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Dynamics - Fluid / Liquid

Updated: Mar 28, 2022

After this point, simulations such as liquid/gas are usually obtained with some plug-ins or external software that are not included in 3d software. X-Particles, Real Flow, FumeFX, Turbulence FD, Phoenix FD etc. All the liquid simulations are based on particles. Liquid-fluid simulations have wide and variable parameters depending on the plugin. Some of the main important and common parameters are as follows:


Emitter :

This is the most important parameter. It determines the source, exit and formation point of the particles. As well as primitive shapes such as Circle, Rectangle, you can define any object (called Object, Model, Mesh, Spline) as an emitter.

With the Fill Emitter, you can start the simulation with an object of your choice as it is filled.

Let's explain some parameters over the simple Circle Emitter:

Speed : As the name suggests, determines the emission speed.


V Random : It gives random speed to the emitted particles on the vertical axis. It provides more organic distribution instead of circled particle emission in certain frequencies.


H Random : Gives randomness on the horizontal axis. Particles protrude beyond the radius of the Circle from which they originate.

Side Emission : Emits particles from side of circle to the side axes instead of emitting from inner area of circle.


Ring Ratio : By creating another circle inside the circle, it provides emission from the region between these two.


Object Emitter also has some unique features:

Body/Object : Where you assign your emitter object.

Randomness : H/V Random combination we saw above.

Distance/Threshold : It determines distance between object and emission start point.

Jittering : It gives the particles small vibrations.

Emission Type: It determines whether the emission will be made from surfaces (face, surface) or points (vertex, point) or volume etc.


Some parameters of Fill Emitter are as follows:

Body/Object : Where you assign the object will be filled with particles.

Fill Ratio X/Y/Z : Adjusts the fullness of particles in certain axes. In other words, it reduces the particles in the relevant axis, slice by slice.

Surface Offset : Determines the distance of the particles from the object.

Fluid :

This section focuses on particles and simulation rather than source.

Resolution : It is the most important parameter. It determines the amount of particles. The more they are, the more detailed and realistic simulations we can get. Calculation time is the cost.


Density : It determines the density of the particles. It has no visible effect on a single emitter, but when different emitters are used to mix, oil/water mixed behavior can be simulated by adjusting the density.


Internal Pressure : Separates the particles from each other. Larger volumes can be filled.


External Pressure : Limits the expansion of the liquid, allowing the particles to approach each other and move together.


Surface Tension : Similar to external pressure. Increases the tendency of particles to form drop and branching structures (tendrils like).


Vorticity : It provides more realistic movements by adding a kind of turbulence. Extreme values ​​will cause the particles to go crazy. Don't try.


Damping : Slows and smooths down the velocity of the liquid. Higher values introduces the viscosity which is opposite of velocity which is resistence against movement. Honey, melted chocolate etc...


Initial State : Defines the simulation you are playing as the initial state at a particular frame. It deletes all the simulation before the specified frame and returns that frame to the first frame of the simulation. Especially useful in situations like Fill etc.


Solver :

Simulation engine settings. General simulation effects and details can be adjusted in this section. If the detailed calculations you want do not appear in the simulation, you should visit here.


Time Scale : It determines the overall speed of the simulation. 1 is the standard value. When the value changes, overall simulation slows down or speeds up.


Min/Max Substeps : It determines the calculation detail of the simulation in general. Normally 1-2 calculations per frame can do the trick. However, in detailed simulations, many problems may occur depending on the speed of the particles. The most important of these is that collision detection doesn't seem to be working properly, with particles passing through objects. In such a case, it is necessary to increase the number of interframe calculations, namely the Min/Max Substeps sufficiently. The cost is time like Resolution.

Use GPU : A powerful next-generation graphics card can increase simulation calculation speed by 5-10 times, depending on the situation. It is recommended to use if you have a good card. Generally, there is no compatibility problem.



Daemons/Forces

Essentially, we will examine the forces affecting all simulations in upcoming posts, but here it is useful to list some forces specific to fluid simulations roughly:


Crown : This daemon-force assists in creating the crown-shaped splash of water that appears on the surface with the drop of water. The shape of the crown, the number of spikes, etc. can be set with this deamon.


Sheeter : This daemon detects holes in the fluid simulation and fills them without changing the resolution of the simulation.


Surface Tension : We explained it in the Fluid section.


Most of the other deamons will be explained in the forces section.


Mesher :

This process turns particles to the meshes and make them ready to the render or export.


Topology : Forms surfaces as quads or triangles. Resolution : Quality of the mesh.


Radius : It is one of the most important parameters. Simulators create a sphere around each particle to create the mesh. Then, these spheres are blended and the final result is obtained. There are a few important points:

If Resolution and Radius are low, you can't see anything but drops or spheres floating around.

If Resolution is low, Radius is high, you will get an integrated mesh but lose detail. A swollen and thick-sided mesh appears.

Ultimately, high Resolution and low Radius are required for the most realistic result.

Smooth : Smooths the mesh and closes some small holes.


Thinning : A filter that improves the overall quality of the mesh.


Relax : It stretches and sharpens the edges of the mesh and provides a more natural appearance. High Relax Iteration causes details to be lost!


Build Mesh : Creates the mesh in the current frame.

Although they may be in different tabs, menus, titles, all of the listed parameters can be found in almost every plugin, . You can visually examine the liquid and gas simulation parameters in more detail from the youtube channel below.




Stay safe.


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