Andrew A. Piacsek

Atmospheric turbulence conditions leading to focused and folded sonic boom wave fronts

The propagation and subsequent distortion of sonic booms with rippled wave fronts are investigated theoretically using a nonlinear time-domain finite-difference scheme. This work seeks to validate the rippled wave front approach as a method for explaining the significant effects of turbulence on sonic booms [A. S. Pierce and D. J. Maglieri, J. Acoust. Soc. Am. 51, 702-721 (1971)]. A very simple description of turbulence is employed in which velocity perturbations within a shallow layer of the atmosphere form strings of vortices characterized by their size and speed. Passage of a steady-state plane shock front through such a vortex layer produces a periodically rippled wave front which, for the purposes of the present investigation, serves as the initial condition for a finite-difference propagation scheme. Results show that shock strength and ripple curvature determine whether ensuing propagation leads to wave front folding. High resolution images of the computed full wave field provide insights into the spiked and rounded features seen in sonic booms that have propagated through turbulence.

SCAMP: Application of Nonlinear Progressive-wave Equation to Sonic Boom Transition Focus

The Nonlinear Progressive-wave Equation (NPE) is one of three numerical models of focused sonic boom prediction being validated as part of the Superboom Caustic Analysis and Measurement Project (SCAMP). Formulated in the time domain, the NPE begins with a far-field acoustic signature and propagates it into the region with caustics, accounting for nonlinear steepening, refraction, dissipative processes, and narrow-angle diffraction. Proper initialization of the NPE requires scaling the sonic boom signature to a rippled wavefront that will generate the desired caustic curvature as it propagates. Details of the scaling and its limitations are described. Numerical results corresponding to SCAMP flight conditions are compared with measurements and results for low-boom conditions are presented.

Using COMSOL multiphysics software to investigate advanced acoustic problems

Numerical simulations provide a valuable tool for students to investigate complicated behavior found in many applications of acoustics. Computational “experiments” can be conducted quickly for a large number of parameter values, enabling students to visualize abstract quantities and to grasp causal relationships not easily recognized when looking at equations. The COMSOL finite-element multiphysics software package provides an integrated workspace in which the user defines a problem, meshes the geometry, and plots the solution(s). A brief overview of COMSOL will be presented, along with three examples of how it can be used to model advanced acoustics problems often encountered by students. The example problems involve musical acoustics, fluid-loaded shell vibrations, and flow resistance in porous materials. Also discussed will be the educational benefit of examining how choices made setting up the model can affect the integrity of the solution.

Nonlinear progressive wave equation for stratified atmospheres.

The nonlinear progressive wave equation (NPE) [McDonald and Kuperman, J. Acoust. Soc. Am. 81, 1406-1417 (1987)] is expressed in a form to accommodate changes in the ambient atmospheric density, pressure, and sound speed as the time-stepping computational window moves along a path possibly traversing significant altitude differences (in pressure scale heights). The modification is accomplished by the addition of a stratification term related to that derived in the 1970s for linear range-stepping calculations and later adopted into Khokhlov-Zabolotskaya-Kuznetsov-type nonlinear models. The modified NPE is shown to preserve acoustic energy in a ray tube and yields analytic similarity solutions for vertically propagating N waves in isothermal and thermally stratified atmospheres.

Environmental noise impact of modern wind farms

Electric power production from wind turbines has increased substantially during the past few years due to the growing emphasis on renewable energy sources and more efficient wind turbine technology. Although modern turbines are significantly quieter than early models, wind farms that are proposed near residential areas generate concern about potential noise issues. The present study consists of two parts: (1) the measurement of sound levels within a 2‐km radius of the existing Nine Canyons wind farm near Richland, WA, and (2) the application of an outdoor sound propagation model to predict noise levels, both at Nine Canyons and in the vicinity of a proposed farm near Ellensburg, WA. At most locations within the Nine Canyon site, recorded sound levels were less than predicted levels, with the exception of some downwind sites that were lower in elevation than the source. Noise levels were greatest downwind from the turbines, but never exceeded 50 dBA beyond 500 m from the nearest turbine. In many cases, wind noise at the microphone exceeded noise levels from the turbines.

Characterization of noise from an isolated intermediate-sized wind turbine

Community-based wind power companies provide subscriptions to individual homeowners and businesses for power generated by a locally installed turbine. Typically, such turbines are of an intermediate size, such as the Vestas V20 120-kW turbines operated by the Cascade Community Wind Company in several locations within Washington state. This model turbine has a tower height of 80 feet with a rotor diameter of 60 ft. Each turbine is installed individually on leased land, with no other turbines nearby. Noise measurements of a turbine located in Thorp, WA, were obtained in a variety of weather conditions. On several occasions with low to moderate wind speeds, the turbine was stopped, enabling the calculation of noise due to the turbine only. Results will be presented showing spectral content and sound pressure level contours for a range of wind speeds.

Time-domain modeling of atmospheric turbulence effects on sonic boom propagation

A 2-D nonlinear time domain computational model of sonic boom propagation has been modified to incorporate the e!ects of atmospheric turbulence. This model, based on the Nonlinear Progressive wave Equation (NPE) of McDonald and Kuperman, is used to propagate N-waves from the upper turbulent boundary layer (TBL) to the ground through di!erent turbulence realizations. The output of the model provides detailed images of the full wave field at arbitrary heights above the ground, as well as shock profiles at specified locations along the wavefront. Preliminary results show multiple scales of spiking and rounding of shock profiles that are clearly correlated with wavefront focusing and defocusing, respectively. These waveform distortions, as well as their spatial variation along the wavefront, qualitatively match those seen in sonic booms recorded at the ground. These results are also consistent with earlier studies that applied the NPE to weak shocks with rippled wavefronts propagating through a homogeneous medium; these studies demonstrated that the combination of nonlinear and geometric e!ects arising from focusing wavefronts can produce the variety of distorted sonic boom waveforms observed in test flights. This paper describes the details of the sonic boom propagation code, including the implementation of turbulence e!ects, and discusses its performance on benchmark problems.

Using interactive simulations to build understanding and promote scientific inquiry

Computer simulations of physical phenomena, with graphical output, have long been used to enable visualizations of processes that are inherently invisible, such as sound wave propagation, or that operate on time scales that make direct observation difficult. For students struggling to make sense of an abstract physical concept, such as the relationship between the propagation of a mechanical wave and the associated motion of the medium, an animated simulation of wave motion can accelerate the “aha!” moment of understanding. But to solidify and build on that understanding, students need to be able to formulate and test predictions. This can be accomplished with an interactive simulation, in which the user can adjust certain parameters of the model and view the response in the solution immediately. In this way, students can conduct virtual experiments. This presentation will use three examples of interactive simulations to demonstrate how interactive simulations can be used in a structured way to support sp...

Preliminary estimates of acoustic intensity vorticity associated with a turbine blade rate

We report on measurements recently made at the Wild Horse wind farm, near Ellensburg, WA, as part of short experiment to evaluate a device sensitive to acoustic particle velocity. Measurements were made at range of approximately 60 m, directly in front of (facing) a turbine, at a height 1.5 m above ground using a sound level meter (SLM) and vector sensor positioned within a few meters of each other. The SLM recorded a steady A-weighted SPL of 58 dB and C-weighted SPL between 70 dB and 84 dB, noting some low-frequency variability due to wind noise. The vector sensor measurement consists of 4 coherently-recorded signals, one from an omnidirectional microphone and three from a tri-axial accelerometer embedded in a lightweight 10-cm diameter sphere, which helps immunize this acoustic particle motion measurement from wind noise. Here we focus on combining velocity and pressure measurements to form acoustic vector intensity. Real (active) and imaginary (reactive) components of this field display temporal proper...

Preparing students for undergraduate research experiences in acoustics

At many primarily undergraduate institutions, and some PhD-granting universities, science and engineering faculty are expected to provide research experiences for undergraduates. One of the biggest challenges for faculty who supervise undergraduate research is preparing students to work productively with some degree of independence. In addition to having familiarity with relevant physical concepts and measurement methods, students must know how to use lab equipment safely, responsibly, and effectively. While required lab courses in typical physics and engineering departments expose students to general laboratory skills and measurement techniques, they do not typically include specialized equipment and methods used in acoustics research. This presentation will describe how an upper-division undergraduate course in acoustics can be designed to prepare students for a productive independent research experience in this field. Examples will be drawn from courses offered at Central Washington University and Brig...