The simulation is swirling air flow going through a conical diffuser. Axial inlet speed of 40 m/s, solid body swirl of around 900 rad/s. I'm trying to get time averaged static pressure recovery of the diffuser.

And by "settle down", I was referring more numerically than physically. As in the max CFL for the domain has "settled down" to around certain value. I interpreted that as some part of the domain is changing to a more "steady state" value. In a similar vein to allowing several flow through times to allow the turbulence to "converge" onto a "steady state" time period.

And I think "shouldn't take too long" is a relative term". If I keep it at the same timestep, the simulation is going to take over a week on the cluster system I'm running it on. Compare that to ~12 hours without the velocity perturbations.