Thank you for replying Federico.

I am trying to replicate the reuslts reported in a scientific paper, so the geometry of the channel sticks to that.

In reality, in the paper are not reported precise dimensions about the farfield, so I chose those by myself. Probably it will be more clear in this pic:

So you are suggesting to increase the size of the farfield?

I set pressure outlet 1 as a pressure outlet with half of the pressure I imposed for the pressure farfield bc. I did this since the flow coming from the pressure farfield is tilted of -10[deg], so in order to avoid the flow to be accumuleted I set pressure outlet as bc for those two edges. 

For pressure outlet 2, here is where I set the back-pressure, zero for this specific case.

Finally, something I forgot in the previous post is that when I computed the solution in Fluent, I also got this message: “Reversed flow on 1 face (0.2% area) of pressure-outlet 10”.

I hope to hearing from you soon.