Fluid
The Fluid node adds the continuity, momentum and temperature equations for an ideal gas but omits volume forces and heat sources. Volume forces and heat sources can be added as separate physics features. Viscous heating and pressure work terms are added by default to the temperature equation.
When the turbulence model type is set to RANS, the Fluid node also adds the equations for k and ε, or the undamped turbulent kinematic viscosity, depending on the turbulence model used.
The thermal conductivity describes the relationship between the heat flux vector q and the temperature gradient ∇T as in q = −k∇T, which is Fourier’s law of heat conduction. Enter this quantity as power per length and temperature.
Heat Conduction
Select a Thermal conductivity k (SI unit: W/(m·K)) from the list—Sutherland’s Law (the default), From material, or User defined. For User defined select Isotropic, Diagonal, Symmetric, or Anisotropic based on the characteristics of the thermal conductivity and enter another value or expression in the field or matrix.
Sutherland’s Law
For Sutherland’s Law enter the following model parameters:
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Conductivity at reference temperature kref (SI unit: W/(m·K))
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Reference temperature Tk,ref (SI unit: K)
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Sutherland constant Sk (SI unit: K)
Sutherland’s law describes the relationship between the thermal conductivity and the total temperature of an ideal fluid according to
Thermodynamics
The High Mach Number Flow interface is applicable for ideal gases. Specify the thermodynamics properties by selecting a gas constant type and selecting between entering the heat capacity at constant pressure or the ratio of specific heats. For an ideal gas the density is defined as
where pA is the absolute pressure, and T is the temperature.
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Select a Gas constant type—Specific gas constant Rs (SI unit: J/(kg·K)) or Mean molar mass Mn (SI unit: kg/mol). The default setting is to use the property value From material. For User defined enter another value or expression for either material property. For Mean molar mass the universal gas constant R = 8.314 J/(mol·K), which is a built-in physical constant, is also used.
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From the Specify Cp or γ list, select Heat capacity at constant pressure Cp (SI unit: J/(kg·K)) or Ratio of specific heats γ (dimensionless). The default setting is to use the property value From material. For User defined enter another value or expression for either material property.
Dynamic Viscosity
The dynamic viscosity describes the relationship between the shear rate and the shear stresses in a fluid.
Select a Dynamic viscosity μ (SI unit: Pa·s) from the list—Sutherland’s Law (the default), From material, or User defined.
Sutherland’s Law
For Sutherland’s Law enter the following model parameters:
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Dynamic viscosity at reference temperature μref (SI unit: Pa·s)
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Reference temperature Tμ,ref (SI unit: K)
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Sutherland constant Sμ (SI unit: K)
Sutherland’s law describes the relationship between the dynamic viscosity and the total temperature of an ideal fluid according to
Mixing Length Limit
This section is available for the Turbulent Flow, k-ε model.
The k-ε turbulence model needs an upper limit on the mixing length to be numerically robust. Select a Mixing length limit — Automatic (the default) or Manual.
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For Automatic this limit is automatically evaluated as:
(6-1)
where lbb is the shortest side of the geometry bounding box. If the geometry is a complicated system of very slender entities, for example, Equation 6-1 tends to give a result that is too large. In such cases, define manually.
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For Manual enter a value or expression for the Mixing length limit (SI unit: m).
Distance Equation
This section is available for Turbulent Flow, Spalart-Allmaras since a Wall Distance interface is then included.
Select how the Reference length scale lref (SI unit: m) is defined—Automatic (default) or Manual:
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For Automatic the wall distance is automatically evaluated to one tenth of the shortest side of the geometry bounding box. This is usually quite accurate but it can sometimes give a too high value if the geometry consists of several slim entities. In such cases, define the reference length scale manually.
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For Manual it defines a different value or expression for the length scale. The default is 1 m.
lref controls the result of the distance equation. Objects that are much smaller than lref are effectively be diminished while the distance to objects much larger than lref are accurately represented.