Help with Fluent Expressions

jm623 · February 13, 2020, 06:28

Hi

Im conducting a transient analysis as part of my final year project on a bimetallic strip heat capture device.The model comprises of only the strip (made from two materials) inside a domain with an inlet and outlet for cooling.The strip is heated from the bottom applied as a temp to the bottom faces, and then cooled by a blast of air from the front. The aim is to investigate how fast different materials can react to various temperature changes. I have the model set up and it works as expected with static inputs. However, i need to write an expression so that when the strip reaches a certain average temperature, the inlet velocity will 'switch on' or increase to a certain velocity and cool the strip.

An ANSYS tutorial (http://storage.ansys.com/webinar/190502-0716.mp4) does almost the same thing @28:56 except I'm changing the input velocity at a certain temp instead of heating a face. However with my setup it always seems to return the 'false' part of the IF statement. Ive setup up a few reports to try monitor the average temp of the strip so i can see the 'air on' temperature being reached, and also one to view the velocity of the inlet so i can see whether it is being changed to the 'air on' velocity.

The expression InletVelocity is used to switch 'on' or 'off' the airflow and is used in the velocity magnitude field of the inlet. AirOn and AirOff are constant values on 2m/s and 0.001 m/s respectively, InletTemp and StripTemp are also constants of 300K and 472.15K. All expressions minus InletVelocity seem to working and are inputing values into the correct places.

https://imgur.com/a/jGea9kf

Does anybody have any idea why its failing to change velocity to the 'AirOn' velocity when the temp is over 300K?

I can provide a link to the archived analysis if anybody is wiling to take a look, if nothing seems wrong from the code provided.

Thank you very much for reading i will await your answers, cheers!

vinerm · February 13, 2020, 10:53

Since expressions are rather new, most of the users are not very experienced with their behavior in Fluent. The expression looks alright but to debug, could you try using either min or max function instead of average?

LuckyTran · February 13, 2020, 11:31

Try hard-coding the boolean true in order to force it to execute the true-IF branch and this should force it to use the AirOn, e.g. IF(true,AirOn,AirOff)

My concern is nested expressions. Instead of IF(blahblah,AirOn,AirOff) try hard-coding the values, e.g. IF(blahblah,2,0,001)

Finally, try putting names in string quotes. IF(blahblah,"AirOn","AirOff")

February 13, 2020, 06:28	Help with Fluent Expressions	#1
jm623 New Member John Mayor Join Date: Feb 2020 Posts: 1 Rep Power: 0	Hi Im conducting a transient analysis as part of my final year project on a bimetallic strip heat capture device.The model comprises of only the strip (made from two materials) inside a domain with an inlet and outlet for cooling.The strip is heated from the bottom applied as a temp to the bottom faces, and then cooled by a blast of air from the front. The aim is to investigate how fast different materials can react to various temperature changes. I have the model set up and it works as expected with static inputs. However, i need to write an expression so that when the strip reaches a certain average temperature, the inlet velocity will 'switch on' or increase to a certain velocity and cool the strip. An ANSYS tutorial (http://storage.ansys.com/webinar/190502-0716.mp4) does almost the same thing @28:56 except I'm changing the input velocity at a certain temp instead of heating a face. However with my setup it always seems to return the 'false' part of the IF statement. Ive setup up a few reports to try monitor the average temp of the strip so i can see the 'air on' temperature being reached, and also one to view the velocity of the inlet so i can see whether it is being changed to the 'air on' velocity. The expression InletVelocity is used to switch 'on' or 'off' the airflow and is used in the velocity magnitude field of the inlet. AirOn and AirOff are constant values on 2m/s and 0.001 m/s respectively, InletTemp and StripTemp are also constants of 300K and 472.15K. All expressions minus InletVelocity seem to working and are inputing values into the correct places. https://imgur.com/a/jGea9kf Does anybody have any idea why its failing to change velocity to the 'AirOn' velocity when the temp is over 300K? I can provide a link to the archived analysis if anybody is wiling to take a look, if nothing seems wrong from the code provided. Thank you very much for reading i will await your answers, cheers!

February 13, 2020, 10:53	Max or Min	#2
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	Since expressions are rather new, most of the users are not very experienced with their behavior in Fluent. The expression looks alright but to debug, could you try using either min or max function instead of average? __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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February 13, 2020, 11:31		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,672 Rep Power: 65	Try hard-coding the boolean true in order to force it to execute the true-IF branch and this should force it to use the AirOn, e.g. IF(true,AirOn,AirOff) My concern is nested expressions. Instead of IF(blahblah,AirOn,AirOff) try hard-coding the values, e.g. IF(blahblah,2,0,001) Finally, try putting names in string quotes. IF(blahblah,"AirOn","AirOff")