7.2.5 Defining Mass, Momentum, Energy, and Other Sources

You can define volumetric sources of mass (for single or multiple species), momentum, energy, turbulence, and other scalar quantities in a fluid zone, or a source of energy for a solid zone. This feature is useful when you want to input a known value for these sources. (For more complicated sources with functional dependency, you can create a user-defined function as described in the separate UDF Manual.) To add source terms to a cell or group of cells, you must place the cell(s) in a separate zone. The sources are then applied to that cell zone. Typical uses for this feature are listed below:

• A flow source that cannot be represented by an inlet, e.g., due to an issue of scale. If you need to model an inlet that is smaller than a cell, you can place the cell where the tiny "inlet'' is located in its own fluid zone and then define the mass, momentum, and energy sources in that cell zone. For the example shown in Figure  7.2.6, you should set a mass source of $\frac{\dot{m}}{V} = \frac{\rho_j A_j v_j}{V}$ and a momentum source of $\frac{\dot{m} v}{V} = \frac{\dot{m} v_j}{V}$, where $V$ is the cell volume.

• Heat release due to a source (e.g., fire) that is not explicitly defined in your model. For this case, you can place the cell(s) into which the heat is originally released in its own fluid zone and then define the energy source in that cell zone.

• An energy source in a solid zone, for conjugate heat transfer applications. For this case, you can place the cell(s) into which the heat is originally released in its own solid zone and then define the energy source in that cell zone.

• A species source due to a reaction that is not explicitly included in the model. In the above example of simulating a fire, you might need to define a source for a species representing smoke generation.

Figure 7.2.6: Defining a Source for a Tiny Inlet

Note that if you define a mass source for a cell zone, you should also define a momentum source and, if appropriate for your model, energy and turbulence sources. If you define only a mass source, that mass enters the domain with no momentum or thermal heat. The mass will therefore have to be accelerated and heated by the flow and, consequently, there may be a drop in velocity or temperature. This drop may or may not be perceptible, depending on the size of the source. (Note that defining only a momentum, energy, or turbulence source is acceptable.)

Sign Conventions and Units

All positive source terms indicate sources, and all negative source terms indicate sinks. All sources must be specified in SI units.

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 dialog box or Solid dialog box, 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 dialog box 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  7.2.5.

• 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 dialog box.

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 ( this equation in the separate Theory Guide), 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, ANSYS 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 ( this equation in the separate Theory Guide), 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 ( this equation in the separate Theory Guide), 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 ( this equation in the separate Theory Guide), 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 Turbulent Kinetic Energy or Turbulent 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., this equation in the separate Theory Guide).

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

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 ( this equation in the separate Theory Guide), 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 Turbulent 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 ( this equation in the separate Theory Guide).

The defined $\omega$ source will appear in the $S_\omega$ term on the right-hand side of the specific turbulent dissipation rate equation ( this equation in the separate Theory Guide).

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 ( this equation in the separate Theory Guide).

The defined $k$ source will appear in the $S_k$ term on the right-hand side of this equation in the separate Theory Guide.

The defined $\epsilon$ will appear in the $S_\epsilon$ term on the right-hand side of this equation in the separate Theory Guide.

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 ( this equation in the separate Theory Guide).

The defined mixture fraction variance source will appear in the $S_{\rm user}$ term in the transport equation for the mixture fraction variance ( this equation in the separate Theory Guide).

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 this equation in the separate Theory Guide.

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 this equation in the separate Theory Guide.

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 this equation in the separate Theory Guide , this equation in the separate Theory Guide , and this equation in the separate Theory Guide.

User-Defined Scalar (UDS) Sources

You can specify source term(s) for each UDS transport equation you have defined in your model. See Section  9.1.3 for details.