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

[Christoph Federrath]

Dear Shadab,

Yes, the TurbGen is what’s basically in StirFromFile in the public FLASH version (although a somewhat earlier version without auto amplitude adjustment for instance). The TurbGen repos (GitHub or BitBucket) contain a FLASH plugin that can be used to upgrade the public version code any time you wish.

A constant (averaged over time) energy injection rate is achieved by setting the appropriate combination of “velocity” (target turbulent velocity dispersion) and “ampl_factor” (amplitude adjustment factor) in TurbGen.par (the parameter file that is read by FLASH, which controls all settings of the turbulence driving). We can chat offline to discuss the detail — please let me know any time.

Best regards,
Christoph


> On 14 May 2024, at 8:36 PM, Alam Shadab TU Ilmenau <shadab.alam at tu-ilmenau.de> wrote:
> 
> 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.):
> Federrath - Codes
> mso.anu.edu.au
> 
> 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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