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"Vorticity" has to be activated in the → fluid container → fluid container (all fluid types, except "Dumb" have a "Compute Vorticity" option) before the simulation starts, otherwise the condition will never be fulfilled and there won't be any filtered particles.
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This field becomes active when “Condition” is set to “Expression”. RealFlow constantly checks against the given expression during a simulation and decides whether the condition is fulfilled or not. There is a long → list → list of keywords that can be used to create a certain condition.
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This option is suited for the creation of trails, for example. It should only be used with → "Dumb" particles or "Override Target" set to "Yes". Otherwise you might see instabilities and particles explosions.
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