Home / Examples / Acoustic Analysis [Mach] / Example 9: Better Accuracy of the Open Boundary

Example 9: Better Accuracy of the Open Boundary

General

The analysis domain other than the sphere is solved with the 2nd-order open boundary condition.
The analysis domain near the driving source is solved by FEM. The outside of the domain is solved using the directivity calculation method. As the 2nd-order absorbing open boundary is applicable only to the sphere, Femtet creates the sphere inside the analysis domain and the 2nd-order absorbing open boundary is applied thereto to perform analysis.

Results will vary depending on Femtet version and the PC environment.

Analysis Conditions

Item	Settings
Solver	Acoustic Analysis (Mach)
Analysis Space	3D
Analysis Type	Harmonic Analysis
Model unit	mm

Harmonic Analysis tab and Open Boundary tab are set as follows.

Tabs	Setting Item	Settings
Harmonic Analysis	Sweep Type	Single Frequency
	Frequency	5×109 [Hz]
	Fast Sweep Setting	Discrete Sweep
Open Boundary	Types	Absorbing Boundary
	Order of Absorbing Boundary	2nd-order
	Radius of the Open Boundary	0.3 [m]

The inside of the radius of the open boundary is solved by FEM. The outside is solved by the directivity calculation.

Graphical Objects

Default value (50) is used for the general mesh size. The left figure below is the analysis object. The area encircled by the dots is analyzed by FEM. The area includes the respiratory sphere as a sound source. By utilizing the symmetry of the analysis model, it is made to be 1/8 model

Body Attributes and Materials

Body Number/Type	Body Attribute Name	Material Name
2/Solid	AIR	000_Air(*)

* Available from the material DB

Boundary Conditions

Boundary Condition Name/Topology	Tab	Boundary Condition Type	Setting
SYM_X/Face	Symmetry/Continuity	Symmetry	Reflective
SYM_Y/Face	Symmetry/Continuity	Symmetry	Reflective
SYM_Z/Face	Symmetry/Continuity	Symmetry	Reflective
Respiratory sphere/Face	Sound Wave	Pressure	1 [Pa]
Outer Boundary Condition *	Sound Wave	Rigid Wall

*Although the rigid wall is selected for the outer boundary condition, it does not function as the rigid wall because it is out of the domain for the finite element method.

Results

The sound pressure distribution is shown in the contour below left. The change of the sound pressure between A and B is shown in the graph below right.

To show the contour of the face including the respiratory sphere, [Full Model] is deselected. The division setting is changed from [Automatic] to [Division Number] and 100 is set for the Division Number.