## Chapter 3: Initial Condition Modeling

Initial values for all solved variables need to be set before the solution can begin.

If your calculation is a steady-state calculation, the initial variable values serve to give the CFX-Solver a flow field from which to start its calculations. Convergence is more rapidly achieved if sensible initial values are provided. However, converged results should not be affected by the initialization.

If your calculation is a transient (unsteady) calculation, then initial values provide the flow field at the time when the CFD calculation starts. The values specified should be the actual flow field present at the beginning of the time of the simulation.

There are three options available for setting the initial values:

- automatically generated default values

- explicitly specified values

- read from an existing results file.

The first two options are specified by setting initialization parameters for the analyses (global) or domains in CFX-Pre. For details, see Initialization in the CFX-Pre User's Guide and Initialization Tab in the CFX-Pre User's Guide, respectively). If you want to read initial conditions from an initial values file (an existing results file from a previous run) then this is done on the Run Definition tab for a simulation execution control or a configuration in CFX-Pre or the Run Definition tab of the Define Run dialog box in the CFX-Solver Manager. For details, see Run Definition Tab in the CFX-Pre User's Guide, Run Definition Tab in the CFX-Pre User's Guide and Run Definition Tab in the CFX-Solver Manager User's Guide, respectively. If you chose to specify an initial values file then the initial conditions read from that file will override any settings made in CFX-Pre.

Automatically generated default values are generally only recommended for steady-state runs. They are specified by choosing the `Automatic` option on the Initialization tabs in CFX-Pre. In this case, the CFX-Solver will calculate its own initial values, based on the physics and boundary conditions in the set-up. Details of how the initial values are calculated can be found in Initialization Parameters. You can set the initial conditions explicitly by choosing the `Automatic with Value` option on the Initialization tabs in CFX-Pre. Both of these options are discussed in Setting the Initial Conditions in CFX-Pre.

Reading results from an existing results file is useful in many cases, including:

- To restart a steady-state run after a physics change.

  When restarting a run with, for example, a new turbulence model, save time and resources by using a set of results from a previous run as the basis for an initial guess (even if the run was only partially converged). These initial guess values are likely to be far more accurate than using an automatic initial guess.

- To begin a transient analysis, using the results of a steady-state analysis as the initial conditions.

  Transient simulations require values to be set for all variables during initialization. Starting a transient run with results from a converged steady-state analysis can lead to good convergence and solution robustness, especially if it is difficult to provide an accurate estimate of the initial conditions.

- To continue a transient run with a new mesh.

  A transient moving mesh case might need to be stopped and restarted with a new mesh, if the mesh quality is poor. The run with the new mesh should continue using the values calculated by the solver when the run with the old mesh was stopped.

In addition, reading results from an existing results file is done implicitly whenever one configuration from a multi-configuration run is set to use initial values from a previous configuration (where the solver will use the results file produced by the previous configuration as its initial values file), and when a run is interrupted and restarted using the automatic remeshing capability. For details, see Remeshing Tab and Remeshing Guide.

The process and limitations for initializing a run with values from a previous results file is described in Reading the Initial Conditions from a File.

This chapter describes:

- Setting the Initial Conditions in CFX-Pre

- Initialization Parameters

- Reading the Initial Conditions from a File

- Using the CFX-Interpolator

Release 18.2 - © ANSYS, Inc. All rights reserved.