Solver aborts due to high Ma-Number

Spray_Ansys · June 5, 2018, 13:42

Hello everybody

I have a question about the error message "Overflow" and have already read the FAQ sheet. My problem is, that when I solve a certain simulation, the residuals do a quite good job and everything seems "normal". After a certain iteration (around 50 to 80) the Mach Numbers becomes high (Ma>2 in most cases) and in the following iteration the solver aborts. The region of high Ma-number could be located in the hexa mesh and the timescale was adjusted that in this region a Courant Number of (more or less) 1.0 exists. So, it seems that this might be a numerical problem. Therefore I would like to ask, if someone already had this kind of error and have a solution / suggestion, what this problem could cause.

Thank you everybody for help!

Opaque · June 5, 2018, 14:13

I assume you are running Total Energy model.

Have you also activated the Viscous Work Term?

How about the High Speed Wall Function Model?

Spray_Ansys · June 5, 2018, 15:02

Quote:

Originally Posted by Opaque

I assume you are running Total Energy model.

Have you also activated the Viscous Work Term?

How about the High Speed Wall Function Model?

Thank you for your reply, Opaque.
That is correct, I'm running the Total Energy model.
Yes, I've activated the Viscous Work Term.
No, currently I've disabled the High Speed Wall Function Model. Do you suggest, that this might help me solving my problem and is the reason for the solver's abort?

It also has to be said, it is a matter of multiphase flow and the velocities are faraway from being in a "critical" mach region.

Opaque · June 5, 2018, 17:45

Are you running with a rotating frame?

If you are running in the stationary frame and multiphase flow, I would start with the Thermal Energy model.

I would not use the High Speed Model unless you know why it is needed, and what the benefits are.

The typical advice for complex simulations, multiphase flow qualifies as such, is to start simple.

Gert-Jan · June 5, 2018, 19:14

Remember that in multiphase situations, the speed of sound is usually much lower than for single phase situations. So your Critical Mach Region might be closer than you think.

Follow the advice of Opaque: Start Simple.
After a succesfull simple run, you can always proceed with more complexity

Spray_Ansys · June 6, 2018, 13:04

Thank you guys for your answers and advices!

Quote:

Originally Posted by Opaque

Are you running with a rotating frame?

If you are running in the stationary frame and multiphase flow, I would start with the Thermal Energy model.

I would not use the High Speed Model unless you know why it is needed, and what the benefits are.

The typical advice for complex simulations, multiphase flow qualifies as such, is to start simple.

No, I'm not running with rotating frame. The analysis type is stationary and also the frame is a stationary frame. Due to the air is compressible, I thought I should air let be in the Total Energy Model. Water, my second fluid, is setup with the Thermal Energy model.

Opaque · June 6, 2018, 13:16

How much of the compressibility effects do you need to model?

As said before, simple. I would start with Air @ 25C (incompressible), Thermal Energy. If the model runs w/o a problem, check the flow solutions and estimate the Mach number present in the solution, and decide the next step.

Recall the goal is to obtain a base solution so you can build with confidence from there.

June 5, 2018, 13:42	Solver aborts due to high Ma-Number	#1
Spray_Ansys Member Join Date: Mar 2018 Posts: 30 Rep Power: 8	Hello everybody I have a question about the error message "Overflow" and have already read the FAQ sheet. My problem is, that when I solve a certain simulation, the residuals do a quite good job and everything seems "normal". After a certain iteration (around 50 to 80) the Mach Numbers becomes high (Ma>2 in most cases) and in the following iteration the solver aborts. The region of high Ma-number could be located in the hexa mesh and the timescale was adjusted that in this region a Courant Number of (more or less) 1.0 exists. So, it seems that this might be a numerical problem. Therefore I would like to ask, if someone already had this kind of error and have a solution / suggestion, what this problem could cause. Thank you everybody for help!

June 5, 2018, 17:45		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,869 Rep Power: 33	Are you running with a rotating frame? If you are running in the stationary frame and multiphase flow, I would start with the Thermal Energy model. I would not use the High Speed Model unless you know why it is needed, and what the benefits are. The typical advice for complex simulations, multiphase flow qualifies as such, is to start simple. Last edited by Opaque; June 5, 2018 at 19:36.

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June 5, 2018, 14:13		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,869 Rep Power: 33	I assume you are running Total Energy model. Have you also activated the Viscous Work Term? How about the High Speed Wall Function Model?

June 5, 2018, 19:14		#5
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,910 Rep Power: 28	Remember that in multiphase situations, the speed of sound is usually much lower than for single phase situations. So your Critical Mach Region might be closer than you think. Follow the advice of Opaque: Start Simple. After a succesfull simple run, you can always proceed with more complexity

June 6, 2018, 13:16		#7
Opaque Senior Member Join Date: Jun 2009 Posts: 1,869 Rep Power: 33	How much of the compressibility effects do you need to model? As said before, simple. I would start with Air @ 25C (incompressible), Thermal Energy. If the model runs w/o a problem, check the flow solutions and estimate the Mach number present in the solution, and decide the next step. Recall the goal is to obtain a base solution so you can build with confidence from there.