Hierarchical Design of Phononic Materials and Structures

Within periodically heterogeneous materials and structures, wave scattering and dispersion occur across constituent material interfaces leading to a banded frequency response. A novel multiscale dispersive design methodology is presented by which periodic unit cells are designed for desired frequency band structures, and are used as building blocks for forming fully or partially periodic structures, typically at larger length scales. Structures resulting from this hierarchical design approach are tailored to desired dynamical characteristics without the necessity for altering the overall geometric shape of the structure nor employing dissipative damping materials. Case studies are presented for shock isolation and frequency sensing.

Predicting Turbulent Boundary Layer Wall Pressure Fluctuations in Adverse Pressure Gradients

A physics-based approach is employed and a new prediction tool is developed to predict the wavevector-frequency spectrum of the turbulent boundary layer wall pressure fluctuations for subsonic airfoils under the influence of adverse pressure gradients. The prediction tool uses an explicit relationship developed by D. M. Chase, which is based on a fit to zero pressure gradient data. The tool takes into account the boundary layer edge velocity distribution and geometry of the airfoil, including the blade chord and thickness. Comparison to experimental adverse pressure gradient data shows a need for an update to the modeling constants of the Chase model. To optimize the correlation between the predicted turbulent boundary layer wall pressure spectrum and the experimental data, an optimization code (iSIGHT) is employed. This optimization module is used to minimize the absolute value of the difference (in dB) between the predicted values and those measured across the analysis frequency range. An optimized set of modeling constants is derived that provides reasonable agreement with the measurements.

Finite Element Analysis of the Distributed Vibration Absorbers for Structural Noise Control

This work presents an extensive analysis of the properties of distributed vibration absorbers (DVAs) and their effectiveness in controlling the sound radiation from the base structure. The DVA acts as a distributed mass absorber consisting of a thin metal sheet covering a layer of acoustic foam (porous media) that behaves like a distributed spring-mass-damper system. To assess the effectiveness of these DVAs in controlling the vibration of the base structures (plate) a detailed finite elements model has been developed for the DVA and base plate structure. The foam was modeled as a poroelastic media using 8 node hexahedral elements. The structural (plate) domain was modeled using 16 degree of freedom plate elements. Each of the finite element models have been validated by comparing the numerical results with the available analytical and experimental results. These component models were combined to model the DVA. Preliminary experiments conducted on the DVAs have shown an excellent agreement between the results obtained from the numerical model of the DVA and from the experiments. The component models and the DVA model were then combined into a larger FE model comprised of a base plate with the DVA treatment on its surface. The results from the simulation of this numerical model have shown that there has been a significant reduction in the vibration levels of the base plate due to DVA treatment on it. It has been shown from this work that the inclusion of the DVAs on the base plate reduces their vibration response and therefore the radiated noise. Moreover, the detailed development of the finite element model for the foam has provided us with the capability to analyze the physics behind the behavior of the distributed vibration absorbers (DVAs) and to develop more optimized designs for the same.

Triply Coupled Vibration Band Gaps in Periodic Thin-Walled Open Cross Section Beams

Triply coupled vibration through periodic thin-walled open cross section nonsymmetrical beams composed of two kinds of material is studied in this paper. Based on the triply coupled vibration equation, plane wave expansion method for the thin-walled beams is provided. If the filling fraction keeps constant, the lattice is one of the factors that affect the normalized gap width. If the lattice and filling fraction keep constant, the Young’s modulus contrast plays a fundamental role for the band gap width, but not density contrast. Finally, the frequency response of a finite periodic binary beam is simulated with finite element method, which provides an attenuation of over 20dB in the frequency range of the band gaps. The findings will be significant in the application of phononic crystals.

Acoustic Cloud Condensation Nuclei Counter

Analysis and simulation of an acoustic cloud condensation nuclei counter is presented. The instrument is capable of accurately counting the number of micro scale water droplets impacting a water surface. The sound produced underwater by the water droplets is determined when the droplets strike the water surface with an impact velocity equal to either their terminal or maximum velocity. First, the terminal velocities of the droplets are calculated using Stoke’s law and compared to measured velocities from Gunn and Kinzer. Then the maximum velocities that these droplets can sustain without breaking are calculated as a function of droplet diameter. Second, the sound due to droplet impact is estimated. Due to their size and water surface tension, there is no bubble formation at impact when the droplets are falling with terminal velocities. However conditions for regular bubble entrainments are established and limit velocities are calculated. Assuming that the micro water droplets can be accelerated, the maximum velocities for no bubble entrainments are calculated. The results show that the level of the sound produced by individual micro scale droplet falling with terminal velocity is so small that experimental verification is not possible. However, reasonable level of acoustic energy can be obtained by increasing the impact velocities of the droplets or by measuring the sound radiated by a group of impacting droplets. Finally, the droplets counting process is simulated using a water surface of one centimeter squared and a vertical growth chamber.

Acoustically Induced Mass Flow in Partially Saturated Porous Media: Applications to Fuel Cells

In this work we aim to develop a theoretical framework for evaluating the feasibility of attaining significant improvement of fuel cells performance and stability by enhancing the transport processes in porous partially-fluid-filled cathode compartments through applying acoustic and structural excitations. A generic unified model has been derived of the structural/acoustic wave propagation in the porous media with consideration of its coupling with mass transfer. It has been demonstrated that the phase saturation has a strong impact on the wave dynamics in porous media. Explicit expressions have been obtained for the generalized multiphase Biot-type coefficients. A generalized filtration equation has been derived that takes into account the effects on mass transfer of dynamic loading, varying saturation, and solid structure distortion in this complex system. For model calibration a series of tests has been conducted to measure water flows through porous media with and without acoustic excitations. It has been demonstrated that the excitations may result in a net change of the saturation inside the porous medium and the applied structural/acoustic loading can intensify the transportation process. Based on the numerical and experimental results, certain recommendations have been made in regards to the selection of materials and the optimization of performance regime.

Measurement and Analysis of Noise Characteristics for Diesel Engine

The vehicle interior noise is mainly caused by irregularity of road, engine and transmission. So it is important to study on the noise characteristics for diesel engine and identify the main noise source and investigate the effective methods to control noise. In this paper, by using the measurement and analysis system, we measured the sound pressures levels and their curves the noise of a military diesel engine, then made spectral analysis and coherence analysis. The results provide bases for low noise design of this diesel engine.

Band Gap Formation in Composite Models With Quasi-Random Fiber Arrangements

Large-scale deterministic simulations are performed in order to observe the band gap formation in composite models having quasi-random fiber arrangements. Composite plates are modeled in two-dimensions with various unidirectional fiber arrangements. The quasi-random fiber arrangements can be qualified as essentially regular with slight randomness. Simulation results are compared with the corresponding case of ideally regular fiber arrangement. The most interesting observation is that the slight randomness in the fiber arrangements enhances the band gap phenomenon by introducing a few secondary band gaps adjacent to the primary band gap. An attempt is made to relate the band gap characteristics to the statistical parameters of fiber arrangements.

Development of an Analog Controller for Tuning an Adaptive-Passive Control Device

Adaptive-passive devices such as adaptive Helmholtz Resonators (HR) and tunable vibration absorbers have been shown to be suitable for controlling both narrowband disturbances and lightly damped structural/acoustic modes driven by broadband disturbances. In order to track changes in the disturbance or changes in the modes, the natural frequency of the absorber, ωn, is tuned to match the observed signals. This is achieved by altering some physical parameter of the control device such as the stiffness of a vibration absorber or the neck cross-sectional area of a Helmholtz resonator. In order to automatically adjust these devices, control systems and tuning algorithms have been developed, most of which involve a digital controller. However, this paper looks specifically at the development of a simple analog controller used to drive a DC motor in order to tune a mechanical device. A two sensor dot product method is employed where one sensor is placed inside of the control device, such as a Helmholtz Resonator, and the other on/in the system under control, such as in a room. The outputs from the two sensors are multiplied together and subsequently low passed in order to extract a low frequency “DC” voltage which acts as an error signal. The error signal is related to the relative phase of the two sensor signals and determines the direction in which the device should be tuned. When the two signals are 90° apart, the system is tuned (i.e. the inner product produces zero DC level). If the drive frequency ω is different than the tuned frequency, then the system is mis-tuned. The relationship between the mis-tuning, ωn-ω, and the error is not linear, but for small perturbations a linear approximation can be used to investigate the stability and performance of the system. The gradient of the function is shown to be largest when the mis-tuning error is zero and is inversely proportional to the damping level in the control device. Once stability of the system has been ensured the ability of the system to track changes in drive frequency is investigated experimentally. The control system is demonstrated using an adaptive Helmholtz resonator which has a variable cross-sectional neck via an iris diaphragm. The iris is controlled using a small DC motor; two microphones (one mounted internally and one externally) are used to supply the driving signal to the circuit.

Noise Abatement From Large Size Electric Generators

Growing environmental sound concerns and recognition that lengthy unprotected exposure to high industrial noise levels can be detrimental to man have resulted in increased attention to reducing industrial noise. In the United States, it is required by law that all turbomachinery manufacturers must provide acoustic guarantees to their customers. For instance, for majority of generators, the near field sound pressure level is usually guaranteed not to exceed 85 dBA. To accomplish this goal, a number of methods of noise reduction have been developed in power industry. As one of the most practical and cost-effective solutions, acoustic blankets have been designed and tested for using on large size electric generators to efficiently reduce their sound pressure levels. This work has successfully demonstrated the potential of acoustic blankets for improve the passive acoustic transmission characteristics from generators. The acoustic data obtained from a field test have shown that the blankets can reduce the overall sound pressure level from large size generators about 4 to 6 dBA.