KNOWN ISSUE: Using 1-way FSI with Fluent fails when the Mechanical system is solved


SYMPTOM:
If a 1-way FSI case is set up using a Fluent system connected to a Mechanical system in ANSYS Workbench, then sometimes the Mechanical system will fail to solve with an error such as:
"Time specified for an imported load does not match any of the step end times. Please enter a valid time value so that the imported load time matches a step end time."
even though the Imported Load seemed to have imported correctly.

PROBLEM:
Behind the scenes, the Fluent data is imported into the Mechanical system using CFD-Post (even though the user never sees CFD-Post). When CFD-Post processes the data provided to it, it is treating the Analysis Time provided to it by the Mechanical system as a floating point number. When the data is returned to the Mechanical system, the Analysis Time then does not match the Analysis Time expected by the Mechanical system (due to the rounding error on the floating point number) and the solution fails.

WORKAROUND:
The only known workaround is to fix the Analysis Time to match the floating point number returned by CFD-Post. Once you have seen the error, then you need to locate the file named
"Import_Output_<number>_<letters>.xml"
which will be in the project directory named "<project_name>_filesdp<N>SYS-<M>MECH" where the quantities <number>, <letters>, <project_name>, <N> and <M> will depend on your specific project and system.

In this file you need to locate the section of the XML which contains the Time specification:

<ParameterDetails id="pa1">
<Name>Time</Name>
<Units description="Seconds" name="s">
<Unit>
<Name>s</Name>
</Unit>
</Units>
</ParameterDetails>

The crucial parameter you need to identify is the ParameterDetails id, which in this case is "pa1". Having determined the ParameterDetails id, then you need to locate the corresponding data section:

<ParameterValue parameter="pa1">
<Data format="float" type="real">1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003,
[...]
1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003, 1.00000005e-003</Data>
</ParameterValue>

You can read off from the data values what time CFD-Post is returning to the Mechanical system (in this case 1.00000005e-003, instead of the expected 1.0e-03).

Now in the Mechanical system, you need to use this time (1.00000005e-003 in the example given) in two places:
a) In the Imported Load part of the tree, if you click on the load in question (e.g. "Imported Pressure") then in the Worksheet view you can see the table of values where you have specified the Source Time. In this table, change the Analysis Time to be the time value returned by CFD-Post (e.g. 1.00000005e-003 in the example provided). Make sure that you enter the number exactly how it appears in the XML file.
b) In the Analysis settings, also set the Step End Time for the relevant load step to be the time value returned by CFD-Post. Again, make sure that you enter the number exactly how it appears in the XML file.

Now the Mechanical system should solve correctly.

FIXED IN:
ANSYS CFX Release 12.1.



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