1. Computer Graphics Imaging Ying Zhu Georgia State University
Lecture 28
Fluids

2. Computational Fluid Dynamics
Blender uses the Lattice Boltzmann method (LBM) to simulate fluids
For more details see

3. Creating fluids The process Model the scene (objects, materials, etc.)
Specify the “Domain” in which the fluid will flow
Select the objects that may collide with the fluid and specify their functions
Create the fluid sources and specify their parameters
Bake the fluid simulation
Play back the fluid simulation (with Alt + A, etc.)

4. Domain A “domain” is a bounding box for fluid simulation
Use the bounding box of the object regardless of its shape
Only one fluid domain is allowed for each scene
All fluid objects must be in the domain
No fluid will be simulated outside the domain
Multiple fluids should not collide with each other in their initial position

5. Domain
Add a cube to the scene, move and resize the cube to cover the area that you want the fluid to be simulated
In “Object”, “Physics buttons” panel, “Fluid” tab
Press “Fluid” button
Press “Domain” button

6. Domain Parameters

7. Domain parameters: resolution
Setting the resolution higher will give you better visual quality
But it will require more memory usage
Also the computation will be slower
Adjust the resolution number based on your computer’s memory capacity
Otherwise the computer may crash
You can see the “Req. BAKE Mem.” number changes as you adjust Resolution

8. Domain parameters: time
Start and End Time
There are two time periods related to fluid simulation
Simulation time: the Start and End time (in seconds) in the “Fluid” tab
Animation time: the Start and End frames in the Anim(ation) tab under the Scene (F10) panel

9. Simulation time and Animation Time
The “simulation time” is used by fluid physics solver to calculate the behavior of fluid
The simulation is then spread over the frame numbers specified in the Anim(ation) tab in the final rendering
Animation Time == Simulation Time: Slow motion
Animation Time > Simulation Time: Slow motion
Animation Time < Simulation Time: Fast Forward

10. Fluid Baking and Animation Time Frames
When baking fluids, Blender will ignore the Start frame in the Anim tab
Blender always bakes fluid from frame #1 to the End frame number specified in Anim tab

11. Simulation time and Animation Time
Suppose in the Fluid tab (under Domain) you have Start Time = 0.0 (second), End Time = 10.0 (seconds)
The fluid solver will simulate fluid for 10 seconds
This means the simulation lasts 250 frames long
The standard frame rate is 25 frames per second
If in the Anim tab, the End frame number is 250, then the simulation will be played in regular speed when you play the animation

12. Simulation time and Animation Time
If in the Anim tab, the End frame number is larger than 250, then the simulation will be played in slow motion when you play the animation
If in the Anim tab, the End frame number is smaller than 250, then the simulation will be sped up when you play the animation

13. Domain parameters: bake
Press the “BAKE” button to bake the fluid simulation
Unlike particles and clothes, fluid baking is not automatically started when you press Alt + A
You must BAKE first, and then press Alt + A to play back the animation
You should specify the Path for storing files created during baking in the Fluid tab (default is /tmp)
The baking progress is displayed in header bar

14. Domain Parameters: viscosity and real-world size
Viscosity: resistance to flow in a fluid
In the Fluid tab, under “Domain”, press “Ad” button, and adjust the viscosity
Can choose predefined settings for water, oil, and honey
Can manually adjust viscosity
“Realworld-size”: the size of the Domain box in meters
Adjust this number to match the size of the fuild in your virtual world

15. Domain parameters: Particles
In the Fluid tab, under “Domain”, press “Par” button
Can adjust
Tracer Particles: generate particles to trace where the water level used to be
Generate Particles: generate particles when water is splashed
Compare the results with and without particles at

16. Fluid parameters Volume init
Volume: the space enclosed by the object is treated as fluid (only works for closed mesh)
Shell: the surface (and a thin shell surrounding it) of the object is considered as fluid (works for both closed and open mesh)
Both (volume and shell)

17. Interacting with obstacle
Select the object to be collided with fluid, in Fluid tab, press “Fluid” and then “Obstacle”
Volume init: which part of the object will be colliding with the fluid
Boundary type
Noslip: the obstacle is sticky
Free: the obstacle is slippery
Part: somewhere in between
Moving object must be “Noslip”

18. Inflow object
Select an object, in Fluid tab, press “Fluid” and then “Inflow”
There is no need to create a separate Fluid object
This object will constantly inject fluid into the domain
Can be used to simulate tap, fountain, etc.
Adjust Inflow velocity (e.g. Z = -0.2) to change the volume of inflow fluid
Enable “Local Coords” if the Inflow object is animated

19. Outflow object
Select an object, in Fluid tab, press “Fluid” and then “Outflow”
The fluid will disappear when it touches the Outflow object
In other words, the Outflow object “kills” the fluid
Can be used to simulate sink, etc.

20. Control object Make an object attract or repel fluid
E.g. object with magic force
Can make the simulation really slow
Time: the time period (in seconds) the control object is activated
Attraction force
Velocity force
How much influence of the control object velocity has on the fluid

21. Control object
Example

22. How to create material for fluid?
Select the Domain object
Go to “Material Buttons” panel
In “Mirror Transp” tab, press “Ray Transp” and set IOR (Index of Refraction) to 1.33 (one tutorial suggests 1.17)
Press “Ray Mirror” and set “RayMir” to 0.11
Set Alpha to 0.109
Set Color to R=0.17, G=0.50, B = 0.89

23. Animating fluid parameters
Blender has a special IPO Curve for Fluid Simulation
Select the fluid Domain object, open an IPO Curve Editor window
Select FluidSim type
FluidSim IPOs cannot be keyframed by simply using the I key
You must manually set values by pressing Ctrl + LMB in the IPO window.

24. Parameters that can be animated
For fluid Domain object
Fac-Visc: changes fluid viscosity
Fac-Time: change the speed of the simulation
0 freezes the simulation
1.0 is normal speed
< 1.0 slow motion
> 1.0 fast forward
GravX/GravY/GravZ: change gravity over time
E.g. in space or in a speeding car (?)

25. Parameters that can be animated

26. Parameters that can be animated
For Fluid, Inflow, or Outflow objects
VelX, VelY, VelZ: animate fluid velocity
Active: takes two values -- Zero or non-Zero
Zero: the fluid disappear
Anything above zero: the fluid appears
Can simulate dripping by animating the Active parameter

27. How to make fluid simulation start later than frame #1?
Note that fluid baking always starts from frame #1
Create a FluidSim IPO curve for the Fluid object’s “Active” parameter
Make the fluid Active at the desired frame number

28. Parameters that can be animated
For Control objects
AttrForceStr: attraction force strength
AttrForceRa: attraction force radius
VelForceStr: velocity force strength
VelForceRa: velocity force radius

29. Tutorials