The Courant number is used in the numerical flow simulation for the discretization of time dependent partial differential equations:

 

 

Here Co is the Courant number, v the velocity, the discrete time step and L is the longest dimension of an element in the flow direction.

Since the upper limit for L is already given by the Peclet number and usually is relatively small, so the time step size has to be chosen sufficiently small depending on the velocity in order to fulfil the Courant criterion.

The following figure shows the influence of the Courant number on the calculated concentration distribution in otherwise identical model parameters:

Influence of the Courant number