Flip fluid stream from fast moving object

[Zybrand]

Hi guys,

 

I'm working on I shot fast moving character emits a trail of water. I got the cache from animation and it is moving really fast, so fast that there are large gaps in my emission. Even with motion blur on the fluid source I can't get a continuous stream. Sub steps don't seem to help because the cache still only changes frame by frame.

 

I don't have a lot experience with flip fluids in Houdini. Any ideas on how to get a continuous stream going?

 

thanks,

Zybrand

[Guest tar]

try adding a trailSop to your geo, set to Connect as Mesh, Trail length 2, trail increment 1.

 

It will create the inbetween geo for the fluid sim

Edited March 17, 2014 by tar

[johner]

 Sub steps don't seem to help because the cache still only changes frame by frame.

 

 

Also try a TimeBlend SOP to interpolate the geometry to sub-frames, assuming deforming geometry with constant topology.

[Zybrand]

Hi Marty and Johner,

 

Marty

The Trail SOP didn't work for me, it did some sort of mesh blending but the polygons got messed up and connected in I weird way. Not sure if I am missing something.

 

Johner

The TimeBlend SOP work perfectly to get the blending between cache frames, thanks.

 

thanks for the help guys.

[Sierra62]

In the fluid source sop there is a a geometry samples option under "scalar Volume, motion blur.   Try increasing this value.  For example I am currently working on a long smoke trail following a fast moving plane crash (500mph) and I have the geometry samples set 15 and worked well for a smooth look.

[Zybrand]

Sierra62. So do you only change the geometry samples to 15 and leave the frames to blur and the blur offset to default?

 

At the moment I am using TimeBlend and using sub-frames in my Dop network but your way will probably sim faster if I can avoid the sub-frames.

[Sierra62]

The blur offset is based off the direction you want to blur (-1 is a trailing blur, 0 is centered, and 1 is leading)  I would lean towards -1 or 0.  As for the frames to blur, it would depend on preferences but I have had success with just the defaults.  But upping this may help.

[LinchpinZA]

Hey guys I am trying the same thing with no such luck. Also have some fast moving geo I'm trying to emit fluid from.

But When I add the motionblur settings that Sierra62 suggested on the fluidsource node, my sdf freaks out I get voxels appearing on the edge of the bounding box of the fluidsource in the isosurfaces display.
I was wondering if there is anything I need to do to fix this. I think the caveat here is that my emission geometry is a couple spheres I have copied on to some moving points. They have velocities calculated
before being plugged into the fluid source using a trail sop, but I think the multiple emission sources is somehow confusing the fluid source sop.

[Zybrand]

Hey LinchpinZA,

 

Do you have a sample scene of the problem or a screenshot?

 

For me the Timeblend fixed my problems because I was reading a cache and it wasn't calculating subframe data to blend with. Once I had that source motion blur worked well.