[FLASH-USERS] Interpreting pressure behavior in the SinkRotatingCloudCore test problem

[Christoph Federrath]

Hi Sean,

the effect is a result of the choice of equation of state. In order to keep the dense cloud in pressure equilibrium with the surrounding low-density gas, initially, one needs to increase the temperature outside compared to inside the cloud, by the same factor as the density is increased in the inside compared to the outside. So initially, the cloud is in pressure equilibrium. As it starts collapsing, the gas in the outside is kept warm by switching off cooling for gas less than 1.15e-19 g/cm^3 (see the last few lines in flash.par, just below usePolytrope = .true.). However, everything denser than that is cooled to be isothermal, by using a polytropic EOS, which is the line I just referred to in flash.par and located in physics/sourceTerms/Polytrope; you should have seen that my setup includes this module. Thus, the dense cloud can collapse and at the interface to the low-density ambient medium leaves behind an under-pressured interface region, because some of the ambient gas is being rarefied there. This doesn’t really matter for any of the main dynamics that’s happening inside the collapsing cloud.
I hope this helps.

Christoph

—
Christoph Federrath
http://www.mso.anu.edu.au/~chfeder <http://www.mso.anu.edu.au/~chfeder>


> On 26 May 2019, at 02:59, Lewis,Sean <scl63 at drexel.edu <mailto:scl63 at drexel.edu>> wrote:
> 
> Hello all,
>  
> In my work towards modeling a protoplanetary disk, I have consistently encountered an interesting behavior in gas pressure. Specifically, a region of low pressure around the collapsed cold gas cloud that is generally about 10x lower than the pressure of the outer regions of the cloud as well as the surrounding less-dense gas. I have attached a few .png files from the out-of-the-box SinkRotatingCloudCore test problem to illustrate what I mean. The images are taken at 34kyr and 50kyr taken looking down the z-axis, and another 50kyr snapshot looking down the x-axis to see the side-view of the forming disk.
>  
> How can this effect be interpreted? Something physical that’s expected? Something numerical that is (un)expected? Initially, I thought that the dense gas cloud was contracting towards its center of mass faster than the surrounding halo gas, creating a vacuum of sorts. However, I have seen the same effect in other simulations of mine where the dense gas is nearly relaxed into a disk though to a lesser degree and the same effect is not seen in plots of the gas density. This makes me think this could be an artifact of the equation of state solver in some way.
>  
> With appreciation,
>  
> Sean Lewis
> Drexel University 
>  
>  
> <pres34k.png><pres50k.png><pres50k_x.png>

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