Box van rear end mod

[Engeu1]

Originally Posted by aerohead

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.

[Engeu1]

Originally Posted by aerohead

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?

[Engeu1]

Originally Posted by aerohead

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?

[Engeu1]

Originally Posted by aerohead

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.

Originally Posted by aerohead

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.

Originally Posted by aerohead

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.

Originally Posted by aerohead

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.

[aerohead]

Originally Posted by Engeu1

Is that the horizontal length for a vertically mounted device? Surely it must also depend upon flow speed?

Due to the extremely low velocity necessary to achieve critical Reynolds number, 'flow speed' has nothing to do with body geometry effects until the local MACH number is transonic, up around 250-mph ( 403 km/h )
----------------------------------------------------------------------------------------
And yes, it is the horizontal component of the aft-body, as a fraction of total body length which determines the maximum angle which will support attached flow, according to Buchheim et al..

[aerohead]

Originally Posted by Engeu1

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.

Wish we had those in the United States. We don't have any. General Motors had one, pre- 1988, however it never made it into commercial production.

[aerohead]

Originally Posted by Engeu1

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?

Yes. Sorry!, mine is the 2nd-Edition, with first English translation.

[aerohead]

Originally Posted by Engeu1

Presumably due to ground effect and weather at ground level? Equally, that should not rule out positive results around ideal conditions.



But it's a matter of net cost-benefit.



No, but that doesn't mean there is no advantage in constraining turbulent flow.



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.

1) mounting next to a wall of turbulence, and perhaps fully embedded within the turbulent boundary layer of the trailer van or truck box to begin with, would violate the 'flight conditions' on which an auxiliary airfoil would rely for its performance.It's not on an aircraft, with air moving 'above and below' it.
2) 'Ideal' conditions include an empirically-observed, statistical mean averaged 7-mph ( 11.3 km/h ) crosswind, within Earth's own turbulent boundary layer ( measured in kilometers, not meters, or millimeters, and in train of vehicular turbulence.
3) Physics is physics, and probabilities of yaw-related performance degradation have been measured from real-world observational data and common practice statistical analysis.It doesn't have good days and bad days.
4) I don't know how one conducts a life-cycle-cost-analysis with performance claims of dubious merit.
5) If you can, in your own words explain the drag reduction mechanism associated with 'constraining' turbulent flow it would be greatly appreciated. I'm forever a student of road vehicle aerodynamics.
6) As to consumer behavior in the commercial marketplace, I'm uncertain as to how much product research goes into the sales dynamic. There are no shortages when it comes to charlatans eyeing a consumer's pocketbook. ' Smartway Verified' has absolutely no meaning or credibility that I'm aware of.
7) NASA Ames Research Center, Palo Alto, California, USA, is the only wind tunnel large enough to accurately test 'Rocketail'. Any claim based upon CFD results would have to be verified at a facility such as NASA's.

[aerohead]

Originally Posted by Engeu1

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.

I had to check, but I've had that paper for years.
Again, their work corroborates Hucho's claim that, only box cavities and boat tails produced drag reductions. 18% in the case of their box-cavity, and 30% in the case of the boat-tail.
And both these devices are longitudinally ' non-ventilated'.
The first device incorporates locked-vortices to accomplish flow deceleration and pressure recovery, while the second provides the actual, solid deceleration ramp.
The investigators are from General Motors Corporation, which conceived the ' OPTIMUM TAIL', which graduate student Scott Funderburk and Professor Carver included in Scott's Masters thesis investigations of boat tails in the 1990s.
Unlike the Ahmed body boat tail, the GM Optimum Tail incorporated generous all-edge radii, for vortex-free flow, of special interest to Funderburk. Myself included.

[aerohead]

I GOOGLED this topic, which took me over to a Cal Tech article.
Most of it was for 'contained' 2-dimensional flow within a pipe, in compressible flow, not something germane to automobiles.
What it said with respect to un-bounded, 3-dimensional flow, and a turbulent boundary layer had no bearing on automotive aerodynamics that I could discern.
A parting thought with respect to 'Rocketail', it will be interesting to see what product liability attorneys do when a motorcyclist slides into the back of an over-turned 18-wheeler with 'Rocketail' and their body is bifurcated with the trailing edges of the vanes.