Hi Simen,

I have considered the idea, but I haven't been able to implement it. The
function to be integrated has lots of variables and I still have not found
an easy way to add it. Nevertheless, I'm not happy with Forward Euler so I
definitely will be trying to implement other more stable methods.

A few weeks ago I tried with Backward Euler, which is an implicit method,
(which in theory has excellent stability properties) but had terrible
results.

I'll let you know if I succeed in this stuff.

Thanks for your suggestions!

Fernando.


"Simen Kvaal" <sim### [at] remove_mestudentmatnatuiono> wrote in message
news:3c65be87@news.povray.org...
> This looks better and better! :)
>
> Have you considered using a second, third or fourth order Runge-Kutta
> integration scheme? It is much more stable than Euler-integration (which
is
> intrisically unstable) and only requires a little more programming. Once
> done, it's certainly worth the while...
>
> http://mathworld.wolfram.com/Runge-KuttaMethod.html
>
> Regards,
> Simen Kvaal.

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>
> I turned off air resistance because I had some bizarre problems with it, but
> I'll work on it very soon.

When you drop a real cloth you would expect the cloth to not just stay
flat and drop. All sorts of non-linear behavour should occur.
Or are your problems of a different sort?

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My ICQ crashed!

It seems that ICQ doesn't like terminal services and stuff like that.

At this moment, I'm running Miro$oft's MSN Messenger:
Tatlicisi-at-hotmail-dot-com
My e-mail address: apacheserver2000-at-yahoo-dot-com

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"andrel linnenbank" <a.c### [at] amcuvanl> wrote in message
news:3C66EF3E.9419D143@amc.uva.nl...
> >
> > I turned off air resistance because I had some bizarre problems with it,
but
> > I'll work on it very soon.
>
> When you drop a real cloth you would expect the cloth to not just stay
> flat and drop. All sorts of non-linear behavour should occur.
> Or are your problems of a different sort?

Yes, I agree with you. However, my simulation is a very simplified model
from reality :) I strive to improve that realism, but my lack of knowledge
in physics is a big obstacle. It would be very nice to know more about fluid
dynamics...

Thanks for your comments,

Fernando.

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I subscribed to sc.physics.computational.fluid-dynamics, but I cannot see
any of the posts.

And about that non-linear behaviour of falling fabrics, I think that those
behaviours are caused by air resistance.

Post a reply to this message

>
> Yes, I agree with you. However, my simulation is a very simplified model
> from reality :) I strive to improve that realism, but my lack of knowledge
> in physics is a big obstacle. It would be very nice to know more about
fluid
> dynamics...
>

Complex behaviour can often be incorporated in models by changing the
premises a little bit: Perturb the model a little in one direction, make the
parameters that describe the cloth vary along the surface et.c.

I really think fluid dynamics wouldn't help you very much in this case.
Computational fluid dynamics (CFD) is a vast area, and millions of CPU hours
are required for relatively small models. And modelling air flow along the
surface of a fabric sure is a complex model!

What about trying to implement velocity dependent air resistance? Assuming
that you can (or alredy have) discretizised your cloth in polygons of some
sort, and assumng that the cloth's velocity through air is slow, maybe a
model like this could be a starting point?

        F = - |v| * cos (theta) * A * constant

where v is the polygons velocity, A its area and theta v's angle with the
surface normal. F is the force the polygon "feels" from the air resistance.

This model assumes that the air is uniformly distributed with equal pressure
everywhere, through all time. This is true when there is no wind, the cloth
falls slowly and when two pieces of cloth are not very close to each other.
(In that case, air pressure will change.)

Again, I emphasize the usefulness Runge-Kutta scheme. Assuming that you are
solving with Euler by 1) calculating forces/acceleration on some set of
point masses, 2) adding acceleration * dt to the velocity of each point mass
and 3) adding velocity * dt to position, it really should be a simple task
to extend it to a fourth order scheme. With adaptive time-step, the method
is very robust! I have successfully modelled a lump of jelly (i.e. 3d-mesh
of points connected by (even nonlinear) springs.)


regards,

Simen Kvaal.

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"Simen Kvaal" <sim### [at] remove_mestudentmatnatuiono> wrote in message
news:3c6853de$1@news.povray.org...

Hi Simen!

>         F = - |v| * cos (theta) * A * constant

Yes, I did something like that!!! I also came up with that idea, however I
did not include A, assuming (maybe wrongly) that all triangles' area would
be more or less the same, so it wouldn't be too bad to ignore that factor
and it would help to speed up the cycle.

My results weren't bad, and I just jumped from the idea of air resistance to
the idea of wind. I think it looks very good. I'll post my latest animation
right way. I hope you'll like it.

> Again, I emphasize the usefulness Runge-Kutta scheme. Assuming that you
are
> solving with Euler by 1) calculating forces/acceleration on some set of
> point masses, 2) adding acceleration * dt to the velocity of each point
mass
> and 3) adding velocity * dt to position, it really should be a simple task
> to extend it to a fourth order scheme. With adaptive time-step, the method
> is very robust! I have successfully modelled a lump of jelly (i.e. 3d-mesh
> of points connected by (even nonlinear) springs.)

Thanks for your suggestion, I promise I will try to implement it soon!

Fernando.

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Might I suggest Ian Burgmyer's Infinity Communicator? Can't be any worse
than M!cr0$oft...

Apache wrote:
> 
> My ICQ crashed!
> 
> It seems that ICQ doesn't like terminal services and stuff like that.
> 
> At this moment, I'm running Miro$oft's MSN Messenger:
> Tatlicisi-at-hotmail-dot-com
> My e-mail address: apacheserver2000-at-yahoo-dot-com

-- 
signature{
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> I have successfully modelled a lump of jelly (i.e. 3d-mesh
> of points connected by (even nonlinear) springs.)

I'd like to see that! Could you post a nice gello animation? My little
system *is* capable of that, but I've never been patient enough to let it
simulate it. I'm in serious need of cpu power, you know  :-)

Post a reply to this message

>
> I'd like to see that! Could you post a nice gello animation? My little
> system *is* capable of that, but I've never been patient enough to let it
> simulate it. I'm in serious need of cpu power, you know  :-)
>

Coming up! :)

Me too in need of CPU power. I will start rendering and hopefully post a
nice MPEG in a day or two...

Regards,

Simen Kvaal.

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