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Channel | sm_ch – Controls which channel is affected by changes made to the settings below. The following channels can be smoothed:

Liquid
Smoke
RGB
Fuel
Velocity 

Smooth this channel | enablesmoothsmoke, enablesmoothtemp, enablesmoothuvw, enablesmoothfuel – If checked the channel will be filtered.

Threshold | smoothtemp.x, smoothsmoke.x,smoothuvw.x,smoothfuel.x –  If this value is 0, the entire grid will be smoothed evenly. The higher the threshold is raised, the less voxels will be affected and only the sharpest gradients will be smoothed. The highest value you could use here depends on the range of the values of the smoothed channel - for Liquid it's usually in the [0,1] range, while for Velocity it could go as high as several hundred. If you set this value too high, no voxels will be smoothed at all.

Similarity | smoothtemp.y, smoothsmoke.y,smoothuvw.y,smoothfuel.y –  Increasing this value will allow you to smooth only the finer small-scale noise without changing the areas of the fluid which are already smooth. Note that just like the Threshold option, this value also depends on the range of the selected channel. If you want to remove only some sharp fine disturbances from the simulation without blurring other areas, set the Threshold to 0, increase the Similarity to the highest values the selected channel can take and then start reducing it until the small-scale noise is removed, while the larger fluid shapes are retained. This option won't take effect if the Threshold parameter is raised to the maximum. 

Random Variation | smoothtemp.z, smoothsmoke.z,smoothuvw.z,smoothfuel.z – This parameter introduces noise of uniform scale in the channel before smoothing is applied. This can be useful if you want to give the fluid a more homogeneous pattern and this way it can also help hide grid artifacts. Note that just like the Threshold option, this value also depends on the range of the selected channel. You can use this option to only add noise to the channel without smoothing it - in order to do this, set both Threshold and Similarity to the highest values of the selected channel and this way they will not take effect.