7.2.4 Fixing the Values of Variables

The option to fix values of variables in ANSYS FLUENT allows you to set the value of one or more variables in a fluid or solid zone, essentially setting a boundary condition for the variables within the cells of the zone. When a variable is fixed in a given cell, the transport equation for that variable is not solved in the cell (and the cell is not included when the residual sum is computed for that variable). The fixed value is used for the calculation of face fluxes between the cell and its neighbors. The result is a smooth transition between the fixed value of a variable and the values at the neighboring cells.

You can fix values for temperature and species mass fractions only if you are using the pressure-based solver. You can fix values for velocity components only if you are using the pressure-based segregated solver. (Refer to this section in the separate Theory Guide for information about the pressure-based segregated solver.)

Overview of Fixing the Value of a Variable

The ability to fix the value of a variable has a wide range of applications. The velocity fixing method is often used to model the flow in stirred tanks. This approach provides an alternative to the use of a rotating reference frame (solution in the reference frame of the blade) and can be used to model baffled tanks. In both 2D and 3D geometries, a fluid cell zone may be used in the impeller regions, and velocity components can be fixed based on measured data.

Although the actual impeller geometry can be modeled and the flow pattern calculated using the sliding mesh model, experimental data for the velocity profile in the outflow region are available for many impeller types. If you do not need to know the details of the flow around the blades for your problem, you can model the impeller by fixing the experimentally-obtained liquid velocities in its outflow zone. The velocities in the rest of the vessel can then be calculated using this fixed velocity profile as a boundary condition. Figure  7.2.5 shows an example of how this method is used to model the flow pattern created by a disk-turbine in an axisymmetric stirred vessel.

Figure 7.2.5: Fixing Values for the Flow in a Stirred Tank

Variables That Can Be Fixed

The variables that can be fixed include velocity components (pressure-based segregated solver only), turbulence quantities, temperature (pressure-based solver only), enthalpy, species mass fractions (pressure-based solver only), and user-defined scalars. For turbulence quantities, different values can be set depending on your choice of turbulence model. You can fix the value of the temperature in a fluid or solid zone if you are solving the energy equation. If you are using the non-premixed combustion model, you can fix the enthalpy in a fluid zone. If you have more than one species in your model, you can specify fixed values for the species mass fractions for each individual species except the last one you defined. See the separate UDF Manual for details about defining user-defined scalars.

If you are using the Eulerian multiphase model, you can fix the values of velocity components and (depending on which multiphase turbulence model you are using) turbulence quantities on a per-phase basis. See Section  24.2.9 for details about setting boundary conditions for Eulerian multiphase calculations.

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 dialog box or Solid dialog box, 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 profile (see Section  7.6) or a user-defined function for a 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 profile (see Section  7.6.1) for temperature, enthalpy, species mass/mole 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  7.2.1 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  7.2.1 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.)

You can fix values for velocity components only if you are using the pressure-based segregated solver. (Refer to this section in the separate Theory Guide 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 J/kg.

If you specify a radial-type profile (see Section  7.6.1) 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.

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. ANSYS 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 profile (see Section  7.6.1) 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.

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 profile (see Section  7.6.1) 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.