve global reductions in acoustic radiation at low frequencies (k(0)a<1.7). A single point-force actuator reduces the radiated power due to a point-force excitation by up to 20 dB at resonance frequencies; between resonance frequencies, more actuators are required because of modal spillover. With multiple control forces, radiation can be reduced by 6-20 dB over the frequency range O
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    A regression approach for simulating feedforward active noise control
    Ruckman, Christopher E.; Fuller, Chris R. (Acoustical Society of America, 1995-05-01)
    Regression analysis is used to examine feedforward active noise control from a statistical point of view. Since numerical techniques for simulating feedforward active noise control in the frequency domain are mathematically similar to linear least-squares regression, two regression-based numerical methods can be applied to control problems. The first uses regression diagnostics such as the F-test, the t-test, and confidence intervals to model the effects of error sensor measurement noise. The second uses collinearity diagnostics to address a form of numerical ill conditioning that can corrupt the results. The regression diagnostics allow realistic modeling of random measurement error; the collinearity diagnostics help avoid numerical difficulties that might otherwise go undetected. Numerical results are given for a structural-acoustic control problem involving a fluid-loaded cylindrical shell. 1995 Acoustical Society of America
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    A regression-based approach for simulating feedfoward active noise control, with application to fluid-structure interaction problems
    Ruckman, Christopher E. (Virginia Tech, 1994)
    This dissertation presents a set of general numerical tools for simulating feedforward active noise control in the frequency domain. Feedforward control is numerically similar to linear least squares regression, and can take advantage of various numerical techniques developed in the statistics literature for use with regression. Therefore, an important theme of this work is to look at the control problem from a statistical point of view, and explore the analogies between feedforward control and basic statistical principles of regression. Motivating the numerical approach is the need to simulate active noise control for systems whose dynamics must be modeled numerically because analytical solutions do not exist, e.g., fluid-structure interaction problems. Plant dynamics for examples in the present work are modeled using a finite-element / boundary-element computer program, and the associated numerical methods are general enough for us with many types of problems. The derivation is presented in the context of active structural-acoustic control (ASAC), in which sound radiating from a vibrating structure is controlled by applying time-harmonic vibrational inputs directly on the structure. First, a feedforward control simulation is developed for a submerged spherical shell using both analytical and numerical techniques; the numerical formulation is found by discretizing the integrations used in the analytical approach. ASAC is shown to be effective for controlling radiation from the spherical shell. For a point-force disturbance at low frequencies, a single control input can reduce the radiated power by up to 20 dB (ignoring the possibility of measurement noise). A more general numerical methodology is then developed based on weighted least-squares regression in the complex domain. It is shown that basic regression diagnostics, which are used in the statistics literature to describe the quality and reliability of a regression, can be used to model the effects of error sensor measurement noise to produce a more realistic simulation. Numerical results are presented for a finite-length, fluid-loaded cylindrical shell with clamped, rigid end closures. It is shown that when the controller reduces the radiated power by less than 2 dB, the control simulation is usually invalid for statistical reasons. Also developed are confidence intervals for the individual control input magnitudes, and prediction intervals which help evaluate the sensitivity to measurement noise for the regression as a whole. Collinearity, a type of numerical ill-conditioning that can corrupt regression results, is demonstrated to occur in an example feedforward control simulation. The effects of collinearity are discussed, and a basic diagnostic is developed to detect and analyze collinearity. Subset selection, a numerical procedure for improving regressions, is shown to correspond to optimizing actuator locations for best control system performance. Exhaustive-search subset selection is used to optimize actuator locations for a sample structure. Finally, a convenient method is given for investigating alternate controller formulations, and examples of several alternate controllers are given including a wavenumber-domain controller. Numerical results for a cylindrical shell give insight to the mechanisms used by the control system, and a new visualization technique is used to relate farfield pressure distributions to surface velocity distributions using wavenumber analysis.
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    Simultaneous structural/acoustical design of composite panels
    Ruckman, Christopher E. (Virginia Polytechnic Institute and State University, 1986)
    Since advanced composite materials generally experience coincidence at lower frequencies than metals when used in aircraft fuselage sidewalls, they may allow more transmission of airborne noise thereby requiring heavier acoustical treatments. A sequential design approach of addressing first structural and then acoustical design does not take advantage of structural/acoustical coupling. A simultaneous approach is expected to help minimize the total sidewall mass. This thesis uses numerical optimization to examine structural/acoustical interactions and compare the sequential and simultaneous design approaches. Acoustical performance is defined in terms of the infinite panel transmission loss at frequencies surrounding the coincidence region (1600 Hz - 12800 Hz for the panels studied.) Impedance transfer theory is used to predict the acoustical properties of a flat unstiffened anisotropic panel treated with a fibrous acoustic blanket, airgap, and limp-mass septum. Structural analysis is based on a fatigue damage resistance criterion. Sequentially designed treated composite panels exibit transmission losses 15 dB - 45 dB higher (transmitted pressure is 6 - 180 times smaller) than a structurally equivalent, equal-mass aluminum panel. Depending on the type of acoustic excitation (specific incidence direction or diffuse source) and the acoustic frequency considered, the simultaneous approach alters the sequential minimum-mass panel in order to 1) improve low frequency performance by raising coincidence frequencies, 2) improve high frequency performance by lowering coincidence frequencies, or 3) make the coincidence region as narrow as possible. Since these structural alterations require that more mass be allotted to the panel and less to the treatment, they only occur for strong structural/acoustical interactions (i.e. near coincidence.) The simultaneous design approach can achieve a moderate improvement (TL increased up to 10 dB, transmitted pressure decreased by a factor or 3) over a sequential design for a particular acoustic performance index, although computation time is increased and acoustic performance may be sacrificed in other regions.