July 14, 2023 at 7:38 pmNavid GoudarziSubscriberWhen we run Fluent and try using the expression for the turbulence intensity, we keep getting this error: "expression is not single-valued". The expression is correct. Any tips?
July 17, 2023 at 8:59 amRobAnsys Employee
It may be correct but is it single valued?
July 17, 2023 at 9:00 amSRPAnsys Employee
You are getting this error because given expression calculates the values at all cells.
July 17, 2023 at 1:58 pmNavid GoudarziSubscriber
Thank you for the response.
It is not single-valued. It is the TI value as a function of "y" coordinates.
The TI expression is defined on the inlet. We do not get a similar error for the inlet velocity expression. Any thoughts?
July 17, 2023 at 2:07 pmRobAnsys Employee
If you work out the values using Excel or the like what does the profile look like?
July 17, 2023 at 2:48 pmRobAnsys Employee
Having checked it looks like you can only have a single value for Turbulent Intensity.
July 17, 2023 at 3:00 pmNavid GoudarziSubscriber
The profile follows a power law polynomial (similar to the velocity profile with a different power). To validate the CFD against experimental results, we need to insert TI values as expressions rather than single values as there is a significant change with "y" coordinates, changing from 40% to less than 5%. Is there any other way that we can apply it?
e.g., should we change the inlet condition from "turbulent intensity and turbulent viscosity ratio" to "turbulent kinetic energy and dissipation rate" and then define expressions for them?
July 17, 2023 at 3:52 pmRobAnsys Employee
For profiles we usually use k and epsilon/omega depending on the model(s) used. I've asked internally whether there's something we're missing for TI, but may not be able to feed much back depending on the answer: it may just be "don't".
July 17, 2023 at 3:54 pmNavid GoudarziSubscriber
Thank you! We will try the K-epsilon (we still need to define an expression for it) and I look forward to hearing back from you.
July 17, 2023 at 3:56 pmRobAnsys Employee
How were you creating the TI profile expression?
July 17, 2023 at 4:05 pmNavid GoudarziSubscriber
It seems that TI might be able to be defined as a UDF rather than an expression: Enter expression for Turbulent intensity or turbulent kinetic energy (ansys.com) would you confirm? (we did not get the ANSYS reply on this trend where it says the "C" value should be positive as the equation will be defined based on the experimental reference case --- please see our equation below where the "C" value is negative).
Here is the TI profile expression: 0.2*(y/0.28 [m])**-0.3594
July 18, 2023 at 7:44 amRobAnsys Employee
We're not sure why the Expression isn't working, but a UDF DEFINE_PROFILE should be fine.
The profile must return a positive value, and the "y" position is against the inlet location in the CFD model. Not sure if Aitor is on the new Forum, but we tended to correct their replies on the very rare occasion there was anything missing: our goal is to moderate rather than answer everything but Community is still quite young.
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