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Old 06-29-2020, 05:14 AM   #61 (permalink)
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Originally Posted by aerohead View Post
I believe that the ability to generate the ring-vortex is predicated upon having the semi-elliptical body cross-section. I have no confidence that a rectangular section could support such annular vorticity.
...Need more data.
See figure 12 in

2012 Khalighi et al. Unsteady Aerodynamic Flow Investigation Around a Simplified Square-Back Road Vehicle With Drag Reduction Devices J. Fluids Eng. Jun 2012, 134(6)

for a very good approximation of a rectangular body induced vortex ring.

In any case, to some degree a rear tail could transition the flow away from rectangular -some tails are well radiused.

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Old 06-29-2020, 05:18 AM   #62 (permalink)
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For prismatic bodies like a truck the angle will be a function of the horizontal length of the ramp, as a percentage of the overall length of the vehicle.
Is that the horizontal length for a vertically mounted device? Surely it must also depend upon flow speed?
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Old 06-29-2020, 05:28 AM   #63 (permalink)
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There are some graphic drag tables dedicated to this relationship, developed by Buchheim et al., in Hucho. I don't have that with me or I'd give directions.
From my notes, it's on page 153, Figure 4.59
I think that's not the 2013 German language version you're referencing [translated] 'Hucho - Aerodynamics of the automobile: fluid mechanics, heat engineering', I see online that the page is about emissions.

Which version is in your notes?
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Old 06-29-2020, 05:59 AM   #64 (permalink)
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1) Rocketail is a fixed auxiliary-wing, high-lift device...
6) From Hucho: ' guide vane's drag exceeded any base pressure-related drag reduction. All attempts to apply the results achieved on two dimensional aerofoils to three dimensional bodies in close ground proximity have so far failed.
Presumably due to ground effect and weather at ground level? Equally, that should not rule out positive results around ideal conditions.

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8) Rocketail adds frontal area
9) Rocketail adds its own profile drag
10) Rocketail adds its own interference drag
But it's a matter of net cost-benefit.

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12) Injecting a high-velocity curtain of air into a horizontal avalanche of turbulence does not undo the turbulence
No, but that doesn't mean there is no advantage in constraining turbulent flow.

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15) ...It's incumbent upon the creators of Rocketail to better explain the mechanism by which it lowers drag.
In the academic realm that makes sense. In the commerical realm, if something works satisfactorily and sells, there may be no advantage to explaining it further. The product is SmartWay Verified for a modest benefit. A better approach would be to look at the EPA verification method and diligence.

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