View Single Post
Old 05-20-2020, 05:49 PM   #2 (permalink)
JulianEdgar
Master EcoModder
 
Join Date: Nov 2017
Location: Australia
Posts: 496
Thanks: 36
Thanked 528 Times in 291 Posts
Quote:
Originally Posted by aerohead View Post
The 'ideal' shape for 2-D flow of struts/symmetrical wing sections was obtained by S. Goldstein,in 1938, and is presented in Fig. 2.28, page 244, of 'AERO-HYDRODYNAMICS OF SAILING, INTERNATIONAL MARINE PUBLISHING,Camden,Maine, copyright 1979, 1988, by C.A. Marchaj.
The drag is governed by surface friction and pressure drag. There exists a single iteration of the struts which embodies the least drag.If it is lengthened,the surface drag increases,along with an overall drag increase.
If the strut is shortened,pressure drag increases,increasing the overall drag.
This one geometry of struts is in the bottom of the drag 'bucket.'
--------------------------------------------------------------------------------------
The same holds true for 3-D flow streamline bodies of revolution. Both surface friction and pressure drag bottom out with a body of Length-divided by Diameter = 2.5. Yielding Cd 0.04. As a half-body,in ground reflection,this body produces Cd 0.08. Adding wheels increases drag to Cd 0.13.
According to Hucho,this drag can be reduced by integrating the wheels into the body. According to Goro Tamai of M.I.T.,a proper set of wheel fairings can reduce the wheel drag approximately 70%, allowing in the neighborhood of Cd 0.095. Honda achieved Cd 0.10 in 1994 with their 'Dream' solar racer.
The 2013 Cambridge University Eco Racer CUER solar car was measured at Cd 0.11.
-----------------------------------------------------------------------------------
The 'template' is intentionally fashioned after the Cd 0.04,2.5 L/D, streamline body of revolution.
Hucho says aerodynamics is about pressure drag.
Hucho says pressure drag is about separation.
The 'template' is incapable of producing separation,by default.
Without separation it achieves the drag minimum recognizable,as Hucho puts it.With tighter wheel fairings it's quite likely that she would have mimic'd CUER's Cd 0.11.
================================================
If you want to believe that aerodynamics is a 'black art',I can't stop any of you. The evidence suggests that it's quite easy to construct really low drag automobiles with out any further research or expenditure. Off - the -shelf technology if you will.
And this argument would date to 1922.
Not such a good post. Starting to:

- use rules of thumb (eg wheel fairings can reduce drag by 70 per cent and then applying that figure to another body)

- make conceptual jumps from theory to real road cars (eg solar race cars are not practical road cars)

- develop strawman arguments (who said aerodynamics was a black art?).
__________________
Modifying the Aerodynamics of Your Road Car

A really good book that should be added to the library of everyone working in automotive aerodynamics, as well as those making car aero modifications at home. - Rob Palin, former Tesla aerodynamicist

Last edited by JulianEdgar; 05-20-2020 at 08:08 PM.. Reason: grammatical error
  Reply With Quote
The Following 2 Users Say Thank You to JulianEdgar For This Useful Post:
aerohead (05-22-2020), Vman455 (05-20-2020)