5.2.1 Definitions
This section explains the following:
Geometry related variables
Area: Area (m2) of a boundary or interface.
Normal: Normal vector 
to a boundary or interface.
Velocity
Velocity: The components of velocity are computed with respect to the coordinate system 
as 
. The unit for velocity of the fluid is meters per second (m/s).
Velocity Magnitude: Magnitude of the velocity in meters per second (m/s) expressed as:
|
5.3 |
where 
are the velocity components in the 
directions respectively.
Axial Velocity Component: Fluid velocity (m/s) in the direction of the Rotational Axis Vector.
Normal Velocity Component: Fluid velocity (m/s) normal to a boundary.
Speed of Sound: Speed of sound (
) is given as:
|
5.4 |
where
|
Ratio of the specific heat at constant pressure ( ) to specific heat at constant
volume ( ).
(1.4 for air at STP) |
|
Gas constant (287 J/kg-K for air). It equals the universal gas constant divided by the molecular weight. |
|
Absolute temperature (K) |
Mach Number (M): The ratio of the fluid velocity to the speed of sound.
Vorticity: Magnitude of the local angular rate of rotation of the fluid (1/s). Mathematically, it is the curl of the local velocity.
|
5.5 |
The velocity used to compute the vorticity is with respect to the stationary coordinate system 
.
Vorticity Magnitude: Magnitude of the vorticity (1/s), expressed as:
|
5.6 |
where 
are the vorticity components in 
directions respectively.
Pressure
Pressure (
): Static pressure in the fluid. The unit for static pressure is Pa. Pressure is always specified as absolute value.
Total Pressure (
): Sum of the static pressure in the fluid and the dynamic pressure. The unit for total pressure is Pa.
The total pressure relations are different for incompressible and compressible flows. They are expressed as follows:
Incompressible Flow
|
where 
is the velocity magnitude:
Compressible Flow
Compressible flows are typically characterized by the total pressure 
and total temperature 
of the flow. For an ideal gas, these quantities can be related to the static pressure and temperature by the following relation:
|
For constant 
, equation 5.8 reduces to the isentropic conditions in aerodynamics:
|
5.9 |
|
5.10 |
where
|
Gas constant (J/kg-K). |
|
Specific heat at constant pressure (J/kg-K) |
|
Ratio of the specific heat at constant pressure ( ) to specific heat at constant
volume ( ).
|
|
Mach Number |
|
Speed of sound (m/s) |
Compression Factor: Compression factor (
) is the ratio of molar volume of a gas to the molar volume of an Ideal Gas. It is used to account for real gas effects which become important the closer the gas gets to a phase change, i.e. low temperatures and/or high pressures. 
for an ideal gas.
Pressure Force: Force due to the normal pressure on a boundary or interface. The unit for pressure force is N.
Average Static Pressure: The average static pressure in a selected fluid volume or boundary. The unit for average static pressure is Pa.
Average Total Pressure: Area averaged value of the total pressure at a boundary or interface, given as:
|
|
5.11 |
where the summations are over all the faces associated with the boundary. The unit for average total pressure is Pa. It may also be expressed as mass averaged total pressure, in which case the mass flux is used as the weighting function.
Flow quantities
Viscosity: Dynamic viscosity (Poise or Pa-s) as used in the Flow module formulation.
Viscous Force: Force due to the fluid shear on a boundary or interface. The unit for viscous force is N.
Gravity Force: The gravity vector expressed in terms of 
components relative to the coordinate system 
. The magnitude of each component has the unit m/s2.
Blanked: Does not solve for a primary variable in a selected volume.
Rotation
Rotational Axis Vector: Rotational axis vector is used to specify the axis of rotation. The direction is specified in terms of a vector relative to the stationary coordinate system 
.
Rotational Center: Rotational center is specified as 
coordinate lying on the Rotational Axis Vector.
Rotational Direction: The direction of rotation is specified as Clockwise or Counterclockwise relative to a Rotational Axis Vector, using the convention that a positive clockwise rotation is based on the vantage point with the Rotational Axis Vector pointing at the viewer.
Rotational Speed: Denotes the speed of rotation. The Rotational Center, Rotational Direction and Rotational Speed are used together with the Rotational Axis Vector to determine the motion and can apply to a Rotating Reference Frame, initial condition, boundary condition, or a remeshing (moving) volume.
Output quantities
Mass Flow Rate: Mass per second crossing a boundary or interface. The unit for mass flow rate is kg/s.
Volumetric Flow Rate: Volume per second crossing a boundary or interface. The unit for volumetric flow rate is m3/s.
Momentum: Momentum force is the product of the mass flux (
) and the velocity, 
.The unit for momentum is N.
Power: Power about the axis of rotation for a rotating boundary. The sign of the power is power of the fluid on the boundary, such that a positive power corresponds to work done by the fluid on the boundary. The unit for power is W.
Revolution Average Mass Flux: Running average of the mass flow rate crossing a boundary or interface over the last revolution. The unit for revolution average mass flux is (kg/s).
Revolution Average Power: Running average of the power applied to a rotating boundary or interface over the last revolution. The unit for revolution average power is W.
Revolution Average Volumetric Flux: Running average of the volumetric flow rate crossing a boundary or interface over the last revolution.
Torque: 
components of the torque about a user provided reference center for a rotating boundary. The signs of the torque components are the torque of the fluid on the boundary, using the right-hand rule relative to the respective 
axis.
Torque in Axis Direction: Torque about the axis of rotation for a rotating boundary. The sign of the torque in axis direction is the torque of the fluid on the boundary, using the right-hand rule relative to the Rotational Axis Vector.