Drag Coefficient Convergence Problem

John

I am modeling a Tesla turbine with Fluent.

The tesla turbine has rotating disks inside and the disks rotate by drag force of injected air.

I have problem with drag coefficient convergence: the drag coefficient fluctuates a lot.

I guess two explanations for that situation.

1. Size of the turbine is too big (>million mesh)

2. The drag coefficient can fluctuates under some selected options.

Q1. I think first explanation looks more plausible. I am wondering if second explanation is possible.

Though the drag coefficient fluctuates, pressure and velocity distribution and power output are converged.

Q2. I am wondering if I can use the power and the pressure distribution though the drag coefficient fluctuates.

I appreciate your favors.