Fluid surface tension too strong, how to relax it?

So after figuring out the bug in Quick Fluid, scaling every fluid object to 1.0 fixes all the baking problems.

But now I’m having a problem with the surface tension being too strong, see:



Here I have a stream that is very large, and I want the surface tension to be almost unnoticeable, but it’s so apparent (like giant beads). I tried adjusting the real world size (10.0 and 1.0) and increasing the resolution to 350 but it still does not fix the problem, the surface tension always seems too strong.

With fluids, the only thing that can fix this is to increase the resolution even higher… But sadly this gets exponencially slower. :frowning:

I’ve already tried resolution at 350, I have a fast computer so it’s not too much of a problem, but I don’t see the surface tension getting any smaller. The Real World Size can only go up to 10.0, so I’m at a loss at what to do.

Is Blender’s fluid system only capable of simulating small volumes of liquid? like water in a cup?

What happens if someone wants to simulate a lake or ocean?

You use the ocean modifier for lakes and oceans.

The real world size only goes up to 10 meters, but the resolution must be increased accordingly if you go that big. 350 is probably enough for a cubic meter or two to look good.

Fluid resolution works similarly to picture resolution. The resolution has to be high enough to fit the real world size. If you try to stretch it over too much space it won’t hold enough detail to look good. In the case of fluids you’re in a 3 dimensional space, so the resolution requirements increase according to the square-cube law. For 10 cubic meters you need an almost unworkably high resolution. 350 isn’t even the right order of magnitude.

Increasing the real world size to its maximum is rarely worth the effort, unless you can spare half a terabyte for the simulation data, and two or three days to bake it. If you can’t afford to go much higher than 350 in resolution, the solution is perhaps a bit counterintuitive. You need to decrease the real world size value so that the resolution can match it. Bring it down to 2 meters, maybe lower. The fluid likely won’t have enough detail on the surface, because as far as the computer knows that surface is much smaller with less room for detail, but the overall flow will look better. You can fake the missing detail with displacement textures after you have the general fluid behavior working properly.

For best results, you should also make sure that your domain cube is as small as possible to contain the fluid simulation elements. If it’s out of the camera for the whole shot, get rid of it. The closer the cube is to the fluid within it, the more accurate the results will be.

Ok, here is a rendering as an animation with the Real World Size set to 1.0 meters and Resolution at 350, I also re-segmented everything so that the domain cube encompasses only the fluid in the camera’s field of view. But I still don’t think this looks good, let me know what you think.

It looks jagged, and it’s a bit hard to see in that grainy render. Resolution is probably ok, but are you using any subdivisions? Subdivision in the Fluid Boundary panel may help. Just 1 or 2. Also make sure that domain object has Smooth Shading turned on.

I have been playing with the fluid system. Have you tried to reduce the amount of Partial Slip? To me it seems like it is what causes the “surface tension” issue. In effect the water is sticking to the ground. Perhaps lowering it, or setting it to No Slip might be the solution. Just a suggestion.

Thanks everyone, and yes I did set the Slip to Free Slip.

But it was increasing the Fluid Boundary’s Subdivisions to 2 that relaxed the surface tension, that did the trick! :slight_smile:

I’m 75% satisfied now. :wink: Because it still has a bit of surface tension still but now its tolorable.