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Airflow vs. Inertia...


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#1 Boerseun

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Posted 14 October 2006 - 02:45 PM

An interesting thing my fellow Hypographers might help me out with...

Sitting by the side of the road in your car at 0km/h, and another car passes you, you can feel the car shaking. The faster the passing car, the more violent the shake to your car. This is particularly noticable when a big truck passes you. The only reason for this, is that there's a high-pressure area in front of the passing vehicle, "pushing" your car away, which is immediately followed by a low-pressure area "sucking" your car right back. This is all good and dandy. But why don't I notice it at all when my car is moving, as well?

I've been thinking about this, and one of the possible answers would seem to be that my car would resist any change in its attitude, resist any "rocking and rolling" of the bodywork because of my speed, where inertia would resist the bodywork rolling when passing another car.

But then I thought to myself that that couldn't be it, because there's no common frame of reference between my car and the passing car, and even at rest, the effects of inertia should be the same in dampening the motion from a passing car. So then I hypothesized that the only factor that could contribute to this would be that when at rest, air pressure is even around my car. When at speed, I have my own high-pressure cell area in front, and a low-pressure area in the rear. Could it be that the "pushing" from the front and the "pulling" from the rear would dampen any effects felt from passing cars, instead of it being inertia?

The other possibility, of course, would be that when at speed, I've effectively got four rotating gyros which would absorb any change in direction through the conservation of angular momentum. But that can't be it either, because all four rotating gyros are mounted on a flexible suspension sytem... So what's the deal, here?

Any airodynamics boffs have any ideas here?

#2 Turtle

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Posted 14 October 2006 - 05:31 PM

Any airodynamics boffs have any ideas here?


In a word, symmetry. When you are driving your vehicle the air flow is directed more-or-less symmetrically around the vehicle, wheras when you have it parked and another vehicle passes much of the turbulence strikes it from the side.
For a nice rock-n'-roll experience while driving, wind gusts validate your ticket.:cocktail:

#3 CraigD

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Posted 14 October 2006 - 06:26 PM

Sitting by the side of the road in your car at 0km/h, and another car passes you, you can feel the car shaking. The faster the passing car, the more violent the shake to your car. This is particularly noticable when a big truck passes you. The only reason for this, is that there's a high-pressure area in front of the passing vehicle, "pushing" your car away, which is immediately followed by a low-pressure area "sucking" your car right back.

Correct, I think.

This is all good and dandy. But why don't I notice it at all when my car is moving, as well?

I do notice this, particularly when passing at about 120 km/hr a big truck moving about 80 km/hr.

I think the reason a lateral force on your car is more noticeable for greater differences in speed when passing, is that the transition from high to normal to low-pressure areas occurs faster, analogous to someone physically pushing the side of your vehicle with a certain force either gradually, or quickly.

#4 HydrogenBond

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Posted 18 October 2006 - 11:39 AM

There are several factors; similar speeds will create similar pressures. But it also has to do with turbulence. If one has even driven behind a 18 wheeler, there is a distance one will be in their wash, i.e, turbulence. Most autos are designed for lamina flow of air to reduce turbulence causing less wash affect. Large trucks will also have lateral wash causing pressure differentials when they pass.

Although not recommend, an interesting experiment is to approach a 18 wheeler from the back and get close enough to be inside their wash. This will reduce the air pressure at the front of the car, causing gas mileage to increase.

#5 CraigD

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Posted 18 October 2006 - 12:24 PM

Although not recommend, an interesting experiment is to approach a 18 wheeler from the back and get close enough to be inside their wash. This will reduce the air pressure at the front of the car, causing gas mileage to increase.

I used this trick once to exceed 100 MPH in a stock 1800cc displacement 1974 VW bug, which in open air topped out at about 80.

I don’t think the driver of the truck was very happy about it.

Some 18-wheelers are capable of amazing speeds!

#6 Lord Hakk

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Posted 26 October 2006 - 09:16 AM

An interesting thing my fellow Hypographers might help me out with...

Sitting by the side of the road in your car at 0km/h, and another car passes you, you can feel the car shaking. The faster the passing car, the more violent the shake to your car. This is particularly noticable when a big truck passes you. The only reason for this, is that there's a high-pressure area in front of the passing vehicle, "pushing" your car away, which is immediately followed by a low-pressure area "sucking" your car right back. This is all good and dandy. But why don't I notice it at all when my car is moving, as well?

now, I dont know much of this stuff but maybe you are focusing on your driving rather than the cars passing you. when im in my parents vehicle i do notice if a big truck or a fast car passes by and it does shake the vehicle