[ghorrocks]

Time for me to ask a question for a change

I am doing a particle tracking simulation and I am getting almost half my particle tracks end with fate "Integration error". So far I have not been able to find what the problem is. Does anybody know what is causing this or how to fix it?

• The simulation is in incompressible air
• Driven by a momentum source.
• The simulation is transient.
• The simulation is well converged and with a mesh and time step which has been shown adequate for the accuracy I require.
• The particles are on-way coupled to the flow.
• The residence time of most particles is very short (2 or 3 time steps)

Below is a snippet from the output file. As you can see, about half the particles disappear with fate "Integration error".

======================================================================
 |                      Timestepping Information                      |
 ----------------------------------------------------------------------
 |       Timestep       |  RMS Courant Number  |  Max Courant Number  |
 +----------------------+----------------------+----------------------+
 |      5.0000E-05      |          1.49        |         16.88        |
 ----------------------------------------------------------------------

 ======================================================================
 TIME STEP =   865 SIMULATION TIME = 4.3250E-02 CPU SECONDS = 1.095E+05
 ----------------------------------------------------------------------
 COEFFICIENT LOOP ITERATION =    1              CPU SECONDS = 1.095E+05
 ----------------------------------------------------------------------
 |       Equation       | Rate | RMS Res | Max Res |  Linear Solution |
 +----------------------+------+---------+---------+------------------+
 | U-Mom                | 0.98 | 9.3E-05 | 8.2E-03 |       1.6E-02  OK|
 | V-Mom                | 0.99 | 1.0E-04 | 4.8E-03 |       2.6E-02  OK|
 | W-Mom                | 0.98 | 1.1E-04 | 6.6E-03 |       1.8E-02  OK|
 | P-Mass               | 0.97 | 2.2E-05 | 2.7E-03 |  5.1  9.4E-02  OK|
 +----------------------+------+---------+---------+------------------+
 ----------------------------------------------------------------------
 COEFFICIENT LOOP ITERATION =    2              CPU SECONDS = 1.095E+05
 ----------------------------------------------------------------------
 |       Equation       | Rate | RMS Res | Max Res |  Linear Solution |
 +----------------------+------+---------+---------+------------------+
 | U-Mom                | 0.41 | 3.8E-05 | 3.9E-03 |       4.0E-03  OK|
 | V-Mom                | 0.60 | 6.2E-05 | 4.7E-03 |       4.0E-03  OK|
 | W-Mom                | 0.63 | 6.9E-05 | 3.8E-03 |       4.0E-03  OK|
 | P-Mass               | 0.60 | 1.4E-05 | 2.2E-03 |  9.2  4.3E-02  OK|
 +----------------------+------+---------+---------+------------------+

 ======================================================================
                          Particle Diagnostics
 ======================================================================

 +--------------------------------------------------------------------+
 |                      Particle Fate Diagnostics                     |
 +--------------------------------------------------------------------+
 | Particle type          | Fate type                       Particles |
 +--------------------------------------------------------------------+
 | Ink                    | Entered domain                :      2560 |
 |                        | Continue from last time step  :     10676 |
 |                        | Left domain                   :         1 |
 |                        | Collected on walls            :      1361 |
 |                        | Waiting for next time step    :     10669 |
 |                        | Integration error             :      1205 |
 +--------------------------------------------------------------------+

The documentation is not very helpful on this. It just says "This message indicates that the tracking of these particles was terminated due to an unexpected error in the particle tracking integration."

[bharath]

hi ghorrocks,
the reason may be if the particles trapped in recirculation zone for a long time this kind of error occur otherwise the time step won't suits well for the problem.

[ghorrocks]

I see. There are no recirculations where the particles are travelling, but I do have an unusually small geometry. I will look into tolerances on minimum particle movements.

Thanks for the tip!

[altano]

Hi ghorrocks

Increasing the value of "number of integration steps per element" could help. Integration error problem solved by that way in my case.

It appears it's late response for your work but it may help others

[ghorrocks]

Thanks for the comment. I had not tried that one.

But unfortunately that piece of work is long finished and I doubt I will get to play with it again. But you never know.

[Julian K.]

Originally Posted by altano

Increasing the value of "number of integration steps per element" could help.

If you increase the "Number of Integration Steps per Element" (nispe), you may have to increase the "Maximum Number Integration Steps" (mnis), as well:
mnis = nsipe * "number of elements a particle travels through"

Thus, if nispe = 1e3 and mnis = 1e6, a particle can travel trough 1e3 elements, before it "Exceeded Integration Limit".

Another reason for a high number of particle loss due to an "Integration Error", may be that the initial velocity of the particles is set to zero. I released particles from a sphere with a velocity of 0 m/s and ca. 50% of the particles got lost due to an integration error. After assigning an initial velocity of 1 m/s, no particles experienced this error.