i am working on a simulation for 2 gases injecting at different time in a tube. I have set the inlet velocity at one opening and another opening as atmosphere pressure. 

1) Helium at the inlet and the velocity is 0.00371 (m/s) for 2 minutes

2) And then, blow in Methane at the inlet and the velocity is 0.00371 (m/s) for another 2 minutes

I wanted to check with the interaction of helium and methane at time dependent. How can i set that up?

Best Regards,

Timothy