In dimensionless numbers in fluid mechanics, the Reynolds number is the inertial force divided by the viscous force, while the Peclet number is the thermal force divided by the inertial force.
Reynolds numbers are an important dimensionless quantity in fluid mechanics. The Reynolds number is the ratio of inertial forces to viscous forces within a fluid which is subjected to relative internal movement due to different fluid velocities. The Reynolds number measures the ratio of advection and diffusion effects on structures in the velocity field, and is therefore closely related to Peclet numbers, which measure the ratio of these effects on other fields carried by the flow, for example temperature and magnetic fields. 
