Physics

In the Physics submodule we can configure different values for the behavior of our fluid. Particle mass, viscosity, stiffness... We can also configure the subframes, which are necessary to give more stability to our simulation. The Timestep value allows us to slow down the particle velocity and get interesting SlowMotion effects. We can also configure different values of the particle behavior and the weight of the forces.

We have options like "Adaptative" that allows to automatically adapt the Subframes to increase them in case the particles have very high velocities to ensure a good behavior of the simulation.

Size Deflect allows the particles to stay on the surface of a collider, so that they do not pass through the geometry. It is very useful if we want to simulate sand or viscous fluids. But for fluids like Water, it is recommended to have it disabled, for a more realistic result of water on the surface.

Integration

In the Integration tab we can set the Timestep and Subframes of our particles. The Timestep is the amount of simulation time in seconds that passes during each frame. We can manipulate it to slow down or increase the speed of the particles and get interesting effects. It is also a property that we can animate from the Animation submodule and allows us to create amazing effects. Let's see an example.

Timestep 0.04

Timestep Animated

The lower the Timestep value, the slower the particle motion. Keep in mind that the slower the movement, the slower the particles will tend to accumulate in the emission zone, so this type of thing must be taken into account when designing the simulation.

Subframes are the number of simulation steps per frame. To improve the stability of the simulation, the value of Subframes should be increased. For example, in fluids with viscosity, it is always important to increase the subframes.

Viscosity 10, Subframes 4, simulation does not work well.

Viscosity 10, Subframes 15, simulation works fine.

Physics

In the Physics tab we can configure some important values of the fluid. The most important are Stiffness and Viscosity. The Drag and Damp values are used to add some air resistance to our fluids, depending on the result we are looking for.

Let's see a simple example of the difference between the Viscosity values.

Water - Oil - Honey

The higher the Viscosity value, the higher the subframes must be for the simulation to work correctly.

Stiffness is a "classic" value in SPH particle systems that serves to indicate how "Incomprehensible" a fluid is. For example, if we want to simulate water, we can set a relatively high value ( like 0.5 ) but for fluids like oil or honey, we can set lower values ( 0.1 ). We can manipulate it as we need to make the behavior of the particles more or less chaotic.

Stiffnes 0.1

Stiffnes 0.5

Advanced Behavior

In this tab we will find 4 options to change the behavior of our particles.

Repulsion Factor: With which force the fluid tries to avoid accumulation (stiffness factor). The check box sets the repulsion as a stiffness factor.
Stiff Viscosity: Creates viscosity for the expanding fluid. The check box sets this to be a normal viscosity factor.
Interaction Radius: Radius of fluid interaction. It also works similar to the constraints when using the Springs + Viscoelastic options, see below.
Rest Density: Density of the fluid at rest.

Field Weights

In this tab we can configure the weight of the forces. How much and in which collection of forces we want to affect the forces we have in scene. It is very useful to be able to work by collection of forces, since it allows each emitter to interact only with the forces we want.

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