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Thread: Aerodynamics of the new Mirage (0.27-0.31 drag coefficient varies with options)

  1. #71
    Optimal VG placement depends on the depth of the boundary layer, height & angle of the VG's. You place them close to the point of separation, but far enough ahead of it that the top of the VG can mix the faster flowing air in the upper part of the layer with the slower bottom layer. You've probably seen this: Mitsu wrote an entire research paper on the size & placement of VG's on the EVO's roof...

    http://mirageforum.com/downloads/16E_03.pdf

    You can see why getting it right is hard to guess at.

    ---

    If flow is actually separated on the 2012-2015 Mirage's rear spoiler, then VG's might help a tiny amount... if they're placed right.

    Changing the angle of the spoiler itself would be better since it avoids the drag penalty of generating the vortices in the first place, which are really just a band-aid.

    I'd love to see a tuft test of the '12-15 spoiler.


        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


  2. #72
    That's not necessarily true about flow characteristics being the same at all speeds...
    Not at "all speeds", but at typical road vehicle speeds.

    Quote Originally Posted by BRagland View Post
    In fact, at around 65 MPH, I would expect flow separation from our spoiler to become almost an inherent issue.
    But again, this implies that Cd is changing with speed, and that's something you never see in discussion of road vehicles at normal road speeds.

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


  3. #73
    Moderator inuvik's Avatar
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    Me thinks it's probably far simpler to bolt a 2017 spoiler on the current gen Mirage.

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage ES 1.2 automatic: 40.5 mpg (US) ... 17.2 km/L ... 5.8 L/100 km ... 48.6 mpg (Imp)


  4. #74
    I'll see if I can dig up the thread on this topic at EcoModder... Maybe I've got it all wrong.

    EDIT... found it: http://ecomodder.com/forum/showthread.php/drag-coefficient-really-constant-24813.html

    My "10 MPH" lower end example was wrong (it's 20), but the general point stands:

    *From 20-mph,to about 250-mph,compressibility effects are non-existent and the drag coefficient is constant and stable within this velocity regime.
    Source is HUCHO, Aerodynamics of Road Vehicles.

    There's more related info in that thread.

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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    cyclopathic (12-08-2015)

  6. #75
    Quote Originally Posted by inuvik View Post
    Me thinks it's probably far simpler to bolt a 2017 spoiler on the current gen Mirage.
    Yep.

    Raise the current one to match its angle, and fill the gaps on the sides that creates. More cost effective.

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


  7. #76
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    Quote Originally Posted by MetroMPG View Post
    Not at "all speeds", but at typical road vehicle speeds.



    But again, this implies that Cd is changing with speed, and that's something you never see in discussion of road vehicles at normal road speeds.
    Cd is also a fudge number, which is actually averaged at various speeds in wind tunnels. Though it can be calculated, a lot of that published number takes into consideration data from actual simulations or real-world air-tunnels in addition to various factors in a calculation, and hence-forth, comes our number.

    Cross-winds, which are so rarely talked about, can have a huge impact on when and where flow separation occurs. So can heading into a head-wind, where your relative velocity may be different from the airflow's. Heading into even a modest head-wind at ~10 MPH going 70 MPH on a US Interstate, means our actual airflow is nearing ~ 80 MPH. Of course, the opposite is true too with a tail wind.

    I'm not going to go out on a limb and say the Cd is changing, though it could be changing at levels so minuscule it doesn't matter here, or in any published numbers.

    With the way our spoilers are designed, separation is possible. Even if it isn't, there's a much better reason they reduce the angle other than "lift" as generally, if we are going to provide that spoiler with lift, it would actually help the vehicle's MPG by reducing weight on those tires, and subsequently, rolling resistance. Kind of like faking a lighter weight vehicle.

    The reason spoilers are added to performance vehicles is to reduce lift, and push the vehicle's rear wheel downwards to enhance stability, handling, and traction. It costs some MPG too. We would actually benefit MPG wise from lift here.

    The lift would be small anyways. So, reducing that lift is not the reason they reduced the angle.

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage ES 1.2 automatic: 51.7 mpg (US) ... 22.0 km/L ... 4.6 L/100 km ... 62.0 mpg (Imp)


  8. #77
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    Quote Originally Posted by Mitz View Post
    So, VG's would be better placed on the roof than at the spoiler as cyclopathic did?
    I was thinking to place Airtabs on the spoilers, at the body side rear, and substitute them as rear tire air dams.
    Mine are actually placed on the roof. 1st piece you see in the picture is spoiler, and 2nd crack separates rear hatch door from roof.

    The VGs are angled to create a 45deg cone, and when you extend it you want to have 100% coverage for the bend point in profile. From distance btw VGs it works out ~1-1.5' from the edge in C. Since in Mirage the bend point is at rear hatch door/roof, it looks like more or less "optimum" location would be at rear antenna (not sure if you have antenna in the same spot)

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage DE 1.2 manual: 46.4 mpg (US) ... 19.7 km/L ... 5.1 L/100 km ... 55.7 mpg (Imp)


  9. #78
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    Quote Originally Posted by MetroMPG View Post
    If flow is actually separated on the 2012-2015 Mirage's rear spoiler, then VG's might help a tiny amount... if they're placed right.

    Changing the angle of the spoiler itself would be better since it avoids the drag penalty of generating the vortices in the first place, which are really just a band-aid.

    I'd love to see a tuft test of the '12-15 spoiler.
    The spoiler on C is located at right angle (actually at no angle) and after running a tank full I am seeing MPG increase from 56.6 to 58.4, which works out to 3% gain. This is over ~500mi, and the temperatures has dropped comparing to what we had prior to Thanksgiving.

    I had no expectations for VGs, so while this is encouraging there is no baseline to compare. C keeps MPG for 3 previous month, so in a week or two I should have enough data to compare month to month, but then the temperature drop would skew results.

    I have a set of VGs coming for Mirage, so I will do the tests prior and after install using the spreadsheet.

    Agree VGs are a kludge, a band-aid, but very useful one at that. I do not want to significantly alter looks, so building kammback tail is out of question. If VGs improve MPG by even 1%, than with 20,000mi/annual with 40mpg, we are saving 5gal of gas, which works out to ~$10, about as much as I paid for the set of VGs.

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage DE 1.2 manual: 46.4 mpg (US) ... 19.7 km/L ... 5.1 L/100 km ... 55.7 mpg (Imp)


  10. #79
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    Quote Originally Posted by MetroMPG View Post
    Yep.

    Raise the current one to match its angle, and fill the gaps on the sides that creates. More cost effective.
    That if you can get it for $10 from junk yard

    Quote Originally Posted by BRagland View Post
    The reason spoilers are added to performance vehicles is to reduce lift, and push the vehicle's rear wheel downwards to enhance stability, handling, and traction. It costs some MPG too. We would actually benefit MPG wise from lift here.

    The lift would be small anyways. So, reducing that lift is not the reason they reduced the angle.
    ACR Viper has Cd = 0.54(!) most of it to create downforce

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage DE 1.2 manual: 46.4 mpg (US) ... 19.7 km/L ... 5.1 L/100 km ... 55.7 mpg (Imp)


  11. #80
    Senior Member Mitz's Avatar
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    Quote Originally Posted by cyclopathic View Post
    Mine are actually placed on the roof. 1st piece you see in the picture is spoiler, and 2nd crack separates rear hatch door from roof.

    The VGs are angled to create a 45deg cone, and when you extend it you want to have 100% coverage for the bend point in profile. From distance btw VGs it works out ~1-1.5' from the edge in C. Since in Mirage the bend point is at rear hatch door/roof, it looks like more or less "optimum" location would be at rear antenna (not sure if you have antenna in the same spot)
    This is what I have in mind and intend to do..
    Name:  VG's proposed location.jpg
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