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<tt>Jesus -<br>
<br>
What you most likely want to resolve is the characteristic
pressure scale height in your problem.<br>
<br>
Hope this helps.<br>
<br>
Tomek<br>
</tt>--<br>
On 3/17/2011 6:20 PM, Jesús Zavala Franco wrote:
<blockquote
cite="mid:AANLkTimsZOOxhs4KRHt08s207BOX=JLfmVqsz_MngcQw@mail.gmail.com"
type="cite">Hi John,
<div><br>
</div>
<div>Thank you for your reply, I have tried different
configurations and I always get the same situation: the gas
spreading outwards with velocities of ~20 km/s after 300 Myrs of
evolution. This happens whether I spread the dark matter
particles or not. By the way, I tried using the point mass
potential that is implemented as an option in FLASH as you
suggested. So I removed the particles completely. I get exactly
the same situation, so it is not related to the particles.<br>
<br>
</div>
<div>I'm thinking that the problem is related to what Colin
mentioned in his answer (check following e-mail)</div>
<div><br>
</div>
<div>Cheers,</div>
<div>Jesus</div>
<div><br>
<div class="gmail_quote">2011/3/17 John ZuHone <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:jzuhone@cfa.harvard.edu" target="_blank">jzuhone@cfa.harvard.edu</a>></span><br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<div style="word-wrap: break-word;">Jesus,
<div><br>
</div>
<div>I would actually not expect this setup to work all
that well. Most certainly the central cell with the
particles and the gas is not going to be in hydrostatic
equilibrium. This would be true of the central cell even
if you had the dark matter spread throughout, but in
that case it would not be much of an issue since what
you would get is some minor flattening of the gas
density profile near the center.</div>
<div><br>
</div>
<div>But if I read you correctly you have the particles
all dropped into this central zone, and the mass of this
central zone alone is much larger than the mass of gas
in the entire system, which means each of your particles
is very massive. What velocities do you have them set
to? Even if you set them initially to zero and they are
all at the center you're going to get some spurious
velocities. Since the particles are so massive the
potential is probably changing a lot and this is
throwing things out of equilibrium in the center.</div>
<div><br>
</div>
<div>Since the HSE is definitely broken in this cell and
probably in a few cells surrounding it, I don't think
it's surprising that you're seeing what you are. You are
trying to simulate a point mass but severely
underresolving it both in terms of spatial and mass
resolution. </div>
<div><br>
</div>
<div>Is there a particular reason you're trying to
represent a point mass in this way? There is a point
mass gravitational acceleration option in FLASH that
would probably be better suited for this purpose. </div>
<div><br>
</div>
<div>Best,</div>
<div><br>
</div>
<font color="#888888">
<div>John ZuHone</div>
</font>
<div>
<div>
<div><br>
</div>
<div>
<div>
<div>On Mar 17, 2011, at 12:55 PM, Jesús Zavala
Franco wrote:</div>
<br>
<blockquote type="cite">Dear all,
<div><br>
</div>
<div>I'm having problems setting up a sphere of
gas in hydrostatic equilibrium under its own
gravity + the gravity of a particle
distribution.</div>
<div><br>
</div>
<div>
In the general case, I would like to have this
particle distribution with the same density
profile as the gas distribution (so I'm aiming
at setting up a dark matter halo with a
particle distribution, with gas inside), but
to describe the problem I'm having, I will
avoid distributing particles in the sphere and
simply:</div>
<div><br>
</div>
<div>1) put 1000 particles in the centre of the
sphere</div>
<div>2) put gas distributed spherical around
this centre with a radial density profile (a
NFW profile), and pressure given by the
condition of hydrostatic equilibrium:</div>
<div><br>
</div>
<div>dP(r)/dr = -(GM(<r)/r^2) * rho_gas(r)</div>
<div><br>
</div>
<div>where M(<r) is the total enclosed mass,
and rho_gas(r) is the gas density. M(<r)=
M_DM+Mgas(<r), with M_DM the total mass of
the 1000 particles, which by the way exceeds
the total mass of the gas by almost an order
of magnitude. To solve the equation I impose a
boundary condition outside the sphere setting
a pressure and a density which are reasonable
according to the problem and the density
profile I'm putting.</div>
<div><br>
</div>
<div>So in short, what I have is a sphere of gas
within a gravitational potential dominated by
essentially a point source in the centre and
with a pressure that should give support
agains the gravitational collapse.</div>
<div><br>
</div>
<div>I'm using a gird of 8^3 with 4 levels of
refinement, an ideal gas equation of state, a
Pfft Multigrid gravity solver, I'm using the
default operator splitting technique to
advance the solution.</div>
<div><br>
</div>
<div>What I notice is that since the first time
step, regardless of my choice of dtinit, the
cells just next to the central cell (the one
containing the 1000 particles), acquire a
radial outwards velocity, whit a size
depending of the time step, and that once the
time evolution reaches the typical times of
the problem (~100 Myrs), this results in the
gas in the inner parts propagating outwards,
reducing the density in the core, after 1Gyr
or so, this propagating gas reaches the
boundaries of the sphere. In other words, the
equilibrium set at first is broken.</div>
<div><br>
</div>
<div>I have tried using the option
ppm_modifystates=.true. since as described in
the user guide:</div>
<div><br>
</div>
<div>
<div style="margin: 0px;">"The version of PPM
in the FLASH code has an option to more
closely couple the hydrodynamic solver</div>
<div style="margin: 0px;">with a gravitational
source term. This can noticeably reduce
spurious velocities caused by the operator</div>
<div style="margin: 0px;">
splitting of the gravitational acceleration
from the hydrodynamics"</div>
</div>
<div><br>
</div>
<div>I thought this could help, but it didn't.
I'm aware that if I wouldn't put the particles
in the centre, this behaviour is just a
resolution problem since the core is not being
properly resolved and the density and enclosed
mass is underestimated there, the pressure is
therefore too high (since is initially set by
solving the hydrostatic equilibrium equation
with the assumed analytical profile) and it
pushed the gas outwards. However, since I'm
setting a significant gravity source in the
center I wouldn't expect this to be the case.
I have tried actually increasing the mass of
the particles by a factor of 10, and not
considering this mass increase in the solution
to the pressure equation, this sets the
overall pressure too low and you would expect
a collapse. Even though when at first the
velocities of the contiguous cells to the
centre, point inwards, after a while, they are
overwhelmed by an outward flow that develops
in the cells next to these, and to my
surprise, the sphere expands as well.</div>
<div><br>
</div>
<div>I'm running out of ideas on the initial
conditions setup, so I'm thinking this could
either be a resolution issue, or a bad choice
of hydro, gravity solver.</div>
<div><br>
</div>
<div>Any hel will be much appreciated. </div>
<div><br>
</div>
<div>Cheers,</div>
<div>Jesus Zavala</div>
<div>Department of Physics and Astronomy</div>
<div>University of Waterloo</div>
</blockquote>
</div>
<br>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<br>
</div>
</blockquote>
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