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Aero and Thermal dynamics

Discussion in 'Body / Exterior / Interior / Cosmetics' started by K2e2vin, Apr 30, 2005.

  1. K2e2vin

    K2e2vin Senior Member

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    i read a few articles and stuff and noticed that most OEM cars have the hot air from the radiator going under the car, and some cars with aftermarket parts have it going over the car(mainly RX-7s). well, according to bernoulli's law, wouldnt the the car with air going over the car suffer from increased lift? this also brings up does the "fluid" temp effect bernoulli's law? like having high-velocity hot air go over the car is better than vs. hot air under the car.
     
  2. jamesA

    jamesA Well known pissed off telephone guy VIP

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    All I know is you want faster moving air moving under the car to help create downforce. Of course the underside of street cars aren't very aerodynamic at all, so it really makes no difference.
     
  3. MikeBergy

    MikeBergy Blah blah blah....

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    The underbodies of cars are not in any way aerodynamic, unless you own a ferrari, or some supercar not attainable by ordinary human beings. Contrary to some peoples thoughts, the upper sides of 99% of the cars out there do not in anyway produce any downforce or lift in comparison to the weight of the car, unless you are going REALLY fast. Go to the Stupid Video website, and look for a video called like "215mph" or something like that. A near stock body fd3s rx7 gets lifted off the ground and thrown around like crazy at the salt flats. I have heard of old vw bugs lifting off the ground at high speeds as well, like over 100mph. As for your reference to bernoulli, that only applies to 1-d, inviscid flow; not relevant to what you are talking about. As for hot air coming off the radiator, the heating of the air going through the radiator is negligible to the amount of mass flow flowing underneath the car. I highly doubt that the air coming out of the radiator heats the ambient conditions over or under the car even 2-3 degrees R. Not a significant heat source. If you have any fun aero questions feel free to shoot them my way. I'll be graduating in December with a BS in aero engineering, and I'm sure I can answer anything you might need to know.

    Edit: and it's thermOdynamics (one word) if you want to be correct in referring to it. :)
     
  4. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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  5. K2e2vin

    K2e2vin Senior Member

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    yea i know its thermaldynamics but i seperated it because i was referrering to aerodynamics and thermaldynamics.

    so having more air go over the car wouldnt really matter? im just curious because you see lots of road-based race cars have that setup(hot air venting over the car), but in those articles about the use of spoilers and such they talk about how the high velocity air over the car induces lift.

    btw; the 350z has a under-body diffuser or something and they said it creates zero lift, so that means without the under-tray that there is lift?
     
  6. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Venting the hot air out over the hood doesn't have anything to do with reducing lift... it's just the easiest way to get the hot air away from the engine. Like Mike said, it's nowhere near a significant amount of airflow- unless the engine bay has a duct going through it from front to rear, and in that case, the duct is usually shaped to not generate lift.

    On cars with an underbody diffuser like the 350Z- they're generally there to reduce lift. On almost all production cars, there is a good bit of lift at speed- it's nothing that will really screw you in daily driving, but on the race track at the limits of adhesion it matters. On cars that might see track duty, it makes sense to try to minimize lift or even try to generate downforce at speed in street trim, plus it's also good for advertising.

    :)
     
  7. MikeBergy

    MikeBergy Blah blah blah....

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    Like I said before thermOdynamics, not thermaldynamics, unless you are talking about volcanos or hot springs or some crap like that :p

    Any good suspension setup will take care of 90% of any float issues you might have at speed in any car. Period. If your car is creating so much lift at anything below 100mph that your steering feels light, I'd like to hear about it. Regarding the 350z underbody, that is nothing new, and ferrari does it a lot better. I'd like to see some comparisons between the car with and without the diffuser. If you did a test like that, I'd guarantee that the diffuser won't make a significant difference below probably 75mph. If it did, you'd see a lot of fun stuff called induced drag, translating into poor gas mileage. With lift, comes drag; you can't separate the two, unfortunately, so most aero packages are only meant to work where it counts the most - at really high speeds, where the pucker factor around a closing radius turn is enough to make you crap your pants. :)

    The reason racing applications throw the hot air from the top of the car as opposed to bottom, is because there is always a lower pressure above the car than below it, and this causes the air to be sucked up through the hood scoop, or grille, through the radiator, and out of the top of the hood, not because the hot air causes anything to happen.
     
  8. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    :thumbsup:

    Ferrari definitely does it a lot better than Nissan.
     
  9. n1mr0d

    n1mr0d Senior Member

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    No doubt! That's why Nissan makes cars and Ferrari makes race preped Automobiles.
     
  10. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    :werd:

    And for 5-10 times the price, they'd better.
     
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