Quote:
Originally Posted by Talos Woten
Hmmm... that doesn't jibe with what I know about aerofoils on planes. Longer, thinner aerofoils do reduce drag, up until skin friction begins to dominate. However, real engineering is often about tradeoffs, in material, size, etc. That's why truncated aerofoils are often used; they are a good balance between weight and performance.
Hehe. Well, that might be what you were thinking, but it's not what I was thinking. I include everything at the front of the car to be part of the nose, including the radiator and wheels.
We can also improve thermal exchange by slowing the flow, giving the air mass more time to transfer heat.
Oh ho! The race is on! Here's my prediction about how the 20% fuel economy increase is going to breakdown:
~3% Nose re-shaping
~5% Air Curtain
12%+ Radiator rerouting (smaller inlet, ducting, etc.)
I agree with you that (sadly) limited space makes the outlet ducting unfeasible. So I'm going to focus on the inlet duct, which there will be plenty of space for once I extend the nose by 8"+. No change in the location of the radiator.
So... would you be interested in a wager whether I can achieve that? I'm predicting that it will be the +8 mpg from 60-something-ish mpg to just over 70. I'm particularly fond of gentlemen's wagers that involves eating crow. (coughs out old feathers).  |
1) 'Aircraft-type bodies do have optimum fineness ratio values ( for which drag is a minimum ), and our data suggest that road vehicles do also.'
William H. Bettes, Graduate Aeronautical Laboratory California Institute of Technology ( GALCIT ), ' The Aerodynamic Drag of Road Vehicles Past, Present, and Future, Engineering & Science, January 1982, page-8.
--------------------------------------------------------------------------------------
' low drag can only be achieved when the separation at the rear is eliminated.' Hucho, 2nd Ed. page- 16.
' (T)he optimum shape in terms of drag is a ( streamline ) half-body, which forms a complete ( streamline ) body of revolution together with its mirror-image produced through reflection from the roadway.' Hucho, 2nd-Edition, page 15.
' The fineness ratio of...5.53...corresponds to an effective ratio in free air of 2.27. This approaches the drag minimum recognizable... With a greater fineness ratio, the drag would increase again as a result of the increasing friction drag.' Hucho, 2nd-Ed. page 210.
' With a lesser fineness ratio, the drag would increase again as a result of increasing pressure drag.' Aerohead, corollary to the above.
' If we conservatively limit aft-body downward/ inwardly sloping surfaces to respect W. A. Mair's 22-degree limit, then we're limited to streamline bodies of revolution no 'shorter' than 2.5:1 fineness ratio. Cd 0.04.
Hucho, 2nd Ed. Page-61, Table 2.1, 3rd from bottom, ( from S. Hoerner's Fluid-Dynamic Drag, 1965 ).
The more you study, it will become obvious that, it'd a major mistake introducing aeronautical engineering into road vehicle aerodynamics.
Airfoils have no place.
2) Aerodynamic detail optimization is much more granular than with lumping the nose, cooling system, and air curtains, etc. together in a broad sweep. It's not done.
3) If the pump volume is to remain constant, and the object is to increase residence time within the heat exchanger, then we're talking about adding more core, or enlarging passages. Just engineer for the worse-case scenario.
If you add core, your now increasing the porosity of the car, kind of a backwards move, aerodynamically speaking.
4) We weren't talking about 'prediction.' You spoke as if referring to a bird-in-hand, prima facie evidence.
* there's little probability that reshaping the nose will net 3%. 14% was the limit, and you were already there.
* The BMW air-curtain is attributed with a 0.01% drag reduction if my notes are correct.
* If you removed the cooling drag completely, you're only looking at delta- Cd -.025. That would put you at Cd 0.235. Only a 9.6% drag reduction, and maybe a 3% mpg improvement.
5) A wager is problematic, as your testing protocol is also problematic. The hybrid system makes nonsense of conventional methodologies. Regen compounds it further. Any 'hypermiling' throws it completely into outer space.
I don't know how we'd ever sort it out into anything definitive.
This is why we have wind tunnels. And my science background dictates that this is the only appropriate venue in which to adjudicate the exercise.$$$$$$
A little salsa makes crow a fine meal! I have no reservation about the contest. We'd all learn something. It's the 'measuring' and 'quantifying' that would be the challenge.
The fellas did a crowd-funding for me back in 2014, to help defray some of the wind tunnel costs. I'm indebted to them all. In 2017 I paid for another member's wind tunnel time. I'm no longer in a position to do that.
Maybe we could pass the hat another time, and help get Champrius into the A2 Wind Tunnel. They tested the 2013 Prius and we know something about intra-tunnel blockage-ratio data variability compared to Toyota's wind tunnel. Something to think about.