Port
The Port boundary condition is used to excite and absorb specific acoustic modes that enter or leave waveguide structures, like a duct or channel, in a linearized potential flow model. A given port condition supports one specific propagating mode. To provide the full acoustic description, combine several port conditions on the same boundary. Make sure that all relevant propagating modes in the studied frequency range are included (among the modes that have a cutoff frequency in the frequency range). By doing this, the combined port conditions provide a superior nonreflecting or radiation condition for waveguides to, for example, the Plane Wave Radiation condition or a perfectly matched layer (PML) configuration. Modal decomposition is also used in jet turbofan noise modeling, where the noise source from the rotating turbofan is decomposed into modes, see Ref. 24 and Ref. 25.
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 boundary condition exists for 3D and 2D axisymmetric models.
On a given boundary, a combination of ports will define the total acoustic field (sum of incident and outgoing waves). For a linearized potential flow model the total velocity potential ϕtot is then
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 the phase of the incident field, ϕin is the mode shape of the incident mode, and ϕout is the mode shape of the outgoing mode. In the presence of a background flow, incident and outgoing modes shapes are not identical for the same mode. The mode wavenumber will also be shifted due to the convective effects. The mode shapes are normalized to have either a unit maximum amplitude, a unit power, or a unit intensity (see the normalization option in the Global Port Settings section). 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 multimode expansion of the solution on the given port. The scattering parameters are automatically calculated when an acoustic model is set up with just one port exciting the system.
When working with ports in the linearized potential flow interface, it is important to remember that computed duct modes (for example, using the The Linearized Potential Flow, Boundary Mode Interface) do not represent an orthogonal basis set when a background flow or an impedance boundary condition is introduced, see Ref. 26. This fact is taken into account in the formulation of the port equations, by including so-called cross-terms between the modes.
In practice, this means that energy based postprocessing variables like the incident or outgoing power of a given mode cannot be used directly. The energy based expressions are given by the product of acoustic variables, so when modes are no longer orthogonal, energy is also carried by the cross-terms that exist in the model. Analyzing the total incident or outgoing power can instead be done by integrating the intensity variables defined by the linearized potential flow model: lpff.Ix, lpff.Iy, etc.
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 port sweep functionality. The port name is automatically incremented by one every time a port condition is added. The Type of port only has the User defined (the default) option.
Port Outgoing Mode Settings
Select if the Outgoing mode is On (the default) or Off, letting you define if an outgoing mode is handled by the port and a scattering parameter solved for. For certain combinations of background flow, boundary conditions, and frequency a situation can occur when no outgoing mode exists.
•
Enter an expression for the Outgoing mode shape (SI unit: m2/s).
•
Enter an expression for the Outgoing mode wave number (SI unit: rad/m).
Port Incident Mode Settings
Select if the Incident mode is On or Off (the default), letting you define if an incident mode is exciting the system and handled by the port. In the same manner as for the outgoing mode, a situation can occur when no incident modes exist.
•
Enter an expression for the Incident mode shape (SI unit: m2/s).
•
Enter an expression for the Incident mode wave number (SI unit: rad/m).
The mode shape and the mode wave number can be computed using the The Linearized Potential Flow, Boundary Mode Interface. The boundary mode study will typically produce a solution with many modes and mode wave numbers. The simplest way to use a specific computed mode in the port is to use the withsol() operator.
For example, to select the 3rd eigenvalue mode shape and out-of-plane wave number, type:
•
withsol('sol1',phi,setind(lambda,3))
•
withsol('sol1',lpfbm.kz,setind(lambda,3))
where 'sol1' refers to the solution of the boundary mode study. This tag is easily found looking at the datasets. It is also assumed that the dependent variable is here called phi, and that the tag to the boundary mode interface is lpfbm.
Optionally, if, for example, a parametric sweep is run over a given parameter, say a Mach number M, the withsol() operator can be called with the additional argument setval(M,0.5), to retrieve the solution for the parameter M equal to 0.5.
Port Geometry
The settings in this section ensure that the correct port area (including symmetries) is used for computing important quantities like power, volume flow, and transfer matrix elements, associated with the port
Select the Port area as Use symmetries (the default) or Selected boundaries.
•
When Use symmetries is selected, symmetry conditions adjacent to the port will automatically be taken into account if the Port area multiplication factor is set to Automatic (the default); if set to User defined, enter the area multiplication factor Ascale manually.
•
When Selected boundaries is selected, the port will have the area of the selected boundaries, without taking any symmetry conditions into account.
Constraint Settings
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.