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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...
Air pressure is the force exerted by the weight of air molecules in the Earth's atmosphere. It is measured in units of pressure, such as pounds per square inch (psi) or pascals (Pa).
As a car moves forward at 60 miles per hour (mph), it creates a disturbance in the surrounding air, pushing air molecules in front of it and creating an area of higher air pressure. This is known as the "bow wave" effect. At the same time, the air behind the car experiences a decrease in pressure, creating a low-pressure area known as the "wake". These changes in air pressure are caused by the car's movement and the resulting friction with the air.
Yes, air pressure is directly affected by the speed of an object moving through it. As the speed increases, the air molecules are pushed closer together, resulting in an increase in air pressure. This is why air pressure is higher in front of a moving car and lower behind it.
Air pressure can have both positive and negative effects on a moving vehicle. On one hand, it can provide a force that helps the car move forward, known as aerodynamic lift. On the other hand, it can also create drag, which can slow the car down and decrease fuel efficiency. Properly designed vehicles take into account these effects of air pressure to optimize performance.
Yes, air pressure can affect tire pressure while driving at 60mph. As the car moves, the air inside the tires also moves and can experience changes in pressure due to the car's speed and the surrounding air pressure. It is important to regularly check and adjust tire pressure to ensure safe and efficient driving.