Pressure Acoustics
The Pressure Acoustics node adds the equations for solving the Helmholtz equation, defined by Equation 2-1, in the frequency domain using the boundary element method. The material properties have to be constant valued for each Pressure Acoustics node added.
Each Pressure Acoustics node adds the necessary BEM equations to the adjacent boundaries of the selection (with the material properties defined). The domain features in the BEM interface can be applied to all types of domain selections: solid domains, finite voids, and the infinite void. Only the exterior boundaries of the selections as well as applied interior boundary conditions need to be meshed. Finite voids are selections of domains that have no solids in the geometry but are only defined by a closed set of boundaries. The Infinite void represents the exterior most infinite domain. This selection is the typical used for radiation or scattering problems. Remember that the materials defined in the Materials node need to be applied to the same selections.
See also Selections: Infinite Void and Finite Voids in the Modeling with the Pressure Acoustics Branch (BEM-Based Interface) section.
In the Settings window, define the properties for the acoustics model and model inputs including the absolute background pressure and temperature.
Model Inputs
The values of the quiescent (or background) Temperature T (SI unit: K) and Absolute pressure pA (SI unit: Pa) are entered in the Model Inputs section. The values of the model inputs can influence the material properties like, for example, the density and speed of sound, depending on their definition in the Materials node. In the BEM formulation these two values have to be constant valued for each Pressure Acoustics node added.
Pressure Acoustics Model
To define the properties of the bulk fluid, select a Fluid model from the list: Linear elastic (the default), Linear elastic with attenuation, Viscous, Thermally conducting, Thermally conducting and viscous, or Ideal gas. The settings for the fluid models are the same as for Pressure Acoustics in The Pressure Acoustics, Frequency Domain Interface.
The fluid models represent different bulk loss or attenuation mechanisms (applied in a homogenized way) or ways of defining the fluid properties of the fluid. Some of these models are sometimes referred to as an equivalent fluid model. The loss model can be a theoretical model or a model based on measurement data for the attenuation in the fluid.
The theory for the fluid models is in the section Theory for the Equivalent Fluid Models.