Amanda D. Hanford

Evolution of statistical properties for a nonlinearly propagating sinusoid.

The nonlinear propagation of a pure sinusoid is considered using time domain statistics. The probability density function, standard deviation, skewness, kurtosis, and crest factor are computed for both the amplitude and amplitude time derivatives as a function of distance. The amplitude statistics vary only in the postshock realm, while the amplitude derivative statistics vary rapidly in the preshock realm. The statistical analysis also suggests that the sawtooth onset distance can be considered to be earlier than previously realized.

The direct simulation of acoustics on Earth, Mars, and Titan.

With the recent success of the Huygens lander on Titan, a moon of Saturn, there has been renewed interest in further exploring the acoustic environments of the other planets in the solar system. The direct simulation Monte Carlo (DSMC) method is used here for modeling sound propagation in the atmospheres of Earth, Mars, and Titan at a variety of altitudes above the surface. DSMC is a particle method that describes gas dynamics through direct physical modeling of particle motions and collisions. The validity of DSMC for the entire range of Knudsen numbers (Kn), where Kn is defined as the mean free path divided by the wavelength, allows for the exploration of sound propagation in planetary environments for all values of Kn. DSMC results at a variety of altitudes on Earth, Mars, and Titan including the details of nonlinearity, absorption, dispersion, and molecular relaxation in gas mixtures are given for a wide range of Kn showing agreement with various continuum theories at low Kn and deviation from continuum theory at high Kn. Despite large computation time and memory requirements, DSMC is the method best suited to study high altitude effects or where continuum theory is not valid.

Simulations and Measurements of the Vibroacoustic Effects of Replacing Rolling Element Bearings With Journal Bearings in a Simple Gearbox

The effects of replacing rolling element bearings with journal bearings on the noise and vibration of a simple gearbox are computationally and experimentally evaluated. A modified component mode synthesis (CMS) approach is used, where the component modes of the shafting and gearbox housing are modeled using finite element analysis (FEA). Instead of using component modes with free boundary conditions, which is typical of CMS, the shafting and gearbox are coupled using nominal impedances computed for the different bearing types, improving convergence of the solution. Methods for computing the actual bearing impedances, including the high damping coefficients in journal bearings, are summarized. The sound radiated by the gearbox is computed using a boundary element (BE) model. The modeling results are validated against measurements made at the NASA Glenn Research Center. Both simulations and measurements reveal that the journal bearings, although highly damped, do not necessarily lead to strong reductions in gearbox vibration and noise.

The propagation of sound on Titan using the direct simulation Monte Carlo

With the recent success of the Huygens lander on Titan, a moon of Saturn, there has been renewed interest in exploring the acoustic environment of the only moon in the solar system with a significant atmosphere. The direct simulation Monte Carlo (DSMC) method is used here for modeling sound propagation in the Titan atmosphere. DSMC is a particle method that describes gas dynamics through direct physical modeling of particle motions and collisions. DSMC is based on the kinetic theory of gas dynamics, where representative particles are followed as they move and collide with other particles. The validity of DSMC for the entire range of Knudsen numbers (Kn), where Kn is defined as the mean free path divided by the wavelength, allows for the exploration of sound propagation in the Titan atmosphere for all values of Kn. DSMC results for the absorption of sound have shown that sound absorption depends heavily on Kn and deviates significantly from the continuum classical assumption for large Kn. This talk present...

The absorption of sound on Mars using the direct simulation Monte Carlo

The physical properties that govern the absorption of sound on Mars are very similar to those on Earth: classical losses associated with the transfer of acoustic energy into heat, and relaxation losses associated with the redistribution of internal energy of molecules. The difference in molecular composition between Earth and Mars as well as the lower atmospheric pressure on Mars results in larger values for the absorption coefficient on Mars. The direct simulation Monte Carlo (DSMC) method is the simulation tool used for modeling sound propagation in the Martian atmosphere. DSMC describes gas dynamics through direct physical modeling of particle motions and collisions. DSMC is based on the kinetic theory of gas dynamics, where representative particles are followed as they move and collide with other particles. The validity of DSMC for the entire range of Knudsen numbers (Kn), where Kn is defined as the mean free path divided by the wavelength, allows for the exploration of sound propagation in the Martia...

Predicting absorption and dispersion in acoustics by direct simulation Monte Carlo: Quantum and classical models for molecular relaxation

In the current study, real gas effects in the propagation of sound waves are simulated using the direct simulation Monte Carlo method for a wide range of frequencies. This particle method allows for treatment of acoustic phenomena at high Knudsen numbers, corresponding to low densities and a high ratio of the molecular mean free path to wavelength. Different methods to model the internal degrees of freedom of diatomic molecules and the exchange of translational, rotational and vibrational energies in collisions are employed in the current simulations of a diatomic gas. One of these methods is the fully classical rigid-rotor/harmonic-oscillator model for rotation and vibration. A second method takes into account the discrete quantum energy levels for vibration with the closely spaced rotational levels classically treated. This method gives a more realistic representation of the internal structure of diatomic and polyatomic molecules. Applications of these methods are investigated in diatomic nitrogen gas in order to study the propagation of sound and its attenuation and dispersion along with their dependence on temperature. With the direct simulation method, significant deviations from continuum predictions are also observed for high Knudsen number flows.

Coupled FE/BE for periodic acoustic systems

While coupled finite element and boundary element (FE/BE) codes are used regularly in acoustics—particularly structural acoustics—and periodic boundary conditions are common, the combination of the three is rare. However, such calculations have been performed in the electricity and magnetism community for quite some time. This talk presents a fully coupled FE/BE framework for acoustics with doubly-periodic boundary conditions, accomplished by way of the Ewald transformation of the appropriate periodic Green’s function. This method is likely of primary interest to the acoustic metamaterials community and it is used to analyze an example problem drawn from this field. Both the internal acoustic field within the FE mesh as well as the system’s radiation characteristics are examined.

Underwater acoustic ground cloak development and demonstration

Acoustic ground cloaks, which conceal an object on a rigid surface, utilize a linear coordinate transformation to map the flat surface to a triangular void by compressing space into two triangular cloaking regions consisting of a homogeneous anisotropic acoustic metamaterial. Transformation acoustics allows for the realization of a coordinate transformation through a reinterpretation of the scale factors as a new material in the original coordinate system. An underwater acoustic ground cloak was constructed from perforated steel plates and experimentally tested to conceal an object on a pressure release surface. The perforated plate acoustic ground cloak successfully cloaked the scattered object. There was excellent agreement between the phase of the surface reflection and cloak reflection with a small amplitude difference. Above 15 [kHz], the cloaking performance decreased as the effective material parameters of the perforated plate metamaterial deviated from the required material parameters.Acoustic ground cloaks, which conceal an object on a rigid surface, utilize a linear coordinate transformation to map the flat surface to a triangular void by compressing space into two triangular cloaking regions consisting of a homogeneous anisotropic acoustic metamaterial. Transformation acoustics allows for the realization of a coordinate transformation through a reinterpretation of the scale factors as a new material in the original coordinate system. An underwater acoustic ground cloak was constructed from perforated steel plates and experimentally tested to conceal an object on a pressure release surface. The perforated plate acoustic ground cloak successfully cloaked the scattered object. There was excellent agreement between the phase of the surface reflection and cloak reflection with a small amplitude difference. Above 15 [kHz], the cloaking performance decreased as the effective material parameters of the perforated plate metamaterial deviated from the required material parameters.

Development of a multi-material underwater anisotropic acoustic metamaterial

Previous work in the open literature has described three potential ways to create an acoustic metamaterial with anisotropic mass density and isotropic bulk modulus: (1) alternating layers of homogeneous isotropic materials, (2) perforated plates, and (3) solid inclusions. The primary focus of this work will be to experimentally demonstrate the anisotropic behavior of a metamaterial comprised of a multi-solid inclusion unit cell in water. The two material design of the unit cell consists of one material more dense and one less dense than the background fluid, which results in an effective mass density tensor for the unit cell where one component is more dense and one component is less dense than the background fluid. Successful demonstration of an anisotropic metamaterial with these effective parameters is an important step in the development of structures based on transformational acoustics.

Metamaterial ground cloak with anisotropic solid inclusions

There has been much research on the use of metamaterials within ground cloaks to hide objects on a rigid surface. Within the literature, ground cloaks made with metamaterials based on unit cells with solid inclusions have required the use of at least two distinct materials within the unit cell to achieve the necessary anisotropic material properties calculated from a transformation method. Here, we present a method to calculate the material properties of an anisotropic solid inclusion for use in a metamaterial ground cloak. The generated materials properties are then shown to be realized through an additively manufactured anisotropic solid inclusion. Finally, numerical results of cloaking performance of the metamaterial ground cloak are presented.