[FLASH-USERS] Question regarding velocity update in Stir.F90

[Alam Shadab TU Ilmenau]

Dear Prof. Federrath,

Thank you very much for providing the reason for subtracting the bulk momentum. At present, I have opted to maintain this term activated in my simulations. If it prove effective, excellent. However, if any issues arise, I will not hesitate to seek your help.


I also thank you for sharing the link for your TurbGen code. I am sure it will be helpful. If I am not wrong, I believe it is the same code in 'StirFromFile' directory of Flash code.


In my simulations, ensuring a fixed and known energy supply rate is crucial. Could you provide insight into whether achieving this is feasible and, if so, what parameters would need to be controlled? Any guidance on this matter would be highly appreciated.



Best regards,

Shadab


________________________________
From: Christoph Federrath <christoph.federrath at gmail.com>
Sent: Tuesday, May 14, 2024 4:50:32 AM
To: Alam Shadab TU Ilmenau
Cc: flash-users at flash.rochester.edu
Subject: Re: [FLASH-USERS] Question regarding velocity update in Stir.F90


Dear Shadab,

I implemented this. The reason for subtracting the bulk momentum is that we normally use continuous turbulence driving in periodic box simulations, such that the gas remains in the zero-momentum rest frame. It is usually a tiny correction, so switching this off mostly likely wouldn’t matter, but I chose to do it to make the momentum perfectly zero in x, y, z. If you have a different application in mind that would require you to keep the momentum, I’d be happy to chat further or make this a user parameter, for instance. Btw: the most recent TurbGen code is available here (which includes some extra features, such as automatic amplitude adjustment, etc.):
<https://www.mso.anu.edu.au/~chfeder/codes/turbgen/turbgen.html>
Federrath - Codes<https://www.mso.anu.edu.au/~chfeder/codes/turbgen/turbgen.html>
mso.anu.edu.au<https://www.mso.anu.edu.au/~chfeder/codes/turbgen/turbgen.html>
        <https://www.mso.anu.edu.au/~chfeder/codes/turbgen/turbgen.html>


Best regards,
Christoph

—
Professor Christoph Federrath
Research School of Astronomy & Astrophysics
Australian National University, Canberra, ACT, Australia
Tel: +61-2-6125-0217<tel:+61-2-6125-0217>
Web: https://www.mso.anu.edu.au/~chfeder

On 13 May 2024, at 10:08 PM, Alam Shadab TU Ilmenau <shadab.alam at tu-ilmenau.de> wrote:

Hi,


I have a query about why the 'momentum/mass' is subtracted during the velocity update in the forcing unit. Here's the snippet from Stir.F90 for reference.

<Screenshot from 2024-05-13 13-51-41.png>
Could anyone please clarify this?

Best regards,
Shadab
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