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

[Sasha Tchekhovskoy]

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>
>
>
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