26.17.5 Checking the Solver Settings

Figure 26.17.7: The Solver Tab in the Case Check Dialog Box

The following recommendations appear under the Solver tab (Figure  26.17.7):

• Enable the unsteady solver option when selecting moving mesh for the fluid boundary.

  If the motion type of the fluid boundary condition is specified as Moving Mesh, then your case should be specified as Transient in the General task page. Visit Section  11.2.2 for steps on setting up moving mesh problem.

  General

• Assign LSQ cell-based gradient reconstruction.

  The least squares cell-based averaging scheme is known to be as accurate as the node-based gradient for irregular unstructured meshes, but less expensive to compute than the node-based gradient. Therefore, it is recommended that least squares cell-based gradient reconstruction is used. See this section in the separate Theory Guide for more information on gradient options.

  Solution Methods

• Change the under-relaxation factor for the energy equation to at least 0.90.

  It is recommended to set the energy under-relaxation factor between 0.90 and 1.0. If you decide to apply this recommendation, then ANSYS FLUENT will automatically set the energy under-relaxation factor to 0.90. If you want to increase this value, you can manually make the change by going to the Solution Controls task page. See Section  13.2.2 for the under-relaxation of the energy equation.

  Solution Controls

• Review reference values for non-dimensionalized force coefficients computed with force monitors.

  If force monitors are activated, be sure to update the Reference Values task page. See Section  26.13.3.

  Solution Controls

• Increase the NOx under-relaxation factor to at least 0.90.

  If the NOx model is enabled, set the NOx under-relaxation factor to a value of at least 0.90 to fully converge the solution. Note that the under-relaxation factor could be lower at the start of the run, but can then be increased after an initial solution is obtained. If you decide to apply this recommendation, then ANSYS FLUENT will automatically set the NOx under-relaxation factor to 0.90. If you want to increase this value, you can manually make the change by going to the Solution Controls task page. See Section  21.1.1.

  Solution Controls

• Increase the Discrete Ordinates under-relaxation factor to at least 0.90.

  If the Discrete Ordinates (DO) radiation model is enabled, set the radiation under-relaxation factor to a value of at least 0.90 to fully converge the solution. Note that the under-relaxation factor could be lower at the start of the run, but can then be increased after an initial solution is obtained. If you decide to apply this recommendation, then ANSYS FLUENT will automatically set the radiation under-relaxation factor to 0.90. If you want to increase this value, you can manually make the change by going to the Solution Controls task page. See Section  13.3.7.

  Solution Controls

• Increase the P1 under-relaxation factor to 1.0.

  If the P1 radiation model is enabled, set the radiation under-relaxation factor to 1.0 to fully converge the solution. Note that the under-relaxation factor could be lower at the start of the run, but can then be increased after an initial solution is obtained. If you decide to apply this recommendation, then ANSYS FLUENT will automatically set the radiation under-relaxation factor to 1.0. See Section  13.3.7.

  Solution Controls

• Increase the species and energy under-relaxation factors to at least 0.90.

  For a case with species transport and energy defined, set the species and energy under-relaxation factors to a value of at least 0.90. If you decide to apply this recommendation, then ANSYS FLUENT will automatically set the species and energy under-relaxation factors to 0.90. If you want to increase this value, you can manually make the change by going to the Solution Controls task page. See Section  15.1.7.

  Solution Controls

• Assign a value of 1 for the under-relaxation factor for unsteady DPM with 1 DPM update per time step.

  It is recommended that the DPM under-relaxation factor be set to 1 for unsteady DPM with 1 DPM update per time step.

  Solution Controls

• Increase the mean mixture fraction under-relaxation factor to at least 0.90.

  If the non-premixed or partially premixed combustion models are enabled, then it is best to set the mean mixture under-relaxation factor to a value of at least 0.90 to ensure full convergence. If you decide to apply this recommendation, then ANSYS FLUENT will automatically set the mean mixture under-relaxation factor to 0.90. If you want to increase this value, you can manually make the change by going to the Solution Controls task page. See Section  16.10.4.

  Solution Controls

• Consider using higher order discretization for improved accuracy of the final solution. First-order discretization may be used in the initial solution.

  It is generally advisable to obtain an initial solution using first-order accurate discretization, however, second order discretization is recommended for improved accuracy of the final solution. See Section  26.2 for more information on discretization schemes.

  Solution Methods

• Select the absolute reference frame for initializing cases when using the MRF model.

  When using the MRF model, always use the absolute reference frame while initializing the solution. Select Absolute under Reference Frame in the Solution Initialization task page. If the Relative to Cell Zone option is selected, which is the default option, the initial flow field can contain discontinuities, which can cause convergence problems in the first few iterations. Refer to Section  26.9.1 for more information.

  Solution Initialization Initialize

• Choose PRESTO! for the pressure discretization scheme.

  When using the VOF model, it is recommended that you use PRESTO! as the pressure discretization scheme. This scheme is recommended for flows with high swirl numbers, a high-Rayleigh-number natural convection, high-speed rotating flows, flows involving porous media, and flows in strongly curved domains. See Section  26.2.3 for more information.

  Solution Methods