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Adaptive Time Stepping with a Variable as criterion |
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March 17, 2016, 04:20 |
Adaptive Time Stepping with a Variable as criterion
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#1 |
Senior Member
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Location: Berlin, Germany
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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" |
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March 17, 2016, 21:24 |
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#2 |
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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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.
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March 18, 2016, 03:11 |
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#3 |
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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! |
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March 18, 2016, 20:33 |
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#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,871
Rep Power: 144 |
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.
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March 22, 2016, 03:46 |
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#5 |
Senior Member
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Location: Berlin, Germany
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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. |
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March 22, 2016, 17:30 |
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#6 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,871
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Try using just pressure, not absolute pressure. I suspect that will be available.
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April 20, 2016, 10:05 |
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#7 |
Senior Member
Join Date: Jul 2011
Location: Berlin, Germany
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Indeed, using the variable names given in the "CFX- Reference Guide" Chapter "Variables in ANSYS CFX " it works.
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Tags |
adaptive time step, time stepping |
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