Name: Erin Hafla
Address: Department of Mechanical and Industrial Engineering, Montana State University, Bozeman, Montana, 59718, USA
Marine hydrokinetic (MHK) devices generate electricity from the motion of tidal and ocean currents, as well as ocean waves, to provide an additional source of renewable energy available to the United States. These devices are a source of anthropogenic noise in the marine ecosystem and must meet regulatory guidelines that mandate a maximum amount of noise that may be generated. In the absence of measured levels from in situ deployments, a model for predicting the propagation of sound from an array of MHK sources in a real environment is essential. A set of coupled, linearized velocity-pressure equations in the time-domain are derived and presented in this paper, which are an alternative solution to the Helmholtz and wave equation methods traditionally employed. Discretizing these equations on a three-dimensional (3D), finite-difference grid ultimately permits a finite number of complex sources and spatially varying sound speeds, bathymetry, and bed composition. The solution to this system of equations has been parallelized in an acoustic-wave propagation package developed at Sandia National Labs, called Paracousti. This work presents the broadband sound pressure levels from a single source in two-dimensional (2D) ideal and Pekeris wave-guides and in a 3D domain with a sloping boundary. The paper concludes with demonstration of Paracousti for an array of MHK sources in a simple wave-guide.
This work was sponsored by Sandia National Laboratories Water Power Technology department and by the Department of Energies Wind and Water Power Technologies Office.
- To define velocity-pressure equations used to predict the sound propagation in the marine environment
- To present the numerical implementation of these equations
This paper presented a coupled, linearized system of equations in the time-domain to solve for the perturbed velocities and pressures that result from sound sources in the marine environment. The velocity-pressure equations were derived from Cauchy's equations of motion and discretized into a fourth-order spatial and second-order time finite-difference program. Furthermore, Paracousti has the capability to calculate the propagated sound field from multiple sound sources with unique profiles, and these sources can have monopole and/or dipole contributions. As each finite-difference grid point allows for separate density and sound speed values, real-world domains can be easily represented and evaluated. Through comparison to analytical solutions and the literature, it was shown that Paracousti reproduces expected results. Although Paracousti was developed for modeling MHK deployments, it may be used to understand any underwater sound-source and its propagation. Additional modeling information provided by Paracousti will allow for an improved understanding of how an MHK device, or array of devices, will affect an environment. This understanding can be used to drive design decisions and deployment locations of MHK devices to reduce negative impacts to any fish or marine mammals in an area of interest.
Halfa, E.; Johnson, E.; Johnson, C.; Preston, L.; Aldridge, D.; Roberts, J. (2018). Modeling underwater noise propagation from marine hydrokinetic power devices through a time-domain, velocity-pressure solution. Journal of the Acoustical Society of America, 143, 3242–3253. https://tethys.pnnl.gov/publications/modeling-underwater-noise-propagation-marine-hydrokinetic-power-devices-through-time