COMSOL 6.3 - Poroacoustics

Poroacoustics

The feature allow modeling of porous materials in the time domain, using an equivalent fluid model. The properties of the equivalent fluid, representing the porous material, are known (measured or simulated) in term of its real and imaginary part of the (isentropic) compressibility βs(f) and density ρ(f), both functions of frequency. To capture the behavior in the time domain, the feature has built-in functionality for setting up and defining the compressibility and density expressions through a rational approximation (or rational expansion) of the frequency dependent data. The approximation has an analytical inverse Fourier transform and can thus be used to define the same frequency dependency in the time domain, by setting up a system of memory equations. These equations are automatically defined by the feature and solved in the porous domain. Fitting of the frequency domain data can be performed using the Partial Fraction Fit function.


	Porous Absorber with Local and Extended Reacting Approximations for Time-Domain Modeling. Application Library path Acoustics_Module/Building_and_Room_Acoustics/porous_absorber_time_domain


	For more information see Partial Fraction Fit under User-Defined Functions in the COMSOL Multiphysics Reference Manual.

Poroacoustics Model

The only option for the is .

Isentropic Compressibility

Select the fit as (the default) or .

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For the option enter all the necessary parameters that are present in the rational approximation (rational expansion), that is, the β∞ (SI unit: 1/Pa), enter the (R and ξ) into the table, and enter the (Q and ζ) into the table (note that the conjugate terms are automatically added). The quantities can also be loaded from a file (remember to include the number column). Select if the rational approximation function is scaled for frequency data or angular frequency data, by selecting the option (selected per default).

•

For the option select the , which is a Partial Fraction Fit function defined in the model, then click the icon (

) to copy the fitted data to the compressibility field, and the residues and poles tables. To go to the function source click the icon (

Density

Select the fit as (the default) or .

•

For the option enter all the necessary parameters that are present in the rational approximation (rational expansion), that is, the ρ∞ (SI unit: kg/m3), enter the (R and ξ) into the table, and enter the (Q and ζ) into the table (note that the conjugate terms are automatically added). The quantities can also be loaded from a file (remember to include the number column). Select if the rational approximation function is scaled for frequency data or angular frequency data, by selecting the option (selected per default).

•

For the option select the , which is a Partial Fraction Fit function defined in the model, then click the icon (

) to copy the fitted data to the density field, and the residues and poles tables. To go to the function source click the icon (


	The necessary data, the complex-valued and frequency-dependent data for the compressibility and the density, can be generated from a small frequency domain model that uses the Poroacoustics in the The Pressure Acoustics, Frequency Domain Interface. Define a domain with the desired Poroacoustics model, add the necessary material properties, and solve the model for the range of frequencies to be resolved (no sources are needed). Then add an Evaluation Group and a Point Evaluation (select a point in the porous domain). Evaluate real(acpr.rho_c) and imag(acpr.rho_c) for the density; and real(1/acpr.K_eq) and imag(1/acpr.K_eq) for the compressibility. The Partial Fraction Fit function can point directly to the Results table or the data can be stored in a .txt file and referred to.