Friction between two colliding gass clouds

I am starting a project, university computer science, in which I want to
model the formation of a solar system from an initial cloud of gass/dust.
In doing so I plan on using smoothed particle hydrodynamics and consider
forces from mutual attraction and friction between colliding clouds of
particles.

I have been unable to find any good documents describing how I shoud go
about calculating the friction force between two such clouds, so I post here
hoping for some good references.

Seeing that I come at this from a computer science background rather than
from astronomy, I might as well write a little extra about my thoughts and
let you tell me if I am missing some important point entirely.

I have made simple tests already with a thousand particles which all attract
each other through gravity. The gravity is not calculated as GMm/r^2 which
goes to infinity as r goes to zero. Instead I use the smoothed version
representing gravitational pull between two objects which are not points but
rather clouds: GMmr/(r^2+epsilon^2)^(1.5). Here epsilon0 is a softening
factor which ensures that the force inscreses as r shrinks, but only to a
certain point after which the force shrinks and reaches zero at the same
time as r does. Two clouds of particles do not pull each others centers when
they are exactly on top of eachother.

This lets the simulation run fine without having particles ejectes when they
collide.

First extra question is if it, considering it is compressible gass, is
correct to let a number of particles become one in that they have same
velocity vector and position. They have grown into one larger cloud.
It seems correct to me. Calculating for example the density of the space
occupied by two such collided clouds will show it to be double of one cloud
and the gravitational pull is also double. Any thoughts on that?

Second extra question is if it will be correct to model a particles friction
against other gass by calculating the average velocity vector of the other
gass in the continium and its density and then simply calculate friction
based on delte velocity and density and decelerate the paricle based on
this. After changing velocity the change in energy could then, based on mass
and density, be converted into heat and thereby updating the heat in the
simulation.

Any references, pointers, hints ect. will be greatly appresiated since I
have had a hard time finding good info on this subject. Though SPH started
out doing exactly what I am trying, it seems 99% of the texts dealing with
it now are all about simulating water.

On Apr 18, 1:11*pm, "Joe Taicoon" wrote:
I am starting a project, university computer science, in which I want to
model the formation of a solar system from an initial cloud of gass/dust.
In doing so I plan on using smoothed particle hydrodynamics and consider
forces from mutual attraction and friction between colliding clouds of
particles.

I have been unable to find any good documents describing how I shoud go
about calculating the friction force between two such clouds, so I post here
hoping for some good references.

Seeing that I come at this from a computer science background rather than
from astronomy, I might as well write a little extra about my thoughts and
let you tell me if I am missing some important point entirely.

I have made simple tests already with a thousand particles which all attract
each other through gravity. The gravity is not calculated as GMm/r^2 which
goes to infinity as r goes to zero. Instead I use the smoothed version
representing gravitational pull between two objects which are not points but
rather clouds: GMmr/(r^2+epsilon^2)^(1.5). Here epsilon0 is a softening
factor which ensures that the force inscreses as r shrinks, but only to a
certain point after which the force shrinks and reaches zero at the same
time as r does. Two clouds of particles do not pull each others centers when
they are exactly on top of eachother.

This lets the simulation run fine without having particles ejectes when they
collide.

First extra question is if it, considering it is compressible gass, is
correct to let a number of particles become one in that they have same
velocity vector and position. They have grown into one larger cloud.
It seems correct to me. Calculating for example the density of the space
occupied by two such collided clouds will show it to be double of one cloud
and the gravitational pull is also double. Any thoughts on that?

Second extra question is if it will be correct to model a particles friction
against other gass by calculating the average velocity vector of the other
gass in the continium and its density and then simply calculate friction
based on delte velocity and density and decelerate the paricle based on
this. After changing velocity the change in energy could then, based on mass
and density, be converted into heat and thereby updating the heat in the
simulation.

Any references, pointers, hints ect. will be greatly appresiated since I
have had a hard time finding good info on this subject. Though SPH started
out doing exactly what I am trying, it seems 99% of the texts dealing with
it now are all about simulating water.

As far as I know, the velocity of a supernova shockwave that's
migrating through a sufficiently dense molecular cloud of mostly
hydrogen, usually doesn't exceed 0.1c, and I don't believe that
singular event alone is sufficient to start the stellar creation
process.

What public funded supercomputer do you have access to?

Computer simulations should more than do the trick, although don't
expect much constructive help from within Usenet/newsgroups (aka
Google Groups).

~ BG

In article ,
"Joe Taicoon" writes:
I am starting a project, university computer science, in which I want to
model the formation of a solar system from an initial cloud of gass/dust.

People have been constructing star formation models since the 1960's
at least, and it is still a subject of very active research. One
paper I just happened to find in a quick search is by Banerjee &
Pudritz (2006 ApJ 641, 949). One key sentence from the Abstract
reads "Here we report on our three-dimensional, adaptive mesh,
magnetohydrodynamic simulations of collapsing, rotating, magnetized
Bonnor-Ebert spheres, whose properties are taken directly from
observations." This will give some idea of the state of the art in
this field.

Of course if all you want is an interesting computer science project,
it doesn't have to be useful for current research.

In doing so I plan on using smoothed particle hydrodynamics and consider
forces from mutual attraction and friction between colliding clouds of
particles.

This (and much of the rest of your message) sounds much closer to
galaxy collisions than star formation. That is, if anything, an even
more active "industry" today than star formation modelling. As an
example of the sort of thing that might make a good project, Antunes
& Wallin (2007 ApJ 670, 261) constructed a model of a specific pair
of interacting galaxies. An extract of their Abstract reads "In
N-body/smoothed particle hydrodynamics (SPH) simulations of
AM 0644-741, we recreate the star formation features, as well as the
underlying kinematics." In other words, they are modelling
separately stars, gas, and (probably) dark matter, and they are
superposing on their model some prescription for how many stars form
at a given gas density. This is a very simple project by today's
standards.

I have been unable to find any good documents describing how I shoud go
about calculating the friction force between two such clouds, so I post here
hoping for some good references.

The classic paper on the subject is by Toomre & Toomre (1972 ApJ 178,
623). For more, you can do an ADS search either for papers citing
that one or for Abstract keywords. (ADS is at
http://adsabs.harvard.edu , and I think there's a mirror site in
Europe.)

....
This lets the simulation run fine without having particles ejectes when they
collide.

The "softened" potential is important, but particles are ejected in
real galaxy collisions.

Though SPH started
out doing exactly what I am trying, it seems 99% of the texts dealing with
it now are all about simulating water.

The astronomy literature on the subject is vast. ADS can help you
navigate it, but you would be better off consulting an expert in the
field. You might look at author affiliations on recent papers and
see whether any authors are near you. Or go ask at the astronomy
department if your university has one. If not, you check whether
someone in the physics department is doing astrophysics.

Good luck with your project.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

People have been constructing star formation models since the 1960's
at least, and it is still a subject of very active research. One
paper I just happened to find in a quick search is by Banerjee &
Pudritz (2006 ApJ 641, 949). One key sentence from the Abstract
reads "Here we report on our three-dimensional, adaptive mesh,
magnetohydrodynamic simulations of collapsing, rotating, magnetized
Bonnor-Ebert spheres, whose properties are taken directly from
observations." This will give some idea of the state of the art in
this field.

Yas, I have also seen models from the danish Niels Bohr Institute now, which
are impressive. Aparently now the models are at a place where they match
observations perfectly.
Is the term adaptive mesh also used for SPH, or do they in fact reconstruct
a tetrahedron mesh continously, and in that case, why?

Of course if all you want is an interesting computer science project,
it doesn't have to be useful for current research.

It is reallyjust an interesting computer science project. In general people
tend to simulate water and I thought it would be interesting to move away
from that and simulate nbody with the different considerations needed :-)


This (and much of the rest of your message) sounds much closer to
galaxy collisions than star formation. That is, if anything, an even
more active "industry" today than star formation modelling. As an
example of the sort of thing that might make a good project, Antunes
& Wallin (2007 ApJ 670, 261) constructed a model of a specific pair
of interacting galaxies. An extract of their Abstract reads "In
N-body/smoothed particle hydrodynamics (SPH) simulations of
AM 0644-741, we recreate the star formation features, as well as the
underlying kinematics." In other words, they are modelling
separately stars, gas, and (probably) dark matter, and they are
superposing on their model some prescription for how many stars form
at a given gas density. This is a very simple project by today's
standards.

Interesting. So they do not simulate the starformation as such but rather
the gas and then they impose some expectations about starbirth... that seems
managable. I was aware that for a plausible starbirth simulation I would
probably need to take quite a few extra factors into account.
I will look at the reference.

The classic paper on the subject is by Toomre & Toomre (1972 ApJ 178,
623). For more, you can do an ADS search either for papers citing
that one or for Abstract keywords. (ADS is at
http://adsabs.harvard.edu , and I think there's a mirror site in
Europe.)

Thanks!

The "softened" potential is important, but particles are ejected in
real galaxy collisions.

Yes, I realize that ejection is not an error in itself. The ejection I was
thinking about was the result of large timesteps in the integration combined
with very close passages of point masses. That will create ejections even
for a 2-body system.

Or go ask at the astronomy
department if your university has one. If not, you check whether
someone in the physics department is doing astrophysics.

Good idea. We have astronomy somewhere here. I should probably go talk with
them about the relevant factors to include in the model as well as things
such as support radius and good particle size etc for this kind of
simulation.

Good luck with your project.

Thanks :-)

In article ,
"Joe Taicoon" writes:
Aparently now the models are at a place where they match
observations perfectly.

That may be going a little far :-), though I'm far from an expert on
this subject. Models are certainly good enough to be compared with
observations, and no doubt in some cases they match well.

Is the term adaptive mesh also used for SPH, or do they in fact reconstruct
a tetrahedron mesh continously, and in that case, why?

No idea; sorry.

It is really just an interesting computer science project.

One way to proceed might be something like:

1. Duplicate the classic Toomre & Toomre result using their exact
method as closely as possible.
2. Increase the number of mass points and investigate how the
computing time changes and whether the result is different. (I don't
think it should be.)
3. Perform the same calculation using a modern method (perhaps SPH)
and see whether the result differs and how the computing time
compares.

Obviously this is only one of a great many possibilities; I have no
idea how closely it might fit your needs.

As for simulating the Moon-creating collision, which you mentioned in
another message: I'd think you would need some way to model solid-
body forces, but I don't have a good understanding of such models.

So they do not simulate the starformation as such but rather
the gas and then they impose some expectations about starbirth

Right. Modelling the details of the collapse of each individual
molecular cloud is far too hard for a galaxy-collision simulation.
In fact, it's a hard problem just on its own because a cloud collapse
model has to take account of chemistry, radiative transfer, and
magnetic fields, not just gravity.

You might get better responses at sci.astro.research. As you have no
doubt noticed, there is a huge amount of nonsense in the unmoderated
sci.astro group.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

One way to proceed might be something like:

1. Duplicate the classic Toomre & Toomre result using their exact
method as closely as possible.
2. Increase the number of mass points and investigate how the
computing time changes and whether the result is different. (I don't
think it should be.)
3. Perform the same calculation using a modern method (perhaps SPH)
and see whether the result differs and how the computing time
compares.

This is Taicoon again from different account.
Thanks for your insight. I read the article and it does hold quite a lot of
details so I might try duplicating the results to some degree.
I do think however that there might be a little too much astronomy in the
project for me, seing that I need to focus somewhat more on the computer
science part of it, so I was hoping for an model of astronomical phenomena
which was in itself simple, but which were perhaps hard to implement and
test numerically.
I now realize that it was incredible naive of me to think that star
formation would fit that description.

As for simulating the Moon-creating collision, which you mentioned in
another message: I'd think you would need some way to model solid-
body forces, but I don't have a good understanding of such models.

That is luckily something I do have some experience in. In general, when
modelling high velocity impacts, using SPH you consider the solid a fluid. I
would consider the gigantic impact into the proto earth as high velocity...
into a soft object even. Besides, would you not in general consider even
Earth today as a soft object? Gently placing a big asteoroid on the surface
of Earth would, I would imagine, cause it to sink into the Earth untill it
"floats".

You might get better responses at sci.astro.research. As you have no
doubt noticed, there is a huge amount of nonsense in the unmoderated
sci.astro group.

I will do that as soon as I have something concrete to ask again. I now see
how very vague I was in here so thanks a lot for your helpfullness :-)

In article ,
"Jason Who" wrote:

snip

I do think however that there might be a little too much astronomy in the
project for me, seing that I need to focus somewhat more on the computer
science part of it, so I was hoping for an model of astronomical phenomena
which was in itself simple, but which were perhaps hard to implement and
test numerically.

You may be interested in the MilkyWay@home project at Rensselaer
Polytechnic, New York:

http://milkyway.cs.rpi.edu/milkyway/

They're collaborating with volunteers on the programming, so their
(GPL'd) source code is available -- there should be a link in a message
from an administrator somewhere in the forums.

--
Odysseus

You may be interested in the MilkyWay@home project at Rensselaer
Polytechnic, New York:

http://milkyway.cs.rpi.edu/milkyway/

That does look interesting. I will take a closer look soon.

In article ,
"Jason Who" writes:
...I was hoping for an model of astronomical phenomena
which was in itself simple, but which were perhaps hard to implement and
test numerically.

Not easy to find, I'm afraid. Not all astronomers are bad
programmers. :-)

I wonder whether there's something to be done in N-body work
simulating the long-term stability of the solar system. That was
popular about 15 years ago, but I haven't seen anything lately. One
group even constructed a special-purpose computer, the "digital
orrery." I don't know whether this area could turn into a project,
but there may be something to consider.

Oops... I see there's work being done on this after all. A quick
check with ADS shows a paper by Batygin & Laughlin (2008 ApJ 683,
1207) that forward-integrates the planets for 20 Gyr. (Bottom line:
the solar system is stable or at least there are no "severe"
instabilities. Phew!) There may still be a project here; I haven't
looked at details.

In general, when
modelling high velocity impacts, using SPH you consider the solid a fluid. I
would consider the gigantic impact into the proto earth as high velocity...

Yes, "liquid forces" would probably have been a better phrase for me
to use. Anyway, you need something to keep the simulated Earth and
incoming body from collapsing to point masses and also to allow
"drops" to stick together after the collision but have finite radius.

Gently placing a big asteoroid on the surface of Earth would, I
would imagine, cause it to sink into the Earth untill it "floats".

In fact, one can estimate the size of mountains by knowing the
average strength of crustal rock and the Earth's surface gravity. So
indeed any object bigger than a mountain should do as you say.
Everyday experience is not a good guide at large scales!

Feel free to email me off-list if you wish.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Steve Willner wrote:
In article ,
"Jason Who" writes:
...I was hoping for an model of astronomical phenomena
which was in itself simple, but which were perhaps hard to implement and
test numerically.

Not easy to find, I'm afraid. Not all astronomers are bad
programmers. :-)

I wonder whether there's something to be done in N-body work
simulating the long-term stability of the solar system. That was
popular about 15 years ago, but I haven't seen anything lately. One
group even constructed a special-purpose computer, the "digital
orrery." I don't know whether this area could turn into a project,
but there may be something to consider.

Oops... I see there's work being done on this after all. A quick
check with ADS shows a paper by Batygin & Laughlin (2008 ApJ 683,
1207) that forward-integrates the planets for 20 Gyr. (Bottom line:
the solar system is stable or at least there are no "severe"
instabilities. Phew!) There may still be a project here; I haven't
looked at details.


Sure is. Can an accretion disk accrete into a non-stable solar system? Average
length of stability? Does stability depend upon the size of the star? Does it
depend on the original size and density of the cloud? At what rate do the
ejections occur? What fraction of the disk is ejected? Mass distribution for
the ejected material? Speed distribution for the ejected material?

Might need to do a million simulations to the age of the universe to get a good
grip on the distributions. A nice distributed computing project.