This page provides information on the Dynamics rollout for a FireSmokeSim object.


Page Contents



This rollout controls the FireSmoke dynamics parameters. It can be accessed in the Modify panel when a FireSmokeSim object is selected.

 UI Path: ||Select Fire Smoke Simulator | FireSmokeSim|| > Modify panel > Dynamics rollout






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.

Gravity | gravgmulextgrav – 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.

Time Scale | timescale – Specifies a time multiplier that can be used for slow motion effects. 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. And of course, all animated objects in the scene (moving objects and sources) must be adjusted as well.

Cooling | cooling – This parameter controls the cooling of the fluid due to radiation. It gradually decreases the temperature until it reaches 300.

Phoenix FD temperatures are in Kelvin, so 300 is room temperature - the temperature where the smoke neither ascends nor descends. A Cooling value of 1.0 corresponds approximately to the speed real smoke with a half thickness of 4 meters cools down. The real cooling is a very complicated process similar to the Global Illumination rendering, so here a simplified formula is used.

Smoke dissipation | smdiss – Used in simulations where the smoke needs to disappear, for example steam, clouds, etc. If you set this parameter to the maximum value of 1, the smoke will disappear immediately.

Smoke buoyancy | smoke_bcy – The buoyancy of the smoke. Positive values make the smoke move upwards. Negative values make it move downwards.

Fuel buoyancy | fuel_bcy – Specifies the buoyancy of the fuel.




Classic Vorticity | vorticity – Adds small-scale detail that is dissipated naturally by the grid-based simulation. Prevents the simulation from becoming smooth and laminar. Unlike Turbulence, which does not care about the fluid's motion at all, the Classic Vorticity algorithms works depending on the velocity of the simulation and changes the velocity field in order to reinforce vortices and add more detail to the simulation. For more information, see the Vorticity example below.

Massive Vorticity | velvort – This mode generally improves the vorticity detail and behavior. Unlike Classic Vorticity which reinforces vortices throughout the grid, the Massive Vorticity gets applied more strongly only to some parts of the fluid, making a difference between thin air with no smoke or temperature, the smoke and temperature's surface, and the inside volume of fluids. The added detail is equally strong for both the fast and the slow moving parts of the fluid. Massive Vorticity also does not produce a boiling effect that is present in the Classic Vorticity algorithm, which tears up the slower moving parts of the fluid excessively and stops the fluid's motion. For more information, see the Vorticity example below.

Smoke Surface/Temp. Surface | vorticitySurfSmvorticitySurfT – These options create vortices only on the smoke/temperature channel's surface without disturbing the flow in the inner volume of the fluid. This way massive vorticity won't block the fluid's general motion and break the fluid apart as the classic vorticity does. It's split into separate smoke and temperature multipliers for additional flexibility. For example, in a fire-only simulation, you need to use Temp. Surface, while in a smoke-only simulation, you need Smoke Surface instead.

Large Scale | vorticity_low – The large-scale multiplier reinforces the big vortex flows in the inner part of the fluid. The effect is similar to what higher Conservation quality can achieve, with the difference that the Large Scale vorticity does not respect obstacles, so it might not be suitable in a more complicated setup. For more information, see the Vorticity example below.


Example: Vorticity



The following example shows the difference between the Classic and Massive types of Vorticity.


Classic Vorticity = 0


Classic Vorticity = 0.3


Massive Smoke Vorticity = 0.3
Large Scale = 0.3




These options add random fluctuations in the fluid's velocity for each grid voxel. It works in combination with the Vorticity parameters.


Amount | rand – The amount of randomization. Only the velocity length is changed; the velocity direction remains the same.

Dynamics | rndfrq – The speed of change of the fluctuations. The higher the value, the faster the random noise will change over time.


The conservation process gives the fluid its characteristic swirling motion. It transforms straight-line movement of the fluid into swirling vortices. The higher the strength of the conservation, the farther the motion forces will be propagated throughout the container, so a movement in one point will cause the fluid to start moving at a distance too. The conservation directs smoke and fire into realistic shapes and helps liquids to support their own weight when at rest, and to fill up a volume they are poured into.

Internally, the conservation updates the directions and magnitudes of the velocities of each cell in the grid, preparing them for the Advection step when the content will be moved between cells. Basically it tries to equalize the velocities coming in and going out from each cell, and does this in many passes, getting closer to the perfect equilibrium. The number of passes is the conservation strength (quality). In nature, conservation has an infinite strength and is always perfect. In Phoenix FD, the better the quality of the conservation is, the farther the movement from one point will be propagated, making the simulation more realistic, but at the expense of longer simulation time for each frame. There are a number of conservation methods in Phoenix FD that you can choose between, depending on the type of your simulation. Each of them comes with pros and cons for the given situation.



Method | refltype – Specifies the method for simulating the conservation. For more information, see the Conservation Method Types example below.

Direct Symmetric – This method has a long range and can transfer movement and pressure from one point of the grid can to distant parts of the grid. It also achieves strong conservation without having to boost the quality too much, and thus the fluid can roll and swirl quite well. Thus it is well suited for simulation of explosions with shockwaves.

Direct Smooth – The smooth method is quite similar to Direct Symmetric. It produces a smoother velocity field and is stronger than Direct Symmetric, but doesn't keep the symmetry that well and it will produce less detail.

Buffered – This method has the weakest conservation strength and shortest range, but produces detail that works well when simulating fire.

PCG Symmetric – This method provides a very strong conservation, but works at a shorter range than the Direct Symmetric method. It actually produces better symmetry than Direct Symmetric and so it can be used to simulate nuclear mushrooms or other effects where high symmetry is crucial. It is also the best method to use for smoke or explosions in general. The Uniform Density option is ignored in this mode, so simulating only fire with this mode may not produce as good results as using the other methods that allow Uniform Density to be enabled.

Quality | reflprec – Increases the strength of the conservation. It will help cases when the liquid or smoke is losing volume, or the swirling of the smoke needs to be increased. Beware that increasing the quality will slow down the simulation. For more information, see the Quality example below.

Uniform Density | uniform_mass – When enabled, the mass of the fluid will not be considered. Fire simulations work better with this option on, which ignores the mass. However, unchecking this option might be useful for pure smoke simulations or explosions (for fire/smoke simulations the inverse of the temperature is considered the mass; the hotter fluid will be lighter and the cooler fluid will be heavier, just as in nature).


Example: Conservation Method Types



The following example shows the difference between the Symmetric, Smooth and Buffered Conservation Method types.


Symmetric conservation


Smooth conservation


Buffered conservation




Example: Conservation Quality



The animation below demonstrates how to cope with losing the volume of smoke. A smoke source is placed in an almost closed room, where the only exit is near the floor. In the real world the smoke will fill the room and then reach the exit, just because once created, the smoke is accumulated and does not disappear. However, when the conservation parameter is low, the smoke does not keep its amount and disappears, never reaching the hole.

Note that the smoke reaches the hole with conservation quality at about 200, and this value is not accidental. The vertical size of the grid is 100, and if it was smaller, the conservation value that allows the smoke to reach the hole would have been smaller too. You can think of this parameter as a distance scope in which the conservation spreads the velocity influence.


Quality = 8


Quality = 50


Quality = 200





Material transfer (Advection)

The advection process moves the fluid along its velocity inside the grid. A problem of all grid-based simulators is that moving the content of one cell to a new place in the grid will blur the result when the destination lies between cells (and it usually does), thus losing the fine details with each new frame. Phoenix FD has a number of advection methods that battle this problem in different ways, each one with its own pros and cons depending on the situation.

Phoenix FD may perform advection more than once per frame, or once in a number of frames, depending on the Steps per frame (SPF) parameter. To get the best detail for smoke and fire, it is best to keep the SPF low. On the other hand a higher SPF works better to keep liquids steady and smooth, and is better for quickly moving fluids in general.



Method | advtype – Specifies the algorithm used for calculating the advection. For more information, see the Advection Method Types example below.

Classic (Semi-Lagrangian) – This method has good stability, but does not strictly conserve the quantity of the transferred material and your fluid may start to gain or lose volume in certain situations. In such cases increasing the Steps per frame (SPF) will help preserve the volume.
Forward transfer – Good conservation abilities, but less stability compared to the classic method. Tends to produce cross-like artifacts.
Multi-Pass – This less dissipative method produces more fine details and keeps the smoke interface sharper compared to other methods. This method is recommended for large scale explosions, veil-like smoke, pyroclastic flows, and all other situations where sharpness is important.
FLIP (Particle based) – Used for liquids.

Steps per frame (SPF) | spf – Determines how many calculations of the simulated grid are performed between two consecutive frames of the timeline. For more information, see the Steps per Frame example below.

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.


Example: Advection Method Types



The following example shows the difference between the Classic and Multi-Pass advection types.


Classic Advection


Multi-pass Advection




Example: Steps Per Frame (SPF)



The first series in this example shows the differences in a Fire/Smoke simulation when the Steps Per Frame is set to 12, and 8.


SPF = 1


SPF = 2


SPF = 8