Don't take my words, just put a flip tank in a new scene and playblast. In my opinion you should not do that as you will loose particles. From your pics, I notice that you are reducing the Particle Radius Scale when you are reducing Particle Sepration to get more volume. Thanks Jeff for the great explaination and ideas to achieve the effects. Hi Vidit, Look what I said, a picture tells a lot then words. Good luck!įlip_small_waterfall_sloped_with_ (2.4 MB) Parent constrain pumps and sinks (with or without velocity sculpting) before and after the raft and in strategic parts of the river to get the raft to the height you want it to and to plow a path through the water churn so that it can at least look plausible. Add velocities to the river bed in an upward direction of course but tilted back a bit in the direction of the water flow to kinda simulate a wee bit of drag between the river bottom and the fluid interacting in that localized region. This can be done by adding/sculpting collision velocities to the river bed for example. Add counter velocities in strategic locations along the river bed in the body of the fluid to keep the fluid in check without affecting the look. Make sure the net new water has counter velocities to push back the water but not so much as to create too much turbulence. So what cheats can you employ to give you some control back? - Use Pumps to emit net new water in strategic places along the waterway with creative velocities mostly with the velocity in Y magnitude determinging fluid emission rate. Use a different camera location and lens focal length to make the river look steeper than what it really is. This is a plausible scenario if the river bed consists of very hard and very soft layers of rock that was eroded over time. What to do to keep things real yet look exciting? - Add flatish shelves with deep catch basins along the path of the river to try to create pools of slower water to impede the progress of the water moving over the thinner parts of the river bed. The steeper the slope, the thinner the water gets given that the same amount of water is entering the falls at the top. Notice how thin the water gets and adding more fluid at the top will just create more of a frothy mess and not a nice thick fast moving sheet of water.
Check out the 1:14 mark and that is pretty much the scenario in reality. So here's the scenario: you need to add a terrific amount of fluid down a steep set of rapids which will create a simulation with an incredible amount of energy then send a raft down in to what will be an extraordinary set of class 6 rapids which no raft could navigate with occupants remaining in said vessel at the end of the ride. FLIP can do decent water sims without divergence these days. This will cause increased memory requirements and decreased sim speed for not much gain. I recommend you stay away from divergence as it requires a divergence field and a divergence pass in the flip solver.
Yes you can carry more particles but in the end it will thin out pretty fast and just delay the effect until further down the steep river. In fact with more accurate velocities present at higher grid values may allow for even greater acceleration and velocities to be supported. The recommendations regarding bulking the sim by simply reducing the volume resolution wrt the particle scale size is fine but it will definitely change the look and not really counter the thinning of the fluid. Your reference image shows a nice calm flat river babbling over a small shelf in to a tiny pool and then slowly carrying forward and you want your fast moving water down a steep slope to have a similar feel. It will look like muddy water with low acceleration/velocity thresholding to sludge with high acceleration/velocity thresholding. If you clamp velocity you will get unrealistic motion for the fluid affected by the velocity as you described. The fluid will thin out as the simulation progresses as your fluid accelerates just like in reality. Flip doesn't have built-in checks to acceleration and velocity by default. Gravity has accelerating effect on the sloped water to the point where it thins out and hits a terminal velocity which can be due to faster water colliding against slower water, friction against the collision surface, internal viscosity forces when the water essentially turns in to a violent mix of air and water and in extreme cases such as a waterfall, air resistance itself.
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