Background Pressure Field

Add a node to model an incident pressure wave or to study the scattered pressure field ps, which is defined as the difference between the total acoustic pressure pt and the background pressure field pb:

This feature sets up the equations in a so-called scattered field formulation where the dependent variable is the scattered field p = ps. In a model where the background pressure field is not defined on all acoustic domains (or it is different) continuity is automatically applied in the total field pt on internal boundaries between domains.

The background pressure field can be a function of space and, for The Pressure Acoustics, Transient Interface using a Time Dependent study, a function of time.

	For a Frequency Domain study type, the frequency of the background pressure field is the same as for the dependent variable p.

Select a : (the default), , , or .

The option defines the background pressure field pb of the type:

where p0 is the wave amplitude, k is the wave vector with amplitude ks = ω/c and wave direction vector ek, and x is the location on the boundary.

For enter values for the p0 (SI unit: Pa) and ek (dimensionless). Select to define the c (SI unit: m/s), of the fluid defining the field, either or .

The option defines the background pressure field pb as a predefined cylindrical wave:

where p0 is the amplitude given at the reference distance rref = 0.548/ks (the distance where the Hankel function is one), ks = ω/c is the wavenumber,

is the Hankel function of the second kind (representing an outgoing cylindrical wave), rs is the distance from the source axis, esa is the direction of the source axis, x0 is a point on the source axis, and x is the location on the boundary.

For enter a p0 (SI unit: Pa), the c (SI unit: m/s) of the medium defining the incident wave, a x0 (SI unit: m), and a esa (the vector does not need to be normalized).

The option defines the background pressure field pb as a predefined spherical wave (only for 2D axisymmetric and 3D):

where p0 is the amplitude given at the reference distance of 1 m, ks = ω/c is the wavenumber, rs is the distance from the source, x0 is the source location of the spherical wave, and x is the location on the boundary.

For enter a p0 (SI unit: Pa), the c (SI unit: m/s) of the medium defining the incident wave, and a x0 (SI unit: m).

	For both the cylindrical and the spherical wave options the source and/or source axis should be located outside the domain where the background pressure field feature is defined.

For the option enter an expression for the pb (SI unit: Pa). This is the only option in the transient interface.