Box van rear end mod

[freebeard]

There is some optimal angle— for various crosswind conditions.

The fill piece for whatever angle can, apparently, be flat-angled curved or stepped.

[aerohead]

Originally Posted by Engeu1

I have radii on the sides at the top, but all rear edges are square.



If fairly or unfairly we put the following in the category of vanes, how would you put the respective fuel economy or drag reduction claims in context vis-a-vis Hucho?

See II Air Foils | School Buses

https://rocketail.com

https://www.researchgate.net/publica...al_Guide_Vanes

(there are full copies of the last paper available online via search)

1) the school bus foil is for back-soiling issues and would mitigate that issue at the expense of higher drag.
2) the rocketail adds frontal area.
3) the rocketail is lowering the drag of a rocketail-induced-larger -frontal area vehicle, compared to the unmodified tractor trailer. When the data is adjusted for the original frontal area of the trailer van, it is this drag that we'd be interested in.
- they use phenomena from axisymmetrical flow which does not exist for land vehicles.
- at flight conditions, crosswind, as it exists for road vehicles would not exist.
- statistically, a truck would always be driving in a crosswind.
- the rocketail cannot alter the total kinetic energy of the airstream ( conservation of momentum)
- it is the turbulent boundary layer ONLY! which can follow a disadvantageous pressure gradient, not the inviscid outer flow.
- since base pressure is governed by the local pressure at the line of separation, it follows that the rocketail is injecting faster, lower pressure air towards the wake, exasperating the base pressure, pressure drag, and overall total drag.
I don't know how rocketail's quanta were arrived at,but I find their claims quite dubious.
And they're comparing to a boat-tail that's not really an aerodynamic boat-tail in the classic sense of the word.
I'd like to see rocketail tested at NASA'a Palo Alto wind tunnel,where the Continuum Dynamics box-cavity was tested under SBA/ DARPA grant.
4) as to the other lateral vanes, they smell of the same thing Hucho commented upon.

[Engeu1]

Originally Posted by freebeard

There is some optimal angle— for various crosswind conditions.

The fill piece for whatever angle can, apparently, be flat-angled curved or stepped.

I've also seen 10 degrees to allow for crosswinds.

Either way I understand you gain some additional amount with v.g.s. I suppose how much will just have to be determined during test.

I suppose generally the closer together the trailing edges, without having the airflow break away, the better. Some other optimum arrangement may exist though.

[Piotrsko]

Originally Posted by Engeu1

I suppose generally the closer together the trailing edges, without having the airflow break away, the better. Some other optimum arrangement may exist though.

Irv Culver was a fan of the "coke bottle" effect which he described as 1.5 times the height equalled the distance for minimum interference

[aerohead]

Originally Posted by freebeard

This is what I thought works before I started lurking on EcoModder. A chamfer, sort of a 'bullet-tail'.


https://ecomodder.com/forum/showthre...ail-33283.html

This is from Morelli. Instead of max camber at 30% of the length it has it at the rear putting the whole body in a favorable pressure gradient. Pending aerohead's agreement.

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.
Morelli's ring vortex borrows heavily from projectile jet-pumping action, seen only in axisymmetric flow of free flight, which would be constrained in ground proximity, as there would be zero active flow from the ground to participate in the phenomena. A perfect belly pan, inclination, and some threshold for ground clearance might be a necessary prerequisite for its operation.Need more data.

[aerohead]

Originally Posted by Engeu1

I've read that the convergence angle can't go much beyond 16 degrees without breakaway, hence the v.g. idea.

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. 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
The drag is dependent upon C-Pillar shape, but you wouldn't worry about that.

[aerohead]

1) Rocketail is a fixed auxiliary-wing, high-lift device, with ancestry back to G. Lachmann & Handley Page Aircraft Company.
2) It is a 'staggered vane' device, borrowing from a slotted, Fowler-Flap wing of high camber.
3) outside a very narrow design window, these can be very high drag, and very low efficiency.
4) they are designed for 'flight' conditions, away from Earth's ambient turbulent boundary layer, and certainly, away from any anthropogenic turbulence.
5) as reported by Hucho , Schlichting, and Hoerner, staggered vanes were investigated by Frey, and reported on in ' Reduction of Drag by Means of Guide Vanes, Forschung Ingenierwesen, 1933 p. 67, 1934, p. 105, and March 1983, pp 67-74.
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.
7) Rocketail could simply be a more elaborate and sophisticated version of the same wrongness.
8) Rocketail adds frontal area
9) Rocketail adds its own profile drag
10) Rocketail adds its own interference drag
11) Rocketail affects only 43% of the wake. 57% is unaffected at all
12) Injecting a high-velocity curtain of air into a horizontal avalanche of turbulence does not undo the turbulence
13) the energy required to create the curtain creates an energy deficit at its source.
14) The pressure at Rocketail is no higher than at the separation line.
15) Injecting a low-pressure, shearing jet into a low pressure turbulent wake, and increasing base pressure would defy the laws of physics?
It's incumbent upon the creators of Rocketail to better explain the mechanism by which it lowers drag.
If staggered vanes previously failed, then, by adding whipped cream and a cherry on top and expecting better results, what alteration to the laws of physics have undergone change to allow this discrepancy?

[freebeard]

https://rocketail.com/technology/

Made me look. It might keep dust off the 'how's my driving?' sign.

[j-c-c]

Originally Posted by aerohead

1) the school bus foil is for back-soiling issues and would mitigate that issue at the expense of higher drag.
2) the rocketail adds frontal area.
3) the rocketail is lowering the drag of a rocketail-induced-larger -frontal area vehicle, compared to the unmodified tractor trailer. When the data is adjusted for the original frontal area of the trailer van, it is this drag that we'd be interested in.
- they use phenomena from axisymmetrical flow which does not exist for land vehicles.
- at flight conditions, crosswind, as it exists for road vehicles would not exist.
- statistically, a truck would always be driving in a crosswind.
- the rocketail cannot alter the total kinetic energy of the airstream ( conservation of momentum)
- it is the turbulent boundary layer ONLY! which can follow a disadvantageous pressure gradient, not the inviscid outer flow.
- since base pressure is governed by the local pressure at the line of separation, it follows that the rocketail is injecting faster, lower pressure air towards the wake, exasperating the base pressure, pressure drag, and overall total drag.
I don't know how rocketail's quanta were arrived at,but I find their claims quite dubious.
And they're comparing to a boat-tail that's not really an aerodynamic boat-tail in the classic sense of the word.
I'd like to see rocketail tested at NASA'a Palo Alto wind tunnel,where the Continuum Dynamics box-cavity was tested under SBA/ DARPA grant.
4) as to the other lateral vanes, they smell of the same thing Hucho commented upon.

Care to "dumb down" that comment, before I start chasing my tail?

[Engeu1]

Originally Posted by Piotrsko

Irv Culver was a fan of the "coke bottle" effect which he described as 1.5 times the height equalled the distance for minimum interference

IMHO, that's more length than many would be comfortable allowing, but those tails are a great concept and a good starting point for designs.