View Full Version : Scale / Working units in fluids?

02 February 2011, 09:24 AM
I'm making some tests to decide how to model my geometry to work with fluids...
as I read in documentation: [User Guide > Dynamics and Effects > nDynamics > About nDynamics > Maya Nucleus solver properties], nucleus works as if 1 unit = 1 meter, but I can't find anything about the units in fluids.
If I modeled my scene in centimeters, I have to scale it to meters?
I made tests with fluids in scenes with a geometry scale of 1 unit = 1 meter and had problems making a simple cigar smoke. Obviously in this scale proportion, my container is 0.3x0.3x0.3 units (meters) and the emitter less than 0.01 units (meters). The resolution is 150x150x150 and I tried with 1, 2, 5 and 10 substeps of simulation (because the motion of the fluid is faster at this scale). There is no way to make it work, it explodes even with a fluid with default sim attributes. I have my scene working units set to centimeters because I read that there are a lot of problems and bugs derivated of changing this parameter and people recommend to keep it untouched.

02 February 2011, 10:47 AM
First, when simulating with Maya fluids, don't think about scaling everything in a realistic manner. As long as it's looking great, scale don't matter at all. Fluid simulation for CG is by no means physically accurate, so finding the correct scale depends on your feeling for a beautyful, natural behaviour and shape of the's all just smoke and mirrors.

Scale your geometry/ models down so they will be working better with your fluid containers (if needed).The container's are working best when using Maya's default scale settings (cm)- this is at least my own highly predjudiced experience. If your containers are really large, they are using a lot of memory and simuklation times will be way too long for working in a sensible manner. Besuides, you might need tons of memory to do those sims using the resolution you need.

Speeding up/ down a simulation can be achieved quickly by using the simulation rate scale, but be careful with that setting.

02 February 2011, 04:27 AM
You can scale your fluid up or down, but your resolution and other values have to be multiplied accordingly ...don't remember the factor , but there is a in the money multiplier to get values working.

Personally I if my geometry has not been modeled metrically accurate...and it is too big or too small for the fluid container, I just scale the geometry to fit inside the container not the other way around. I do change the container size but generally just to give it shape....that is..making it taller ..or shorter...or longer....or thicker...depending on the type of fluid I'm trying to create. As for speed....stick with using simulation scale rate on the container properties. Above 1 makes your fluid flow act like a smaller scaled cigar smoke....values below slow down your fluid....this increases the visual scale of a fluid...for example watching a big plume of smoke from afar.

Just remember that no one value makes a fluid rise slower or faster....there are many ways to do this with fluids.

02 February 2011, 09:59 AM
Thank you for your responses! :) I'll try grouping and scaling my geometry if necesary.

CGTalk Moderation
02 February 2011, 09:59 AM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.