[Sidefx-houdini-list] FLIP'pin out

Craig Hoffman craigleehoffman at aol.com
Fri Aug 5 12:52:49 EDT 2011


Wow!   Thanks for all the info.  

 

I am going to take your suggestion and look into SPH first.  I like to see
what I can get "out of the box" before spending too much time of my limited
spare time on developing things.  J

 

 

But it would be great if there were some common 'recipes' (or even presets)
for settings for common fluid types (water/alcohol/milk/etc.).

 

-Craig

 

 

-----Original Message-----
From: sidefx-houdini-list-bounces at sidefx.com
[mailto:sidefx-houdini-list-bounces at sidefx.com] On Behalf Of Nick Zutphen
Sent: Friday, August 05, 2011 2:25 AM
To: sidefx-houdini-list at sidefx.com
Subject: Re: [Sidefx-houdini-list] FLIP'pin out

 

Hi Craig,

 

To refer to your question about ending up with less fluid than expected,
which you can also define as 'volume loss'. Since I'm not familiar with your
knowledge about the inner workings of the FLIP solver, a short intro /
reminder: To get around the instability of SPH the particle's velocity get
projected on to a volume field to handle the non divergence which is
projected back to the particles for advection. In order to get the speed
advantages the resolution of the volume field is drastically lower that the
particle count or resolution. Think of an average of 8 particles per voxel.
So each voxel adjusts multiple particles at once. 

 

What happens with 'volume loss' is that individual particles get to close
together, which results in the problem where the volume non divergence is
not able to push them apart any more and the fluid 'collapses' a bit every
step, since it controls multiple particles at once instead of individual
particles. To get around this, particle separation is used in an attempt to
separate individual particles. This works until a certain extend, the fact
that this hasn't been resolved lead into the advice to use FLIP for rivers,
waterfalls and that sort of effects where the volume loss is less of an
issue.

 

At a certain point I've attempted to create my own particle separator, which
turned out to be rather useless since I was heading towards a similar
solution as the existing one but  slower since I wasn't using C++. (a
typical case of: 'you're young and you think you can do better that a very
experienced developer' ;-)

 

A hack, which also has it's own pitfalls, but which seemed to work in some
cases is to calculate your own divergence attribute. By doing a point cloud
lookup on the particles you can determ the distance to it's closest
neighbor, if you store this on the particles in a -1 to 1 range and toggle
'diverge by attribute' on the solver you can force the non-divergence to
actively push fluid apart which gets to close together. The pitfall is that
you add energy in to the mix, which can result in a restless fluid and
you'll hit the issue of streaming fluid along collision edges.

 

I guess one can conclude that fluids aren't that easy, my advice is either
live with it, use SPH or attempt to get the divergence volume separation to
work. Alternatively you can try to increase the resolution of the volume
fields by adjusting the 'division size' expression on the FLIP Configure
object, but this will give you a speed penalty and I'm not sure if it will
work in the first place since I haven't tested it my self.

 

Cheers,

Nick van Zutphen

 

----

 <http://www.nickvanzutphen.com> http://www.nickvanzutphen.com

 

 

 

 

 

 

 

 

 

On Aug 5, 2011, at 3:21 AM, Craig Hoffman wrote:

 

> 

> 

> 

> Hi all,

> 

> I am creating an animation of a mixed alcoholic drink with multiple
different colored liquids pouring into a glass (one at a time) and am using
FLIP fluids and everything is working okay.  I am getting pretty good
results fairly quickly but can't quite get that last bit to make it feel
really realistic, so I have a couple questions:

> 

> 

> The amount of liquid that ends up in the glass seems like a lot less than
what is poured in- like around half to my eyes.  Maybe it is just an
illusion, but I am just going off experience with liquids in real life and
how much I expect to be in a glass after a nice thick strong stream pouring
in for a couple seconds.  Is there a way to fix/cheat this so that I get a
result more like what I expect?  (Didn't think about animating 'particle
separation' until now- and I am away from my PC, so haven't tried it yet-
though I am not sure if that would work or be stable, etc.)

> 

> The liquid is a little too "active" with individual particles going off on
their own, etc. and it takes a little too long to settle down.  I started
with "0" for both bounce and friction for the liquid and played around with
friction settings for a while, but haven't found a setting that makes it
really "feel" like real liquid.  Should I venture into viscosity to settle
it down quicker- or is that more for thicker fluids than water or alcohol?  

> 

> Is there any reference for good FLIP fluid settings for real-world fluids
(as well as 'collision object' settings for glass/ice/etc.)?

> Lastly, can anyone drop me a hint if I should just table this project
until after Siggraph (assuming H12 releases then..) because all of this is
so drastically improved/sped up/revolutionized/etc.?   :)

> 

> Any help or suggestions are greatly appreciated.

> 

> Thanks,

> Craig Hoffman

> 

> 

> 

 




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