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.