Hi every body!

I have a weird problem in flip simulations.. during tests when I using high particle separation ( low res sim ) , I set every thing to give me the nice sim I want.. then when I am happy with the results I want to up res the sim, so I increase the res ( decrease the particle separation ) .. but the results are weird .. every thing changes .. (like: you may have higher splashes).. and its hard to do the primary test on high res sim .. its become so slow .. and this happens also when you try to sim in a slow motion.. it gives different results not like the normal speed sim..

is that a common thing?

and is there any ideas to mach the end results ( hi res sim) with the tests (low res sim)?

Efforts are always made to minimize the effect, but I'm afraid that's pretty much the nature of using any kind of fluid sim.

Effectively, the wider the separation and the fewer timesteps you use, the more “inaccurate” the result will be (the ideal real-world situation would be infinite timesteps and atomic-level resolution, while the opposite end of the resolution scale is just one giant immobile sphere representing your entire sim - obviously that will be a very poor indicator of the true sim behaviour)… so a preview-resolution sim can be pretty wide of the mark, and you should always mix in a few moderate-res tests along the way to ensure you're heading in the right direction. Going straight from super-low-preview-res to super-high-production-res will almost always leave you wondering why everything changed so much.

Depending on the speed of your fluid, one of the biggest causes can be from not increasing the substeps on the FLIP Solver node. If you double the resolution, it will be attempting to derive an accurate pressure solve while particles are moving across double the number of grid cells per substep… you'll end up getting dense mini-ripples all over your sim if the substeps are too low for a given resolution/speed combination, and they can really alter the behaviour of the sim.

Quite often, lower res sims tend to bulk out the fluid too much (as the fewer particles you have, the fatter their radius needs to be to compensate), so increasing the resolution can end up making the fluid look thinner and feel like it's covering less volume. You'll need to factor that in and sometimes emit more fluid for a high-res sim to keep it looking as full as the test.

You'll get a feel for what sort of results to expect with experience. It does require a certain amount of trial-and-error.