The advanced fluid models are defined using individual physics feature nodes: Poroacoustics, Narrow Region Acoustics, and Anisotropic Acoustics. In the time domain nonlinear effects can be included using the Nonlinear Acoustics (Westervelt) node.
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Details about the Model Input and the Default Model Inputs are found in the Global and Local Definitions chapter of the COMSOL Multiphysics Reference Manual.
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Linear elastic (the default) for defining the classical lossless fluid: Go to Linear Elastic Fluid Model.
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User-defined attenuation for defining losses through an attenuation coefficient: Go to User-Defined Attenuation Fluid Model.
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Atmosphere attenuation defines the standard attenuation of atmospheric (moist) air:
Go to Atmosphere Attenuation Fluid Model. |
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Ocean attenuation defines attenuation in the seawater of the ocean: Go to Ocean Attenuation Fluid Model.
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Viscous, Thermally conducting, and Thermally conducting and viscous defines the classical thermoviscous attenuation model: Go to Thermally Conducting and/or Viscous Fluid Model.
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General dissipation defines losses through the sound diffusivity: Go to General Dissipation Fluid Model.
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Ideal gas to define the properties of an ideal gas: Go to Ideal Gas Fluid Model.
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The fluid models represent different bulk loss or attenuation mechanisms (applied in a homogenized way) or ways of defining the properties of the fluid. Some of these models are sometimes referred to as equivalent fluid models. The loss model can be a theoretical model or a model based on measurement data for the attenuation in the fluid like the atmosphere or the ocean.
Losses in porous materials are defined in Poroacoustics. Thermoviscous boundary layer losses in narrow regions of constant cross section (like waveguides) can be modeled using Narrow Region Acoustics.
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For Density and speed of sound, define the Speed of sound c (SI unit: m/s) and Density ρ (SI unit: kg/m3).
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For Impedance and wave number, define the Characteristic acoustic impedance Z (SI unit: Pa·s/m) and enter a Wave number k (SI unit: rad/m).
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For Bulk modulus and density, define the Effective bulk modulus K (SI unit: Pa) and Density ρ (SI unit: kg/m3). Selecting User defined is well suited for entering the properties of a user-defined porous material fluid model. Predefined porous models exist in the Poroacoustics domain feature.
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Do not confuse the attenuation coefficient α with the attenuation constant m most often defined in room acoustics (it is also known as the intensity attenuation coefficient). The two are simply related through 2α = m. The difference stems from how the intensity scales in the propagation direction: I(x) = I0·exp(-2αx) = I0·exp(-mx).
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Speed of sound c (SI unit: m/s).
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Ratio of specific heats γ (dimensionless).
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Thermal conductivity k (SI unit: W/(m·K)).
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Dynamic viscosity μ (SI unit: Pa·s).
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It is possible to assess the magnitude of the losses due to thermal conduction and viscosity, that is, the power dissipation density (SI unit: W/m3). This is done during the analysis process by plotting the variables for:
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Select a Gas constant type: Specific gas constant Rs (SI unit: J/(kg·K) (the default) or Mean molar mass Mn (SI unit: kg/mol). For Mean molar mass, the molar gas constant (universal gas constant) R = 8.314 J/(mol·K) is used as the built-in physical constant.
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From the Specify Cp or γ list, select Heat capacity at constant pressure Cp (SI unit: J /(kg·K)) (the default) or Ratio of specific heats γ. For common diatomic gases such as air, γ = 1.4 is the standard value.
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