Airflow - pressure - windshield - production

[roadtrip]

Knowing that there is a high-pressure area that develops at the base of a 'typical' production car windshield ......... and knowing that there are many variabales having to do with the shape of the hood and angle of the windshield, etc. .......... can anyone describe the shape of that pressure area in general terms?

I'm thinking in terms of a flat hood, not a factory or aftermarket cowl-induction hood (seems to work pretty well for NASCAR).

In my head I see it as an enlongated football shape with pressure above and below the intersection of cowl and windshield, tapering left and right, but how far forward, left/right, does the area extend?

DS

 

[Ratliff]

http://us1.webpublications.com.au/static/images/articles/i1086/108677_2lo.jpg

Put tufts of yarn on the windshield, drive down the highway at about sixty, and see which way they point.

[RichFox]

Don't forget NASCAR guys use cowl induction air intake because they are required to. Not because they think it's the hot setup

[roadtrip]

You're right ............ probably not the most efficient way to induct air, but the idea of maintaining a stock configuration is where I'm going ........... not that NASCAR and stock have anything to do with one another.

Tuft arrangement plotted ......... now if I can find an hour to myself to play with it.

DS

[Ratliff]

For applications where you don't have a tunnel ram and twin carbs sticking up above the hood, taking air from the base of the windshield usually works pretty good.

[roadtrip]

Results from an earlier post and experiment ...... if anyone cares .............

I tried several arrangements. The pic posted is the most typical.

With duct tape covering the gap from edge of hood to windshield, air flowed pretty much like you would think with no major changes in direction.

With no tape, as in the pic, when you give the air somewhere to go, it wants to wrap itself around the trailing edge of the hood right away - 40 mph or less.
Tufts A and B layed on the glass in a shallow ledge atop the cowl piece. Tuft C, about 1" up on the cowl piece, was drawn down into the high pressure (high vacuum?) area but at a slightly higher speed.

Probably not news to you aerodynamically experienced guys but it was a lesson for me.
"I hear ...... I forget ......
 I see ....... I remember."

DS

[Seldom Seen Slim]

Thanks for the empirical information.  That is - you tested and now know what happens on your vehicle, not just what someone says would happen.  Good photo, good information, good for you.

[Super Kaz]

Slim,
Back when I was racing my 106mm Turbo in SS/O in the NMRA {I had The 1st Pro 5.0 Big Turbo motor in a super street outlaw car,They Banned it the Nrext Year } on my best run{8.20@170 COASTING FROM 1000FT } the Hood "a 6-inch Cowl back" sucked off at 170mph and we had to add 4 more hood pins and a role of 200mph Tape to keep it on ! Never got to get the stock suspension ,AND real 28/10.5{little boots} to get me in the 7's,but this was 8 or 9 years ago !
Now thier in the 6's!!!
I lost that on my 3rd Divorce ,and everything else :!
I sure do miss that BEAST ! o h well.
Hoods are allot more then something to cover your engine.
Do your home work and do it right.
If you{or anyone of the Forum} ever need a Test Pilot I'll Drive/Ride/ Race anything thats safe,and would love to help,any team I can.
Kaz..................

[John Noonan]

Kaz,

Based on what the original poster asked about what are your feelings or knowledge that pertains directly to his question?  What do you think of the picture posted and the results he may have with either ...awe, forget it I was just messing with you...since neither of us posted anything relevant to his question/post I did not want you to feel all alone..

[Stainless1]

What's up with that John?  Don't you have an irrelevant picture of a jet powered drag boat with cowl induction and a driver parachute pack to throw in here... 
See ya on the salt 

[roadtrip]

I don't know him but I'm assuming SK's use of the word BEAST .... was in reference to the car.

[Super Kaz]

I don't know him but I'm assuming SK's use of the word BEAST .... was in reference to the car.

1700 hp on a 10inch Tired ,street type suspension car was Scary Fast ! never got to use the Boost controller or the Big Turbo as they banned them the next year . I Know it would have gone 250+ setup for Maxton as the B#tch Never Stopped Pulling !
Remember John who's little Buddy !
Your Motor's been talking too me ,and it wants to go home:evil:........

[hotrod]

can anyone describe the shape of that pressure area in general terms?

On the sort of body style you discribed it would form a  semicircular wedge of high pressure air that extends at the body mid line from about the center of the windshield sloping forward to about  1/3 the length of the hood forward from the cowl. It is roughly shaped like 1/2 of an ellipse with the long axis extending from A post to A post.

If you have ever parked a car in a snow storm and had a small snow drift form on the cowl and lower windshield area you know what the approximate area is on your car. The reason the drift forms is that the air flow slows down as it transitions from the hood and up the windshield and allows the snow to settle out of the flow.

It rapidly disappears if there is much side wind (on the older corvettes it would vanish when the relative air flow got to about 15 degs off the hoods axis if I recall correctly).

If you remember how you draw an ellipse using string and a pencil you can get a very rough approximation by taking a piece of string and fixing the ends at points on the base of the windshield about 1/3rd the width of the windshield from the A posts, and make the string long enough so if you pull the slack out toward the front of the car at the mid line it forms a triangle that goes about 1/3 the length of the hood. Now place a grease pencil in the loop of string at that apex and keeping the string taut move it to both the right and left edges of the hood. Repeat the same process on the windshield and you will have layed out a ball park perimeter of that wedge of higher pressure air. Obviously pressures will be highest right at the center of the windshield and gradually drop back to local air pressure as you move toward any of the edges.

Hard to describe a geometric shape in words but I hope that makes sense. I will try to find an illustration.

Larry

[roadtrip]

Very graphic description, and easy to visualize. Pretty much the stretched football shape I imagined, and all very logical, but didn't understand how effective it was 'till I watched the yarn disappear around the back edge of the hood ..... quickly. I expected more turbulence or eddying in the area.

I just wanted fresh air without cutting the hood or a bunch of curved ducting from the front end, so I cut a 10 by 3 inch hole in the firewall that opens into the fresh air duct at the base of the windshield. I made a sheet metal duct with the same sized opening and mated it to a 9 inch air filter base and lid, creating the shortest possible path from outside air to throttle body. I made a cover plate for the hole in the firewall and the assembly can be removed and returned to all-weather mode in a few minutes.

Thanks
DS

 
 

[jdincau]

Since you now have a direct connection to the carb, make sure you close off any openings that let that fresh air into the interior of the car. You may run into a firewall integrity issue at tech.
Jim