[medaouarwalid]

Originally Posted by BlnPhoenix

So did you find some average temperature in your experiment? Have you measured exactly after one hour? Try to describe your experimental findings. Is the temperature constant over time, if yes what do you expect if you would measure after five minutes? Same temp as after one hour?

If you cannot afford the simulation to run to one hour, i would concider redoing your measurements and run it till you have experimental data to compare.

Mass imbalance means that the mass flow in the systems equals the mass flow out of the system. It's very important to ensure mass balance in every time step. So please check for that and as mentioned set your target residual values accordingly. Also for temperature and velocities as i described above. Your residuals sound good but try to look for the actual change in values to see what they actually mean.

My case is a dynamic and thermal study of air jets .Temperature was not constant over time because i was dealing with hot air jets in a room, and with time of corse the temperature increases , for each station ( axialy and radially) i mesured the temperature at that station to calculate the reduced temperature ( ti- ta)/(tmax- ta) where ti is the temperature at each station , ta is the ambiant temperature and tmax is the maximum température at the inlet. Until now i didnt find a numerical way (in postprocessing) to get the température the same way like i got it in experiance. Because experimantaly the mesurements of temperature was not with a constant time step, it depended to when the velocity on the device's screen became constant, so i waited until i get a velocity mesurements then i get a temperature mesurement (ti and ta) .
If there is no solution, i think i will just do a dynamic study, because it is impossible for me experimentaly to get température mesurements in a constant time step, because i dont know if in that moment the velocity will be stable on the device's screen.

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[medaouarwalid]

Originally Posted by FMDenaro

Your case will be stiff using a compressible flow model. Are you preconditioning the equations? Could you switch to a pure incompressible solver? Try to evaluate the normalized residuals

I did a small search about your questions, well first, fluent hide the different solvers from the users, when you select different density specifications, it will activate different solvers. I am setting density constant where there is other choices like ideal gas, incompressible-ideal-gas..... so the i guess that i am using an incompressible solver.
For the preconditioning, what i understand is that it has un effect of reducing the acoustic speed of the system to accelerate the convergence in incompressible flows as the mach number goes to zero wright? But i dont know how to put that in the software

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[FMDenaro]

Originally Posted by medaouarwalid

I did a small search about your questions, well first, fluent hide the different solvers from the users, when you select different density specifications, it will activate different solvers. I am setting density constant where there is other choices like ideal gas, incompressible-ideal-gas..... so the i guess that i am using an incompressible solver.
For the preconditioning, what i understand is that it has un effect of reducing the acoustic speed of the system to accelerate the convergence in incompressible flows as the mach number goes to zero wright? But i dont know how to put that in the software

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http://www.afs.enea.it/project/neptu...th/node378.htm

but you can use also the NITA

[medaouarwalid]

Originally Posted by FMDenaro

http://www.afs.enea.it/project/neptu...th/node378.htm

but you can use also the NITA

I am using a bounded second order implicit for transient formulation in a pressure based solver

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[medaouarwalid]

Originally Posted by FMDenaro

http://www.afs.enea.it/project/neptu...th/node378.htm

but you can use also the NITA

Okay, now i set non iterative time advancement. Any other quggestions befor i restart calculation sir ?

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[FMDenaro]

Originally Posted by medaouarwalid

Okay, now i set non iterative time advancement. Any other quggestions befor i restart calculation sir ?

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Have you infos about a physical initial condition? Otherwise, if you set an arbitrary initial condition you have to wait enough time to let the solution correlating in a physical way from the arbitrary numerical one.

[medaouarwalid]

Originally Posted by FMDenaro

Have you infos about a physical initial condition? Otherwise, if you set an arbitrary initial condition you have to wait enough time to let the solution correlating in a physical way from the arbitrary numerical one.

No, only boundary conditions. About residaul normalize, i let the default iteration number (5) and normalization factor (0) for all ???

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[medaouarwalid]

Divergence detected in AMG solver: x-momentum

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[arjun]

Originally Posted by medaouarwalid

Divergence detected in AMG solver: x-momentum

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This must be NITA of fluent. It is its main problem.

[FMDenaro]

Originally Posted by medaouarwalid

Divergence detected in AMG solver: x-momentum

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This is a very general advice and can be due to several issues...

Does it appear after the first time step or during the transient?
It could be to an error in the setting of the BC.s or a numerical instability ...

[FMDenaro]

Are you setting the fractional step or the PISO? Did you set a second order time-advancing?

[medaouarwalid]

Originally Posted by FMDenaro

This is a very general advice and can be due to several issues...

Does it appear after the first time step or during the transient?
It could be to an error in the setting of the BC.s or a numerical instability ...

It appeared after the 4th time step

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[medaouarwalid]

Originally Posted by FMDenaro

Are you setting the fractional step or the PISO? Did you set a second order time-advancing?

I was using The piso one, i will try FSM then i will tell you + yes bounded second order implicit

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[medaouarwalid]

- i used hybrid initialization , a warning displays
Warning: convergence tolerance of 1.000000e-06 not reached
during Hybrid Initialization.

-i started calculation
nb : there is also warnings from the begining of the calculation:
reversed flow in 4884 faces on pressure-outlet 6. reversed flow in 4884 faces on pressure-outlet 7.
- after incrementing to the 3rd time step :
# Divergence detected in AMG for x-momentum: protective actions enabled!
# Divergence detected in AMG for x-momentum, temporarily solve with BCGSTAB!
Error: Divergence detected in AMG solver: x-momentum
Error Object: #f

[arjun]

my advise is that run first few time steps with implicit method without worry about cost.

This is reason, FVUS have a setting that says fraactional solver after this many time steps.

In the start it is not converged and NITA has problems.

[medaouarwalid]

Originally Posted by arjun

my advise is that run first few time steps with implicit method without worry about cost.

This is reason, FVUS have a setting that says fraactional solver after this many time steps.

In the start it is not converged and NITA has problems.

What implicit methode ? I am already using it i guess

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[FMDenaro]

Check your BC.s, maybe they do not satisfy the incompressibility constraint

[medaouarwalid]

Originally Posted by FMDenaro

Check your BC.s, maybe they do not satisfy the incompressibility constraint

How is so ? 7 Hot air jets blowing of 7 nozzles in atmospher in a room with velocity 7 m/s for each jet. I dont think it is cmplicated BCs .
The roof is wall, nozzles wall also, and the rest i set it pressure outlet because i wanted to limite the domaine, should i put the real dimensions of the room and set every thing as wall ?

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[FMDenaro]

Originally Posted by medaouarwalid

How is so ? 7 Hot air jets blowing of 7 nozzles in atmospher in a room with velocity 7 m/s for each jet. I dont think it is cmplicated BCs .
The roof is wall, nozzles wall also, and the rest i set it pressure outlet because i wanted to limite the domaine, should i put the real dimensions of the room and set every thing as wall ?

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Pressure outlet must allow the mass flow rate entering from the nozzles to exit. If that does not happen, the continuity equation is not satisfied, the mass increases and the error in the divergence-free constraint acts as a source driving to divergence.

[medaouarwalid]

Originally Posted by FMDenaro

Pressure outlet must allow the mass flow rate entering from the nozzles to exit. If that does not happen, the continuity equation is not satisfied, the mass increases and the error in the divergence-free constraint acts as a source driving to divergence.

i noticed experimentaly that velocitie at let saying 2 meters in axial direction is close to zero , So to reduced the dimensions of the room i set pressure outlet at 2 meter instead of putting the real dimension of the room and set it as wall. I will change the geometri by setting the real dimensions and real boundary conditions . After that I will share with you the result.
A question, could the temperature of the jet in my case have an influence in compressibility ?

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