I’m making a list of all the ways a Blender artist might mess up a fluid simulation. It’s common, according to comments I see in YT fluid tutorials, to set up everything, tap the space bar, and nothing at all happens. If that ever happened to you, or anything else bizarre or disappointing happened in a fluid simulation, I’d like to know, and put it in my list.
Someday I’ll make an article, maybe a video, a sort of checklist of things to check if your fluid simulation is messing up in some way. (Unless someone else has already done that? I’ll do mine anyway.)
The default resolution of the domain is only 32. This means if you create an emitter that’s too small, it might be smaller than a single cell and fail to emit fluid.
Misunderstanding the different behavior of the 3 cache types. “Replay” (the default) creates the cache in real time while playing the animation, but that cache is only a temporary preview for the viewport and you sometimes need to tweak a setting on the domain to force that cache to reset. “Modular” allows baking the base simulation and the noise separately, but you have to press the bake button to get anything, and you have to bake both parts before you can see the end result. “All” is like modular, but it does both parts at once.
The timeline isn’t set to “play every frame”. This setting is found on the timeline, in the “playback” menu. When doing a simulation, it’s important that you aren’t on a mode that can skip frames, or it can make a physics preview fail after frames were missed.
You used a smoke inflow in a liquid domain or a liquid inflow in a smoke domain.
You are in material preview mode (which uses Eevee) and Eevee’s volume start/end settings don’t match the scale of the scene, causing the smoke to be out of visible range.
Other problems with fluids:
In liquid simulations, the “fractional obstacles” button is very important. Some types of obstacles (like diagonal slopes) work poorly without it and have fluid stuck on them in ugly patterns.
In liquid simulations, the mesh generation has a default “upres factor” of 2. This is on the low quality side. A value of 3 will give a much more detailed result without changing anything else.
In liquid simulations, it’s difficult to make the motion blur work. You have to go to the mesh tab and check the “use speed vectors” button. Then, you have to go to the cache settings and set the format to “uni cache”. As far as I can tell, openVDB caches don’t support motion blur for liquids (unless I missed an option).
Common shading problems
Sometimes, begginers don’t realize they need a special volume material for a smoke simulation to render.
If you want white smoke in Cycles, you need to add some volume bounces in the render settings.
Liquids need some shader work to render correctly. By default, in Cycles, a glass shader will look too dark and fail to do proper caustics. You need to either use a shader trick to let more light through or use path guiding. Also, you need to increase the glossy bounces for proper internal reflections. Finally, realistic water is colored by adding a volume shader, not by changing the color or the glass surface.