[medaouarwalid]

hi, excuse me for my questions, i am new in simulations, i'am using fluent to simulate an air jet, i use transient time study and K-epsilon model, when i set the time step to 0.0001s the solution converged rapidally , and every 20 iterations i get the message "solution converged" , with 10^-4 for continuity , 10^-6 for velocities and 10^-8 for energie , but when i increased the time step to 0.01 it stills not converged until now. could any one explain to me please !

[FMDenaro]

The time step has a great relevance on the solution, both in terms of accuracy and in the numerical stability.

[medaouarwalid]

Originally Posted by FMDenaro

The time step has a great relevance on the solution, both in terms of accuracy and in the numerical stability.

please more details , How do I know in advance choose the right time step

[piu58]

You should understand the concept of the Courant number. Some of the solvers control the time step by this number, which is a fine idea.

Than you have to think about the relation between Co number and convergence

All in all it can be said: The finer the mesh the lower the max Co number (or the time step) need to be. A smaller time step may take less wall time for calculating a solution.

[medaouarwalid]

Originally Posted by piu58

You should understand the concept of the Courant number. Some of the solvers control the time step by this number, which is a fine idea.

Than you have to think about the relation between Co number and convergence

All in all it can be said: The finer the mesh the lower the max Co number (or the time step) need to be. A smaller time step may take less wall time for calculating a solution.

Thank you, what i am learning from this forum is larger than what i learnd in my entire years of study . What a great forum

[juliom]

Besides what professor Denaro said, the time step is critique in explicit schemes but not in implicit scheme. The latter is stable at high time step. However, is not as accurate as the explicit one

[medaouarwalid]

Originally Posted by juliom

Besides what professor Denaro said, the time step is critique in explicit schemes but not in implicit scheme. The latter is stable at high time step. However, is not as accurate as the explicit one

I am using a bounded secpnd order implicit fomulation + i found that best time step need to be less that ( min size/ velocity) , in my case it means less than 0.0001 !!! So to see the result of 1 hour i need 36 milion iterations !?!?

[piu58]

> So to see the result of 1 hour i need 36 milion iterations !?!?

Look at your mesh and at your boundary conditions. Something is wrong here.

[medaouarwalid]

Originally Posted by piu58

> So to see the result of 1 hour i need 36 milion iterations !?!?

Look at your mesh and at your boundary conditions. Something is wrong here.

Why? I have the smalest size of the mesh is 0,000875m, velocity is 7m/s so the recomanded time step will be less than 0.0001 . This time step will incrementes every time the solution converge ( in my case it was every 9 iterations when time step was 10^-5) so to reach 1 minutes​ i need 6 milions time step, true or not ?? Or i dont need to wait until 1 minute to see the result of the velocities??

[piu58]

> Why?

Because it is unnormal that a well formulated case converges such slowly. There may be skewed cells or an inappropriate balance between cell size and velocity.

Nonphysical boundary conditions lead often to convergence problems.

That are the directions I go first.

[medaouarwalid]

Originally Posted by piu58

> Why?

Because it is unnormal that a well formulated case converges such slowly. There may be skewed cells or an inappropriate balance between cell size and velocity.

Nonphysical boundary conditions lead often to convergence problems.

That are the directions I go first.

Balance between cell zize and velocity ? I didnt hear this befor, can you explain to me ?
Boundry conditions are simple in my case , i have a room of dimensions 2 meter heigh, 4meter length and 4 meter width, and an air jet blowing vertically through a nozzle placed in the ceiling, so the ceiling i set it wall, the inlet is in the liddle of the ceiling , walls of the room as pressure outlet because the room is big , floor also as pressure outlet .

[piu58]

Originally Posted by medaouarwalid

Balance between cell zize and velocity ? I didnt hear this befor, can you explain to me ?
Boundry conditions are simple in my case , i have a room of dimensions 2 meter heigh, 4meter length and 4 meter width, and an air jet blowing vertically through a nozzle placed in the ceiling, so the ceiling i set it wall, the inlet is in the liddle of the ceiling , walls of the room as pressure outlet because the room is big , floor also as pressure outlet .

What means pressure outlet? Constant pressure?
Waht velocity b.c. did you set?

Do you have a compressible medium? If not it may be you move in volume without any way to let it out.

[medaouarwalid]

Originally Posted by piu58

What means pressure outlet? Constant pressure?
Waht velocity b.c. did you set?

Do you have a compressible medium? If not it may be you move in volume without any way to let it out.

I did pressure outlet at the walls of the room because the velocity near to it was close to zero, also the pressure was constant in the room (atmospherique pressure) , the inlet like i said is a nozzle placed in the middle of the ceiling, i set it velocity inlet and give it 7m/s intensity 4% and hydraulique diameter 0.004m