The Pressure Acoustics, Asymptotic Scattering Interface
The Pressure Acoustics, Asymptotic Scattering (paas) interface (), found under the Acoustics>Pressure Acoustics branch () when adding a physics interface, is used to model scattering problems at high frequencies using the Kirchhoff–Helmholtz integral formulation. The acoustic field is assumed to be locally plane such that the scattered field can be expressed analytically. The scattering object has to be acoustically large; the wavelength should be much smaller than important geometric features as well as the radius of curvature of important features, see Ref. 70, 71, and 72. The scattering analysis is only valid for open domains. The harmonic variation of all fields and sources is given by eiωt using the +iω convention.
The scattering object surface can be treated as perfectly reflecting or having absorbing properties by defining a surface normal impedance, a reflection coefficient, or an absorption coefficient. The latter two can depend on the angle of incidence. The interface can model scattering from spherical and plane waves in 3D and 2D axisymmetry, as well as plane and cylindrical waves in 2D. The interface has built-in functionality to compute the visibility factor.
When this physics interface is added, these default nodes are also added to the Model Builder — Pressure Acoustics, Background Pressure Field, Scattering Object (with Wall subfeature), and Exterior Field Calculation. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and source. You can also right-click Pressure Acoustics, Asymptotic Scattering to select physics features from the context menu.
Domain and Boundary Nodes for the Pressure Acoustics, Asymptotic Scattering Interface
Submarine High-Frequency Asymptotic Scattering. Application Library path Acoustics_Module/Underwater_Acoustics/submarine_asymptotic_scattering
Settings
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is paas.
Sound Pressure Level Settings
The settings selected here are only used if the transient solution solved is transformed into the frequency domain using the Time to Frequency FFT study. The zero level on the dB scale varies with the type of fluid. That value is a reference pressure that corresponds to 0 dB. This variable occurs in calculations of the sound pressure level Lp based on the root mean square (rms) pressure prms, such that
where pref is the reference pressure and the star (*) represents the complex conjugate. This is an expression valid for the case of harmonically time-varying acoustic pressure p.
Select a Reference pressure for the sound pressure level based on the fluid type:
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 Use reference pressure for air to use a reference pressure of 20 μPa (20·10−6 Pa).
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 Use reference pressure for water to use a reference pressure of 1 μPa (1·10−6 Pa).
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 User-defined reference pressure to enter a reference pressure pref, SPL (SI unit: Pa). The default value is the same as for air, 20 μPa.
Visibility
The high frequency assumption used in the asymptotic scattering interface requires the computation of the visibility. That is the surface of the scattering object “illuminated” by the background (incident) acoustic field. Select the Method to compute the visibility either Angle of incidence (the default) or Hemicube.
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Angle of incidence: COMSOL Multiphysics evaluates the angle of incidence between the background field and the surfaces directly, without considering which face elements are obstructed by others. This means that shadowing effects (that is, surface elements being obstructed in nonconvex cases) are not taken into account. Elements facing away from each other are, however, excluded from the integrals. The Angle of incidence method is fast and accurate for simple geometries with no or little shadowing.
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Hemicube: The more sophisticated and general hemicube method uses a z-buffered projection on the sides of a hemicube (with generalizations to 2D and 1D) to account for shadowing effects. Think of it as rendering digital images of the geometry in five different directions (in 3D; in 2D only three directions are needed), and counting the pixels in each mesh element to evaluate its view factor.
Its accuracy can be influenced by setting the Radiation resolution of the virtual snapshots. The number of z-buffer pixels on each side of the 3D hemicube equals the specified resolution squared. Thus the time required to evaluate the irradiation increases quadratically with resolution. In 2D, the number of z-buffer pixels is proportional to the resolution property, and thus the time is, as well. Visualizing results will be slower when the Hemicube option is selected.
Discretization
In this section you can select the discretization or element order used for resolving the pressure and its normal derivative at the scattering surfaces. No dependent variable is solved by the interface, but the representation of curved surfaces is influenced by this setting as well as the number of elements needed to resolve the problem.