Measurement of Stress/Charge Coefficients of a Thin Polyvinylidene Fluoride Film

An experimental/analytical approach is used to detemine the piezoelectric coefficients of PolyVinyliDene Fluoride (PDVF). PVDF manufacturers usually publish the values of the charge coefficients in the 1 or 3 directions of the piezoelectric film. However; some applications, such as the use of volume displacement sensors for vibrating plates in active noise control, require the knowledge of the PVDF charge coefficients in the 2 direction. The objective of this work is to determine experimentally and numerically the stress/charge coefficient of a PVDF film in the 2 direction. The method is based on volume displacement measurement of a vibrating beam. Two PVDF volume displacement sensors are fabricated with two different piezoelectric 1 and 2 directions respectively. An accelerometer is used to measure the volume displacement of the vibrating beam. The values are compared with the output charge of the PVDF sensors to determine the unknown stress/charge coefficient. Results obtained at room temperature are presented and compare favorably with existing similar data.

Simulating and Measuring Structural Intensity Fields in Plates Induced by Spatially and Temporally Random Excitation

The structure-borne power in bending waves is well understood, and has been studied by many investigators in ideal beam and plate structures. All studies to date, however, have considered only the structural intensity induced by deterministic, localized drives. Since many structures of practical interest are excited by spatially random pressure fields, such as diffuse and turbulent boundary layer pressure fluctuations, techniques for measuring and predicting the structural intensity patterns in plates excited by such fields are presented here. The structural intensity at various frequencies in a simply-supported, baffled, flat plate driven by a diffuse pressure field is simulated using analytical techniques and measured by post-processing data from a scanning laser Doppler vibrometer and reference accelerometer using finite differencing techniques. The measured and simulated fields are similar, and show intensity patterns different from those caused by deterministic point drives.

A Method for Characterizing Elastic Structures in the Mid-Frequency Region

This work presents a method for characterizing elastic structures when spatially varying properties over the input and output contact regions are considered. Most analytical or experimental approaches, such as the four-pole parameter method, are limited by the inherent use of lumped quantities to represent critical parameters. When the excitation frequency increases, however, the structural wavelength becomes comparable to the dimensions of the contact region. As a result, the point-quantity assumption is no longer valid. To address this limitation, the work described here reformulates the traditional four-pole method in terms of quantities defined over planes. Consequently, spatial variations across the region connecting the structures can be considered. After the method is derived, it is applied to a simplified engine mount model in which two elastic beams are coupled through a set of elastic and inertial elements. Just like for the four-pole method, the formulation approach uses building blocks for simple structures that can be assembled to represent more complex structures. Some of the potential applications for this method are also discussed. By using this method, a meaningful characterization of the dynamic behavior can be obtained for structures when the frequency increases beyond that for which the point quantity approaches become invalid.

Development of an Adaptive Helmholtz Resonator for Broadband Noise Control

This paper presents the development of adaptive Helmholtz resonators aimed at controlling broadband disturbance for the reduction of noise transmission into rocket payload fairing. Helmholtz resonators are commonly used for narrow band control application and so are designed to present the lowest amount of damping yielding maximum impedance. For this particular application however, optimal damping ratios usually superior to 4% are required. This relatively high level of damping permits more lightweight and compact design options to be considered that are not possible for low damping applications. Two design solutions are presented. The first tunes the resonator by varying the length of an accordion neck. The second varies the HR opening using an iris diaphragm. The characteristics of these two devices are measured, and a solution to maintain the damping level relatively constant is also proposed. Finally, experimental result obtained in a large cylinder representative of a payload fairing using 8 adaptive resonators is presented.

Benchmark Solutions for Computational Aeroacoustics (CAA) Code Validation

NASA has conducted a series of Computational Aeroacoustics (CAA) Workshops on Benchmark Problems to develop a set of realistic CAA problems that can be used for code validation. In the Third (1999) and Fourth (2003) Workshops, the single airfoil gust response problem, with real geometry effects, was included as one of the benchmark problems. Respondents were asked to calculate the airfoil RMS pressure and far-field acoustic intensity for different airfoil geometries and a wide range of gust frequencies. This paper presents the validated solutions that have been obtained to the benchmark problem, and in addition, compares them with classical flat plate results. It is seen that airfoil geometry has a strong effect on the airfoil unsteady pressure, and a significant effect on the far-field acoustic intensity. Those parts of the benchmark problem that have not yet been adequately solved are identified and presented as a challenge to the CAA research community.

Review of Two Numerical Approaches to Predict Nonlinear Airfoil Response to High-Amplitude Incident Gust

In this work, two different numerical methods of time-accurate nonlinear analysis are reviewed and compared in application to the problem of nonlinear unsteady aerodynamic and aeroacoustic airfoil responses to a high-intensity impinging gust. The incident perturbation field is of finite amplitude relative to the mean flow so that in general, no assumption of a linear superposition of responses from each individual harmonic can be made. Thus, in addition to providing a comparison of two different approaches in computational aeroacoustics, the paper achieves the objective of obtaining verified solutions determining the limits of validity for linearized methods, universally accepted in studies of unsteady aerodynamics and aeroacoustics. The work investigates nonlinear near- and far-field responses of a Joukowksi airfoil in the parametric space of gust intensity and frequency.

A Parametric Sonic Crystal Modal Analysis Using Finite Element Modeling

Sonic crystals are typically materials with millimeter scale arrays of acoustic resonators embedded in a matrix material. They provide sound attenuation in acoustic band gaps at frequencies approximately two orders of magnitude lower than those predicted by Bragg’s theory of reflection. There are many potential applications of sonic crystals as filters and frequency selective acoustic damping devices. Performance characteristics of single-cell and double cell based sonic crystal structures were computationally evaluated using finite element methods. In this work, the sonic crystal consisted of cylinder inclusions encased in a soft polymer coating and embedded in a block of epoxy matrix material. Parametric studies were performed to evaluate the effects of material properties of the inclusion, coating and matrix. Mode shapes were determined. A preliminary comparison with Local Interaction Simulation Approach (LISA) is presented. The influence of material property variation, without changing geometric features, on single-cell and double-cell sonic crystal performance is discussed.

Effects of Combustor Acoustics on Fuel Spray Dynamics

An experimental liquid fuel LDI combustor, developed to study thermoacoustic instability processes and to test active combustion control systems, was found to demonstrate three distinct stability regimes, with system characteristics not reported in earlier literature. These observations led to a series of further investigations, both in reactive and non-reactive conditions, to gain an insight into effects of combustor acoustics on fuel spray dynamics. This paper presents only the non-reacting flow results, from both experimental and modeling investigations. The experimental setup and construction details of an isothermal acoustic rig are presented. Phase-locked PDA measurements of droplet velocities and diameters from a simplex atomizer spray were acquired, with and without combustor swirl co-airflow, under varying acoustic forcing conditions and spray feed pressures. Measurements made at four locations in the spray are related, in the paper, to these variations in mean and unsteady inputs. The dynamic behavior of the spray is then presented in terms of frequency response characteristics related to acoustic fields imposed on the spray. Finally, results from non-reacting spray modeling, predicting droplet trajectories, are reported. The modeling was done using the deterministic separated flow approach. These trajectories are compared to the reported experimental results to support preliminary explanations for the unique experimental observations of the swirl-stabilized kerosene flame in a single can combustor geometry.

Amplitude-Limiting Feedback Control of Combustion Instability With a High-Momentum Air Jet

A new strategy that integrates low-frequency modulation of a high-momentum air-jet with amplitude feedback is presented for control of combustion oscillations in a swirl-stabilized spray combustor. The oscillations in the combustor of interest are dominated by an acoustic mode (235 Hz) with a low frequency (13 Hz) bulk-mode (of the upstream cavity) superimposed. An effective strategy for control is shown to be achieved through the use of a concept which utilizes low bandwidth modulation of a high-momentum air-jet that penetrates into the regions of positive Rayleigh index. It is shown that with a low frequency modulation (5 Hz) of the high momentum air-jet, the pressure oscillations can be reduced significantly (by a factor of nearly 6). Further improvement in control is achieved with an amplitude-limiting feedback strategy, in which, the valve opening and closing of the control air-jet is driven by the pressure amplitude relative to a specified threshold. The goal of the controller is to maintain pressure oscillations below the pre-set threshold level. With this strategy, the valve frequency and duty cycle are automatically adjusted based on the amplitude of the pressure signal. It is observed that modulation frequencies are typically in the range of 5–30 Hz (although higher frequencies, as high as 130 Hz, are needed occasionally). Duty cycles less than 50% are required for effective control. The amplitude-limiting feedback controller is shown to combine the benefits of low-bandwidth actuation, low-duty cycles, and greater reductions in pressure oscillations.

Weight Optimized Structural and Acoustic Actuators for the Control of Sound Transmission Into Rocket Payload Compartments

The reduction of sound transmission into rocket payload compartments is a challenging application for active control due to the broadband nature of the disturbance, the large structural and acoustic space and the very high acoustic levels required. The exterior acoustic field that drives the payload fairing at liftoff is typically in the order of 145dB and the active control system must be able to counteract this high drive level using lightweight actuators. This paper is concerned with the development of structural and acoustic actuators for this application with the emphasis on maximum output level in the 60–200Hz bandwidth for a given actuator weight. The electromagnetic structural actuators are based on powerful rare earth magnets in a two degree of freedom arrangement. It is shown that a two degree of freedom arrangement allows the output in the bandwidth of interest to be increased over a simple one degree of freedom arrangement. The design is termed a distributed active vibration absorber or DAVA as the second degree of freedom is provided by a light and distributed foam element that allows easy attachment and low stress concentration on the structure. The two degree of freedom arrangement also acts as a natural low pass filter to naturally remove unwanted spillover at higher frequencies. The acoustic component is also based on powerful rare earth magnets, however the two degree of freedom arrangement used for the structural actuator is no longer of interest. The main concern is in the reduction of the speaker and cabinet weight. It is shown that careful design of the speaker and cabinet can lead to large reductions in weight without loss of performance. Data taken from an active control experiment on a large composite cylinder, coupled with data from the characterization of the actuators will be used to determine the total actuator weight needed for control in a typical launch environment.