[monkey1]

Hello Everybody!

I'm dealing with flow Simualtions and interfaces with conditional state changes. My problem is, that when my interface state changes from closed to open I need very small time steps. But to reach that point I'd rather use bigger time steps to save computing time.

Coming to my question:
Is there a possibility for setting the condition for time step adaption in dependency of a flow variable. I would want something like:
If Pressure < Value then adapt the time step till reaching a (high) max value
If Pressure >= Value limit the max value for the time step to a very small value

If I use Courant or coeff loop adaption my system crashes when opening the interface due to too big time steps. Also, I already tried smth. like:
max Time step = if(areaAve(Absolute Pressure)@Interface Location< 1.4[bar],0.01[s],1e-7[s])
But the simulation didn't even start, telling me "Absolute Pressure" is an unknown value at the time of evaluation.

Thx for "listening"

[ghorrocks]

The adaptive time step method should have given you the time step size which can run well when the interface is open or shut. So I would make a user function which uses that time step size when it is steady state and ramps down to a small time step for the open/shut event then ramps back up again afterwards.

[monkey1]

Dear Glenn,

at first thx for the answer.
I'm doing a transient simulation of a vessel equipped with a burst disc. Inside this vessel an gas mixture is ignited and due to the combustion process the pressure rises. When a pressure threshold is reached, the burst disc is failing. That is what I wanted to simulate.

The adaptive time stepping does not give me the time step for the open/close state change moment. While the interface is closed, both domains are fully separated and the solkutions is converging fast and good leading to a big time step. As the adaptive time stepping is not able to "anticipate" what will happen, it runs into the state change with the too big time step leading to a crash. That's why I want to link the time step size to a physical variable controllign the state change.

Your tip with the user function is nice...but will it work when I make a function with input parameter absoulte pressure and output time step?
Another kind of time step adaption does not come to my mind as I do NOT know when the state change will occur.

Best regards!

[ghorrocks]

Oh - you don't know when the state change occurs. Then you can make the function from just about any variables you like, so if pressure is the trigger then use that.

[monkey1]

Ok I will try that.
My first shot to write in the field:
Maximum Timestep
the following expression:
if(areaAve(Absolute Pressure )@REGION:BERSTSCHEIBE>1.38[bar],0.0000001[s],0.01[s])

lead to a direct crash of the solver with the following message:

Error processing expression 'Maximum Timestep'.
The expression is invalid because:
Absolute Pressure is not available for use in this term

So I'm curious to see if this problem disappears when using a user function.

[ghorrocks]

Try using just pressure, not absolute pressure. I suspect that will be available.

[monkey1]

Indeed, using the variable names given in the "CFX- Reference Guide" Chapter "Variables in ANSYS CFX " it works.