# 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?

• It makes a difference when studying ground effect, which for that a conveyor is added below the model to simulate the moving ground. Can you clarify what you mean by "wouldn't stationary/semi wind inevitably fill the tunnel after collision"?
– ymb1
Jan 4 '20 at 18:40
• if fluid is moving in a large tube and you put a box in it's center would you expect it's back to be vacuum forever, b/c any fluid that goes behind the box wouldn't leave. but normally if an object is moving at a high speed depending on it's shape there should a vacuum at it's rear. Jan 4 '20 at 19:03

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.

• not to be rude but i understand your still puzzled by it but given up. hopefully my intuition will be back on track soon, wouldn't wanna argue with myself every time i remember it. Jan 5 '20 at 20:03