| Re: [GMCnet] Better Fuel Economy [message #95834] |
Sat, 14 August 2010 15:01  |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
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Here's one way to visualize it: All air flow causes some drag. The least drag
is when flowing across a smooth surface. So to push air through the radiator,
around all the jagged shapes and out the bottom causes more drag than if that
same air flowed over the smooth outside surfaces. The lowest drag path is then
over the top (unless you fill the roof up with a subdivision of stuff) and sides
- the next is under the sort-of-smooth bottom and the worst is through the
engine compartment. Yes, if you can reduce the amount of air that flows
through the engine compartment you can reduce drag. Of course, you don't want
to block flow through the radiator is it is the ONLY place you want air to go
through. So the idea is to block any flow that DOESN'T go through the
radiator. Any kind of flap, baffle or bulkhead should help. How to get the air
out? Note the shape behind the front wheels on most race cars - it is a gentle
radius and the purpose is to provide a low-drag exit for air.
Just some thoughts,
Gary Casey
'73 23 in Colorado
Quote:
> Every body thats watched a stock car race knows that less air through the
>radiator/grill results in more speed(less drag)
...
I'm having a hard time understanding this physically.
It is like saying a glass window perpendicular a stream of air has less drag
than a window screen of the same size?
How can this be true? If true it is complex because as the mesh of the screen
becomes coarser and coarser the drag must eventually decrease and go away since
when there's no screen there's no drag.
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| Re: [GMCnet] Better Fuel Economy [message #95836 is a reply to message #95834] |
Sat, 14 August 2010 15:23  |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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"" Any kind of flap, baffle or bulkhead should help. How to get the air
out? Note the shape behind the front wheels on most race cars - it is a gentle
radius and the purpose is to provide a low-drag exit for air.
Just some thoughts,
Gary Casey
'73 23 in Colorado
""
Gary, I'm not agreeing nor disagreeing, but it's a good dicussion. I was remotely involved in aero and cooling tests only because I needed to get it done or had to support it. What I realized in a hurry was how some of the techniques are totally counter intuitive. The other thing is that best airflow for cooling may get in the way of best airflow for fuel economy. We had great tools to analyze both, but in the end we had to resort to the wind tunnel and little peices of string and smoke generators. That is still the case and Nascar is a huge customer of GM's wind tunnel in Warren. In many cases it is better to have the air flow actually separate from the vehicle surface to reduce friction, rather than provide smooth surfaces to accomodate the airflow--that's why you see spoilers and sharp and jagged edges on many new designs. The same goes for airflow through the cooling system and the whole issue of recirculation. Most cars have secondary lips behind the visible spoilers or air dams--the frontal ones are usually for appearance and overall vehicle aero, while the uglier vertical spoilers or lips under the rad supports are there to maximize cooing performance. I know trial and error may help find a marginal solution but the system is so complicated and interactive that a wind tunnel is by far the best way to understand what is going on for any specific vehicle configuration.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Better Fuel Economy [message #95865 is a reply to message #95834] |
Sat, 14 August 2010 19:29 |
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mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
Karma: 0
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Senior Member |
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| Gary Casey wrote on Sat, 14 August 2010 13:01 |
Here's one way to visualize it: All air flow causes some drag. The least drag
is when flowing across a smooth surface. ...
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Not necessarily. Air flow isn't that simple. Sometimes a little turbulence will help flow around a shape.
Years ago, when golf balls where smooth, golfers noticed that old scuffed up golf balls flew a little farther. This lead to the development of dimples on the balls. Granted... our GMC's are not a little round ball.
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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