[Santos-Dumont]

Originally Posted by sajad

Hi, Santos
Thanks for your reply,

i just don't understand why we should use M=0.8 for far field Mach number. far from the rotor, air is in stagnation so it's velocity and Mach number is zero. where is my mistake?

Your mistake is a confusion between real life and simulation. In real life the air is considered stagnant and the rotor moves through it. In simulation or in a wind tunnel the simulated object is fixed in space and the air moves around it. This changes nothing for the result and it makes everything much simpler !
It's all about referential.

Think of it this way, if you put a GoPro on an aircraft wing. From your point of view the wing will not move while the air will be flowing towards the wing. That is the case you want to simulate with CFD. (like this : http://youtu.be/0TNV2xRiTK4)

Simulating real life case is possible if you use a boundary condition with zero velocity and make your object move in the domain with variable mesh. This have zero interest obviously.