pook wrote:
If I place that ball in a tube, remove the air from above the ball it will rise up the tube correct?
Correct.
pook wrote:
Is the pressure to lift the ball coming from inside the ball or under the ball or both?
Under. Consider that changing the pressure inside the ball will not change what happens to the ball in this scenario...Not counting mass/density/buoyancy effects.
pook wrote:
Now lets view the car with tunnel/diffuser in profile. The air under is accelerated, lift is created (down force in our case) is that down force coming from the outer skin of the car (roof, hood, trunk) or the top side of the tunnel?
Both, depending on your frame of reference. Globally it's coming from the outer skin of the car. Locally, at the tunnel mounts, it's coming from the top side of the tunnel.
Maybe it would help to put some numbers to this. All units are in gage pressure, which is relative to atmospheric pressure. So 0psi is atmospheric. Let's say you have two identical cars. The driver in Car A has the HVAC on low, creating a cockpit pressure of +1 psi. The driver in Car B has the HVAC on ludicrous, creating a cockpit pressure of +5 psi. Now lets say that the top surface of the car has an average pressure of +1 psi, while the lower surface of the car has an average pressure of -1 psi.
The floor pan of Car A sees a pressure differential of (-1 - +1 = -1-1 = -2) -2 psi, which means the floor is experiencing 2 psi in the downward direction. The roof of Car A sees a pressure differential of (+1 - +1 = 1-1 = 0) 0 psi, which means the roof is experiencing no directional pressure. Now notice that if you add the floor and roof effects you get (-2 + 0 = -2) -2 psi, or 2 psi in the downward direction...Which just so happens to be exactly the same as if you only look at the pressure below and above the car, as if it were filled with foam, which works out as (-1 - +1 = -1-1 = -2). Now is this a coincidence because of everything being only +1 or -1? Let's take a look at car 2 to find out.
If we look at the air above and below the car only for Car B, it is exactly the same math as when we did so for car A. But breaking it down into upper and lower surfaces, rather than the foam filled car, here is how it works out. The floor pan of Car B sees a pressure differential of (-1 - +5 = -6) -6 psi, which means the floor is experiencing 6 psi in the downward direction. The roof of Car B sees a pressure differential of (+5 - +1 = 5-1 = 4) 4 psi, which means the roof is experiencing 4 psi in the
upward direction. Adding the floor and roof effects, you get (-6 + +4 = -6+4 = -2)
So no, it was not a coincidence. The overall (global) effect on the car as a whole is the same no matter what the pressure is inside, because the pressure inside is exerting an equal an opposite force to the roof as it is the floor.
However, also note that the (local) floor pan in Car A only needs to support a 2 psi of pressure. Meanwhile, even though Car B has the same total downforce, the floor pan must support 6 psi of pressure, and therefore must be able to support 3x the load as that of Car A.
Your tunnel mounts are analogous to the floor pan in my example.
pook wrote:
If it's the top side of the tunnel then pretend I fill the interior and every cavity with foam? Does it still generate lift or stop?
As noted above, yes.
pook wrote:
The original question is if a hypothetical tunnel/diffuser can create 500lbs of down force at X speed, does the support apparatus need to be able to hold the 500lbs or is some of that 500lb placed on the body of the car?
If you mean that the tunnel itself creates 500 lbs of downforce relative to the air on the other side of it, yes that is what the mounts need to support...But if you mean the car as a whole simply makes 500 pounds of downforce, that does not provide enough information to determine how much load the tunnel mounts alone are supporting.