mass in is not equal in transient simulation

yaseen wsu · May 3, 2016, 11:40

hello
I simulated free surface by transient simulation with 10 sec total time and time step with 0.06 sec. but the result of mass flowrate (kg/s) at outlet does not equal to inlet. is it better to increase total time or reduce time step
thanks for your suggestion.

ghorrocks · May 3, 2016, 21:27

We cannot tell with the small amount of information you have posted. This FAQ discusses some general issues: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Note that in transient simulations the flow in does not have to equal the flow out. In transient simulations you can gain or loose material by having it build up or reduce in the simulation domain. So an imbalance between flow in and out does not necessarily mean you have a problem.

I would recommend you look at the final results file in CFD-Post and see what is going on. When you understand what is happening you will know whether your simulation is correctly set up, whether it needs to run longer or whether what you have is correct.

yaseen wsu · May 6, 2016, 11:11

Quote:

Originally Posted by ghorrocks

We cannot tell with the small amount of information you have posted. This FAQ discusses some general issues: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Note that in transient simulations the flow in does not have to equal the flow out. In transient simulations you can gain or loose material by having it build up or reduce in the simulation domain. So an imbalance between flow in and out does not necessarily mean you have a problem.

I would recommend you look at the final results file in CFD-Post and see what is going on. When you understand what is happening you will know whether your simulation is correctly set up, whether it needs to run longer or whether what you have is correct.

thanks
but in modeling spillway, after a certain time the flow must become steady (inflow = outflow)

Opaque · May 6, 2016, 15:20

That is only true when the solution reaches a state where all the transient/temporal derivatives are exactly 0. Otherwise, it is difficult to tell.

May 3, 2016, 11:40	mass in is not equal in transient simulation	#1
yaseen wsu Senior Member yaseen Join Date: Oct 2015 Location: Hawler Posts: 174 Rep Power: 10	hello I simulated free surface by transient simulation with 10 sec total time and time step with 0.06 sec. but the result of mass flowrate (kg/s) at outlet does not equal to inlet. is it better to increase total time or reduce time step thanks for your suggestion.

May 3, 2016, 21:27		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	We cannot tell with the small amount of information you have posted. This FAQ discusses some general issues: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F Note that in transient simulations the flow in does not have to equal the flow out. In transient simulations you can gain or loose material by having it build up or reduce in the simulation domain. So an imbalance between flow in and out does not necessarily mean you have a problem. I would recommend you look at the final results file in CFD-Post and see what is going on. When you understand what is happening you will know whether your simulation is correctly set up, whether it needs to run longer or whether what you have is correct. yaseen wsu likes this.

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May 6, 2016, 15:20		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,803 Rep Power: 32	That is only true when the solution reaches a state where all the transient/temporal derivatives are exactly 0. Otherwise, it is difficult to tell.