Steady State Residuals - Repeating Patterns

Mames · June 6, 2016, 08:28

I am using PumpLinx software to look at flow through hydraulic valves, talking in the region of 50 lpm. Currently just running a steady state solution but I get the following residuals, which I understand to be the type of thing you see in a transient solution. Fairly new to the world of CFD I was hoping to get some feedback from more experienced users.

I set the convergence criteria to 1x10-3 yet the residuals (at least not all of them) do not reach that mark. The outputs I am looking at (force and flow) seem to have settled on a final figure (plot attached), would this suggest that not reaching the convergence mark is not an issue?

Residuals.jpg

Spool Force.jpg

Could anyone comment on the residuals graph? Is the repeating pattern a sign that the simulation is incorrect, or is this an issue with boundary condition, or is it even an issue at all?

Any help would be appreciated! Thank you.

fluid23 · June 6, 2016, 10:10

It looks to me like your problem is unsteady, you should try and implicit unsteady solver with appropriate time step and time average results.

A steady solution may be somewhat representative of the transient solution, but to get better accuracy you will want to stick with unsteady solver. That is almost certainly why you see the oscillation in results and residuals.

Mames · June 6, 2016, 10:40

Thank you MBdonCFD. That would make a lot of sense.

Do you know why the residuals seem to be converging before entering an oscillatory pattern, rather going straight into the pattern?

On a side, do you know much about PumpLinx? It doesn't seem to be the most popular piece of software. At least there is very little in the way of guides/examples/information in general floating about on the internet.

Cheers.

June 6, 2016, 08:28	Steady State Residuals - Repeating Patterns	#1
Mames New Member Join Date: Jun 2016 Posts: 3 Rep Power: 9	I am using PumpLinx software to look at flow through hydraulic valves, talking in the region of 50 lpm. Currently just running a steady state solution but I get the following residuals, which I understand to be the type of thing you see in a transient solution. Fairly new to the world of CFD I was hoping to get some feedback from more experienced users. I set the convergence criteria to 1x10-3 yet the residuals (at least not all of them) do not reach that mark. The outputs I am looking at (force and flow) seem to have settled on a final figure (plot attached), would this suggest that not reaching the convergence mark is not an issue? Residuals.jpg Spool Force.jpg Could anyone comment on the residuals graph? Is the repeating pattern a sign that the simulation is incorrect, or is this an issue with boundary condition, or is it even an issue at all? Any help would be appreciated! Thank you.

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June 6, 2016, 10:10		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	It looks to me like your problem is unsteady, you should try and implicit unsteady solver with appropriate time step and time average results. A steady solution may be somewhat representative of the transient solution, but to get better accuracy you will want to stick with unsteady solver. That is almost certainly why you see the oscillation in results and residuals.

June 6, 2016, 10:40		#3
Mames New Member Join Date: Jun 2016 Posts: 3 Rep Power: 9	Thank you MBdonCFD. That would make a lot of sense. Do you know why the residuals seem to be converging before entering an oscillatory pattern, rather going straight into the pattern? On a side, do you know much about PumpLinx? It doesn't seem to be the most popular piece of software. At least there is very little in the way of guides/examples/information in general floating about on the internet. Cheers.