Vortex Cloud Simulation

[874604650]

Why my cloud push away in the few final frame?~  my velocity field is not change direction~~~~~

Thank you~

cortex_cloud.mp4

[874604650]

I solved the problem with divergence~~~but any better solution to do vortec cloud?? thank you

[eimk]

This is likely a result of linear/tangential velocity increasing while the centripetal force remains the same.

The equation F=mv²/R, where F is centripetal force, m is mass, v is linear/tangential velocity, and R is the radius, shows that if F and m remain constant and v increases then the radius must also increase.

Linear/tangential velocity increases because the force(s) acting on the smoke cause an acceleration, which is why it starts moving in the first place.

I haven't tested this, but if you decrease the force(s) causing it to spin it will take longer to get up to the speed you want, but then I think it will settle at a smaller radius.

[ru ti]

Whether to reduce the radius to extend the life？ 

[874604650]

On 2020/7/21 at 12:41 AM, eimk said:

This is likely a result of linear/tangential velocity increasing while the centripetal force remains the same.

The equation F=mv²/R, where F is centripetal force, m is mass, v is linear/tangential velocity, and R is the radius, shows that if F and m remain constant and v increases then the radius must also increase.

Linear/tangential velocity increases because the force(s) acting on the smoke cause an acceleration, which is why it starts moving in the first place.

I haven't tested this, but if you decrease the force(s) causing it to spin it will take longer to get up to the speed you want, but then I think it will settle at a smaller radius.

Wow!thank you,I think I should increase the centripetal force as much as the rotational force increases

[eimk]

I don't know what your setup is, but if you keep the forces constant eventually they will balance out with air resistance and the velocity will become constant too. If the forces keep increasing the sim could get chaotic and difficult to manage.

[874604650]

7 hours ago, eimk said:

I don't know what your setup is, but if you keep the forces constant eventually they will balance out with air resistance and the velocity will become constant too. If the forces keep increasing the sim could get chaotic and difficult to manage.

here is my file and cache thank you. 

odforce.rar

[eimk]

My computer doesn't have enough RAM to run your sim without Houdini crashing, but my best guess for what could help would be decreasing the strength of your velocity field and/or adding a drag force. Alternatively you could look into the vortex force node, which might be easier to work with than your current setup.

https://www.sidefx.com/docs/houdini/nodes/dop/vortexforce
https://www.sidefx.com/docs/houdini/nodes/sop/vortexforceattribs.html

[ziconic]

You are running a pyro sim, which by default aims to make the velocity field divergent free. The velocity field that you have set up is clearly not non-divergent. You are trying to push all the smoke toward the center (i.e. the field's divergence is strongly negative in the middle). So when the pyro sim tries to make the velocity field non-divergent, it has to modify the velocity field near the middle to push the smoke out and away from the center. What you did by specifying the target divergence field is correct. You are telling the pyro sim to make the velocity field's divergence negative in the middle, rather than zero everywhere, which will give you the effect of pushing the smoke toward the middle.

[paranoidx]

On 01/08/2020 at 11:44 AM, ziconic said:

You are running a pyro sim, which by default aims to make the velocity field divergent free. The velocity field that you have set up is clearly not non-divergent. You are trying to push all the smoke toward the center (i.e. the field's divergence is strongly negative in the middle). So when the pyro sim tries to make the velocity field non-divergent, it has to modify the velocity field near the middle to push the smoke out and away from the center. What you did by specifying the target divergence field is correct. You are telling the pyro sim to make the velocity field's divergence negative in the middle, rather than zero everywhere, which will give you the effect of pushing the smoke toward the middle.

Awesome internet took me here while I tried to solve my problem with pyro by using AXIOM solver. I have cross product of N to introduce swirl around a mesh, then do want to increase the speed of v but not make the smoke expand in same acceleration speed magnitude value. I did not know about centripetal force until Hristo Velev from FB Grp Hodini Artist point me to some of this knowledge. Thank you.

My solve for this is create divergence field and push it inward (negative value) but solve by TIME STEP but not solve by every frame, in the same time Velocity field will add in with method PULL to act as a drag slowly accelerate to the max value but not add in right away. That was my case.

Here is cross product result: 

And here is result from add more speed but can not reduce expand