How to access data from previous time-step without udf

Ckcs2025 · February 24, 2025, 21:16

Hi guys!

I'm looking for a way to define a pressure outlet condition of the form P(t) = P(t-1)*(Qdt/C) for a Windkessel style compliant boundary. I would like to do this in the Fluent Expression manager as I have no experience with UDFs, but I cannot find how to access the pressure at the outlet for the previous time-step. Does anyone know how to do this as an expression, or does this have to be a UDF?

MKuhn · February 25, 2025, 05:12

It should work without an UDF, if you will use the result just of the previous time step.

Define a "Report Definition" --> "Surface Report" --> "Area-Weighted Average" and than Pressure as Field Variable of your Outlet
Than you can go to your boundary set up, choose Expression and here you can access the Report Definition from your outlet.

You have to be careful with the units. Everything must be in SI units and it must add up, i.e. you must end up with Pascal (or kg/(m s²)) if you define a pressure.

February 24, 2025, 21:16	How to access data from previous time-step without udf	#1
Ckcs2025 New Member Join Date: Feb 2025 Posts: 2 Rep Power: 0	Hi guys! I'm looking for a way to define a pressure outlet condition of the form P(t) = P(t-1)*(Qdt/C) for a Windkessel style compliant boundary. I would like to do this in the Fluent Expression manager as I have no experience with UDFs, but I cannot find how to access the pressure at the outlet for the previous time-step. Does anyone know how to do this as an expression, or does this have to be a UDF?

February 25, 2025, 05:12		#2
MKuhn Senior Member Moritz Kuhn Join Date: Apr 2010 Location: Germany, Dresden Posts: 236 Rep Power: 18	It should work without an UDF, if you will use the result just of the previous time step. Define a "Report Definition" --> "Surface Report" --> "Area-Weighted Average" and than Pressure as Field Variable of your Outlet Than you can go to your boundary set up, choose Expression and here you can access the Report Definition from your outlet. You have to be careful with the units. Everything must be in SI units and it must add up, i.e. you must end up with Pascal (or kg/(m s²)) if you define a pressure.

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