When running a simulation at a higher resolution (lower voxel size) it will take longer to simulate and may produce different results. Not only will there be more detail, but even the coarse shape of the fluid motion may be different. While the differences won't be huge, they may still be undesirable after you got the overall look right in a lo-res simulation.

Up-res'ing is a simulation post-process that allows you to increase the resolution of a simulation cache without running a full simulation at the higher resolution. The result will look pretty much exactly like the lo-res version but with additional small detail.

In order to be able to up-res an existing simulation cache, it needs velocity. Make sure you enable velocity caching when running the lo-res simulation. When up-res'ing a cache you don't need to cache the velocity again unless you need it for rendering (for sub-grid detail or motion blur).

Note that you can also change the Max. Simulation Steps for the up-res post-process as well as the channel parameters (i.e. the parameters in Simulation/Temperature, Density, Fuel and Burn).

Upres Scale

When running an Up-Res'ing post-process by clicking the Up-Res button, the resolution of an existing simulation will be increased. This parameter specifies how much larger the resolution will be.

Fine Turb. Intensity

The Up-Res'ing post-process also adds small scale turbulence to the fluid that will create additional detail. This parameter specifies the strength of this small scale turbulence.

Fine Turb. Small Power

Specifies how stong small turbulence detail is compared to the next larger size. This works just like the Simulation/Turbulence/Small Power parameter but it's applied only for the additional detail added during Up-Res'ing.