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Motion Inertia | ext_wind, inert_mul – 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 | grav, gmul, extgrav – Phoenix Gravity makes the liquids fall down and makes fire rise up. In fire/smoke simulations it creates velocities depending on the Temperature channel. In voxels where the Temperature is above 300 (in Kelvins, which is room temperature), Gravity will create upwards velocities. Where Temperature is below 300, the opposite will happen - velocities will be created pointing downwards. The hotter the fluid, the faster it will rise, and the cooler the fluid below 300, the faster it will fall down. The Gravity option is a multiplier, so using the default 1.0 will make it behave like real world gravity, setting it to 0.0 will disable its effect completely, and you can also use negative values, which will inverse the gravity effect.

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. Time Scale different than 1 will affect the Buildup Time of Particle/Voxel Tuners and the Phoenix Mapper. In order to get predictable results you will have to adjust the buildup time using this formula:
Time Scale * Time in frames / Frames per second

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

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.

Steps per frame (SPF) is one of the most important parameters of the simulator, with a 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. You cannot take a shortcut to simulate the last frame of a simulation, without first simulating all of the frames that come before it, one by one.

The simulation produces good results if each step introduces small changes to the sim.

For example, if you have an object that is hitting a liquid surface with a high speed, the result will not be very 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 need to introduce intermediate steps, until the object's movement becomes small enough that it happens smoothly across all steps for that frame.

The SPF 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 intermediate steps are simply calculated, but not exported.

Increasing the Steps per frame (SPF) also comes with significant trade-offs to performance and detail.

A higher SPF decreases performance in a linear way. For example, if you increase the SPF twice, your simulation will take twice as long. However, quality does not have a linear relation to SPF.

For maximum detail, it is best to use the lowest possible SPF that simulates without any of the issues described in the tip box below, since each additional step kills fine details. For more information, please refer to the Phoenix Explained docs.

Signs that the Steps per frame (SPF) needs to be increased include:

More often than not, these 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.