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Drag

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by NiMH, Mar 11, 2005.

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  1. Frank Hudon

    Frank Hudon Senior Member

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    lift? try this, a full load in a Cessna 150 on a 95:95 day and see how much runway you need to get airborne, try again on 0:5 day and you will see that they are airborne in short order. warm humid air is very thin. Ask the guy who almost took out the runway lights at the far end one afternoon.
     
    #21 Frank Hudon, Mar 17, 2005
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  2. hyo silver

    hyo silver Awaaaaay

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    I was trying to avoid all the hot air about humidity. I was trying to steer the conversation back to lift and the Prius, thinking it might have something to do with the dangerous handling some people have reported on the highway. Yes, I know it's dangerous to handle people on the highway. Pull over and practice your parking instead. It's a very roomy car, don't you know.
     
    #22 hyo silver, Mar 17, 2005
  3. drash

    drash Senior Member

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    Nice tutorial, jeepien, thanks. Learn something new everyday (slaps hand on forehead).
     
    #23 drash, Mar 18, 2005
  4. NiMH

    NiMH New Member

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    Hmmm...as far as lift is concerned we might need somebody with a large wind tunnel to start a new string on that topic.

    Drag is a pretty straightforward formula. Lift is a consequence of the first law of thermodynamics (energy can neither be created nor destroyed) -- in this case a velocity is imparted on the air travelling on one side of the airfoil (in order to cover the greater distance caused by the curvature) which requires higher kinetic energy and results in lowered potential energy through a pressure drop. In short, faster moving fluids have a lower static pressure.

    So how much lift is generated by the air flowing over the top of the Prius? I have no idea -- a physical model of the car shape and airflow and a bunch of integrating would be needed to calculate such a thing. Formula 1 cars definitely have to worry a lot about lift and go to great lengths to keep the car on the ground -- this is why they also have very poor coefficients of drag.

    But for all you fellow armchair physicists, here are some things to think about (scoff at, or whatever):
    - A key to generating good lift is to have laminar (smooth) air flow across the surface as opposed to turbulent flow. This is why flatter roofs are more likely to get lifted off in a strong wind than steep roofs - laminar flow and lift force. Laminar flow is also key to reducing drag. hmm...
    - A good airfoil has a flat, straight surface on the underside of the airfoil to keep the air velocity as static as possible on the underside versus a large arc (as long as it isn't too large to disturb the laminar flow) on the top. hmm....

    I noticed some posts about the annoying percussion that happens when one window of the car is opened, and I think this is probably also a consequence of having nice laminar flow around the car. Most likely the air flow creates a pressure drop next to the window which drops the pressure inside the car until that pressure drop on the inside breaks the laminar flow on the outside resulting in a pressure rise again until the cycle repeats - causing that sensation that your head is inside a bass drum. Opening another window breaks this "oscillation" because the air flows through the car instead of compressing and decompressing the passenger compartment.

    Hyo Silver could have a point about the instability on the highway -- smooth air flowing around the car is great for reducing drag, but if the flow is disturbed on one side (wind, car turning, etc.) the car could get "sucked" toward the opposite side. I think it stands to reason that an "aerodynamic" car can also be an unstable car.
     
    #24 NiMH, Mar 24, 2005
  5. Dr.Jay

    Dr.Jay New Member

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    I'm gonna be sorry I mentioned this but. . .
    Hey DanMan32, you should get better mileage on light rainy days.
    It lowers the horsepower some, but a small amount of 'Steam' (water vapor) injected into the air intake increases MPG.
    There are devices (mostly sold in Europe) that consist of a copper tube wrapped around the exhaust manifold, a water supply and a liquid/gas separator that is connected to the vacuum port on the air intake.
    The copper tube heats the water to boiling and the steam is sucked into the air stream by the engine vacuum.
    The liquid/gas separator keeps the water from getting 'sucked' into the cylinders until it has been heated to boiling.
    The water supply is just a radiator 'over-flow' bottle.
    Only problem is you have to keep the bottle filled with water or you loose the engine vacuum.
    I hooked it up on some older cars I owned and it actually works.
    Engine ran smoother and 2-3 MPG increase. Pick-Up of car decreased though.
     
    #25 Dr.Jay, Mar 26, 2005
  6. Ray Moore

    Ray Moore Active Member

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    Thank you Gary. It was frustrating to watch this old argument played out once again. Your explanation was good. There were also some good empirical examples of the phenomena. Here is one more. Humid air rises until it reaches an air mass that is cold enough to cause it to condense. That's why the clouds are up there instead of down here.

    As to engine performance, water vapor inhibits combustion and displaces oxygen. This leads to incomplete combustion, loss of power, and lazy piston syndrome. OK, I'm done.

    Please forgive my posts while I'm taking pain medication. They could be more nutty than usual.
     
    #26 Ray Moore, Mar 27, 2005
  7. DanMan32

    DanMan32 Senior Member

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    Foggy or humid days, I do get better mileage, but on rainy days, usually lower probably due to increased rolling resistance.
     
    #27 DanMan32, Mar 28, 2005
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  8. KokomoKid

    KokomoKid Member

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    Humid air is less dense, so there would be less drag.

    Posted via the PriusChat mobile app.
     
    #28 KokomoKid, Sep 26, 2018
  9. KokomoKid

    KokomoKid Member

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    Humid air is less dense, because it has more water H2O, which is about half the atomic weight of N2 and O2 which make up most of "dry" air.

    Posted via the PriusChat mobile app.
     
    #29 KokomoKid, Sep 26, 2018
  10. KokomoKid

    KokomoKid Member

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    Water has a molecular weight of 18. O2 has a molecular weight of 32. N2 has a molecular weight of 28. When you have a lot of gaseous water in the air, it reduces the density sunstantially.

    Posted via the PriusChat mobile app.
     
    #30 KokomoKid, Sep 26, 2018
  11. KokomoKid

    KokomoKid Member

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    You meant moist air is less dense than dry air.

    Posted via the PriusChat mobile app.
     
    #31 KokomoKid, Sep 26, 2018
  12. KokomoKid

    KokomoKid Member

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    If the pavement is actually wet, the extra drag from "moving" the water might more than cancel out the lower aero drag, but with dry pavement, higher humidity gives better mpg. This is all dependent on properly working closed loop fuel systems.

    Posted via the PriusChat mobile app.
     
    #32 KokomoKid, Sep 26, 2018
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