Diffusivity Used for kinematic viscosity (also known as momentum diffusivity) and thermal diffusivity. The Kinematic Viscosity of a fluid is the dynamic viscosity divided by the fluid density. In heat transfer analysis, thermal diffusivity (usually denoted ? but a, ?, and D are also used) is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy. Fluid Mechanics new for ODM2 Encompases both kinematic viscosity and thermal diffusivity ODM2 Units Type Controlled Vocabulary ODM2 Working Group A vocabulary for describing the type of the Unit or the more general quantity that the Unit represents.