In mud (or sand) the resistance of the fluid described by the parameters above will stop you and so keep you from sinking. But for substances like water it isn't the drag that stops you from sinking. Its buoyancy. This parameter is here so as to keep you from sinking when you wish to simulate fluids with low drag (resistance). For fluids like mud or sand you can put it at 0, but it is best to keep it at some minimum value. For fluids with behavior index >=1 it will behave like you are in water. For fluids with behavior index <1 it'll behave like you are in mud.
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