sand ripples simulation

Hi folks,

I am trying to simulate the formation and motion of sand ripples ( sand particles driven with wind). I think that this process may be considered as a two phase flow or a flow of a mixture of sand and air. Specifically, I need to simulate the collision of particles. I have tried particle tracing modules, but the particles do not hit each other- no collision is occurerred. I am also trying the Mixture Model, Laminar Flow Module, and just getting some errors.

I appreciate it if you could share with me your brilliant ideas to help me through the process.

Cheers,
Shayan