Anyone use Porous coverings?

[93tracerwagon]

As I understand it, however, the flow at the rear of square backed bodies is largely two dimensional because there is little vorticity in the third dimension like 30 angle backed vehicle....been reading Hucho...twice. Have to just give it a try eventually.

[aerohead]

Originally Posted by 93tracerwagon

As I understand it, however, the flow at the rear of square backed bodies is largely two dimensional because there is little vorticity in the third dimension like 30 angle backed vehicle....been reading Hucho...twice. Have to just give it a try eventually.

*Road vehicles operate in 3-dimensional flow.
*Road vehicles operate in a full,turbulent boundary layer.
*Road vehicles operate at super-critical Reynolds number.
*Numerical solutions which can predict a 3-D automotive flow must be based on the full Navier-Stokes Equation of spherical coordinate system,requiring days on a supercomputer for a single iteration.Ask Mercedes-Benz.
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The investigators who published the work on porosity haven't a clue about the real world and don't even know what they don't know,blundering ahead on preposterous assumptions which defy reality.
They didn't even get the Ahmed body correct.
Re 300,000 is sub-critical.
They're operating in a laminar boundary layer which is impossible for a road vehicle.
They don't understand separation/base pressure-induced drag.
Without powered, blown, or suctioned artificial architectures,there's nothing the porosity can contribute.
This has been known to anyone who's actually followed aerodynamics for the last 95 years.

[93tracerwagon]

I have read a lot of papers that seem to be just repeating a lot of already known facts....so it appears the depth of most of the "research" out there is pretty poor. It seems the auto industry is just starting to use aero designs that have been known since the 60's....it really is pretty sad. Did you have a chance to look at the other "summary" paper I mentioned? Curious as to your thoughts on some of these ideas....rear plates etc.

[aerohead]

Originally Posted by 93tracerwagon

I have read a lot of papers that seem to be just repeating a lot of already known facts....so it appears the depth of most of the "research" out there is pretty poor. It seems the auto industry is just starting to use aero designs that have been known since the 60's....it really is pretty sad. Did you have a chance to look at the other "summary" paper I mentioned? Curious as to your thoughts on some of these ideas....rear plates etc.

As far as the rear goes,all we have is boat tails and box cavities.
Georgia Tech struggles on with pumped slots.They've never discussed the horsepower requirements to deliver the air volumes/pressure necessary to make that work.(on aircraft,the jet turbines are enlisted to supply suction or pressure)
Hucho thought we might revisit the inflatable tail,invented in 1930's Germany.


Here's an early bean-bag chair proof of concept

[93tracerwagon]

I am not sure exactly what these are. Is this the cavity created by recessing the rear of the vehicle like has been recommended for car mirrors and like the hatch glass of the Original VW Rabbit (about a 9% savings in drag IIRC)?

[freebeard]

Did you have a chance to look at the other "summary" paper I mentioned? Curious as to your thoughts on some of these ideas....

You read Hucho twice, I downloaded that PDF twice. It looks to me like they are using porosity at the front corners to internalize a turning vane.

I suspect the problems with their work that aerohead alludes to is that the correct Reynold number for the gross shape of the vehicle is different than the correct Reynold number for the fine details (at the same temperature and barometric pressure ). As skyking said.

I think you are right that a long box van will have sufficient length for the mayhem that happens at the front end to be ameliorated by reattachment and merging. The air is pressinginward along thr sidesand top

The Richard Wood infographic allows passive porosity into the club, but not base plates for that last 25%. It probably goes boattail—>box cavity—>base plate/porosity. In addition to adding 'Georgia Tech' to you search terms, try 'Richard Englar' he's the guy there. And 'Coventry University'. Here's a PowerPoint slide from them that addresses box cavities:


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I question those shallow cavities in the paper you cited. My own thought was to take a go-kart frame and a surfboard. Slice the surfboard in half raise the top half 2' and wrap a 2' strip of trampoline mesh around it. Raise the top half an extra inch to put the mesh in tension. It would stretch into a hyperboloid. With black mesh it would be like a tinted window (or those pin-hole reading glasses), with rapid air changes inside, but no buffeting. I'm not sure how you would get in or out.

You can get free samples: FREE Trampoline Fabric Sample Pack - SLO Sail and Canvas



The black mesh at the bottom looks to be some thing over 25% open. Much stouter than window screen.

With an August join date, you may not have seen this before. aerohead's inflatable boattail was 1:1. This is all I got before I decided (what was I thinking?) to sell my panel van.



It has a solid diverter, and where I thought to make it inflatable, now I lean toward bent fiberglass rods.

[aerohead]

Originally Posted by 93tracerwagon

I am not sure exactly what these are. Is this the cavity created by recessing the rear of the vehicle like has been recommended for car mirrors and like the hatch glass of the Original VW Rabbit (about a 9% savings in drag IIRC)?

Here is an example of a proper box cavity.
If properly inset,and of proper length,they will capture an optimized vortex,of which the inviscid flow will skip over,briefly reattach onto the trailing edge before separation.
As the flow diverges,it decelerates,gains pressure,imparting a higher base pressure,reducing pressure drag.

From Hucho's published drag tables,the maximum drag reduction is 7.9% at a length equal to 42% of the body length.(His example was a Volkswagen Transporter)

[freebeard]

7.9% at a length equal to 42% of the body length.

Typo or full boattail?

[aerohead]

Originally Posted by freebeard

Typo or full boattail?

If you extrapolate Hucho's drag curve, a drag minimum for the box-cavity is 7.9% delta-Cd, which occurs when the length of the cavity is equal to 42% of the VW's overall body length.Any shorter or longer and the drag is greater.
Boat tails work better

[freebeard]

That appears to me to be 12% of body length. I could believe 42% of body width.