Postprocessing BEM Results
When solving a problem with The Pressure Acoustics, Boundary Elements Interface, the resulting solution will consist of the dependent variables on boundaries. Evaluating the solution in a domain is based on a kernel evaluation.
On boundaries, dependent variables exist for the pressure p, up and down pressure-dependent variables on interior boundaries (pabe.p_up and pabe.p_down), as well as a normal flux variable pabe.pbam1.bemflux.
The Pressure Acoustics, Boundary Elements Interface sets up predefined postprocessing variables that combine the properties of the boundary variables, when needed, with variables based on the kernel evaluation.
The BEM solution can be evaluated at a given coordinate using the at3_spatial(...) operator. In a 3D model you can evaluate the sound pressure level in the point (x,y,z) = (1 m,0,0) by typing:
at3_spatial(1[m],0,0,pabe.Lp,’minc’).
This can, for example, be used to normalize the data in a Radiation Pattern plot by typing:
pabe.Lp-at3_spatial(1[m],0,0,pabe.Lp,’minc’)
Domain Evaluation
The BEM solution can in general be evaluated in domains using the Grid 2D and Grid 3D datasets. This sets up an underlying grid for the kernel evaluation of the solution. The grid dataset can, for example, be selected as the input Dataset for a slice plot. A grid dataset and a multislice plot are generated and used in the default plots when a BEM model is solved. Note also that the kernel-based variables can be evaluated on domains that are meshed.
All the Dedicated Acoustics Plots for Postprocessing that exist for the FEM interfaces can be used with the variables defined by the BEM solution. This is, for example, the Radiation Pattern plot or the Directivity plot used for analyzing the spatial radiation pattern of a transducer or a scattered field.
Symmetries and Indefinite Boundaries
There are important aspect on how results are visualized when the Symmetry/Infinite Boundary Condition functionality is used:
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First, when setting the Parameter Bounds in the grid dataset. Say the condition represents a symmetry plane then visualize the solution in the entire domain. On the other hand if the condition represents an infinite sound hard boundary, like the ground or an infinite baffle, then only visualize the solution above that plane by setting the Parameter Bounds in the Grid 3D or Grid 2D dataset in Results.
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Secondly, evaluation on the grid dataset will also show results inside radiating/scattering objects, on the symmetry side. These are unphysical results and can be removed using a Filter subnode with a logic expression.
Boundary Evaluation
When evaluating the solution on boundaries, the plot lists are populated with special boundary variables in a Boundary variables submenu located under, for example, the Pressure and sound pressure level or Acceleration and velocity menu. The boundary variables have a “_bnd” added to their names. For example, the total acoustic pressure on a boundary is pabe.p_t_bnd. On interior boundaries, there are also up and down variants of the variables, with “_up” and “_down” added, respectively. On interior boundaries (where an interior boundary condition has been added), the boundary variable is the average of the up and down variables.
The boundary variables should be used when plotting on surfaces, as they do not rely on the kernel evaluation but are directly related to the DOFs solved for in the BEM problem.