Quote:
Originally Posted by Phase
Any idea why these partial box cavity edge fins are working better than expected?
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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
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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.