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Old 06-01-2023, 12:53 PM   #711 (permalink)
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Quote:
Originally Posted by aerohead View Post
If we have an 'expectation' of some realistic, isolated, drag-related performance enhancement, based upon some fluid mechanics mechanism, and then the observed performance happens to exceed what would be considered the upper limit, constrained by that specific mechanism itself, then it calls into question, either the viability of this chosen mechanism, our our interpretation of the observation attributed to it.
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I looked ahead on your posts, and picked up on the talk about 'temperature.'
I'm guessing that this is being associated with air density?, and it's relation to drag; and that, perhaps you felt that at around 65F-to-60F, the air would be dense enough not to allow you to see the mpgs indicated, for the extended trailing edges added to the car?
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If this jives with your thinking, then I'd want to offer some things to think about:
If you'll go online to 'Air Density Online,' and look at what their data requirements are for an air density calculation, in addition to the dry-bulb temperature, you'd also need to provide the:
* Raw barometric pressure reading
* Relative humidity ( RH )
* Elevation
* and technically, you'd also need to provide the latitude of the location where you're testing, as 'gravitational constant' is a myth ( you'll never see the accepted 32.2 feet per second/per second ) in either hemisphere on Earth, and local 'g', which is used in the air density calculation varies with latitude and should at least be recognized.
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So why is this important? Here's a bona fide example, courtesy of Volkswagen AG:
When Volkswagen took their Jetta Hybrid to the Bonneville International Speedway, in 2013, to set a land speed record, the 'air temperature' was 46-F ( which might imply 'dense' air ).
However, when the barometric pressure of 25.9-inches of mercury, 14% relative-humidity, and 4,236-foot elevation were factored together, we saw that the air density (rho) had fallen to 0.0020565 slugs/ ft-cubed, 13.5% lower than 'Standard Air.'
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I believe that this 'bookkeeping' complication is contributing to what you're experiencing.
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If you had someone riding shotgun during your road tests, they could be online looking at National Weather Service and NOAA sites, plus calling AWOS at nearby airports to get dry-bulb temp., Dew Point, Barometer, and whether it's 'rising' 'steady', or 'falling', altimeter setting, density altitude data, and wind intensity and direction.
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There's a half-dozen other things you're not accounting for as well. It's why car companies don't 'road test.' It's all done inside a computer.
It's tough not being a multi-millionaire.
It had nothing to do with temperature or air temperature. That day was actually the coldest day of the week, even colder since I went up to Seattle and itís about 10-15 degrees colder than Portland. The temps for that day testing were 65 like indicated. Most of my driving around Portland the last month has been in the 70s sometimes 80s. And my numbers never got that high.

I also took off the cavity fins halfway thru the trip at a rest area and continued my trip, my MPG dropped a few points. All within the same few hours of driving

So no, not air density related.

Iím doing another test tomorrow driving from Portland to Salt Lake City. Will see how they fins work for that higher speed drive ( made some new ones an inch or two longer and stiffer)

Iím assuming they reduce drag by giving a smaller wake, increasing base pressure at the rear, and an even sharper and cleaner separation

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Old 06-02-2023, 09:54 AM   #712 (permalink)
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Standard map sensor for air density. Used to be $14 ea in a parts store, making the output human observer useful is a bit fussy
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Old 06-16-2023, 11:30 AM   #713 (permalink)
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Congratulations on your latest mpg numbers. If you donít mind the look and they are attached solidly enough, I would say keep driving with your ďfinsĒ on and see if the numbers hold up. It is not totally scientific but it may show a trend over time.
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The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
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Old 06-16-2023, 10:44 PM   #714 (permalink)
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Congratulations on your latest mpg numbers. If you donít mind the look and they are attached solidly enough, I would say keep driving with your ďfinsĒ on and see if the numbers hold up. It is not totally scientific but it may show a trend over time.
I donít think I can get a smooth finish with all the tape crumbles. Itíll cause too many vortexes maybe.
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Old 06-16-2023, 10:45 PM   #715 (permalink)
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South Carolina doesnít care about aerodynamic wheels or tire sizes I see
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Old 07-05-2023, 01:34 PM   #716 (permalink)
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I would think that causes vortices on the back side of the cardboard.

Better or worse? I dunno.
Yes you are right, it does create vertical vortice which reduces seperation (making the back of the car appear smaller to the air) by an inch or so (on average, not accounting sidewind etc). It's a good thing.
Velcro is a good idea too

Also I think it does look sporty even if it didn't work
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Old 07-06-2023, 09:34 AM   #717 (permalink)
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The hilarious thing is that there are wing sections that are more efficient ( less drag) when the boundary layer is turbulent, notably if the section was optimized for that flow ttpe
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Old 07-06-2023, 07:12 PM   #718 (permalink)
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The hilarious thing is that there are wing sections that are more efficient ( less drag) when the boundary layer is turbulent, notably if the section was optimized for that flow ttpe
Thatís probably why I still saw an mpg benefit with the fins even if the tape section was building out because of the energized vortices. Later this month Iíll be putting abs plastic on it permanently and will be able to flare/taper the edges a little more inward for a smaller wake.
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Old 07-14-2023, 04:56 PM   #719 (permalink)
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instead of running CFD simulations, i may make some molds or models of my car shape and put it in a river. fluid dynamics is still fluid dynamics. and then can do mods and then drop the car mold back into the water and observe the flow and wake and turbulance. i see it all the time with rocks or my sluicebox when gold panning
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Old 07-14-2023, 05:28 PM   #720 (permalink)
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Do you know about graysgarage? ecomodder.com/forum/showthread.php/automotive-aerodynamics-video-series-31047.html

He employed an open-top test section and hydrogen bubbles. Upside down.

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