Multi-Configuration in ANSYS CFX: Automatically Initializing a Transient Simulation from a Steady-State Simulation

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In reality, most fluid mechanics problems are unsteady in nature. Steady-state approximations have been used since the advent of CFD as they provide adequate accuracy in many cases while the computational demands are quite moderate compared to a full transient, unsteady simulation. Current advances in multi-core desktop workstations and accessibility to high performance computing clusters allow CFD engineers to simulate larger problems with more detailed physics than ever before. This has allowed CFD engineers to increase the fidelity of their simulations by modeling the unsteady, transient nature of the flow.

Typically when running a transient simulation, best practices would dictate use of a a steady-state simulation as an initial guess. In the past this would require you to run two separate simulations, one steady state and one transient.  In ANSYS CFX software, it is now possible to do both in a single simulation by using a multi-configuration setup.

First, start by setting up a steady-state simulation in CFX-Pre in the normal manner.  In the Outline Tree the simulation will be named Flow Analysis 1 by default.  Right click on this and select “Duplicate” as shown below.

After this is done you will have a Copy of Flow Analysis 1 that is identical at the moment to Flow Analysis 1.  Rename these to Steady State and Transient respectively by right clicking on each analysis and selecting “Rename”.  Under the transient simulation, modify the settings so that the setup is transient instead of steady state.  Now we have a steady- state and transient setup in CFX-Pre.

The next step is to define in which order to run the setups. This is done under the Simulation Control section in the Outline Tree. Right click on “Configurations” and select “Insert > Configuration” and name the configuration Steady. Under the details, keep the default setting to allow the steady-state simulation to begin at the start of the simulation as shown below.

Next, make a new configuration, this time named Unsteady  In the details of Unsteady, select “Transient” for the flow analysis, and activation condition option  ”End of Configuration” and set the configuration to be “Steady” as shown below.

To use the steady-state simulation as an initial guess, go to the Run Definition tab and check the Configuration Execution Control check box and the Initial Values Specification check box.  You can then set the Steady configuration to be used as an initial guess for the Unsteady configuration as shown below.

Once the CFX Solver is executed, it will automatically run the steady-state case, followed by the transient case using the steady state solution as an initial guess.

There are many other applications for multi-configuration runs such as:

  • Mesh sensitivity studies
  • IC Engine simulations
  • Turbulence model studies
  • Reacting flow – initializing reacting flow with cold flow
  • Many more….
Keep in mind that if the changes are parametric in nature, you are better off to create parameters and run multiple design points using Workbench. But for changes that are not parametric in nature, multi-configuration simulations are a great way to set up a string of simulations to run in a particular sequence and use previous results to initialize where applicable.