Date:         Mon, 29 Apr 1996 12:54:00 -0700 (PDT)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         "Maher, Steve (SD-MS)" <SMAHER@gi.com>
Subject:      Re: aerodynamics (Seriously...)


>This post will provide lotsa opportunity for well deserved ribbing.......

I for one would never consider taking advantage of this...    ;)


>While driving around the salty/muddy roads this winter, I noticed that the
>air tended to separate from the surface of the van near the area of the aft 

>body panel seam, or the seam just infront of the cooling air intakes at the 

>back of my vanagon.  This was indicated by the wet body panels infront of
>the seam and behind the seam the surface of the van was dry.  Why is this
>important?  If my observations and hypothesis are correct, then the cross
>sectional  area contributing to separated pressure drag is the cross
>sectional area outlined by the transition between wet to dry body panels.

Any truth to the rumor that this seam also marks the boundary between
the hot air in the engine compartment and the cool air forward of it?
And that water on the rear sections dries off a lot faster than the front?


> If the separation point could be moved rearward on the van, the cross
>sectional area for pressure drag would be reduced.  By putting vortex
>generators near the transition line (experimentation would be required) the 

>spearation point could be moved rearward, the cross sectional area reduced
>and drag decreased.
>
>If you are unfamliar with vortex generators, look at the wing of an 
airplane
>next time you travel somewhere without driving your van.  The little metal
>tabs sticking up on the wing at (usually) regularly spaced intervals at an
>angle to the airflow on the outboard leading edge or upstream of the flaps
>or ailerons are vortex generators.
>
>This is similar to why golf balls are dimpled.  If hit the same, a smooth
>golf ball will not travel as far as a dimpled golf ball.   If this worked,
>the drag would be reduced (somewhat) and mpg increased.

Any of you midwesterners, who once parked your bus outside during a 
hailstorm,
care to comment on the advantages of dimpled surfaces vis-a-vis gas mileage?


>Experimentally, the placement of the vortex generators (vg's) could be
>optimized for a driving speed by getting a bunch of the adhesive backed
>magnets and gluing the metal tabs to them.  Before placing the vg's on the
>van surface, tape a grid of yarn tufts on the van surface and drive at your 

>normal cruising speed with somebody in a vehicle beside you video taping 
the
>behavior of the tufts in the airflow.  This will provide baseline data to
>judge the effect of the vg's when placed at different locations on the van. 

> Then place the tabs in a line parallel to separation line.  Do the
>driving/video taping thing again.  By using the magnetic tabs, the location 

>of the tabs will be easily changed without using paint damaging adhesives.
> Start the tests with the line of vg's in front of the separation line and
>repeat the test several times.  Each time, incrementally move the line of
>vg's rearward.  Additional experiments could/should be done with size, 
angle
>and spacing of the vg's (i'm not an aerodynamacist, so I can't provide
>guidance here, infact my whole theory may be flawed, but thot experiments
>are fun).

And you'll be a shoe-in for next January's Rose Parade.   :^)

Seriously, Fred describes solid, legitimate methods of testing for boundary
layrer seperation and turbulence, which have been used as he describes
for years on aircraft, both in research wind tunnels and on real aircraft
in flight.

But I would guess that the main reason for buses getting lower gas mileage
than a same-weight sedan, and especially that little round sedan first
brought out in '47, is that large, tall, flat front; and especially the
large, tall, flat rear end. Aircraft can often get away with having a fairly
blunt front, but the rears are always tapered to avoid the low-pressure
area that literally sucks the bus backward as you cruise down the freeway.
Vortex generators may help a teensy bit, but you're still driving a vehicle
that's "streamlined as a brick".
 ______________________________________________________________________
           _                                            ______________
    ______//________    Steve Maher    smaher@gi.com   //__][__||____\\
   /o _   |  -| _   \   San Diego, CA     75461,1717  (o _-|       _ o|
   `-(_)=======(_)---'                                 '(_)-------(_)-'
 '66 Mustang Coupevertible      '89 Son Sherwin        '80 VW V6anabrick
 w/windshield like a spoiler                        w/o vortex generators
 on a glider wing

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