Hi everyone,
We have a masterclass coming out soon that will explain how to set up a re-sim of a river simulation. However, I will do my best to explain the workflow here and answer any questions that come up.
The re-sim idea comes from the addition of the “Boundary Layer” in the FlipSolver node. With the Boundary Layer, we can maintain a surface and velocity in a thin layer around the volume limits of the simulation based on SOP volumes that we link in. This is valuable because we can now prescribe in-flow and out-flow velocities at the volume limits, whereas before the limits were closed (i.e. zero velocity) or open (i.e. any outflow velocity). By setting the velocity to exactly match our linked velocity volume, we have much more control over our simulation. The surface volume controls how the geometry is maintained in the boundary layer strip. Any voxels below our surface will be reseeded with particles and any voxels above our surface will have particles removed. This allows us to maintain fluid volume in the simulation when we want fluid flowing into the simulation through one region and out through another. For this reason, the Boundary Layer option is used heavily in the Oceans toolset.
For rivers, we need to bake out our low-res simulation first. We then load the baked geometry (surface and velocity) in to our up-res simulation through a File SOP and wire the surface and velocity into the Boundary Layer. In the example from the launch video, the up-res FlipSolver's volume limits were adjusted to focus on a small section of the river to be up-res'd. We also modified the scene by adding a collision object. Finally, we need to seed new particles into the first frame. In the same Geometry node as the File Sop, we used Points from Volume to seed particles into the low-res surface (only within the volume limits of the new simulation) and sampled the velocity from the baked velocity to assign to the particles. Theses particles used the new, high res particle separation. We then wired these particles into the FlipObject's intial data. This allows us to start the simulation with right lo-res velocity.
It is also possible to drive the up-res with GAS Guiding Volume (which is used primarily for the Guided Ocean Layer). But BUYER BEWARE we found that turbulence tends to form at the seam between the guiding volume and the free fluid volume. Because of this, we do not currently endorse this approach for doing an up-res, but we are researching how to solve this in a future version.
If you want to try anyway, GAS Guiding Volume is similar in spirit to the Boundary Layer but with arbitrary volumes. The surface of the guiding volume will force the fluid simulation to match the guiding velocity, much like how the Boundary Layer forces the fluid simulation to match its velocity at the volume limits. Therefore, you will need to modify the low-res baked surface to only be “plugs” on either end of the simulation before you link it to the guiding volume. Finally, it's essential to make the appropriate modifications to the “Solver” tab in the FlipSolver so the guiding volume is visible to the solver (see http://www.sidefx.com/docs/houdini/nodes/dop/gasguidingvolume [sidefx.com] for instructions).
I hope this helps!
Ryan
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Houdini Lounge » Houdini 16 - Flip Fluid Upresing ?
- rgoldade
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