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EuniceC

FLIP simulation problem

Hi,

I have been trying to learn Houdini and have been doing a flip simulation of a boat moving across an ocean. So far, it has been okay, until I saw that spray particles were being emitted through the boat. It was then I realised that there are default water particles inside the boat even in frame 1. I have checked my boat collision geometry (the blue collision geometry and the purple collision guide inside the guidedoceanlayer node) thoroughly and I really do not know what's wrong. I thought that the collision geometry is what allows the particles to interact properly with the geometry, so does having a thin collision geometry affect the collision? If so, how can I increase the collision geometry thickness in the inside of the boat? I tried polyextruding outside but there will be an imaginary space between the outside of the boat and the water particles.

I've attached the video here and my houdini file. I've cached some stuff out to make the simulation run faster. Can someone please help me with this?? I have been trying to fix this for the longest time. Nothing I've done worked and I've ran out of ideas on what to do.

boatcollisiongeometry.PNG

waterinboat.PNG

BoatGeo_Test4.mov

OceanBoatSim2.hip

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Here, the purple wireframe is the collision geometry of the boat and the black mesh is the particle fluid surface. I do not understand why does the black mesh follow the collision geometry to a certain extent and then completely ignores it. 

 

D2OtKT1.png

 

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What I have done in this case is to create a boolean version of the starting fluid source. Your attached file does not have the boat geometry locked so I can't test with your posted .hip file. But looking it over I would suggest subtract the boat from /obj/guidedoceanlayer_initial/OUT_INITIAL. This way the initial has a hole in it that is the same size as the boat hull. This way the boat is not immediately intersecting the fluid on the first frame.

Remember, don't view flip collision shapes using the Static Object (i.e. blue). Flip collisions are directly constructed from particle separation. So turn on the Collision guide on the FlipObject (i.e. lavender) to view the actual flip collision object.

Particle Separation Controls Fluid Collision Resolution.

I would not worry too much about a minor amount of fluid on the inside of the boat shape. Setup your camera move and focus on what needs to look good for the final shot.

Edited by Atom

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9 minutes ago, Atom said:

What I have done in this case is to create a boolean version of the starting fluid source. Your attached file does not have the boat geometry locked so I can't test with your posted .hip file. But looking it over I would suggest subtract the boat from /obj/guidedoceanlayer_initial/OUT_INITIAL. This way the initial has a hole in it that is the same size as the boat hull. This way the boat is not immediately intersecting the fluid on the first frame.

Remember, don't view flip collision shapes using the Static Object (i.e. blue). Flip collisions are directly constructed from particle separation. So turn on the Collision guide on the FlipObject (i.e. lavender) to view the actual flip collision object.

Particle Separation Controls Fluid Collision Resolution.

I would not worry too much about a minor amount of fluid on the inside of the boat shape. Setup your camera move and focus on what needs to look good for the final shot.

Thanks for your reply!

I will try out your method. Here is also a hip file with the locked geometry and the geometry file itself. I didn't really worry about the fluid underneath the ocean that is within the boat until I saw that it gave me problems when I have big collisions with the waves.

I have simulated a stormy ocean, so as the boat collides with the big waves, some spray particles are leaking from inside the boat and it looks pretty obvious in my point of view because it occurs on both sides of the boat, so no matter which side I twist my camera to, I can still see the leak.

I attach the link of the test I did here. 

 

OceanBoatSim2.hip

boat_v001.bgeo

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I basically took the boat provided and recreated what I think you are trying to accomplish. I animated the boat along a transform and added velocity. On the assemble make sure to forward that attribute, along with any others, so they reach the simulation. I used the Wave Tank tool with the moving boat as the subject to create the initial simulation. Inside that FLIP sim I set the Static Object of the boat to Use Deforming Object and upped the collision resolution from 30 to 96. I also lowered the Particle Separation a bit, down to 0.3. The Wave Tank tool also takes care making sure particles do not exist inside the collision object.

I then used the whitewater tool and choose the FLIP Object, as requested by the prompt in the 3D viewport, as the fluid source.

 

This gets the basic sim up and running. White Water seems to be behaving but the wave crests are not as dramatic as shown in your video.

 

boat_waves.gif

ap_OceanBoatSim2.hipnc

Edited by Atom

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In the previous version the boat was in motion and the wave tank followed along. In this version the boat is a RBD Hero object that just bobs around in a wave tank. I added a lot more density to the RBD Hero object so I had to turn up the Feedback Scale quite a bit on the solver to keep the boat from sinking. Play around with the relationship between object Density and Feedback Scale to determine how high the boat rides on the water.

 

boat_waves.gif

ap_OceanBoatSim3.hipnc

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