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7.28.1 Procedure for Defining Sources

To define one or more source terms for a zone, follow these steps (remembering to use only SI units):

1.   In the Fluid panel or Solid panel, turn on the Source Terms option.

2.   Set the appropriate source terms under the Source Terms tab, noting the comments below.

  • To specify a source, click the Edit... button next to the mass, momentum, energy, or other source. The sources panel will open where you will define the number of sources. For each source, choose constant, user-defined, or none in the drop-down list.

  • To specify a constant source, choose constant in the drop-down list and then enter the constant value in the field.

  • To specify a temperature-dependent or other functional source, you can use a user-defined function (see the separate UDF Manual).

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

  • Remember that you should not define just a mass source without defining the other sources, as described in Section  7.28.

  • Since the sources you specify are defined per unit volume, to determine the appropriate value of your source term you will often need to first determine the volume of the cell(s) in the zone for which you are defining the source. To do this, you can use the Volume Integrals panel.



Mass Sources


If you have only one species in your problem, you can simply define a Mass source for that species. The units for the mass source are kg/m $^3$-s. In the continuity equation (Equation  9.2-1), the defined mass source will appear in the $S_{m}$ term.

If you have more than one species, you can specify mass sources for each individual species. There will be a total Mass source term as well as a source term listed explicitly for each species (e.g., h2, o2) except the last one you defined. If the total of all species mass sources (including the last one) is 0, then you should specify a value of 0 for the Mass source, and also specify the values of the non-zero individual species mass sources. Since you cannot specify the mass source for the last species explicitly, FLUENT will compute it by subtracting the sum of all other species mass sources from the specified total Mass source.

For example, if the mass source for hydrogen in a hydrogen-air mixture is 0.01, the mass source for oxygen is 0.02, and the mass source for nitrogen (the last species) is 0.015, you will specify a value of 0.01 in the h2 field, a value of 0.02 in the o2 field, and a value of 0.045 in the Mass field. This concept also applies within each cell if you use user-defined functions for species mass sources.

The units for the species mass sources are kg/m $^3$-s. In the conservation equation for a chemical species (Equation  14.1-1), the defined mass source will appear in the $S_i$ term.



Momentum Sources


To define a source of momentum, specify the X Momentum, Y Momentum, and/or Z Momentum term. The units for the momentum source are N/m $^3$. In the momentum equation (Equation  9.2-3), the defined momentum source will appear in the ${\vec F}$ term.



Energy Sources


To define a source of energy, specify an Energy term. The units for the energy source are W/m $^3$. In the energy equation (Equation  13.2-1), the defined energy source will appear in the $S_h$ term.



Turbulence Sources


Turbulence Sources for the $k$- $\epsilon$ Model

To define a source of $k$ or $\epsilon$ in the $k$- $\epsilon$ equations, specify the Turbulence Kinetic Energy or Turbulence Dissipation Rate term. The units for the $k$ source are kg/m-s $^3$ and those for $\epsilon$ are kg/m-s $^4$.

The defined $k$ source will appear in the $S_k$ term on the right-hand side of the turbulent kinetic energy equation (e.g., Equation  12.4-1).

The defined $\epsilon$ source will appear in the $S_\epsilon$ term on the right-hand side of the turbulent dissipation rate equation (e.g., Equation  12.4-2).

Turbulence Sources for the Spalart-Allmaras Model

To define a source of modified turbulent viscosity, specify the Modified Turbulent Viscosity term. The units for the modified turbulent viscosity source are kg/m-s $^2$. In the transport equation for the Spalart-Allmaras model (Equation  12.3-1), the defined modified turbulent viscosity source will appear in the $S_{\tilde{\nu}}$ term.

Turbulence Sources for the $k$- $\omega$ Model

To define a source of $k$ or $\omega$ in the $k$- $\omega$ equations, specify the Turbulence Kinetic Energy or Specific Dissipation Rate term. The units for the $k$ source are kg/m-s $^3$ and those for $\omega$ are kg/m $^3$-s $^2$.

The defined $k$ source will appear in the $S_k$ term on the right-hand side of the turbulent kinetic energy equation (Equation  12.5-1).

The defined $\omega$ source will appear in the $S_\omega$ term on the right-hand side of the specific turbulent dissipation rate equation (Equation  12.5-2).

Turbulence Sources for the Reynolds Stress Model

To define a source 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 terms. The units for the $k$ source and the sources of Reynolds stress are kg/m-s $^3$, and those for $\epsilon$ are kg/m-s $^4$.

The defined Reynolds stress sources will appear in the $S_{\rm user}$ term on the right-hand side of the Reynolds stress transport equation (Equation  12.7-1).

The defined $k$ source will appear in the $S_k$ term on the right-hand side of Equation  12.7-29.

The defined $\epsilon$ will appear in the $S_\epsilon$ term on the right-hand side of Equation  12.7-32.



Mean Mixture Fraction and Variance Sources


To define a source of the mean mixture fraction or its variance for the non-premixed combustion model, specify the Mean Mixture Fraction or Mixture Fraction Variance term. The units for the mean mixture fraction source are kg/m $^3$-s, and those for the mixture fraction variance source are kg/m $^3$-s.

The defined mean mixture fraction source will appear in the $S_{\rm user}$ term in the transport equation for the mixture fraction (Equation  15.2-4).

The defined mixture fraction variance source will appear in the $S_{\rm user}$ term in the transport equation for the mixture fraction variance (Equation  15.2-5).

If you are using the two-mixture-fraction approach, you can also specify sources of the Secondary Mean Mixture Fraction and Secondary Mixture Fraction Variance.



P-1 Radiation Sources


To define a source for the P-1 radiation model, specify the P1 term. The units for the radiation source are W/m $^3$, and the defined source will appear in the $S_G$ term in Equation  13.3-5.

Note that, if the source term you are defining represents a transfer from internal energy to radiative energy (e.g., absorption or emission), you will need to specify an Energy source of the same magnitude as the P1 source, but with the opposite sign, in order to ensure overall energy conservation.



Progress Variable Sources


To define a source of the progress variable for the premixed combustion model, specify the Progress Variable term. The units for the progress variable source are kg/m $^3$-s, and the defined source will appear in the $\rho S_c$ term in Equation  16.2-1.



NO, HCN, and NH $_3$ Sources for the NOx Model


To define a source of NO, HCN, or NH $_3$ for the NOx model, specify the no, hcn, or nh3 term. The units for these sources are kg/m $^3$-s, and the defined sources will appear in the $S_{\rm NO}$, $S_{\rm HCN}$, and $S_{\rm NH3}$ terms of Equations  20.1-1, 20.1-2, and 20.1-3.



User-Defined Scalar (UDS) Sources


You can specify source term(s) for each UDS transport equation you have defined in your model. See Section  9.3.3 for details.


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