How can I time velocity flow-field input to be after particle creation?

I'm studying fluidized bed behavior, and I'd like it so that I can start the inlet gas to only activate after particle creation. Particle creation is partly limited to time because of collisions with other particles, geometries, etc. so how can I go about this?

I hinged a lot of my project on the CFD-DEM coupling method used in this windshifter tutorial.