Re: [sparta-users] [EXTERNAL] Re: Injected pressure larger than input pressure when using subsonic model (and unstable for larger pressures)

[Gallis, M. A <ma...@sa...>]

Cosimo,

I just wanted to add to what Savio said.

The subsonic BC was really meant for an outflow BC. For inflow, the standard “supersonic” inflow BC with a substantial number of simulators per cell will work well.

Michael

From: "Poovathingal, Savio" <sav...@uk...>
Date: Tuesday, April 5, 2022 at 10:29 AM
To: Cosimo Livi <cos...@as...>
Cc: "spa...@li..." <spa...@li...>
Subject: [EXTERNAL] Re: [sparta-users] Injected pressure larger than input pressure when using subsonic model (and unstable for larger pressures)

Cosimo,

The issue is because of fluctuations inside the interior cells, which can be fixed by having a lot of particles inside the cells, taking smaller time steps (1/20 or 1/30 of mean collision time), and initializing particles inside the domain before the simulation starts through create_particles. Alternatively, you can also try the sub-relaxation averaging that could help.

Savio

On Tue, Apr 5, 2022 at 11:01 AM Cosimo Livi via sparta-users <spa...@li...<mailto:spa...@li...>> wrote:
Dear SPARTA community,
I have been using SPARTA to address internal flow problems quite extensively in the past months and I have found some issues related to the available boundary conditions in this context. Since I wanted to present the problem in a systematic (and reproducible) way, I prepared a simple report (pdf in the attachments) to quantitively and qualitatively explain such issues. Besides the report, I also have an attachment that allows to reproduce the simulations (surface and input files).

All cases are run with SPARTA (7 Jan 2022). Code is compiled with kokkos using the following commands: cd sparta/src; make yes-kokkos; make kokkos_cuda. The src/MAKE/Makefile.kokkos_mpi_only has been edited on line 32 with the following: SPARTA_INC = -DSPARTA_GZIP -DSPARTA_BIGBIG

I will use the rest of this email to briefly introduce the topic.

The main points I’d like to discuss in this post are the following:

  1.  Fix emit/face subsonic boundaries tend to inject more pressure than what is specified from input (see Fig. 3 in pdf).
  2.  For large values of the specified pressure, I observe unphysical and unstable behavior of the macroscopical fields. I observe this in both simple (rectangular Poiseuille, Fig. 3) and complex geometries (Figs. 4-5 in pdf)
  3.  By removing the subsonic keyword, results are fine, but now it is impossible (up to my knowledge) to precisely impose a specific pressure due to the unknown outflux of particles at the inlet

I have the feeling that most of the problems I have experienced with the subsonic condition are related to the fact that it uses the instantaneous values of the macroscopical fields to apply the Fang & Liou method. To make this statement more solid, I compared some cases with the results of a commercial DSMC solver (FM_DSMC, http://www.flow-matters.com/<https://gcc02.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.flow-matters.com%2F&data=04%7C01%7Cmagalli%40sandia.gov%7C3ecc8c877e6448e1be8a08da17217290%7C7ccb5a20a303498cb0c129007381b574%7C1%7C0%7C637847729669162643%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=NMeL8f3ZBUvXVw0yaA%2BrV0L%2BQxt0ape5tT%2FxWGXnTfw%3D&reserved=0>) that indeed uses averages of the flow macroscopic values to apply the Fang & Liou method.

I would be very interested if someone of the community experienced similar issues, or if someone can point out to some mistake that I am systematically doing in the setup. It goes without saying that any comment from the developers is greatly appreciated!

Thank you for your attention and best regards,
Cosimo


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Savio J. Poovathingal,
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