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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If From material is selected (the default), the equilibrium density, and its dependence on the equilibrium pressure p0 and temperature T0, is taken from the defined material. Make sure that the Thermal Expansion and Compressibility settings are correct.
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For Ideal gas also select the Gas constant type — select Specific gas constant Rs (SI unit: J/(kg·K) or Mean molar mass Mn (SI unit: kg/mol)
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For User defined enter a value or expression for the Equilibrium density ρ0(p0, T0) (SI unit: kg/m3). The default is ta.p0/(287[J/kg/K]*ta.T0), which is the ideal gas law.
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Dynamic viscosity μ (SI unit: Pa·s).
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Bulk viscosity μB (SI unit: Pa·s). The bulk viscosity parameter describes the difference between the mechanical and thermodynamic pressures. It is associated with losses due to expansion and compression.
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Thermal conductivity k (SI unit: W/(m·K)).
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Heat capacity at constant pressure Cp (SI unit: J/(kg·K)). This is the specific heat capacity or heat capacity per unit mass.
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If the material is air, the From equilibrium density option works well as the equilibrium density ρ0 = ρ0(p0,T0) is defined as a function of both pressure and temperature.
For the water material the coefficient of thermal expansion is well defined as ρ0 = ρ0(T0), while the compressibility should be defined using the default From speed of sound option.
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Speed of sound c (SI unit: m/s).
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Ratio of specific heats γ (dimensionless). The default is 1.
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See the Theory Background for the Thermoviscous Acoustics Branch section for a detailed description of the governing equations and the constitutive relations.
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In certain cases it can be interesting not to include thermal conduction in the model and treat all processes as adiabatic (isentropic). This is, for example, relevant for most liquids where the thermal boundary layer is much thinner than the viscous. Not solving for the temperature field T also saves some degrees of freedom (DOFs). This is achieved by selecting the Adiabatic formulation option under the Thermoviscous Acoustics Equation Settings section.
See also Solver Suggestions for Large Thermoviscous Acoustics Models for suggestions on how to select an iterative solver for large problems.
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