Run a DES¶

A DES in Bramble is a RANS initialisation followed by a transient solve for a fixed physical time. You set the solution stages in the Template Case and choose a DES workflow for each Attitude.

To start the DES from a finished RANS solution in the same Run, use a Donor Solve workflow instead. See use donor workflows.

Make a DES Template Case¶

1. Copy the RANS Template Case: in the Template Cases table, click the copy icon ("Create a copy of this template case"). Give the copy a name that says it's for DES, and click Create Copy. See copy a Template Case.

2. Open the copy. In the header card, click Edit. In Update Template Case, set Workflows to the DES workflow, remove any RANS-only workflows, and click Update.

   Why the workflows matter

   The Solution options depend on the Template Case's Workflows. If every workflow listed is a RANS or steady workflow, Transient isn't offered. If every workflow is a DES or transient workflow, only Transient is offered. With no workflows selected, all options are offered.

3. Go to the Solutions tab. In Solution Stage, set Solution to Transient. The table's fields change to:

   Field	Meaning	Example
   RANS Iterations	Iterations of the RANS initialisation run before the DES	500
   DES Solution Time	Physical time to solve the DES for, in s	2
   DES Time Step	Time step of the DES, in s	0.0002

4. In Average Window, enter the number of time steps to average over: the averaging time divided by the time step. For example, to average over the last 0.5 s with a 0.0002 s time step, enter 0.5 ÷ 0.0002 = 2500.

5. Delete every row in the Convergence Criteria table, and in Convergence Criteria Monitors if you use it. A DES runs for a fixed physical time, so it doesn't use convergence criteria. Once the rows are gone, the Start Time and Number of times unconverged fields disappear.
6. Click Save in the header.

For every field on the tab, see the Template Case editor reference.

Set up and launch the Run¶

1. Create a Run for the DES. If you choose the RANS Run as its Baseline Run, you can compare the two. See create a Run.
2. On the Run's CFD Setup tab, click the edit icon next to an Attitude's name.
3. In CFD Attitude Setup, choose the Methodology, the DES Workflow and your DES Template Case.
4. Click Save, or choose Save to All from the Save menu.
5. Launch the Run, review the pre-checks, then start Mesh & Solve.

Check the solution and run on¶

For a DES, Check Convergence produces force plots but doesn't judge convergence. Look at the View plots on the Run Checks tab to decide whether the averaging window has settled.

If you need more time, use Run-on. It's shown in the Mesh & Solve row of the Summary tab when the workflow supports it, and either the post-run checks have failed or post-processing has finished and the Attitude hasn't been signed off. For DES workflows, the Run-on dialog asks for:

• Iteration Time (default 1)
• Averaging Period Time (default 1.5)

Start from another solution (fast restart)¶

If Fast Restart is enabled for your Account, an Attitude can start its solve from another Attitude's latest solution, instead of initialising from scratch. Contact support@bramblecfd.com to have it enabled for your Account.

It only works for methodologies in the OpenFoam Methodology Group. The donor must be an Attitude from the same Map entry (a matching Attitude), in any Project, and not archived.

1. On the CFD Setup tab, click the edit icon next to the Attitude's name.
2. Tick Enable Mapped Solution.
3. Under Mapped Solution Donor Case, choose the donor's Programme ("Select a programme"), Project ("Select a project") and Attitude ("Select an attitude").
4. Click Save. Save to All isn't available while Enable Mapped Solution is ticked, because each Attitude has its own donor.

The Mapped Solution Donor Case row on the CFD Setup tab then links to the donor.

Until the donor has finished Mesh & Solve, the Attitude's Mesh & Solve button is disabled ("Waiting for donor case to complete Mesh and Solve stage.") and the Summary tab shows "Mapped Solution Donor Case has not completed solve stage." The pre-check Mapped Solution Donor Status also fails. The Attitude doesn't start by itself: once the donor has finished, click Mesh & Solve.

To stop using a donor, clear the Attitude selection under Mapped Solution Donor Case and click Save.

Related: Use donor workflows · MeanCalc · Template Case editor reference · CFD setup options · Checks reference