david66ad
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If I place a 5' L x 12" W open box at front and a 1/8" hole at the back of the box and drove my car at 60 mph how much psi would I generate at the 1/8" hole?
If I replaced the box with 5' diameter funnel down to a 1/8" spout, what would the outgoing PSI be from the spout?erobz said:It's probably difficult to say without determining the drag coefficient of the box. A small hole like that it isn't going to alter the drag much.
I think you would basically have:
$$ P = \frac 1 2 C_D \rho_{air} v^2 $$
It would be the same. A pressure is independent of area.david66ad said:If I replaced the box with 5' diameter funnel down to a 1/8" spout, what would the outgoing PSI be from the spout?
I don't know. Others are saying the pressure is independent of the shape, size, so the drag coefficient is irrelevant. So, it would seem the same as the box. I'm still a bit suspicious to what degree of an approximation that is.david66ad said:If I replaced the box with 5' diameter funnel down to a 1/8" spout, what would the outgoing PSI be from the spout?
Stagnation pressure is essentially the maximum drag, or a 1.0 drag coefficient. Can't do better than that, and it isn't hard to achieve. That's the point of a pito-static tube for airspeed measurement.erobz said:Its interesting that it isn't dependent on the drag coefficient? The drag force varies with it. The drag coefficient encapsulates form drag and pressure drag. I guess the pressure drag portion ( which is what is of interest here) must be independent of the form drag.
The only problem I'm seeing that optimization is apparently automatic independent of shape?russ_watters said:Stagnation pressure is essentially the maximum drag, or a 1.0 drag coefficient. Can't do better than that, and it isn't hard to achieve. That's the point of a pito-static tube for airspeed measurement.
The OP appears to be under the common but false belief that you can do better with a funnel.
I don't understand what you mean.erobz said:The only problem I'm seeing that optimization is apparently automatic?
You can't alter the shape to reduce the pressure. Its apparently automatically maximized.russ_watters said:I don't understand what you mean.
Yes. The key feature is just a featureless hole. There's not much that can be done to make it better or worse.erobz said:You can't alter the shape to reduce the pressure. Its apparently automatically maximized.
The cone and the box will have very different coefficients of drag, but the pressure acting on them is the same; independent of the drag coefficient.
The pressure acting at the stagnation point is the same, yes. The overall pressure distribution (and of course the overall drag) is going to highly depend on the shape of course.erobz said:You can't alter the shape to reduce the pressure. Its apparently automatically maximized.
The cone and the box will have very different coefficients of drag, but the pressure acting on them is the same; independent of the drag coefficient.
[Google] I stand corrected!cjl said:Also, interestingly, it is possible for drag coefficient to exceed 1...