Ricardo A. Burdisso

Reliability based optimization: A safety index approach

Abstract An alternative approach to the classical chance constraint optimization technique is presented. The proposed method employs an advanced second-moment method in evaluating the probabilities of violating the constraints. The approach is applied to the optimal design of a simple structure. The results are compared to those obtained using the classical chance constraint optimization technique. Using the proposed optimization approach the basic drawbacks of the classical chance constraint optimization technique are shown to be overcome. The improved accuracy of the new optimazation method is verified using Monte-Carlo simulation.

Active control of aircraft engine inlet noise using compact sound sources and distributed error sensors

An active noise control system using a compact sound source is effective to reduce aircraft engine duct noise. The fan noise from a turbofan engine is controlled using an adaptive filtered-x LMS algorithm. Single multi channel control systems are used to control the fan blade passage frequency (BPF) tone and the BPF tone and the first harmonic of the BPF tone for a plane wave excitation. A multi channel control system is used to control any spinning mode. The multi channel control system to control both fan tones and a high pressure compressor BPF tone simultaneously. In order to make active control of turbofan inlet noise a viable technology, a compact sound source is employed to generate the control field. This control field sound source consists of an array of identical thin, cylindrically curved panels with an inner radius of curvature corresponding to that of the engine inlet. These panels are flush mounted inside the inlet duct and sealed on all edges to prevent leakage around the panel and to minimize the aerodynamic losses created by the addition of the panels. Each panel is driven by one or more piezoelectric force transducers mounted on the surface of the panel. The response of the panel to excitation is maximized when it is driven at its resonance; therefore, the panel is designed such that its fundamental frequency is near the tone to be canceled, typically 2000-4000 Hz.

Optimal Placement of Piezoelectric Actuators for Active Structural Acoustic Control

This paper presents a general formulation of the optimization problem for the place ment and sizing of piezoelectric actuators in adaptive LMS control systems. The selection of objec tive function, design variables and physical constraints are separately discussed. A case study for the optimal placement of multiple fixed size piezoelectric actuators in sound radiation control is pre sented. A solution strategy is proposed to calculate the applied voltages to piezoelectric actuators with the use of linear quadratic optimal control theory which is to simulate the LMS feedforward control algorithm. The location of piezoelectric actuators is then determined by minimizing the ob jective function, which is defined as the sum of the mean square sound pressure measured by a number of error microphones. The optimal location of piezoelectric actuators for sound radiation control is determined and shown to be dependent on the excitation frequency. Particularly, the opti mal placement of multiple piezoelectric actuators for on-resonance and off-resonance excitation is presented. The results show that the optimally located piezoelectric actuators perform far better sound radiation control than arbitrarily selected ones. This work leads to a design methodology for adaptive or intelligent material systems with highly integrated actuators and sensors. The optimiza tion procedure also leads to a reduction in the number of control transducers.

Design approaches for shaping polyvinylidene fluoride sensors in active structural acoustic control (ASAC)

A design methodology is presented for shaping structural polyvinylidene fluoride sensors for active structural acoustic control applications. A structural PVDF sensor yielding a con trolled response equivalent to that obtained when implementing a microphone error sensor perpen dicular to the surface of a simply supported beam was designed and tested. In addition, the equiva lent PVDF sensor design approach for the simply supported plate is presented; however, practical limitations in achieving the desired three-dimensional polarization profile prevent physical imple mentation. Results from tests conducted on the simply supported beam demonstrate that PVDF structural sensors can be designed to achieve directional acoustic control of structure-borne sound. While this design approach is currently limited to structures whose response can be described as a function of one dimension, the potential for applications in more complex structures is readily achieved upon determining a practical method of varying the polarization profile of the material as a function of two dimensions.

A coupled tire structure/acoustic cavity model

The dynamic characteristic of the tires is a key factor in the road-induced interior noise in passenger vehicles. The tire acoustic cavity is a very important factor in the tire dynamics and it must be considered in analyses. This paper describes a closed form analytical model for tire-wheel structures. In order to incorporate the dynamics of the cavity on the tire response, the tire acoustic-structure coupled problem is solved simultaneously. The tire is modeled as an annular cylindrical shell where only the outside shell is flexible, i.e. tire sidewalls and wheel are assumed rigid. From the analytical solution of the eigenproblems, both the tire structure and cavity acoustic responses are expanded in terms of their eigenfunctions. The main objective of the model is to have an efficient tool to investigate the physical coupling mechanisms between the acoustic cavity and the tire structure without the need of complicated numerical model such as finite elements. The result shows that the proposed model captures the main mechanisms of the effect of the tire air acoustic on the tire dynamics.

Active control of fan noise from a turbofan engine

A three-channel active control system is applied to an operational turbofan engine to reduce tonal noise produced by both the fan and the high-pressure compressor. The control approach is the feedforward filtered-x least-mean-square algorithm implemented on a digital signal processing board. Reference transducers mounted on the engine case provide blade passing and harmonics frequency information to the controller. Error information is provided by large area microphones placed in the acoustic far field. To minimize the error signal, the controller actuates loudspeakers mounted on the inlet to produce destructive interference

A new hybrid passive–active noise absorption system

A new hybrid passive–active system is developed for sound absorption over a wide frequency range. The system is comprised of a layer of absorbing material positioned at a distance from an active wall, leaving an air space. The motion of the active wall is based on a new control approach which consists of the minimization of the reflected wave within the airspace which modifies the layer’s back surface impedance so as to match the characteristic impedance of air. This technique is referred here as inducing an impedance-matching condition. Both numerical and experimental results of such a system are presented for normally incident planar waves. The hybrid passive–active system results in a high absorption coefficient of 0.8–1.0 over the frequency range 100–2000 Hz and is insensitive to system parameters such as air space depth and absorbing layer thickness.

Theory of feedforward controlled system eigenproperties

Abstract Adaptive feedforward algorithms have be successfully applied in the active control of sound and vibration. However, the actual mechanisms of control inherent in the technique remain an area of much interest. The main objective of this research is to study the dynamic characteristics of a feedforward controlled elastic system. The structure is assumed to be subjected to a harmonic input excitation, and the resulting vibration minimized by utilizing another force for control. The controller is defined by minimizing the square of the response at a point denoted as error sensor location. A new mathematical approach to predict the dynamic characteristics of the controlled structure is presented. It is shown that the controlled system effectively behaves as having new eigenproperties. The controlled eigenvalues and eigenfunctions are function of the control force and error sensor locations, and independent of the input disturbance. Numerical examples demonstrate the applicability of the proposed formulation. These results are also corroborated experimentally using a feedforward controller implemented on a TMS320C20 DSP processor.

Calibration and Demonstration of the New Virginia Tech Anechoic Wind Tunnel

A unique new removable anechoic system and new acoustic treatment for the Virginia Tech Stability Wind Tunnel is described. The new system consists of a Kevlar-walled acoustic test section flanked by two anechoic chambers. In its new configuration the facility is closed aerodynamically and open acoustically, allowing far-field acoustic measurements with a flow quality comparable to that of a hardwalled wind tunnel. An extensive program of experiments has been conducted to examine the performance of this new hardware under a range of conditions, both to examine the effects of acoustic treatment on overall test-section noise levels and to ascertain the aerodynamic characteristics of the new test section. Noise levels in the test section of the anechoic facility are down by as much as 25 dB compared to the original hard-walled configuration. Lift interference corrections (for a baseline NACA 0012 airfoil) are less than half those expected in an open-jet wind tunnel. Acoustic measurements of airfoil trailing edge noise using a microphone phased array are compared to past experiments conducted on similar airfoils in an open-jet facility.

A broadband passive–active sound absorption system

In recent years, hybrid absorption systems have been implemented which achieve high sound absorption over a broad frequency range. This work is an experimental study of a broadband hybrid absorption system which is comprised of a layer of sound-absorbing material (the passive component) positioned at a distance from a movable wall (the active component) inside an impedance tube. The movable wall is used to impose desired boundary conditions in the cavity behind the passive layer, thereby increasing the absorption of the system at frequencies where the passive material is not independently effective. Both pressure-release (i.e., minimizing the pressure at the back surface of the layer) and impedance-matching (i.e., minimizing the reflected wave from the layer) boundary conditions are studied. The performance of the hybrid system for these two boundary conditions is compared for broadband disturbances over a frequency range of 100–1000 Hz. The unmodified passive system showed absorption coefficients greater...