Units of the drag parameter

[ThomasJannaudCAM]

Hi all,

Quick question that has been bugging me for a bit.
In the documentation of the dust module (https://idefix.readthedocs.io/latest/modules/dust.html) the physical meaning of the drag parameters is explained, relating it to the sound speed, the gas mean free path and sound speed, the grain stopping time, size and density. But there are no units mentioned.
I have assumed that it is all in CGS units, and it seems confirmed by the comments in the FargoPlanet setup, but I wanted to be sure that I am not missing anything.

Thanks in advance,

Best wishes,

Thomas

[glesur]

There is no unit, because it's in code units :-) So no, it doesn't assume CGS. The only thing idefix uses is gamma_i, and this is units of 1/density units*time units, so whatever density units and time units you choose, gamma will be in those units. Now how you construct gamma from your favourite grain model is up to you, but in the end, it should be put in code units (i.e. your choice of density unit, time unit, length unit)

[ThomasJannaudCAM]

Oh I did not know this, thank you !

[neutrinoceros]

Off topic, but I'm sure you guys will love to learn that GitHub markdown actually supports a subset of LaTeX : https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions

[birnstiel]

I did use it in my answer already 😃

[ThomasJannaudCAM]

Yes and I was wondering how you did that, thanks for the tip Clément :)

[neutrinoceros]

Indeed you did... 😅
I just learned that it doesn't render properly on GitHub Mobile. 🤦🏻‍♂️