is there difference b/w moving object in a fluid and static object in a moving fluid? wind tunnel is usually based on static object in a moving fluid, but wouldn't stationary/semi wind inevitably fill the tunnel after collision, but in the real world the moving object is always meeting a new particle and the previous ones always leave the field.

if indeed there is an actual difference wouldn't that render turbulence simulation impossible?, and also incorrect CFD simulations?.

since wind tunnel do exist there can't be any difference but why?
That's called Galilean invariance. There's no difference whatsoever between a steadily moving object in a stationary fluid, and a stationary object in a steadily moving fluid.

You seem to be thinking that in a wind tunnel, some air will always get trapped downstream of (that is, behind) the object; and that for an object flying outdoors, air will never get trapped behind (that is, downstream of) the object. Neither of those things is true.

I see that you're thinking about supersonic flight; you're saying that


it seems like if an object is flying supersonically, it will leave a vacuum in its wake, but
it also seems like if an object is stationary in a supersonic wind tunnel, it will not have a vacuum in its wake. 


I actually agree with your intuition here; I also think it seems like these two things will happen. 

But when my intuition contradicts a law of physics, it doesn't mean that the law of physics is wrong; it means that my intuition is wrong.