– As you probably know the (differential) equations describing the motion of all the particles are resolved by using numerical methods.

– Honestly I don’t know wich algorithm Blender uses, but I suppose it uses the standard **v*(Δt/Δx)** for the CFL Number. If you know another formula is used please write me!

– What does the CFL Number represent? Look at this image (voxels are 3-dimensional and this is a one-dimensional example but this reasoning can be extended to a 3D voxel):

– If **v** is too hight, we loose the information that should be stored in the second voxel. The solution is to insert extra Time Steps.

– **x** is the size of a voxel and **t** the time the particle needs to run it, **v=x/t** the particle’s velocity.

– If we reduce the velocity, that’s to say **v≤x/t**, the particle will run one FULL voxel (or LESS).

– If we think in terms of incremental values: **v≤Δx/Δt ⇒ v*(Δt/Δx)≤1**

– **CFL≤1** is the condition, for a particle, to travel only from a voxel to a adiacent one.

– Why does the default Blender CFL Number is set to 4.0 instead of 1.0 (or less) ? I think it’s to reduce baking time.

– **How to choose the right CFL Number for your simulation?** Blender is not “real simulations” oriented. Remember we will never obtain a “real” simulation as everything is based on approximate physics that lead to approximate solutions. In general a smaller CFL number means a more ‘realistic’ simulation but if you find your simulation looks better with a greater CFL Number, raise it up!

– In this first example see how different values of CFL Number affects a simulation:

– What about using CFL Numbers greater than one? Consider again the above image: if for a certain velocity your particle cannot travel ,e.g, 3 voxels, no matter if we rise up the CFL Number, because if your particle cannot travel 3 voxels, will not be able to travel 4 or more, so that no extra Time Steps will be used.

– In this second example see how a simulation remains exactly same from CFL Number = 4.2 up.

– Always keep in mind that your simulation is a combination of CFL Number , Timesteps, Resolution Divisions, Video Frame Rate etc..!

**Some considerations**

– The CFL Number is a necessary (but not sufficient) condition for the convergence of the differential equations that describe the motion of all the particles inside the domain.

– “Convergent Solutions” means that when Δx and Δt (used in the numerical method) tend to zero, this numerical solution will tend to the exact solution.

– Sometimes the CFL Number is confused with a ‘Stability Condition’ (that means that as computation proceeds, errors get smaller). Keep in mind that there are methods stable for arbitrary Δt that, however, are not convergent unless a CLF condition is met.

– Smaller CFL Numbers mean bigger computational times (because of the extra Time Steps).

– CFL Number=0 , make it sense? The CFL Number can be equal to zero only if velocity is equal to zero that’s to say for NO moving fluids or NO time flow. (**v*(Δt/Δx)=0 ⇒ v=0 OR Δt=0** ). I think that if you choose CFL NUmber=0 then Blender sets very small value, please let me know if you know the correct answer to this question!