my brick is still in the garage so my remarks are theoretical because i
don't know if mine will overheat or not. i do know abour sailing and
what makes a sailboat sail "upwind". when the sail assumes shape, the
air traveling around the outside has to go further than the air
traveling across the chord. the air on both sides have to meet at the
leech (back) of the sail so the air going around the outside has to
travel further in the same time so it must go faster. the faster the
air moves, the lower the pressure so a pressure differential is
created. the sail on a boat sailing "upwind" is actually being sucked
upwind.
now if we look at the air flow over the hood of a car, i would say that
the air moving over the hood and windshield is moving considerably
faster than the air moving through the engine compartment. that means
that the pressure on the outside is less than the pressure under the
hood. my guess is that the pressure would be less on the top in a wind
tunnel test even if only the hood was used rather than the whole car.
the curvature of the hood would make it act like a sail.
i would be amazed if air goes into the slot at the windshield. i don't
think you could make it do that at high speeds even with a big fan
blowing air down the windshiels from above. there are two simple
experiments to try to get the picture.
take a piece of paper (regular 81/2 x 11) and hold one edge up to your
chin just below your mouth. when you blow air out of your mouth, the
paper will rise. you should be able to get the paper to go totally
horizontal. if it tries to go higher, different forces come into play.
the reason the paper rises is because when you blow, the air is moving
over the top of the paper and that decreases the pressure on the top of
the paper. if no air is being blown over the bottom, the bottom is at
atmospheric pressure. since the atmospheric pressure is higher, the
paper rises.
the other experiment is more direct. you can create and approximation
of a wind tunnel by driving your car. the calmer the day windwise, the
better because all the forces will be a result of the car moving through
the air. tape some string to the back edge of the hood so it can stream
back over the slot to the windshield. maybe three or four over the
width of the hood. now go for a ride and watch the strings. if they
lift up, there is a the air is being sucked out of the engine
compartment. if they get pulled down into the engine compartment, there
is a high pressure area forcing air into the compartment. i think we
know what will happen because this low pressure area on the top of the
hood is what tries to lift the hood if it is not latched.
i just wish i could drive my brick through the air.
bob #319
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