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7.27.2 Procedure for Fixing Values of Variables in a Zone

To fix the values of one or more variables in a cell zone, follow these steps (remembering to use only SI units):

1.   In the Fluid panel or Solid panel, turn on the Fixed Values option.

2.   Fix the values for the appropriate variables, noting the comments below.

  • To specify a constant value for a variable, choose constant in the drop-down list next to the relevant field and then enter the constant value in the field.

  • To specify a non-constant value for a variable, you can use a boundary profile (see Section  7.26) or a user-defined function for a boundary profile (see the separate UDF Manual). Select the appropriate profile or UDF in the drop-down list next to the relevant field.

    If you specify a radial-type boundary profile (see Section  7.26.1) for temperature, enthalpy, species mass fractions, or turbulence quantities for the $k$- $\epsilon$, Spalart-Allmaras, or $k$- $\omega$ model, the local coordinate system upon which the radial profile is based is defined by the Rotation-Axis Origin and Rotation-Axis Direction for the fluid zone. See Section  7.17.1 for information about setting these parameters. (Note that it is acceptable to specify the rotation axis and direction for a non-rotating zone. This will not cause the zone to rotate; it will not rotate unless it has been explicitly defined as a moving zone.)

  • If you do not want to fix the value for a variable, choose (or keep) none in the drop-down list next to the relevant field. This is the default for all variables.



Fixing Velocity Components


To fix the velocity components, you can specify X, Y, and (in 3D) Z Velocity values, or, for axisymmetric cases, Axial, Radial, and (for axisymmetric swirl) Swirl Velocity values. The units for a fixed velocity are m/s.

For 3D cases, you can choose to specify cylindrical velocity components instead of Cartesian components. Turn on the Local Coordinate System For Fixed Velocities option, and then specify the Axial, Radial, and/or Tangential Velocity values. The local coordinate system is defined by the Rotation-Axis Origin and Rotation-Axis Direction for the fluid zone. See Section  7.17.1 for information about setting these parameters. (Note that it is acceptable to specify the rotation axis and direction for a non-rotating zone. This will not cause the zone to rotate; it will not rotate unless it has been explicitly defined as a moving zone.)

figure   

You can fix values for velocity components only if you are using the pressure-based segregated solver. (Refer to Section  25.1.1 for information about the pressure-based segregated solver.)



Fixing Temperature and Enthalpy


If you are solving the energy equation, you can fix the temperature in a zone by specifying the value of the Temperature. The units for a fixed temperature are K.

If you are using the non-premixed combustion model, you can fix the enthalpy in a zone by specifying the value of the Enthalpy. The units for a fixed enthalpy are m $^2$/s $^2$.

If you specify a radial-type boundary profile (see Section  7.26.1) for temperature or enthalpy, the local coordinate system upon which the radial profile is based is defined by the Rotation-Axis Origin and Rotation-Axis Direction for the fluid zone. See above for details.

figure   

You can fix the value of temperature only if you are using the pressure-based solver.



Fixing Species Mass Fractions


If you are using the species transport model, you can fix the values of the species mass fractions for individual species. FLUENT allows you to fix the species mass fraction for each species (e.g., h2, o2) except the last one you defined.

If you specify a radial-type boundary profile (see Section  7.26.1) for a species mass fraction, the local coordinate system upon which the radial profile is based is defined by the Rotation-Axis Origin and Rotation-Axis Direction for the fluid zone. See above for details.

figure   

You can fix values for species mass fractions only if you are using the pressure-based solver.



Fixing Turbulence Quantities


To fix the values of $k$ and $\epsilon$ in the $k$- $\epsilon$ equations, specify the Turbulence Kinetic Energy and Turbulence Dissipation Rate values. The units for $k$ are m $^2$/s $^2$ and those for $\epsilon$ are m $^2$/s $^3$.

To fix the value of the modified turbulent viscosity ( $\tilde{\nu}$) for the Spalart-Allmaras model, specify the Modified Turbulent Viscosity value. The units for the modified turbulent viscosity are m $^2$/s.

To fix the values of $k$ and $\omega$ in the $k$- $\omega$ equations, specify the Turbulence Kinetic Energy and Specific Dissipation Rate values. The units for $k$ are m $^2$/s $^2$ and those for $\omega$ are 1/s.

To fix the value of $k$, $\epsilon$, or the Reynolds stresses in the RSM transport equations, specify the Turbulence Kinetic Energy, Turbulence Dissipation Rate, UU Reynolds Stress, VV Reynolds Stress, WW Reynolds Stress, UV Reynolds Stress, VW Reynolds Stress, and/or UW Reynolds Stress. The units for $k$ and the Reynolds stresses are m $^2$/s $^2$, and those for $\epsilon$ are m $^2$/s $^3$.

If you specify a radial-type boundary profile (see Section  7.26.1) for $k$, $\epsilon$, $\omega$, or $\tilde{\nu}$, the local coordinate system upon which the radial profile is based is defined by the Rotation-Axis Origin and Rotation-Axis Direction for the fluid zone. See above for details. Note that you cannot specify radial-type profiles for the Reynolds stresses.



Fixing User-Defined Scalars


To fix the value of a user-defined scalar, specify the User defined scalar-n value. (There will be one for each user-defined scalar you have defined.) The units for a user-defined scalar will be the appropriate SI units for the scalar quantity. See the separate UDF Manual for information on user-defined scalars.


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