A Partial Fraction Fit function (
) uses a modified adaptive Antoulas–Anderson (AAA) algorithm, AAA
2, to compute a partial fractional fit. Use this function for a rational approximation of frequency-domain responses. This approximation makes it possible to compute its inverse Fourier transform analytically and thus obtain the time-domain impulse response function. Doing so is useful in, for example, models using a Pressure Acoustics, Transient or Pressure Acoustics, Time Explicit interface. You select frequency-dependent impedance data as input and can then use the fitted parameters to set up an impedance condition. See the
Acoustics Module User’s Guide for more details. The Partial Fraction Fit function is, in general, a complex-valued function of a real-valued argument (frequency). The algorithm treats the input data as complex numbers, not separating its real and imaginary parts.
Select a Data source —
File or
Result table to define the data source for the partial fraction fit function.
If you select File (the default), enter the complete network path and name of the data file in the
Filename field, or click
Browse to select a text or data file with spreadsheet data in the
Interpolation Data dialog box. The data should contain a column containing frequency values and columns containing real and imaginary value parts. You can import data files with comma-separated, semicolon-separated, space-separated, and tab-separated data. You can also click the downward arrow beside the
Browse button and choose
Browse From (
) to open the fullscreen
Select File window. Click the downward arrow for the
Location menu (
) to choose
Show in Auxiliary Data (
) to move to the row for this file in the
Auxiliary Data window,
Copy Location (
), and (if you have copied a file location)
Paste Location (
). Also choose a decimal separator from the
Decimal separator list:
Point (the default) or
Comma. Click
Import (
) to import the data into the model; otherwise, COMSOL Multiphysics references the data on your file system. Click
Export to save the data for the partial fraction fit function to a file and reference from there instead of including it in the model. Click the
Discard button to delete the imported data for the partial fraction fit function from the model. Click the
Refresh button (
)to ensure that the file is reread when needed.
If you select Result table, choose the table to use from the
Result table list.
In the Data Column Settings section, you specify how each column in the input data, displayed in the
Columns column should be interpreted in the
Type column — as
Frequency,
Real,
Imaginary, or as an
Ignored column — and their units, if applicable, can be entered or selected from the
Unit list underneath the table and will appear in the
Settings column. The function can only have one
Frequency column, one
Real column, and one
Imaginary column. When you click the
Fit Parameters button (
) at the top of the
Settings window, the AAA
2 algorithm is run on the input data to make a partial fraction fit, which in turn returns poles, residues, and an asymptotic term. Those are used in an analytic expression that defines the function.
If desired, change the relative tolerance for the partial fraction fit in the Tolerance field (default: 1·10
−3). In practice, a higher tolerance results in a greater number of poles and residues and increases the computation time. A too tight tolerance may cause the algorithm to stagnate and yield spurious poles (Froissart doublets).
This section contains the real-valued parameters in the Residue, R and
Pole, ξ columns of the table under
Real residues and poles and complex-valued parameters in the
Residue, Q and
Pole, ζ columns of the table under
Complex residues and poles. Use the
Delete Row (
) and
Clear Table (
) buttons as desired to edit the tables. Use the
Load from File (
) and
Save to File (
) buttons to load or save data to or from the tables. You can also click the downward arrow beside the
Load from File button and choose
Load From (
) to open the fullscreen
Select File window.
Click the Update Residues button (
) after you have modified or discarded existing poles or manually added new poles. The residues and the asymptotic term will be updated. This may be useful if, for example, you discard poles, whose contributions are negligible.