21 - 1 Specifying Fluid Properties
Fluid properties, such as viscosity, describe the fluid model used to create the dataset. These properties are required for many calculations performed by other dialogs. They are set via the Fluid Properties dialog. Values entered must be dimensionally consistent with each other and with your dataset. If you imported your data using the PLOT3D data loader, the default fluid properties will most likely suit your needs.
For a layout with multiple datasets, a separate set of fluid properties is maintained for each dataset. You can copy the settings from one dataset to another using the Save Settings and Load Settings options in the Analyze menu. These actions also transfer the settings made in the Reference Values, Field Variables, Geometry and Boundaries, and Unsteady Flow Options dialogs.
The Fluid Properties dialog is accessed by selecting Fluid Properties from the Analyze menu.
The Fluid Properties dialog allows you to specify properties for a compressible or incompressible fluid. For incompressible (uniform density) fluids, you specify density, specific heat, viscosity and conductivity. For compressible (variable density) fluids, you specify the gas constant, gamma (the ratio of specific heats), viscosity and conductivity.
By default, each fluid property is a constant. However, each property can be overridden by a field (dataset) variable (with the exception of density). When a field variable is assigned, the local value of that variable is used for field calculations using that property, and the constant value is used only for global calculations, such as the calculation of reference (free-stream) quantities. To assign a field variable for a particular property, set the Use Field Variable toggle and click Select to choose a variable from the current dataset from the Select Variable dialog.
• Incompressible - Toggle-on to indicate the fluid is incompressible. For incompressible fluids, you must specify density, specific heat, viscosity and conductivity. For compressible fluids, you must specify gas constant, gamma, viscosity and conductivity.
• Density (for incompressible fluids only) - Density represents the mass of fluid occupied by a unit volume. Its dimensions are [Mass]/[Length]**3.
• Specific Heat (for incompressible fluids only) - Specific heat is the amount of energy required to raise a unit mass of the fluid one degree in temperature. Dimensions are [Length]**2/[Time]**2[Temperature].
• Gas Constant (for compressible fluids only) - The specific gas constant has dimensions of [Length]**2/[Time]**2[Temperature].
• Gamma (for compressible fluids only) - Gamma represents the ratio of the specific heat at constant pressure to the specific heat at constant volume, a non-dimensional quantity.
• Viscosity - The dynamic viscosity's dimensions are [Mass] / [Length] [Time].
• Conductivity - The thermal conductivity's dimensions are [Mass] [Length] / [Time]**3[Temperature].
21 - 1.1 Specifying Incompressible Fluid Properties
When the Incompressible check box is selected, the density of the fluid and its specific heat (Cv), viscosity (m), and conductivity (k) must be entered. Gamma (g), the ratio of specific heats at constant volume and pressure, is unity for incompressible fluids, so the Gamma section is inactive. Gas Constant (R) is also inactive. The thermal and caloric equations of state for incompressible fluids are shown below. r is density, and e represents the internal energy per unit mass.
Since the density entered in the Fluid Properties dialog represents the density of the fluid throughout the physical domain, you are not allowed to enter a reference value for density in the Reference Values dialog, or choose a density field variable on the Field Variables dialog (see Section 21 - 3.2 “Identifying State Variables”).
Specific heat (Cv) is the amount of energy required to raise a unit mass of the fluid one degree. It has dimensions of:
Viscosity (m) represents the dynamic viscosity coefficient, in units of
Conductivity (k) is the thermal conductivity of the fluid, in units of:
21 - 1.2 Specifying Compressible Fluid Properties
When the Incompressible check box is not selected, the specific gas constant, gamma, viscosity and conductivity must be entered. Since density is not a constant property of compressible fluids, the Density text field is inactive, as is the Specific Heat section of the dialog. The thermal and caloric equations of state for compressible fluids are shown below. p is pressure, and e is internal energy per unit mass:
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The caloric equation of state assumes constant specific heats for the fluid. In situations where this assumption is not valid (such as high-temperature flows) The gas constant is the universal gas constant divided by the molecular weight of the fluid:
giving units of:
Gamma is the ratio of the gas specific heats and is non-dimensional:
21 - 1.3 Working with Non-dimensional Data
Consider a case where temperature is non-dimensionalized by dividing it by free-stream temperature:
and pressure is non-dimensionalized with gamma (the ratio of specific heats) and free-stream pressure:
We wish to know what to enter for the gas constant in the Fluid Properties dialog. We plug what we know into the thermal equation of state (where r is density and R is the gas constant):
Since the equation of state must hold for the free-stream conditions, we know:
From this, we see that the product of denominators (1) and (2) in the second-previous equation must equal: 
.
This doesn't entirely answer our question, however, and in the absence of additional information, we simply need to decide how r and R are each individually non-dimensionalized. The requirement we just determined is that the product of the two must be non-dimensionalized by 
. So we may decide to non-dimensionalize density by free-stream density,r¥, which leaves the gas constant non-dimensionalized (that is, divided) by 
. In the Fluid Properties dialog, we enter 
for Gas Constant. If we chose to leave Gas Constant at unity, density would be non-dimensionalized by gamma and free-stream density, 
.