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Art Direction velocity fields on interior of object

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yaniboy
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Hello,

This is my first post to this forum so please let me know if I can do anything to provide more information to assist anyone in answering my questions.

I am looking to create a custom velocity field limited to the interior of an object. I have a couple questions.

I am using a vdb to create my velocity field. Is there a way to limit that velocity field to the interior of my object opposed to the limits of the bounding box?

What would be a good method to procedurally create fields that turn away from the interior surface of my object to create the look of flow inside of the object. I have tried using collision which could work but is very expensive and not very art directable.

Thanks in advance,
Yani

Attachments:
ghvelfield.jpg (535.6 KB)
ghgeo.jpg (419.5 KB)

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Enivob
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You could try something like this. Invert the normals, then get them to lie down along the surface using the cross product. Route those normals to velocity for you output.

Attachments:
Untitled-1.jpg (424.7 KB)
ap_vel_field_iside_shape.hiplc (106.6 KB)

Using Houdini Indie 20.5
Windows 11 64GB Ryzen 16 core.
nVidia 3060RTX 12BG RAM.
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yaniboy
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Thank you very much Enivob for your response. This is helpful. I am now curious if there is a way to merge this velocity with the one I had previously. The effect I am after is an hourglass with sand flowing from the bottom to the top section and I would like the top section to appear turbulent. I have tried various maths in vops to combine the tangential velocity field and a curly cross product velocity field. Actually the initial field I uploaded is a combination of a cross product and the tangential field I generated using an add node in vops but did not yield the result I wanted. I have also tried volume merge in sops but it doesn't appear to work. I feel like I am missing something very obvious.
Edited by yaniboy - March 13, 2021 12:24:45

Attachments:
hgcrossprod.jpg (420.1 KB)
tangentfield.jpg (447.7 KB)

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mestela
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This might give you some ideas.

The curlnoise vop has options to steer noise away from a surface, some more notes here:

https://www.tokeru.com/cgwiki/index.php?title=HoudiniVolumes#Swirly_lines.2C_or_worms_with_volumes_and_curl_noise [www.tokeru.com]

Attachments:
pops_hourglass_swirl.hip (445.8 KB)

http://www.tokeru.com/cgwiki [www.tokeru.com]
https://www.patreon.com/mattestela [www.patreon.com]
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yaniboy
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This is awesome! Thank you Matt! I hadn't thought to use the curl noise in this fashion and I think this will get me on the right path. Also I never realized I could use the alpha attribute like that. I will definitely use that again. Thank you for all the work you have put into the cgwiki.
Edited by yaniboy - March 14, 2021 00:51:40
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snailsnell
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I am wondering if there is a way to do this with a velocity field that doesn't use curl noise!! I am making a velocity field that is directed by some input curves. I need to clip it to the shape of a VDB / constrain it only to the inside of the VDB, but having trouble finding a method
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