Drag Bucket Paradox, sometimes less aero = less drag

[kach22i]

Originally Posted by Tesla

But things still don't add up:
The Dryden van boattail, now they reported significant benefits and yes they did get seperation further down the on the original tail before they truncated it, but they most likely went way past the 22° point then as well.

As I understand the template theory, they should have had seperation on that tail in the first 6", yet flow remained attached for a good 4'.

Excellent observations.

Maybe a case of theory not matching practice or reality?

Originally Posted by Tesla

I have read many threads about the front curvature, but they are usually dismissed under the banner that the big gains are in the rear, but this information seems to suggest this is not quite the whole truth.

I agree the whole bow pressure build up on purpose so that the airflow clings to the aft body is fairly non-template friendly to say the least.

[Tesla]

One of the primary contradictions are the semi truck boat tails, they are consistantly setting their ideal angles at a straight 12-15°, this does not appear to be an arbitrary guess that contradicts boattail theory, they are not doing 22° or 5° as the template theory would suggest as an ideal start.
So something else is happening to suggest that with longer vehicles or shorter boat tails that the optimum diverges from the template, my thoughtsm are that it rests with the boundary layer thickness and its ability to maintain attachment in proportion to it's thickness.
It may well be likely that if the truck boattails extended in length beyond 4' or continued to increase angle that the limit of attachment may well be 4' at 12-15°, but for many vehicles this may be an important distinction as there are very few cases where a full boattail is possible and length seems to be the overwhelming compromise.
The difference been that with a short boattail using the template you get a 20% wake reduction or with a more aggressive short tail you get a 50% wake reduction, that makes for a big difference in FE if the theory holds true, what does it matter if we get seperation just before the end of the steeper angle tail, the vehicle is seperating from the air at that point anyway and we would be leaving a smaller wake to hold the vehicle back.

[aerohead]

Originally Posted by Tesla

One of the primary contradictions are the semi truck boat tails, they are consistantly setting their ideal angles at a straight 12-15°, this does not appear to be an arbitrary guess that contradicts boattail theory, they are not doing 22° or 5° as the template theory would suggest as an ideal start.
So something else is happening to suggest that with longer vehicles or shorter boat tails that the optimum diverges from the template, my thoughtsm are that it rests with the boundary layer thickness and its ability to maintain attachment in proportion to it's thickness.
It may well be likely that if the truck boattails extended in length beyond 4' or continued to increase angle that the limit of attachment may well be 4' at 12-15°, but for many vehicles this may be an important distinction as there are very few cases where a full boattail is possible and length seems to be the overwhelming compromise.
The difference been that with a short boattail using the template you get a 20% wake reduction or with a more aggressive short tail you get a 50% wake reduction, that makes for a big difference in FE if the theory holds true, what does it matter if we get seperation just before the end of the steeper angle tail, the vehicle is seperating from the air at that point anyway and we would be leaving a smaller wake to hold the vehicle back.

*tails on commercial vehicles are 'optimized' for their arbitrary length restriction,overall weight penalty to payload,considerations for manufacturing cost,on-road durability,as well as 'pain' to any driver who must deal with the tail when loading/unloading cargo.
*No thought is given to what a rig would experience with an 'ideal' tail.
*If you're interested in a limited-length tail,I would recommend the GM' 'Optimum' tail,as you will find in the Aero Lab at Texas Tech Universities' College of Mechanical Engineering.It possesses all the features you're interested in.
*Outside of the context of legal length restrictions we can do better.

[Tesla]

I understand what you are saying with the commercial restrictions, but even within that is the fact that all the work that has gone into that area indicates that a flat plate angle of 12-15° is around the optimum in that application, there would be no difference in cost or inconvenience to set flat plate boat tails to 5° if that was found to be the optimum angle in that application.
There is no argument that an extended boat tail along the lines of the template would be a far superior option, but where there is a length restriction there seems to be more effective outcomes by using a more aggressive angle, how much of this is due to the boattail effect, base cavity wake seperation or other effect I don't know, but all the evidence points to a superior outcome when such restrictions are in place.
Admittedly the best results achieved are usually only in the 5-10% range for FE, so there is obviously a lot more scope for improvementsand this may well follow the lines of the dryden van where progressive curvature was used rather than straight panel, but it still appears that if you can't go beyond the 20% mark in length then a more aggressive approach may yield the best improvement in FE.
I still haven't been able to locate any reasonable info on the GM "Optimum" tail you have mentioned a number of times, but still looking around every now and then.

[freebeard]

I still haven't been able to locate any reasonable info on the GM "Optimum" tail you have mentioned a number of times, but still looking around every now and then.

Have you seen this?
Effect of rear body taper on a simple body

It appears some others may apply:
http://wwwm.coventry.ac.uk/researchn...Challenge.aspx

[kach22i]

Originally Posted by freebeard

Have you seen this?
Effect of rear body taper on a simple body

It appears some others may apply:
Coventry University - The Aerodynamics Challenge

Spot on topic, excellent research.

Thanks.

[freebeard]

Spot on topic, excellent research.

Truth be known, I was looking for a paper I saved to disk, unsuccessfully; and I went to Google. It was actually cited on ecomodder here:
http://ecomodder.com/forum/showthrea...tml#post296665

I still haven't found the one I was looking for. Typically, if I stop looking it will pop up on its own a few days later.

[aerohead]

Originally Posted by Tesla

I understand what you are saying with the commercial restrictions, but even within that is the fact that all the work that has gone into that area indicates that a flat plate angle of 12-15° is around the optimum in that application, there would be no difference in cost or inconvenience to set flat plate boat tails to 5° if that was found to be the optimum angle in that application.
There is no argument that an extended boat tail along the lines of the template would be a far superior option, but where there is a length restriction there seems to be more effective outcomes by using a more aggressive angle, how much of this is due to the boattail effect, base cavity wake seperation or other effect I don't know, but all the evidence points to a superior outcome when such restrictions are in place.
Admittedly the best results achieved are usually only in the 5-10% range for FE, so there is obviously a lot more scope for improvementsand this may well follow the lines of the dryden van where progressive curvature was used rather than straight panel, but it still appears that if you can't go beyond the 20% mark in length then a more aggressive approach may yield the best improvement in FE.
I still haven't been able to locate any reasonable info on the GM "Optimum" tail you have mentioned a number of times, but still looking around every now and then.

*The GM 'OPTIMUM' tail is very much like the last boat tail shown in the 15th graphic of #248(permalink),page 25,of the 'boat-tail-trailer' thread.
*This configuration yielded Cd 0.214 for the basic trailer van body sans wheels in ground proximity.
*With wheels,you might expect the drag to rise to Cd o.264 with wheels on the ground.
*Which would leave an additional streamlining potential of 54% on the table.
*Baron von Koenig-Fachsenfeld measured Cd 0.159 for a 1936 Omnibus,intercity passenger bus model with a near-'Template',extensible boat tail.
*The 1987 Renault V.I.R.A.G.E. measured Cd 0.29 with the 'longest' boat tail they tested.
*Walter Korff suggested Cd 0.20 for his more organic design of the early 60s.
*If you'd like to see GM's actual work,I'd contact the university and find out how they got it.Or perhaps the public information office at GM's headquarters.

[freebeard]

This may be the one I was thinking of:

http://bioage.typepad.com/greencarco...on-Project.pdf

Not as much there as I remembered. The popular configurations seem to be a flat taper or an inset box.

[freebeard]

Oh, I don't know, maybe it was this one:

Fuel Savings by Means of Flaps Attached to the Base of a Trailer: Field Test Results

It shows the same inset or slanted flaps, but sizes them at 1/4 the vehicle width.