Size of hood vent

[aerohead]

Originally Posted by NeilBlanchard

Hi,



My interpretation of the drawing you posted is the opposite -- the base of the windshield has some negative pressure, and so it is the best place to exhaust the cooling air.

Neil,the graphic is a bit confusing,but you'll notice that in the yellow portions of the image,the arrowheads are directed "away" from the car (signifying lift,or negative pressure),while within the black portion ,the arrows are directed "at" the car,signifying positive ram pressure.The image is "dyslexic" are far as intuition goes.Maximum lift is occuring over the leading edge of hood and roof while stagnation is occuring at grille and cowl area.

[holypaulie]

This is my modded hood with vents. I've cut original stock hood and place vents from old pontiac. I also tilt radiator about an inch back for better air exhaust and made air ducting to vent directly from fans. My radiator intake is about same size what vents. When I put my hand by vents while radiator fans is blowing, I feel hot air coming out. I like this setup works very good no problem so far.

[NeilBlanchard]

Hi,

If the front edge of the nose is low pressure, I'll eat my hat. I think that drawing is bogus.

[aerohead]

Originally Posted by NeilBlanchard

Hi,

If the front edge of the nose is low pressure, I'll eat my hat. I think that drawing is bogus.

Neil,I know it seems crazy,but you can find these studies going back decades.Dr.Timothy Maxwell of the Texas Tech Aero Lab once explained to me that with the 1st-gen Ford Taurus,that the hole in the front of the car where Ford placed their oval emblem,actually had air coming "out" of it!.Remember,as you displace forward,the air is accelerated from rest,to it's maximum velocity as it rounds the leading edges of the car.The "airspeed" at the leading edge of the hood could be as high as 50% greater than your road speed.Same for over the roof.----------------------

From conservation of mass,and energy,the only way the air can gain velocity, is if it trades off pressure at the same time.------------

It's how a perfume sprayer or carburetor works.Daniel Bernoulli researched all this and published long ago and its in all fluid mechanics text.On race cars,the radiator exit duct may be positioned on the forward radius to both reduce lift,as well as dump air to this low pressure area.Also,this is a race car,and by moving the fully-ducted lightweight cooling system forward,heavier components can be compacted and nested close together,closer to the CG of the car,for lower polar moment.

[Frank Lee]

The drawing is good; you need to think about your interpretation of it some more.

[TestDrive]

Air pressure profile per Aerodynamic flow may have been a better label.

[NeilBlanchard]

Hi,

Velocity means that air is being deflected, or pushed up and over the front of the hood, and this is where drag is most easily created. The air velocity is almost nil at the base of the windshield (I have seen maple seeds just sit there) and so while it may not be generating lift, I don't think that there is much pressure there.

So, the diagram makes sense if you think about velocity (changing direction), and this also means pressure against the surface of the vehicle. Low velocity is closer to still/stagnant air, and therefore is lower pressure against the surface of the car (because it is closer to the way it was before the car pushed it out of the way).

The vectors are showing lift, I guess. They only shade the parts that have a positive Y value. I guess it's the X values that I have a problem with -- why are the vectors below the neutral point (on the front bumper) going the right (positive X) and the vectors above the neutral point go to the left? Is this area of the car pulling the car forward?

I'll draw a picture later that is more helpful, to me at least.

[NeilBlanchard]

Hi,

I've attempted to show the air flow lines, overlaid on the "velocity" image (if that is what it is showing):


And here it is with just the flow lines:


As you can see the greatest velocity occurs where the air is being pushed aside at the greatest angle, and this is what gives it the greatest velocity. I think that the nose pushes the air up above the main part of the hood, and then the upper part of the windshield again pushes it up; creating another higher velocity zone.

[IkoIko]

holypaulie, I like the hood mod. Did holes of that size effect the hood's strength? Does it flex under normal use?

I've played around with the a Magnehelic gauge and the aerodynamic flow profile makes sense to me. I found lower pressure 6 inches back from the front edge of the hood, and higher pressure up by the cowl.

I'm going to next check along the side of the front fenders and further off the center line of the hood as suggested by aerohead.

Thanks again for all the input.

[TestDrive]

Originally Posted by NeilBlanchard

The air velocity is almost nil at the base of the windshield (I have seen maple seeds just sit there) and so while it may not be generating lift, I don't think that there is much pressure there.

There is a pocket of high pressure air there that deflects the flow.
Hence the maple seeds sit there undisturbed.