Mass flow rate imbalance problem

frossi · May 22, 2017, 22:25

Hi,

I first run a simulation with a very coarse mesh, and the imbalance is very low (0.01 kg/s). Then, I refine the mesh and run it again, and the imbalance increases to 250 kg/s!

How is this possible? That imbalance is close to my flow rate in the domain. How can this happen with the fine mesh? Shouldnt it happen when the mesh is coarse?

juliom · May 23, 2017, 08:16

Wow that's impossible. Is the first time I see that in several years doing CFD. Are you using a commercial software? If so; make sure your domain and boundary remains unchanged. Check the output file go the mesh to check that your boundaries remains constant.
That sounds impossible

-Maxim- · May 24, 2017, 03:46

You could check in Post where your mass is going

And I think in order to help you better, you should post your ccl/out-file and some screenshots of your meshes.

frossi · June 6, 2017, 14:35

Hi,

how can i check where my mass is going in POST? Do you mean I should check the velocity streamlines?

My problem is that the residuals, mass flow rate, and my forces (lift and drag) converge around 1000 iterations, and they are almost perfectly converged until 2000. Then, suddenly all these quantities start to oscillate, diverging more and more every time.

Also note that I am doing a parametric study. I keep the wing geometry and mesh the same, while only changing the velocity. My results for lower velocities (from Mach 0.2 to Mach 0.3) are right and stay converged. While when I run Mach 0.35 (the same case as the pictures) you can see how the results diverge. How come this problem arises only at higher velocities?

What should I do? here is my mesh.

juliom · June 7, 2017, 10:11

Ok, lets start from the beginning. What are your boundary conditions, schemes and tpe of solver. ?

frossi · June 7, 2017, 18:06

Pressure based solver, second order schemes, steady state solution.

Boundary conditions:
inlet, symmetry plane at the wing root, and everything else is an opening.
Total pressure and temperature are sea level conditions, and the speed is Mach 0.35.

Thank you for your help.

juliom · June 7, 2017, 18:21

Well Frossi; I might be wrong though.
My guess is that you are solving compressible flows, because I read something about Mach number. Therefore, you have to use denisty based solvers, and your boundary conditions do not match with the right BC, for this type of flows.
I suggest you to do a short proof of this. Run a case with very small Mach number, below 0.3. Assign mass flow rate at the inlet, outlet = outflow and symmetry in the 'z' direction. Try to avoid opening in this in this case.
I have seen people (including myself) using slip boundary conditions at the far field and at the bottom. Symmetry in the normal direction (perpendicular to your screen).
Solve this case, and you will se that it must converge.
Also, since you are using a compressible scheme, my advise is to use a transient approach, because your equations includes density and density changes with time.

ghorrocks · June 7, 2017, 21:30

Just checking: Are you using Fluent or CFX?

May 22, 2017, 22:25	Mass flow rate imbalance problem	#1
frossi Member Ferruccio Rossi Join Date: Jun 2016 Location: Melbourne, FL USA Posts: 91 Rep Power: 10	Hi, I first run a simulation with a very coarse mesh, and the imbalance is very low (0.01 kg/s). Then, I refine the mesh and run it again, and the imbalance increases to 250 kg/s! How is this possible? That imbalance is close to my flow rate in the domain. How can this happen with the fine mesh? Shouldnt it happen when the mesh is coarse?

May 23, 2017, 08:16		#2
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Wow that's impossible. Is the first time I see that in several years doing CFD. Are you using a commercial software? If so; make sure your domain and boundary remains unchanged. Check the output file go the mesh to check that your boundaries remains constant. That sounds impossible frossi likes this.

May 24, 2017, 03:46		#3
-Maxim- Senior Member Maxim Join Date: Aug 2015 Location: Germany Posts: 413 Rep Power: 13	You could check in Post where your mass is going And I think in order to help you better, you should post your ccl/out-file and some screenshots of your meshes. frossi likes this.

June 7, 2017, 10:11		#5
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Ok, lets start from the beginning. What are your boundary conditions, schemes and tpe of solver. ? frossi likes this.

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June 7, 2017, 18:06		#6
frossi Member Ferruccio Rossi Join Date: Jun 2016 Location: Melbourne, FL USA Posts: 91 Rep Power: 10	Pressure based solver, second order schemes, steady state solution. Boundary conditions: inlet, symmetry plane at the wing root, and everything else is an opening. Total pressure and temperature are sea level conditions, and the speed is Mach 0.35. Thank you for your help.

June 7, 2017, 18:21		#7
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Well Frossi; I might be wrong though. My guess is that you are solving compressible flows, because I read something about Mach number. Therefore, you have to use denisty based solvers, and your boundary conditions do not match with the right BC, for this type of flows. I suggest you to do a short proof of this. Run a case with very small Mach number, below 0.3. Assign mass flow rate at the inlet, outlet = outflow and symmetry in the 'z' direction. Try to avoid opening in this in this case. I have seen people (including myself) using slip boundary conditions at the far field and at the bottom. Symmetry in the normal direction (perpendicular to your screen). Solve this case, and you will se that it must converge. Also, since you are using a compressible scheme, my advise is to use a transient approach, because your equations includes density and density changes with time.

June 7, 2017, 21:30		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,853 Rep Power: 144	Just checking: Are you using Fluent or CFX?