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divergance detected in amg solver-coupled

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Old   February 17, 2016, 03:39
Default problem with turbulent model- divergence in amg solver
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arshia
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hi all,
i'm tring to simulate a moving object in a pipe by patching a high pressure and tempreture zone behind the object,
i used a six dof udf for moving object by dynamic mesh scheme ,
i solved this by using fluent 6.3.26 and it work well without any problem
but when i'm tring to use ansys fluent 15 , it diverge at the first time step
with inviscid model it works well on fluent 6.3.26 and fluent 15
viscous model : k-e standard
density based
transient - time step:1e-6
all of the setup are the same
so thanks to help me
after one time step this error apears:
divergance detected in amg solver-coupled- coursening group size
divergance detected in amg solver-coupled- increasing relaxation sweep

i tried to decrease relaxation factor and courant number to 1 but it didn't help
also i tried to fine the mesh but it didn't affected the problem

Last edited by hamed1987; February 17, 2016 at 06:21.
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Old   February 17, 2016, 10:50
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Lucky Tran
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After patching, did you iterate a few times before proceeding to the next time-step? It's important to do a few iterations after patching so that the solution field is updated and consistent with the current state.
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Old   February 17, 2016, 11:40
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Quote:
Originally Posted by LuckyTran View Post
After patching, did you iterate a few times before proceeding to the next time-step? It's important to do a few iterations after patching so that the solution field is updated and consistent with the current state.
thank a lot lucky for your response
after patching the temp and press
i set the time step to 1e-6
and iretation per time step 20
so we have 20 iteration in each time step and so the first time step

and after these 20 iteration for first time step it diverges.
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Old   February 17, 2016, 11:44
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Quote:
Originally Posted by hamed1987 View Post
thank a lot lucky for your response
after patching the temp and press
i set the time step to 1e-6
and iretation per time step 20
so we have 20 iteration in each time step and so the first time step

and after these 20 iteration for first time step it diverges.
You need to enter solve iterate 20 into the TUI in order to iterate without time-stepping
If you simply patch and then press the calculate, it will go to the next time-step and not iterate on the patched fields.

You will go into the next time-step with uniform fields from the previous time-step, which may be "inconsistent"
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Old   February 17, 2016, 12:15
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Quote:
Originally Posted by LuckyTran View Post
You need to enter solve iterate 20 into the TUI in order to iterate without time-stepping
If you simply patch and then press the calculate, it will go to the next time-step and not iterate on the patched fields.

You will go into the next time-step with uniform fields from the previous time-step, which may be "inconsistent"
dear lucky
i tried to solve the problem for some time step with inviscid method without any dynamic mesh( dynamic mesh was off) and then i switched the method to k-e standard and the dynamic mesh on
but it didn't work
so is it diffrent with the method you say??
how can i iterate into the TUI??
thanks a lot
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Old   February 17, 2016, 12:56
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Ok you did several things that all resulted in the same problem. In a transient simulation, after you make any changes you should always do some iterations before proceeding to the next time-step.

When you switch from inviscid to a turbulence model in a transient simulation, the turbulence fields are empty and not initialized. If you press the calculate button, then it will go to the next time-step but it won't have any turbulence on the old field to solve with.

You should feeze all current fields and iterate only the turbulence fields if you want to ensure that no other fields are affected.

Quote:
Originally Posted by hamed1987 View Post
dear lucky
i tried to solve the problem for some time step with inviscid method without any dynamic mesh( dynamic mesh was off) and then i switched the method to k-e standard and the dynamic mesh on
but it didn't work
so is it diffrent with the method you say??
how can i iterate into the TUI??
thanks a lot
TUI (text-user-interface)

just type this in and press enter
solve iterate 20
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Old   February 21, 2016, 07:00
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Quote:
Originally Posted by LuckyTran View Post
Ok you did several things that all resulted in the same problem. In a transient simulation, after you make any changes you should always do some iterations before proceeding to the next time-step.

When you switch from inviscid to a turbulence model in a transient simulation, the turbulence fields are empty and not initialized. If you press the calculate button, then it will go to the next time-step but it won't have any turbulence on the old field to solve with.

You should feeze all current fields and iterate only the turbulence fields if you want to ensure that no other fields are affected.



TUI (text-user-interface)

just type this in and press enter
solve iterate 20
dear lucky
i did what you do,
after patching pressure and tempreture i typed solve iterate 20,
residuals was reach :
continuty e-18
x-vel e-12
y-vel e-12
energy e-19
k e-16
epsilon e-16
after that i run the calculation by time step size e-6,
it didnt diverge but i have a new problem ,
the residuals of continuty and x-vel and y-vel and energy goes to be constant at these values (like the picture)Untitled.jpg
and the object didn't move until now (after 1000 time step)
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Old   February 21, 2016, 14:19
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It looks like your simulation is working.

It looks like you have stationary boundary conditions (constant pressure outlets and some sort of constant inlet) and probably also constant properties. After an initial short transient your velocity and turbulence are stationary and hence the residuals stay constant (because the velocities and turbulence do not change at all).

It will take time for your patched temperature to convect downstream into your domain. You should have a good feel for this. If your time-step is very small then it will take many time-steps for the new inlet conditions to arrive in the domain.

This is probably unrelated to your problem, but I forgot to mention that you should not switch models (from inviscid to turbulent) in the middle of a transient simulation. You may get the wrong physics because your initial state is inviscid and suddenly you activate a turbulence model so the flow evolves from inviscid and transitions (temporally) into a turbulent flow. Unless this is what you are trying to simulate...
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Old   February 21, 2016, 14:59
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Quote:
Originally Posted by LuckyTran View Post
It looks like your simulation is working.

It looks like you have stationary boundary conditions (constant pressure outlets and some sort of constant inlet) and probably also constant properties. After an initial short transient your velocity and turbulence are stationary and hence the residuals stay constant (because the velocities and turbulence do not change at all).

It will take time for your patched temperature to convect downstream into your domain. You should have a good feel for this. If your time-step is very small then it will take many time-steps for the new inlet conditions to arrive in the domain.

This is probably unrelated to your problem, but I forgot to mention that you should not switch models (from inviscid to turbulent) in the middle of a transient simulation. You may get the wrong physics because your initial state is inviscid and suddenly you activate a turbulence model so the flow evolves from inviscid and transitions (temporally) into a turbulent flow. Unless this is what you are trying to simulate...
sorry lucky ,
i did'nt understand completly what you say
when i calculate my problem with turbulent model in fluent 6.3.26 or inviscid model, the object move from the first time step because we have high pressure(350e6) and high tempreture (3000) and the object weight is 0.4 ,
so there is a high force behind the object that it must move at the first time step
but when i tried to do what you said( tui iterations)
after that i calculate it dosent move after 0.001s
after 0.001s it must reach the middle of the barrel
i have two stationary in mesh zone
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Old   February 22, 2016, 06:18
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Quote:
Originally Posted by LuckyTran View Post
It looks like your simulation is working.

It looks like you have stationary boundary conditions (constant pressure outlets and some sort of constant inlet) and probably also constant properties. After an initial short transient your velocity and turbulence are stationary and hence the residuals stay constant (because the velocities and turbulence do not change at all).

It will take time for your patched temperature to convect downstream into your domain. You should have a good feel for this. If your time-step is very small then it will take many time-steps for the new inlet conditions to arrive in the domain.

This is probably unrelated to your problem, but I forgot to mention that you should not switch models (from inviscid to turbulent) in the middle of a transient simulation. You may get the wrong physics because your initial state is inviscid and suddenly you activate a turbulence model so the flow evolves from inviscid and transitions (temporally) into a turbulent flow. Unless this is what you are trying to simulate...
dear lucky
i continued the simulation but the object didn't move at all.
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Old   February 24, 2016, 15:24
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Hi
eny comment?
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