Store information from previous timestep in CFX

WillisH · June 18, 2013, 17:30

I am simulating a transient flow through a simple geometry of a pipe with heated and unheated lengths in CFX 13. I am trying to figure out how to take the average outlet temperature from the previous timestep and set it as the average inlet temperature of the current timestep. I have been trying to use an additional variable with the Update Loop = TRANS_LOOP modification but am getting the following error from the solver manager.

"The expression is invalid because:
PrevOutletTemp is not available for use in this term
Error processing expression: Total Temperature = PrevOutletTemp"

I have tried running iterations with the AV and the inlet condition set to numerical values prior to trying to the pass the temperature value but I get the same error. I have not been able to figure out the correct execution of this method. I want to pass the value equal to areaAve(Temperature)@outlet from the previous timestep to the current inlet temperature condition. What should I set the algebraic equation of the AV to? Do I use the AV directly in the inlet temperature condition or an expression or function? Any help, quidance, or suggestions would be appreciated.

singer1812 · June 19, 2013, 15:21

Not sure if I follow what you are doing or not. Been a while since I used this.

In the past, I think you need to run a few iterations of analysis without this setting to initialize the variable.

Then use this to intialize the run with this setting.

WillisH · June 19, 2013, 18:14

I want to simulate a recirculating flow, The flow is pumped through heated and unheated lengths of pipe and the temperature of the fluid domain should raise through the whole period of the transient simulation. To model the recirculation of the flow I want to set the inlet temperature for each time step equal to the outlet temperature of the previous timestep.

evcelica · June 19, 2013, 19:50

Just use your current outlet temperature, I believe this would be the correct way to do it, and it will not make a difference if your time steps are small enough anyways.
Doing it your way would make it time step dependent.

WillisH · June 20, 2013, 14:14

Thanks for the suggestion, That worked well!

June 18, 2013, 17:30	Store information from previous timestep in CFX	#1
WillisH New Member Join Date: Jun 2013 Posts: 3 Rep Power: 13	I am simulating a transient flow through a simple geometry of a pipe with heated and unheated lengths in CFX 13. I am trying to figure out how to take the average outlet temperature from the previous timestep and set it as the average inlet temperature of the current timestep. I have been trying to use an additional variable with the Update Loop = TRANS_LOOP modification but am getting the following error from the solver manager. "The expression is invalid because: PrevOutletTemp is not available for use in this term Error processing expression: Total Temperature = PrevOutletTemp" I have tried running iterations with the AV and the inlet condition set to numerical values prior to trying to the pass the temperature value but I get the same error. I have not been able to figure out the correct execution of this method. I want to pass the value equal to areaAve(Temperature)@outlet from the previous timestep to the current inlet temperature condition. What should I set the algebraic equation of the AV to? Do I use the AV directly in the inlet temperature condition or an expression or function? Any help, quidance, or suggestions would be appreciated.

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June 19, 2013, 15:21		#2
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	Not sure if I follow what you are doing or not. Been a while since I used this. In the past, I think you need to run a few iterations of analysis without this setting to initialize the variable. Then use this to intialize the run with this setting.

June 19, 2013, 18:14		#3
WillisH New Member Join Date: Jun 2013 Posts: 3 Rep Power: 13	I want to simulate a recirculating flow, The flow is pumped through heated and unheated lengths of pipe and the temperature of the fluid domain should raise through the whole period of the transient simulation. To model the recirculation of the flow I want to set the inlet temperature for each time step equal to the outlet temperature of the previous timestep.

June 19, 2013, 19:50		#4
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	Just use your current outlet temperature, I believe this would be the correct way to do it, and it will not make a difference if your time steps are small enough anyways. Doing it your way would make it time step dependent.

June 20, 2013, 14:14		#5
WillisH New Member Join Date: Jun 2013 Posts: 3 Rep Power: 13	Thanks for the suggestion, That worked well!