Pressure Acoustics
The Pressure Acoustics node adds the equations for time-harmonic and eigenfrequency acoustics modeling in the frequency domain. In the Settings window, define the properties for the acoustics model and model inputs including the background pressure and temperature.
The advanced fluid models are defined using individual physics feature nodes: Poroacoustics and Narrow Region Acoustics
For details about the available fluid models, see Theory for the Equivalent Fluid Models.
For more information about using variables during the results analysis, see Postprocessing Variables.
Model Inputs
If Ideal gas is selected as the Fluid model, enter a Temperature T and an Absolute pressure pA. For User defined enter a value or an expression for the absolute pressure (SI unit: Pa) and the Temperature (SI unit: K) in the field. This input is always available.
In addition, select a temperature field defined by a Heat Transfer interface or a Non-Isothermal Flow interface (if any), for example.
Non-Isothermal Flow requires the addition of the Heat Transfer Module or CFD Module.
If applicable, select a pressure defined by a Fluid Flow interface present in the model. For example, select Absolute pressure (spf) to use the absolute pressure defined by a Laminar Flow interface spf. This makes it possible to use a system-based (gauge) pressure, while automatically including the reference pressure in the absolute pressure.
The input to these fields influences the value of the material parameters in the model. Typically, the density ρ and the speed of sound c in the model depend on the absolute pressure and/or the temperature. Picking up any of those from another physics interface typically results in ρ = ρ(x) and c = c(x) to be specially varying.
Detailed aeroacoustic models that take into account the full background flow (including the movement of the fluid) are available under the Aeroacoustics Interfaces.
Pressure Acoustics Model
To define the properties of the bulk fluid, select a Fluid model from the list: Linear elastic (the default), Linear elastic with attenuation, Viscous, Thermally conducting, Thermally conducting and viscous, or Ideal gas.
The fluid models represent different bulk loss or attenuation mechanisms for (applied in a homogenized way) or ways of defining the fluid properties of the fluid. Some of these models are sometimes referred to as an equivalent fluid model. The loss model can be a theoretical model or a model based on measurement data for the attenuation in the fluid.
Losses in porous materials are defined in Poroacoustics and viscothermal losses in narrow regions are defined in Narrow Region Acoustics.
The fluid models may be roughly divided into these categories:
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General fluids:
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Linear elastic. Go to Defining a Linear Elastic Fluid Model.
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Linear elastic with attenuation. Go to Defining a Linear Elastic with Attenuation Fluid Model.
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Ideal gas. Go to Defining an Ideal Gas Fluid Model.
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Fluids with bulk viscous and/or thermal losses (for plane propagating waves):
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Viscous. Go to Defining a Viscous Fluid Model.
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Thermally conducting. Go to Defining a Thermally Conducting Fluid Model.
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Thermally conducting and viscous. Go to Defining a Thermally Conducting and Viscous Fluid Model.
The theory for the fluid models is in the section Theory for the Equivalent Fluid Models.
Defining a Linear Elastic Fluid Model
Linear elastic is the default. When the material parameters are real values this corresponds to a lossless compressible fluid. From the Specify list, select Density and speed of sound (the default), Impedance and wave number, or Bulk modulus and density. To add user defined losses, in a general way, specify the properties as complex-valued data.
For each of the following, the default values (when applicable) are taken From material or for User defined enter other values or expressions.
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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 Equivalent 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.
Defining a Linear Elastic with Attenuation Fluid Model
The Linear elastic with attenuation model adds a user defined attenuation to the fluid; this data is typically based on experimental data for the attenuation coefficient α. Adding attenuation makes the wave number k complex valued. For example, a plane wave p(x) moving in the x-direction is attenuated according to
When the attenuation is defined in Np per unit length, the wave has a spatial exponential decay governed by the attenuation coefficient.
The default Speed of sound c (SI unit: m/s) and Density ρ (SI unit: kg/m3) are taken From material. For User defined enter other values or expressions for one or both options.
Select an Attenuation type: Attenuation coefficient, Np per unit length (the default) to define an attenuation coefficient α in Np/m (nepers per meter), Attenuation coefficient, dB per unit length to define an attenuation coefficient α in dB/m (decibel per meter), or Attenuation coefficient, dB per wavelength to define an attenuation coefficient α in dB/λ (decibel per wavelength). For any selection, enter a value or expression in the Attenuation coefficient α edit field.
Linear Elastic with Attenuation Fluid Model
Defining an Ideal Gas Fluid Model
For Ideal Gas you can also edit the Model Inputs section. For each of the following, the default values are taken From material. For User defined enter other values or expressions for any or all options.
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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.
Defining a Viscous Fluid Model
The Viscous fluid model adds the attenuation due to bulk viscous losses. This type of model is relevant in highly viscous fluids or when acoustic waves are traveling over large distances (relative to the wavelength). The losses apply for plane propagating waves. This is not a model for viscous boundary layer losses in narrow regions, for this see Narrow Region Acoustics. For each of the following, the default values are taken From material. For User defined enter other values or expressions for any or all options.
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Speed of sound c (SI unit: m/s).
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Density ρ (SI unit: kg/m3).
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Dynamic viscosity μ (SI unit: Pa·s).
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Bulk viscosity μB (SI unit: Pa·s).
Viscous Fluid Model
Defining a Thermally Conducting Fluid Model
The Thermally conducting fluid model adds the attenuation due to thermal conduction effects in the bulk of the fluid. This type of model is relevant in fluids that have high thermal conduction or when acoustic waves are traveling over large distances (relative to the wavelength). The losses apply for plane propagating waves. This is not a model for thermal boundary layer losses in narrow regions, for this see Narrow Region Acoustics. For each of the following, the default values are taken From material. For User defined enter other values or expressions for any or all options.
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Speed of sound c (SI unit: m/s).
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Density ρ (SI unit: kg/m3).
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Heat capacity at constant pressure Cp (SI unit: J/(kg·K)).
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Ratio of specific heats γ (dimensionless).
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Thermal conductivity k (SI unit: W/(m·K)).
Thermally Conducting Fluid Model
Defining a Thermally Conducting and Viscous Fluid Model
The Thermally conducting and viscous fluid model adds the bulk attenuation that is due to the combined effect of viscous losses and thermal conduction. This type of model is relevant in fluids that have high thermal conduction and viscosity or when acoustic waves are traveling over large distances (relative to the wavelength). The losses apply for plane propagating waves. This is not a model for thermoviscous boundary layer losses in narrow regions, for this see Narrow Region Acoustics. For each of the following, the default values are taken From material. For User defined enter other values or expressions for any or all options.
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Speed of sound c (SI unit: m/s).
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Density ρ (SI unit: kg/m3).
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Heat capacity at constant pressure Cp (SI unit: J/(kg·K)).
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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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Bulk viscosity μB (SI unit: Pa·s).
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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the viscous power dissipation density (acpr.diss_visc),
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the thermal power dissipation density (acpr.diss_therm), or
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the combined total power dissipation density (acpr.diss_tot).
Thermally Conducting and Viscous Fluid Model