Dear Zhangshi,
this charU enters the converter. It is only a discretisation parameter and has no influence on the settling of a sphere. You might want to look at the documentation of the converter in our user guide or the doxygen:
https://www.openlb.net/user-guide/
https://www.openlb.net/DoxyGen/html/d6/d92/classolb_1_1UnitConverter.html
Best regards
Robin