There are multiple refinement methods when generating tet volume meshes in Fluent Meshing. Some are legacy and the two most commun options to defines growth rates for tet meshes are :
– Geometric : used when a global growth rate is used everywhere and no Body Of Influence are needed. The growth rate is defined in the Tet panel. –
Size field : used when multiple growth rate are defined at different places of the model and if Body Of Influence are needed. The growth rate is defined in the Sizing (Scoped or Functions) panels.
For both the Refinement Method must be the default : adv-front . For those 2 options the growth rate is based on edge length, not on cell volume.