Periodic Port
The Periodic Port is a dedicated feature used to model transmission, reflection, and scattering problems for periodic structures such as, for example, absorbers and diffusers. The feature is in particular interesting for diffusers as it can split the reflected energy into specular an non-specular directions. The periodic port handles plane wave incidence on the structures as well as all reflected and transmitted diffraction orders (when set up). The condition exists in 3D and 2D. The Periodic Port condition is set up together with a Periodic Condition with the Floquet periodicity (Bloch periodicity) option.
The general setup steps are as follows:
1
Define a geometry representing the periodic structure. The model can contain either only one periodic port for a pure reflection/scattering problem, or two periodic ports for a transmission problem. The model can be either a pure pressure acoustics model or a multiphysics model, for example, of a poroelastic layer.
2
Set up one periodic port where the system is excited. The first periodic port added has incident wave excitation turned on as the default.
3
Add a second periodic port for a transmission problem. The two port conditions are applied on two parallel boundaries. On the second periodic port refer to the first port using the Mode settings from incident periodic port option.
4
Add the periodic conditions and select the Floquet periodicity as Type of periodicity. The k-vector for Floquet periodicity can now point to the periodic port (selected in the drop down). This ensures a consistent model setup.
5
Add the necessary Diffraction Order Port subfeatures to capture all diffraction orders.
To evaluate if enough diffraction orders are included evaluate the imaginary part of the diffraction order port wave-numbers, for example, imag(acpr.pport1.dport1.kn), imag(acpr.pport1.dport2.kn), imag(acpr.pport1.dport3.kn) and so on. This can be done in an Evaluation Group with a Global Evaluation. Do this at the highest studied frequency and ensure that the highest diffraction order defined, only has evanescent waves (imaginary part different from 0) for all incidence directions. This means that no more propagating diffraction orders exist.
Reference Corner
This section is present if Incident wave excitation at this port is set to On and the model is 3D. Select the reference corner used to define a local coordinate system at the port surface for defining the incident wave direction (see details below).
Port Mode Settings
Activate if the given port is excited by an incident plane wave. For the first Periodic Port condition added in a model, the Incident wave excitation at this port is set to On. For subsequent conditions added, the excitation is set to Off per default.
When the Incident wave excitation at this port is set to On, then select how to define the incident plane wave. Set Define incident wave to Amplitude (the default) or Power.
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For Amplitude, enter the amplitude (SI unit: Pa) of the incident wave. This is in general defined as the maximum amplitude for a given mode shape.
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For Power, enter the power (SI unit: W) of the incident wave. This is in general defined as the RMS power of the incident wave.
For both options, enter the phase φ (SI unit: rad) of the incident wave. This phase contribution is multiplied with the amplitude defined by the above options. The Amplitude input can be a complex number.
Define the incidence direction of the plane wave to the port. The polar angles are defined in a local system of coordinates. In 2D models the system is made of the selected edge and the port normal. In 3D models the system is defined as seen in the sketch below. Make sure to define the Reference Corner rref in 3D models.
Figure 2-4: Local port system of coordinates in a 3D model.
In 2D enter the Polar angle of incidence ϕin. In 3D models enter both the Polar angle of incidence ϕin and the Azimuthal angle θin.
When the Incident wave excitation at this port is set to Off, then select the incident port in the Mode settings from incident periodic port combo (default is Not defined). This ensures that the orientation of the local coordinate systems on the two periodic ports are consistent.
Schroeder Diffuser in 2D: Application Library path Acoustics_Module//Building_and_Room_Acoustics/diffuser_schroeder_2d
Constraint Settings
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog. For information about the Constraint Settings section, see Constraint Settings in the COMSOL Multiphysics Reference Manual.