[biro]

I found a paper from Ansys with Information about transient simulation in CFX. There by I have realised the meaning of some values.

To take some experience I will start a transient Simulation and if there are any question after the run I will report another post.


Unfortunately the file size of the paper exceed the maximum file size, so I can't attache for other user.

[biro]

I have set up my first transient simulation. For Time Duration I set 10s (but now I think the value is to high to reach).

There by for me arise three further question:
1. Whats the difference between "Analyse Type" --> "Timestep Adaption" --> "Target Max (Min) Loops" and "Solver Control" --> "Convergence Control" --> "Max (Min) Coeff Loops.
I set Target Max Loops = 5; Target Min Loops = 3 and for Min Coeff Loops (=1) and Max Coeff Loops (=10) I don't change the default values.
Is this right or did I misunderstood the post of ghorocks:

The recommended approach is to use adaptive timesteps. Set it to home in on 3-5 coeff loops per iteration, and make the minimum and maximum allowable time step wide enough that it will never reach it.

2. For transient Results (Output Control) I set a timestep Intervall of 8 ? Is this a good value ? I can't imagine, which value I have to choose.

3. What will I see in the Transient Result Files.


Thank you for your help!

Greets

[ghorrocks]

1. Those values look OK
2. Note that as the time step size will vary the time spacing of the results will vary. But you appear to only be interested in the final result so you might not need transient results files at all.
3. snapshots of the flow at that time step.

[biro]

thank you for the fast reply.

For my opinion in the result file there are no transient Information and with the transient files that all "build" my transient simulation.
But now I think thats wrong.


I will take a look on my simualtion the next days, I think there will be more question

[biro]

The trend of my transient simulation looks very good (for me).
I attache the progress of the residuals and imbalances, pressure at the interesting Monitorpoints and Reynoldsnumber at the interesting region.

With this results:
Can I say, that the shedding region have no effect on the pressure and Reynoldsnumber in the interesting region ?
It's look like a steady state solution from a transient run, is this right ?

I'm just surprised about the fast convergence ?!

[ghorrocks]

Sure looks steady state to me. So it looks like you needed a transient run to get to a steady state solution. This happens sometimes.

[biro]

Now I'm happy, after a long time I have finally a solution.


Thank you for you endurance, ghorrocks. It was a great help!

[biro]

Once again I have a question about how to interpret my transient run.

My tutor said, the fluctuations of the MassFlow (it's MassFlow at inlet) from the steady state run (see picture) have also to occur in the transient run. I don't understood this, because I have no transient boundary.

Can someone explain me, what I really (have to) see when I look at the transient run?

And when I look on the timestep:
The adaptive timestep method decrease the timestep from initial timestep (4e-4) to the minimum timestep (1e-10).
I thought the adaptive timestep method finds the rigth timestep to resolve transient flow changes ?

Now I'm a little bit confused.

[biro]

Here are also the RMS values for the transient run.

[ghorrocks]

What settings did you use for the adaptive time stepping?

[biro]

I made screenshots of the settings I have used

[ghorrocks]

Of course your simulation went bezerk - you have 1e-12 as a residuals convergence criterion. You should replace this with something more sensible, 1e-4 is a good starting point for a transient simulation.

[biro]

I choose the value, because I want to stop the run by my self, equal to steady-state run.

But I will start my simulation with a value of 1e-4 once more.

[ghorrocks]

The whole idea of a transient run is to advance it through time. When you choose 1e-12 residual tolerance that means the adaptive time stepping will just reduce the time steps to nothing and you do not advance in time - which defeats the purpose of the transient simulation you are trying to do.

[biro]

The timestep size (6.6127e-5) is unchanged since 250 timesteps. Can I assume, that a good timestep size is found ?

Is it recommend to start another run with a fix timestep size found by the adaptive Method ? Or should I change the residual convergence criteria ?


You will see, that all flow values begin to oscillate. I want to research the frequencies and the different values at different timesteps.

[biro]

Does no one has an answer ? My simulation is advanced very well.

What can I do for further investigation ? Should I run with fix timestep size ? Or shoud I increase residual tolerance ?

[ghorrocks]

Your results show there is clearly a strong transient effect happening.

But what happens next is entirely up to you. Do you want to investigate the periodic cycle? Or the startup transient? Or are you just interested in the overall averaged behaviour? Each of these will require a different approach.

[biro]

Initially I'm interested in the behaviour of the periodic cycle (frequences and maybe pressure allocation on different times) and on the overall averaged behaviour.
So I will execute the items one by another, but I don't really know how to start for both.

For a start I would disregard the transient startup.