Port
The Port boundary condition is used to excite and absorb elastic waves that enter or leave waveguide structures. A given port condition supports one specific propagating mode. To provide the full description, combine several port conditions on the same boundary, for example, one for longitudinal, one for torsional, and one for transverse modes. Make sure that all relevant propagating modes in the studied frequency range are included (all modes that have a cutoff frequency in the frequency range). By doing this, the combined port conditions provide a superior nonreflecting condition for waveguides to, for example, a perfectly matched layer (PML) configuration or the Low-Reflecting Boundary. The same port boundary condition feature should not be applied to several waveguide inlets/outlets. The port condition supports S-parameter (scattering parameter) calculation but it can also be used as a source to just excite a system.
The Port node is only available with some COMSOL products (see https://www.comsol.com/products/specifications/).
The Port boundary condition exist only for 3D models.
See the tutorial Mechanical Multiport System: Elastic Wave Propagation in a Small Aluminum Plate. Application Library path: Structural_Mechanics_Module/Elastic_Waves/mechanical_multiport_system
On a given boundary, a combination of ports will define the displacement field u (sum of incident and outgoing waves) as
where the summation “i” is over all ports on the given boundary “bnd”, Sij is the scattering parameter, Ain is the amplitude of the incident field (at port “j”), φ is a possible phase, and ui is the mode shape of the i-th port. The mode shape ui is normalized to carry a unit power of 1 W. This definition means that the scattering parameter Sij defines the amplitude of mode i when a system is exited at port j (with mode j). This corresponds to a multi-mode expansion of the solution on the given port. The scattering parameters are automatically calculated when a model is set up with just one port exciting the system. To get the full scattering matrix, The Port Sweep Functionality can be used.
Only one port should be excited at a time if the purpose is to compute S-parameters. The S-parameters are defined as solid.Smatrix11, solid.Smatrix21, etc. and can be used in postprocessing.
In other cases, having more than one port exciting the system might be wanted, but the S-parameter variables cannot be correctly computed. When several ports are excited, the S-parameter output is turned off.
Port Properties
Enter a unique Port name. Only nonnegative integer numbers can be used as Port name as it is used to define the elements of the S-parameter matrix. The numeric port names are also required for the port sweep functionality. The port name is automatically incremented by one every time a port condition is added.
Select a Type of port: Numeric or User defined. Depending on the selection, different options appear in the Port Mode Settings section (see below).
Port Mode Settings
Depending on the option selected in the Type of port (see above):
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In the case of a Numeric port, a boundary mode problem is solved on the port face to compute the desired propagating mode. This option requires the use of a Boundary Mode Analysis step in the study. It should be placed before the Frequency Domain step. In the study, add one Boundary Mode Analysis step for each Numeric port and make sure to reference the proper Port name in the study step. When using the Numeric port, it is not possible to perform a frequency sweep in the Frequency Domain study step. Only one frequency can be used and it should correspond to the Mode analysis frequency entered in the Boundary Mode Analysis step(s). One option is to add a Parametric Sweep and define a parameter for the frequency used in both the steps. In this case, care should be taken when setting up the search criteria in the mode analysis.
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For User defined, enter user defined expressions for the Mode shape un and the Mode wave number kn. The mode shape will automatically be scaled to unit power before it is used in the port condition. Use the user defined option to enter a known analytical expression.
When the Numeric port option is used and the boundary mode analysis is run, the boundary conditions from the Solid Mechanics model are automatically inherited in the boundary problem. For this automatic procedure, there is only support for the Free, Fixed, Roller, Symmetry, Antisymmetry, and Spring Foundation conditions.
Incident Mode Settings
Activate if the given port is excited by an incident wave of the given mode shape. For the first 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. If more than one port in a model is excited, the S-parameter calculation is not performed.
When the Incident wave excitation at this port is set to On, then select how to define the incident wave. Select the Define incident wave: Displacement amplitude or Power.
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For Displacement amplitude enter the amplitude Ain 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 Pin of the incident wave. This is in general defined as the RMS power of the incident wave.
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For both options enter the phase φ of the incident wave. This phase contribution is multiplied with the amplitude defined by the above options. The Displacement amplitude input can be a complex number.
Note that when the Activate port sweep option is selected at the physics level, the options in the Incident Mode Settings section are deactivated. This is because this option automatically sends in a mode of unit power, sweeping through one port at the time.
All ports with incident wave excitation turned off have an arrow symbol, added in the Graphics window, that points outward. When incident wave excitation is turned on, the arrow symbol points inward.
If a waveguide is cut by one or several symmetry planes, care should be taken when setting up the port modes and when postprocessing.
When postprocessing, remember that absolute values like, for example, the outgoing power at port 1, solid.port1.P_out, needs to be multiplied with an appropriate factor. Multiply with 2 if one symmetry plane is used etc.
Constraint Settings
To display this section, click the Show More Options button () and select Advanced Physics Options.
The Port Sweep Functionality
The port sweep functionality is used to reconstruct the full scattering matrix Sij by automatically sweeping the port excitation through all the ports included in the model. When the port sweep is activated, the options in the Incident Mode Settings in the port conditions are deactivated and COMSOL controls which port that is excited with an incident mode.
The port sweep functionality is activated at the main physics interface level by selecting Activate port sweep in the Port Sweep Settings section. Enter the Sweep parameter name, the default is PortName. Create a parameter with the same name under Global Definitions>Parameters 1. This is the name of the parameter to be used in a parametric sweep, here it should represent the Port name integer values (defined when adding the port conditions). Add a parametric sweep study step and run the sweep over the PortName parameter with an integer number of values representing all the ports in the model. Once the model is solved the full scattering matrix can be evaluated using the defined global variables solid.Smatrix11, solid.Smatrix21, solid.Smatrix12 etc.
Use the Global Matrix Evaluation under Derived Values to evaluate the full scattering matrix solid.Smatrix.
Location in User Interface
Context Menus
Solid Mechanics>More>Port
Ribbon
Physics tab with Solid Mechanics selected:
Boundaries>More>Port