tpn

I found Bernouilli's equation that you need to bring into the example. Torque deals with how the object gains velocity, drag applies to how much potential velocity is lost. But you need to take into account the fact that air exerts its own force against an object no matter which is in motion.

Air density is altitude and temp dependent and Static Pressure is the ambient pressure for a given density. I believe that it also assumes that only one or the other (air/object) are in motion.

Static Pressure + Air Density x Velocity squared/2 = Total Pressure
Ps + r * [V^2/2] = Pt

a variation of this equation is used to calculate air speed by solving for Velocity with a pilots tube which measures the total pressure against a known air density and known static pressure thus giving air speed
V^2 = [2 * {pt - ps}] / r

Here is a link that will explain it better
Pilots Tube

This will only give you the pressure at the point of contact. Does not take slip-stream, drag, lift, turbulence or any other aerodynamic variables into account. That's why a 150 mph wind exerts enough force to rip up trees, considerably more force than .5 psi. The same force a car faces at 150 mph +/- all the variables.

All this by no means means that my numbers were exact. I still accept the fact that they are likely squed. As I said before the device we used was not designed to measure the very complicated aerodynamics that go on under the hood. It was only an attempt to try and roughly quantify what was happening with the 2 CAI's we were playing with.

s2oooboy

.....and a bunch of jargon later...(jk,,,,i know what your saying ).....tpn wins...LOL
u had me sold at "Hello" .....or was it "Bernouilli?" LOLOL

ultimate lurker

Actually, s2oooboy, if you or tpn had bothered to actually run the numbers you'd see tpn just proved me right.

Allow me to demonstrate.

Since we are looking for pressure differentials due to speed, we can eliminate the Pstatic term. It will be the same whether we are at rest or travelling at 150 mph. That leaves us with the equation:

1/2 * r * v^2 = P

Referencing our handy physics textbook, we can find that dry air at 20 C (68 F) has a density of 1.29 kg/m^3. Density will of course decrease with increasing temperatures.

V = 150 mph = 220 ft/sec = 67.1 m/sec

Thus

P = 0.5 * 1.29 * 4502.4 = 2904 Pa (The unit of a Pascal is equal to N/m^2, a Newton is 1 kgm/sec^2, so the units for a Pascal are kg/ms^2 which fits with the terms we arrive at).

Now, to convert Pa to psi, 1 Pa = 0.000145 psi so,

P = 2904 Pa * 0.000145 psi/Pa = 0.421 psi

Not bad considering I totally threw out tire drag in my back of the envelope calculation knowing that if anything it would elevate my pressure estimate. I like to solve problems in simple terms and I didn't even have to whip out my physics book to get within 20% of the real number. Now that you've made me get it out, I expect you to come organize my garage :-).

tpn, as for why a 150 mph wind can rip a tree out of the ground, it has to do with surface area. How much surface area does a 30 ft tall tree have in square inches? Multiply that by 1/2 psi and see how many tons of force are created.

UL

tpn

Damn impressive........ you the man UL

I dont know how organized your garage would be after I got thru with it, heck I can't find anything in my own now

ultimate lurker

My only concession to garage organization is to make sure anything near the cars is soft enough not to hurt them if it falls over :-)

If you want some really good data on ram air, try and find a copy/reprint/etc. of the Oct99 and Dec99 issues of Sport Rider magazine. They test a number of bikes by instrumenting the airboxes with a Pi Research datalogger and pressure sensor, and then go ride them up to max speed (160-190 mph). They then went back and duplicated those airbox pressures on the dyno (with some serious air compressors - fans won't do). They found that on average a ram air system was good for 3-5% gains in power, and some started making differences as low as 60-70 mph. While the maximum pressure they recorded was only about 0.35 psi (at 180 mph), you have to take into account that most bikes, when run on the dyno with still air, were recording airbox vacuums of 0.3 psi or so. Thus, even if you get back to ambient, you can make a big difference.

UL