Can anyone say for what type of problems we need dispersion? I visualise for example for a combustion problem where we mix fuel with air then we have a chemical reaction and a change in state the dispersion make sense but for many other applications may be not/ Can somebody show a list where dispersion is a must and another one where dispersion coould be neglected? As you know the dispersion is a random function associated with position in time of a particle trajectory.

You mean diffusion?

No is not diffusion -is something aftecting the mixing is associated with the trajectory- making the fluid to mix-the trajectories to mix is a random function associated with each position doing a variation around x,y,z position-Fluent did it by adding this forced random function therefore not resulting from the basic equations/