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

[Sarah T. Stewart]

Hi Sasha, & co.

I am also observing a strange phenomenon with energy transport in a
rotating, self-gravitating system with strong shear in the velocity field
in a finite volume code.

Can you point me to any papers that discuss errors/problem/issues with
studying shearing rotating systems in finite volume codes?
Thanks,
Sarah

On Mon, May 27, 2019 at 7:08 PM Sasha Tchekhovskoy <
atchekho at northwestern.edu> wrote:

> I do not have much experience with FLASH specifically, but with other
> finite volume methods such as Athena or HARM, one can get very low or
> negative pressure regions along discontinuities (e.g., shear
> discontinuities or shocks), especially if the density contrast at the
> discontinuity is substantial and/or if supersonic flows are involved. One
> way to look at this is near the discontinuity the scheme reduces to low
> (first) order, and the increased numerical truncation error ends up
> contaminating the smallest quantity, which is typically the thermal
> pressure.
>
> I agree that seeing maps of density and velocity could be helpful to get a
> sense of the expected force balance at the discontinuity. Maybe even 1D
> slices in addition to 2D maps.
>
> Best,
> Sasha
>
>
> On Mon, May 27, 2019, 19:42 Ryan Farber <rjfarber at umich.edu> wrote:
>
>> Hi Sean,
>>
>> I'm not familiar with that problem so you'll hopefully get a more useful
>> answer from someone else. But until then, I have some thoughts below which
>> I hope might help.
>>
>> First, this made me think of Jaehan Bae's work regarding the spiral
>> density wave instability. However, I believe that only happens when there's
>> a planet/object orbiting the (proto)star (what happens is that multiple
>> gaps in the disk appear despite there being only one object, implying that
>> HLL Tau's many gaps don't necessarily mean it has that many planets). I
>> only remember seeing movies of density but I would think the low density
>> region would also be low pressure).
>>
>> Speaking of which, it might be useful to see density and temperature
>> plots as well to understand in which (or both) variable causes the low
>> pressure you're seeing.
>>
>> Other thoughts:
>> Is there an analytic solution to your problem (or a simplified version of
>> it) to compare to? Absent that, you could try a different EOS solver. You
>> could also experiment with the hydro solver (if you're doing hydro, trying
>> the (un)split solver; different Riemann solver).
>>
>> If all those look the same then I would think it's something physical. If
>> you have checkpoint files at 34,50 kyr then you can look at ACCX, ACCY,
>> ACCZ (check Flash.h if I spelled them right; I'm traveling currently) to
>> see if your low pressure region is in force balance, explaining it's
>> persistence. It may also help to consider the centrifugal, gravitational,
>> and pressure gradient forces individually.
>>
>> I'm assuming SinkRotatingCloudCore uses self-gravity; is that in fact the
>> case? Does it use radiative cooling? If so, you might want to also try
>> turning cooling off to simplify things a bit.
>>
>> Best,
>> Ryan
>>
>>
>>
>> Sent from my iPhone
>> On May 25, 2019, at 11:59 AM, Lewis,Sean <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>
>>
>> --
Sarah T. Stewart
Professor, Dept. Earth and Planetary Sciences, UC Davis
530.794.8689  sts at ucdavis.edu  @SarahTStewart
sarahtstewart.net
go.ted.com/sarahtstewart
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