26.19.3 Using Solution Steering

Solution steering is disabled by default. However, when the following criteria are met, the solution steering feature will become available for selection:

Feature	Setting
Solver Type	Density Based
Solver Formulation	Implicit
Time Formulation	Steady
Data is valid (either data file has been read
or flow has been initialized)

To activate solution steering, click the Solution Steering check box as shown in Figure  26.19.1.

Figure 26.19.1: The Run Calculation Task Page with Solution Steering Enabled

The Run Calculation task page will then expand to display the solution steering main controls (see Figure  26.19.1). To obtain a flow solution using solution steering, you will need to perform the following:

1.   Select the type of flow.

2.   Select the maximum accuracy desired (first to second order blending).

3.   Click Calculate.

The user can also adjust the number of iterations, or customize the parameters of the solution steering if the default setting is not sufficient for the type of flow problem being solved.

Before using solution steering, you will need to prepare and set up the case as usual as described in the separate Getting Started Manual.

Once Solution Steering is activated, specify the following:

Flow Type   allows you to select the flow type that best describes the flow in the solution domain. Five choices are available: incompressible, subsonic, transonic, supersonic, and hypersonic.

FMG Initialization   when enabled allows for full multigrid initialization before starting stages 1 and 2. FMG initialization is enabled by default.

First to Higher Order Blending   allows you to reduce the desired solution accuracy by selecting a blending factor less than 100%. The default setting is 100%. See this section in the separate Theory Guide for more information. The blending factor will be grayed out if Second Order Upwind discretization for the Flow equations is not selected in the Solution Methods task page. The solution accuracy may be reduced (typical values are 75% or 50%) if it is not possible to obtain a converged solution with the maximum second-order accuracy (i.e. blending = 100%)

Courant Number   in the Run Calculation task page is a non-adjustable field displaying the current CFL number, which allows you to view it during the calculation.

More Settings...   opens the Solution Steering dialog box, providing a host of settings that control the solution steering strategy, as shown in Figure  26.19.2.

Figure 26.19.2: The Solution Steering Dialog Box

The Solution Steering dialog box, shown in Figure  26.19.2, contains two tabs. The Steering Settings tab sets the solution steering parameters and the FMG Settings sets the full multigrid initialization parameters.

In the Steering Setting tab, you can modify the parameters used in Stages 1 and 2.

Stage 1   

Duration is the number of iterations in stage 1. The CFL number used during these iterations is set in the Initial field, in the Courant Number group box.

Stage 2   

The Courant number update in stage 2 can start immediately after the end of stage 1, or after a certain designated number of iterations. If the Courant number update is to start immediately after stage 1 then Immediately should be selected (this is the default option). If the Courant number update is desired after some lagged period of iterations, then After should be selected and the lag in the number of iterations should be entered in the field below it. The frequency at which the Courant number is updated is defined in Courant Number Update Interval field.

Courant Number   

Initial is the starting Courant number and Maximum is the maximum allowed Courant number. The solution steering algorithm will not allow the solver to exceed the maximum Courant number, but will allow the solver to use a Courant number less than the initial Courant number if divergence in the solution has occurred.

Explicit Under-Relaxation Factor   allows the solution to be under-relaxed to improve convergence. The under-relaxation value is determined by the Flow Type that you selected in the Run Calculation task page, when Solution Steering was enabled. In general, you do not need to alter the default value set in this field. Refer to this section in the separate Theory Guide for more information about explicit relaxation.

Default   is available in the Steering Settings tab to reset any changes made to the parameters to their original default values.

In the FMG Settings tab (Figure  26.19.3), the Number of multigrid levels and Number of Cycles in each Level, as well as the FMG Courant Number used in the FMG initialization can be adjusted. The default values used in the multigrid settings are determined from the type of flow that you selected, the size of the mesh, and the flow dimensionality. The Default button is used to reset any changes to the original default values. For more information about FMG initialization, refer to Section  26.10.

Figure 26.19.3: The FMG Settings Tab in the Solution Steering Dialog Box