Aerodynamics and Cooling
By Lawrence M. Cooper (7-21-00)

Webmaster's note : This article was written in response to questions about cooling a Z-car,
but it applies very well to all cars. HIGHLY RECOMMENDED!

blank50.gif (825 bytes) Let me toss out some of my qualifications first, not to come across as a bragger but just to say that I have some experience in this area

(1.Aero. Eng. degree at VPI, 2.Design Engineer at Fairchild, 3.USAF Research Pilot at Wright-Patterson AFB & Edwards AFB etc., 4.Now flying big fat jet through air).

This subject takes a lot of words to explain but here goes.

First, throw away the term laminar flow.You can pretty well bet that due to our close proximity to the ground and the existence of turn signal lenses, bumpers, bugs, wipers, molding, etc that our Zcars are operating well into the turbulent air flow regime.You don't really care.  In fact many aircraft manufacturers purposely use vortex generators to create turbulent flow on their high flying jets.

As air approaches your car, air particles are affected in different ways depending on their relationship to the body of the car.These air particles slow down and build up upon each other, each one trying to find the easiest way around your car.The ones about four feet above the ground simply do a little jog and divert over the top of the windshield and accelerate along the roof and down along the hatch.Air particles about two feet above the ground slow down slightly as they approach the front of the hood, then accelerate over the hood, slow down again at the windshield wipers, and accelerate again as they pass over the windshield and roof. 

Air particles about one foot off the ground are really slowed down and will either try to go up and over, down and under, around, or through the radiator or any other convenient hole in the nose.  By the way, this very slow area of air is called the stagnation point.  Stagnation points are great places to put radiators.  Now, air particles near the ground will slow down slightly then accelerate under the car before twisting and turning due to the complicated affects of the under carriage.

This is how the slowing and accelerating of these air particles applies forces to the car:Simply put, air that is slowed down builds up high pressures, while air that is accelerated lowers pressures.High pressures on the car start at about the front suspension cross member, and run forward to the front bumper on up and over to a point about one foot aft of the front edge of the hood.From this point back to about 6 inches prior to the wipers, low pressure exists.From that point to about half way up the windshield, another high pressure area forms.From the middle of the windshield on up over the roof, a low pressure area exists. Various aerodynamic books as well as fundamentals of flight books show drawings of this pressure gradient on wings.The Z is just a complicated wing.

I think the question was, "how do we use the aerodynamics of the car to increase engine cooling" or something like that. Nissan already attempted this in the 280 with hood louvers.  Notice that the location of these louvers are as aft as possible on the hood but prior to the high pressure area near the wipers.This places them in a low pressure area, allowing the hot air in the engine compartment to vacate.This also allows the air stagnating on the nose to more easily pass through the radiator.

It's been mentioned that some models had a piece of metal starting from the aft edge of the radiator, running aft to the cross member. I am not familiar with this since I only have a 240Z but I have seen similar devices on Mazda's and other hot running cars.These devices were installed to smooth and accelerate the air under the radiator and combined with tiny louvers, extract air out of the engine compartment, much as the 280Z louvers did on top.This only worked while the vehicle was in motion, the faster the better.Contrary to what some might think, this piece of metal does not improve cooling while the car is stationary and in fact, if the cooling fan is not well shrouded, this piece of metal can hurt air flow through the radiator while the car is stationary. This metal panel's effectiveness is further neutralized with the installation of an air dam or spoiler.An air dam will do just that, prevent air from going under the car and therefore not flowing along the surface of the louvered panel.

An air dam has other great benefits, however.  First, it prevents air from compressing under the nose of the car which stops that previously mentioned high pressure area that results in front end lifting and lower grip. Second, it also prevents air from interfering with the undercarriage which causes drag.Third, it funnels air into the radiator improving cooling.  This funneled air can also be used to cool oil , brakes, and even be fed into the intake system on some cars.Of course air dams only work when the car is moving.

What advice would I give to the gentleman trying to cool the V-8?  1)Get a good radiator.I use a huge Griffin radiator. Works great, is light, looks cool, ain't cheap though.  2) Use a 280 Z hood.If you want to modify a  hood with louvers, run the louvers from no forward of the front axles back to about where the 280Z louvers are.   3) Properly shroud the cooling fan so that every last molecule of air that passes through the radiator also passes neatly through the fan.This is the secret for cool stop and go performance. 4)If you won't be using an air dam, a belly pan with louvers running from the radiator to the cross member would help at speed.5)If  you want to use an air dam(my recommendation), don't use a belly pan. 4)Make sure that any path the air could take to bypass the radiator is sealed. Air should be used to cool stuff or feed the pistons, not just aimlessly wandering around frame members.

About NACA ducts.They are not used to evacuate air from enclosed compartments.They are only used to take air flowing with good velocity along a surface and remove some for useful purposes.They can not take slow air from an engine bay and remove it out into the free stream.If you want to do that you have to use louvers.On some aircraft they are refereed to as cowl flaps.Look at the old pictures of the SUNOCO CanAm turbo car to see how they used louvers to suck air out of the top of the wheel wells. This is big time over kill, however.The real secret is a good radiator and a good fan.

Technical Dept Home