Computational aeroacoustic simulations of infrasound signals from volcanic eruptions

Abstract:

Volcanic eruptions are complex phenomena involving multiphase flow, turbulence, entrainment and shocks within the acoustic source region - details of fundamental interest that are usually hidden within the expanding jet of opaque eruptive fluid. A goal of the volcano acoustics community is to link physics and processes occurring within the inaccessible region of erupted material to a remote region where acoustic observations are possible.

Many volcano infrasound studies have utilized, with remarkable success, a very simple monopole point source model that relates the acoustic signal to the rate at which the atmosphere is pushed outward by the eruption. The monopole point source model has rigorous basis in acoustic analogy theories developed by Lighthill (1952), Curle (1955) and Ffowcs Williams and Hawkings (1969) and is appropriate for compact sources and linear wave propagation. These theories have additional terms expressed as volumetrically distributed quadrupoles, which are well studied in terms of turbulence and jet noise, and surface dipoles, which arise from tractions exerted by bounding surfaces. In the volcano acoustic field, however, these terms have received relatively little attention and have been used primarily in a qualitative manner.

As the volcano acoustic community attempts to extract more information from acoustic observations, there is a need to quantitatively revisit the relationships between eruption properties and the observed acoustic signal. Here, we present an integral representation that can be used to calculate acoustic radiation from eruption properties. By defining a surface surrounding the acoustic source region and integrating quantities over the surface, we can compute the acoustic radiation at any point outside of the surface. We validate the integral representation against computational aeroacoustic simulations of short-duration explosions.

The integral representation is well able to handle distributed/non-compact acoustic sources (such as pyroclastic-density currents). This approach enables us to take a complex eruption simulation and compute the acoustic radiation at remote regions where acoustic observations can be made in the field. We discuss how this approach can be used to investigate the relationship between acoustic signals and eruption properties.