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25.22.2 Step-by-Step Solution Processes

One important technique for speeding convergence for complex problems is to tackle the problem one step at a time. When modeling a problem with heat transfer, you can begin with the calculation of the isothermal flow. To solve turbulent flow, you might start with the calculation of laminar flow. When modeling a reacting flow, you can begin by computing a partially converged solution to the non-reacting flow, possibly including the species mixing. When modeling a discrete phase, such as fuel evaporating from droplets, it is a good idea to solve the gas-phase flow field first. Such solutions generally serve as a good starting point for the calculation of the more complex problems. These step-by-step techniques involve using the Solution Controls panel to turn equations on and off.



Selecting a Subset of the Solution Equations


FLUENT automatically solves each equation that is turned on using the Models family of panels. If you specify in the Viscous Model panel that the flow is turbulent, equations for conservation of turbulence quantities are turned on. If you specify in the Energy panel that FLUENT should enable energy, the energy equation is activated. Convergence can be sped up by focusing the computational effort on the equations of primary importance. The Equations list in the Solution Controls panel allows you to turn individual equations on or off temporarily.

Solve $\rightarrow$ Controls $\rightarrow$ Solution...

A typical example is the computation of a flow with heat transfer. Initially, you will define the full problem scope, including the thermal boundary conditions and temperature-dependent flow properties. Following the problem setup, you will use the Solution Controls panel to temporarily turn off the energy equation. You can then compute an isothermal flow field, remembering to set a reasonable initial value for the temperature of the fluid.

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This is possible only for the pressure-based solver; the density-based solver solves the energy equation together with the flow equations in a coupled manner, so you cannot turn off the energy equation as described above.

When the isothermal flow is reasonably well converged, you can turn the energy equation back on. You can actually turn off the momentum and continuity equations while the initial energy field is being computed. When the energy field begins to converge well, you can turn the momentum and continuity equations back on so that the flow pattern can adjust to the new temperature field. The temperature will couple back into the flow solution by its impact on fluid properties such as density and viscosity. The temperature field will have no effect on the flow field if the fluid properties (e.g., density, viscosity) do not vary with temperature. In such cases, you can compute the energy field without turning the flow equations back on again.

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If you have specified temperature-dependent flow properties, you should be sure that a realistic value has been set for temperature throughout the domain before disabling calculation of the energy equation. If an unrealistic temperature value is used, the flow properties dependent on temperature will also be unrealistic, and the flow field will be adversely affected. Instructions for initializing the temperature field or patching a temperature field onto an existing solution are provided in Section  25.14.



Turning Reactions On and Off


To solve a species mixing problem prior to solving a reacting flow, you should set up the problem including all of the reaction information, and save the complete case file. To turn off the reaction so that only the species mixing problem can be solved, you can use the Species Model panel to turn off the Volumetric option under Reactions.

Define $\rightarrow$ Models $\rightarrow$ Species $\rightarrow$ Transport & Reaction...

Once the species mixing problem has partially converged, you can return to the Species Model panel and turn the Volumetric Reactions option on again. You can then resume the calculation starting from the partially converged data.

For combustion problems you may want to patch a hot temperature in the vicinity of the anticipated reactions before you restart the calculation. See Section  25.14.2 for information about patching an initial value for a flow variable.


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Up: 25.22 Convergence and Stability
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