This page provides information on the Dynamics rollout for liquids.

 

Page Contents

 

Overview


This rollout controls the fluid's motion parameters.

UI Path: ||Select Liquid Simulator | LiquidSim object|| > Modify panel > Dynamics rollout

 

Parameters


 

 

Simulate Air Effects | simair – When enabled, turns on the built-in air simulator. Strong sources or fast moving obstacles inside the simulator will create air velocities to carry splash, mist, and foam particles at high speed. Used mostly when realistic mist is needed. The simulation is not expensive, but can dramatically increase the splash and mist quality.

The air effects stop affecting particles once they exit the simulator, thus altering the particle speed and direction around the simulator walls.

Motion Inertia | ext_wind – When enabled, moving the simulator object over a series of frames causes inertial forces in the opposite direction of the movement. This allows you to link the simulator to a moving object and keep the size of the grid relatively small, as opposed to creating a large grid that covers the entire path of the moving object. Motion Inertia can be used for moving ground and water vehicles, torches, fireballs, rockets, etc. When this option is used together with the Initial Fill Up option and Open Container Wall conditions, a simulation of moving an object over a sea surface can be done. For more information, see the Motion Inertia example below.

When running liquid simulations with the Initial Fill Up option and Open Container Wall conditions, the surface of the generated liquid should remain smooth. If you encounter artifacts in the form of horizontal lines perpendicular to the direction of movement, with Motion Inertia enabled, please ensure that the Scene Scale is reasonable considering the type of effect being simulated. Other possible solutions in case tweaking the scale is not possible are to either increase the Steps per Frame, or to reduce the Cell Size of the Simulator.

Liquid artifacts usually appear when the liquid particles move a great distance between frames. Increasing the Scene Scale or the Steps per Frame allows them to stabilize, which in turn keeps the surface smooth.

Gravity | grav, gmul – When enabled, a standard gravity of 9.8 m/s 2  is automatically added. This value is a multiplier and scales the gravity accordingly. You can also use negative gravity.

Initial Fill Up | initfillflevel – When enabled, the container is filled up with liquid when the simulation starts. The numeric field determines the fill-up level, measured in % of the Z size.
When this option is used with Open Container Walls, the container can be moved and the grid will behave as part of an infinite ocean. For liquid simulations using Confine Geometry, you can enable Clear Inside on the geometry and liquid will not be created at simulation startup in the voxels inside the geometry.

The liquid created through the Initial Fill Up option will be initialized with the values set for the Default RGB and Default Viscosity parameters below.

Fill Up For Ocean oceanfill – Change the Open Container Walls of the simulator so that they would act like there is an infinite liquid volume beyond them. Pressure would be created at the simulator walls in order to support the liquid, and if the surface of a wall below the Initial Fill Up level or the bottom is cleared from liquid during simulation, new incoming liquid would be created. In order to eliminate air pockets between Solid geometry and the liquid mesh, this option will automatically set all Solid voxels below the Initial Fill Up level to contain Liquid amount of 1, even if they don't contain any Liquid particles. If you don't want this effect, enable Clear Inside from the Phoenix FD Properties of the Solid geometry.

All simulator walls must be set to Open for Fill Up For Ocean to take effect.

Steps per Frame | spf – Determines how many calculations of the simulated grid are performed between two consecutive frames of the timeline.

One of the most important parameters of the simulator, with significant impact on quality and performance. To understand how to use it, keep in mind that the simulation is a sequential process and happens step by step. It produces good results if each simulation step introduces small changes, but it's also a trade-off between performance and detail, as described below.

For example, if you have an object that is hitting the liquid surface with high speed, the result will be not good if at the first step the object is far away from the water, and at the second step, the object is already deep under the water. You have to introduce intermediate steps until the changes of each step get small enough. This parameter creates these steps within each frame. A value of 1 means that there are no intermediate steps and each step is exported into the cache file. A value of 2 means that there is one intermediate step, i.e. each second step is exported to the cache file while the intermediate steps are just calculated, but not exported.

Signs that this parameter needs to be increased are:

  • Liquid simulations have too many single liquid particles.
  • Liquid simulations are torn and chaotic.
  • Liquid simulations of streams have steps or other periodical artifacts.
  • Fire/Smoke simulations have artifacts that produce a grainy appearance.

More often than not, those issues will be caused by the simulation moving too quickly (e.g. the emission from the source is very strong or the objects in the scene are moving very fast). In such cases you should use a higher SPF.

Keep in mind that higher Steps Per Frame decreases the performance in a linear way, i.e. if you increase the SPF twice, your simulation will go twice as slow. However, the quality does not have a linear relation to the SPF. Each simulation step kills fine details, and thus for maximum detail it's best to use the lowest possible SPF that runs without any of the issues mentioned above. For additional information, please refer to Phoenix FD Explained.

Time Scale | timescale – Specifies a time multiplier that can be used for slow motion effects. For more information, see the Time Scale example below.

In order to achieve the same simulation look when changing the Time Scale, the Steps per frame value must be changed accordingly. For example, when decreasing the Time Scale from 1.0 to 0.5, Steps per frame must be decreased from 4 to 2. All animated objects in the scene (moving objects and sources) must be adjusted as well.

Active Bodies Mult | fluidToSolidInteractionMult - A multiplier for the effect of the Velocity channel on the Active Bodies in the Simulator. To convert a polygon object into an Active Body, enable the Active Body checkbox in the Phoenix FD Properties for that object.

Default RGB | lq_default_rgb - The Simulator is filled with this RGB color at simulation start. The Default RGB is also used to color the fluid generated by Initial Fill Up, or by Initial Liquid Fill from the Phoenix FD Properties of a geometry - both of these options create liquid only at the start of the simulation. During simulation, more colors can be mixed into the sim by using a Phoenix FD Liquid Source with RGB enabled, or the color of existing fluid can be changed over time by using a Phoenix FD Mapper. If a Phoenix FD Liquid Source does not have RGB enabled, it also emits using the Default RGB value.

The RGB Grid Channel has to be enabled in the Output Rollout for this parameter to take effect.

RGB Diffusion | rgbdiff – Control how quickly the colors of particles are mixed over time during the simulation. When it's set to 0, each FLIP liquid particle carries its own color, and the color of each individual particle does not change when liquids are mixed. This means that if red and green liquids are mixed, a dotted red-green liquid will be produced instead of a yellow liquid. This parameter allows the colors of particles to change when the particles are in contact, thus achieving uniform color in the resulting mixed liquid. For more information, see the RGB Diffusion example below.

Default Viscosity | lqvisc – Determines the default viscosity of the liquid. This value is used when no viscosity information for the emitted liquid is provided to the Simulator by the Source. For more information, see the Viscosity example below.

  • All FLIP liquid particles are set to this viscosity value at simulation start. You should use higher viscosity for thicker liquids such as chocolate, cream, etc
  • The Default Viscosity is also used for the fluid generated by Initial Fill Up, or by Initial Liquid Fill from the Phoenix FD Properties of a geometry - both of these options create liquid only at the start of the simulation.
  • If a Phoenix FD Liquid Source does not have Viscosity enabled, it emits using the Default Viscosity value.
  • During simulation, liquids of variable viscosity can be mixed into the sim by using a Phoenix FD Liquid Source with Viscosity enabled.
  • The Viscosity Grid Channel export has to be enabled in the Output Rollout for variable viscosity simulations to work.
  • The viscosity of existing liquid can be changed over time by using a Phoenix FD Mapper in order to achieve melting or solidifying of fluids.
  • You can shade the liquid mesh or particles using the fluid's viscosity with the help of the Phoenix Grid Texture or Particle Texture
  • It's important to note that using viscosity does not automatically make the liquid sticky. For example, molten glass is viscous, but not sticky at all. Stickiness can be enabled explicitly from the Wetting parameters section. If Stickiness is not enabled, even the most viscous fluid would slide from the surfaces of geometries or from the jammed walls of the Simulator.

Viscosity Diffusion | viscdiff -  Phoenix FD supports sourcing of fluids with different viscosity (thickness) values. This parameter specifies how quickly they blend together. A low value will preserve the distinct viscosities, while a high value will allow them to mix together and produce a fluid with a uniform thickness.

Non-Newtonian |  nonnewt – Modifies the viscosity with respect to the liquid's velocity to overcome the conflict between viscosity and wetting, where a high viscosity of real liquids prevents wetting. Non-Newtonian liquids are liquids that behave differently at different velocities. This parameter accounts for this behavior by decreasing the viscosity in areas where the liquid is moving slowly and retains a higher viscosity where the liquid is moving quickly. For example, to cover a cookie with liquid chocolate, high viscosity is needed in the pouring portion of the motion to obtain the curly shape of the chocolate as it lands on the cookie and begins to settle down. On the other hand, a smooth chocolate is needed to settle in over the cookie without roughness and holes. If the viscosity is high enough, the chocolate might look right during the pouring and settling motions but won't settle in to form a smooth thin layer over the cookie. This parameter decreases the viscosity where the liquid is moving slowly (over the surface of the cookie) while keeping the faster-moving stream tight and highly viscous. For more information, see the Non-Newtonian example below.

Droplets Surfing | dsurf – This parameter affects the liquid and the splash particles, controlling how long a particle hovers on the surface before it merges with the liquid. The parameter is used mostly in ocean/wave simulations.

 

 

Example: Motion Intertia

 


 

The following video provides examples of moving containers with Inertial Forces enabled to show the differences between values of 0.10.5, and 1.0.


 

Example: Steps Per Frame

 


 

The following video provides examples to show the differences of Steps per Frame values of 15, and 15.


 

Example: Time Scale

 


 

The following video provides examples to show the differences of Time Scale with values of 0.11.0, and 2.0.


 

Example: Viscosity

 


 

The following video provides examples to show the differences of Viscosity with values of 0.00.5, and 1.0.


 

Example: Non-Newtonian

 


 

The following video provides examples to show the differences of Non-Newtonian with values of 00.1, and 1.0.

 

 

Example: RGB Diffusion

 


 

The following video provides examples to show the differences of RGB Diffusion with values of 0.00.5, and 1.0.


Surface Tension


 

Strength | lqsurft – Controls the force produced by the curvature of the liquid surface. This parameter plays an important role in small-scale liquid simulations because an accurate simulation of surface tension indicates the small scale to the audience. Lower Strength values will cause the liquid to easily break apart into individual liquid particles, while higher values will make it harder for the liquid surface to split and will hold the liquid particles together. With high Strength, when an external force affects the liquid, it would either stretch out into tendrils, or split into large droplets. Which of these two effects will occur is controlled by the Droplet Breakup parameter. For more information, see the Surface Tension example below.

Droplet Breakup | lqstdropbreak – Balances between the liquid forming tendrils or droplets. When set to a value of 0, the liquid forms long tendrils. When set to a value of 1, the liquid breaks up into separate droplets, the size of which can be controlled by the Droplet Radius parameter. For more information, see the Droplet Breakup example below.

Droplet Radius | lqstdroprad – Controls the radius of the droplets formed by the Droplet Breakup parameter, in voxels. This means that increasing the resolution of the Simulator will reduce the overall size of the droplets in your simulation.

 Increasing the Droplet Radius can dramatically slow down the simulation. Please use it with caution.

 

Example: Surface Tension

 


 

The following video provides examples to show the differences of Surface Tension with values of 0.00.51.0 and Droplet Breakup 0.0

 

 

Example: Droplet Breakup

 


 

The following video provides examples to show the differences of Droplet Breakup with values of 0.00.51.0 and Droplet Radius set to 3.0 voxels.

 

Wetting


Simulation of wetting can be used in rendering for blending of wet and dry materials depending on which parts of a geometry have been in contact with the simulated liquid. Wetting can also change the behavior of simulated viscous liquid and make it stick to geometries.

The wetting simulation produces a particle system called WetMap. It can be rendered using a Particle Texture | PhoenixFDParticleTex map which blends between a wet and a dry surface material. The drying info is kept in the particle size channel. To convert the map to grayscale, enable the Mult. by size option for the Particle Texture | PhoenixFDParticleTex map.

 

 

Wetting | wetting – Enables the wetting simulation. The liquid will leave a trail over the surfaces of bodies it interacts with.

Consumed Liquid | lq2wet – Controls how many liquid particles disappear when creating a single wetmap particle. The main purpose of this parameter is to prevent long visible tracks from being left by a single liquid particle. For more information, see the Consumed Liquid example below.

Drying Time (sec)drying – Controls the drying speed in seconds. The WetMap particles are born with a size of 1, and if they are in an air environment, the size decreases until it reaches zero after the time specified with this parameter.

Sticky Liquid | wetdyn – This option produces a connecting force between the WetMap particles at the geometry surface and nearby liquid particles, when the liquid particles have at least a little ViscosityFor more information, see the  Sticky Liquid example below.

Geometry transforming or deforming at a high velocity may cause some or all of the Wetting particles stuck to it to disappear. To resolve this, dial up the Steps Per Frame parameter from the Dynamics tab of the Simulator.

 


 

Example: Consumed Liquid

 


 

The following video provides examples to show the differences of Consumed Liquid values of 00.5, and 1.

 

 

Example: Sticky Liquid

 


 

The following video provides examples to show the differences of Sticky Liquid values of 00.5, and 1.