Initial Conditions-Reset, Apply, OK
The IC Reset, Apply, and OK buttons control the settings for the initial conditions.
• Reset - restores the settings back to the values that the Initial Condition had when it was first selected (effectively undoing any setting operations).
• Apply - saves all of the initial value settings that have been made during the current setting operation. It is important to click Apply, otherwise the settings will not be saved. If you leave this section of the CFD-ACE-GUI with unsaved changes you will receive a warning to remind you to apply the settings.
• OK - saves all of the initial value settings and de-selects the active Initial Conditions.
It is important to click Apply or OK, otherwise the settings will not be saved.
Control Panel-Solver Controls (SC)
Control Panel
Solver Controls Introduction
The Solver Control panel enables you to specify the numerical aspects of the solution and to select the solver output options. Depending on the modules you have selected in the PT panel, other tabs may be displayed. If so, they are described in the Modules section. The following are the common tabs for Solver Controls:
• Iterations
Solver Control Panel
Control Panel
Solver Controls-Iterations
The Iterations tab enables you set criteria to control the iterative solution process.
• Maximum Iterations - Total number of iterations to be performed by the solver.
• Convergence Criteria - The minimum reduction in residuals for each variable. Default value is 0.0001 (four orders of magnitude).Min. Residual: Value of the residual below which the second criteria is not applied.
The solver quits the iterative procedure if any of the two criteria is satisfied. If you need to run a specific number of iterations, set the Convergence Criteria to a very small number (1.0E-10) but larger than the Minimum Residual value.
Solver Controls - Iterations Tab
Control Panel
Solver Controls-Spatial
The Spatial tab represents the spatial differencing scheme and controls the spatial accuracy of the simulation. The options provide several differencing schemes to calculate the convective term in the transport equations.
Solver Controls - Spatial Tab
The panel contains three columns; the first defines the variable, the second defines the type of spatial differencing to be used for that variable, and the third determines the amount of upwind blending to be used (if any).
The variable list will include any variables that are solved for (as determined by the active Modules) as well as any auxiliary variables which may also be used in the transport equations.
Several differencing schemes are available and they are discussed further in the Numerics chapter. The default scheme is the first-order Upwind Differencing scheme. If selecting higher-order schemes, you can also enter a blending factor to blend the higher-higher-order scheme with the Upwind Differencing scheme for added stability.
For some of the higher-order schemes, especially the Central Differencing scheme, obtaining a converged solution may be impossible due to numerical instabilities. In these cases, the blending factor can be used in conjunction with the higher-order scheme to provide a solution which is of greater accuracy than a first-order Upwind Differencing scheme. The blending factor default value of 0.1 results in a differencing scheme that is 10% upwind and 90% higher order.
Control Panel
Solver Controls-Solvers
The Solvers tab enables you to select the linear equation solver used to solve the algebraic equations for each dependent variable and the controlling parameters for that solver.
Solver Controls - Solvers Tab
The panel contains four columns: the first defines the variable, the second defines the linear equation solver to be used for that variable, the third determines the maximum number of solver sweeps (iterations) allowed, and the fourth determines the convergence criteria to be used.
The variable list includes any variables that are solved for (as determined by the active Modules).
For most variables, the default solver selection is the CGS+Pre (conjugate-gradient-squared plus preconditioning) solver. An Algebraic Multigrid Solver (AMG) is also available. Some variables may shown additional solver selections. Recommended solver selections are discussed in the individual Module section of the CFD-ACE+ Modules manual.
For either the CGS + Pre or the AMG solver, you can set the maximum number of sweeps, or iterations, and the convergence criteria. Because these solvers detect their own convergence, you can control the amount of residual drop that you require for the linear equation solver to terminate. The residual drop for the CGS solver is equal to the square of the Criterion given CFD-ACE+. If the solver reaches the maximum number of sweeps before the convergence criteria is met, then the solver will terminate and a warning message will be printed to the output file.
In most cases, you can use the default selections of the CGS+Pre solver with a maximum of 50 or 500 sweeps (depending on variable).
Control Panel
Solver Controls-Relax
The Relax tab sets the under relaxation factors for each of the solved variables and for the auxiliary variables. Under relaxation is a constraint on the change of a dependent or auxiliary variable from one solution iteration to the next. It is required to maintain the stability of the coupled, non-linear system of equations.
Solver Controls - Relax Tab
The panel contains four columns: the first defines the variable, the second contains a slider bar which can be used to adjust the value, the third contains up/down buttons to adjust the order of magnitude of the value, and the fourth is a field for the under relaxation value itself.
The variable list includes any variables that are solved for (as determined by the active Modules) and any auxiliary variables that are used in active equations. The solved variables are listed in the group at the upper side of the panel and the auxiliary variables are grouped at the lower side of the panel.
To increase the under-relaxation, or constraint, on any of the dependent (solved) variables, increase the value. (The default value for most dependent variables is 0.2.) Values greater than one are allowed but not recommended in most cases. To increase the under-relaxation on an auxiliary variable, decrease the value. (The default value for most auxiliary variables is 1.0.) The auxiliary variable under relaxation values are bound between 0 and 1. See the Numerics section for more details on how under relaxation is applied to the equations. See each Module’s section for guidelines on setting under relaxation values.
Control Panel
Solver Controls-Limits
The Limits tab enables you set minimum and maximum values for certain dependent and auxiliary variables.
Solver Controls - Limits Tab
The panel contains three areas: the first defines the variable, the second is an entry field for the minimum allowed value, and the third is an entry field for the maximum allowed value. The variable list includes any variables that are solved for (as determined by the active Modules) and any auxiliary variables that are used in active equations.
The default Variable Limits are generous and should only be changed in special situations. Be aware that any time the solver enforces these limits, (with the exception of Mixture or Species Fractions), it is artificially constraining the solution. This can cause convergence problems or produce a non-physical solution. The results should always be checked to make sure that no variable is hitting a limit. See each Module’s section for advice on setting limits.
Control Panel
Solver Controls-Advanced Settings
The Adv tab (Advanced Settings) provides options for advanced users.
Buffered Output
This option changes program output so that it is written only when internal buffers are full. Default mode is to flush RSL and out file every iteration once at the end of iteration. This is normally the way users want the program to operate as it ensures that, for example, residual records appear in the model.RSL file at the completion of each iteration. For small problems that run for a long time (due to either a large number of iterations or time steps) the default output mode can cause a significant portion of the elapsed time to be devoted just to output with resulting poor CPU utilization. For these cases, selecting the Buffered Output option can result in a noticeable savings in turnaround time for the simulation.
Higher Accuracy
This option activates an alternative numerical discretization scheme in the program that results in a higher accuracy result for a given grid. This capability is added in an experimental form at release of Version 2004. As implemented in that release it is relatively inefficient and can cause significantly longer run times than the default discretization scheme. It has also shown some robustness problems which have not yet been resolved. It may be best to use a solution with the default discretization scheme as initial conditions for a simulation with this option.
The other options displayed depend upon the modules being used in the simulation. For more information on Advanced Settings, see the Modules section.