Rx7 Forum and Owners Club - Fluid Dynamics 101

Fluids take the path of least resistance. Air is a fluid. Air and water will behave in the same way when it comes to flow and resistance.

I'll try to be as simple as I can about this, and mind you I am leaving out a TON of extra qualifiers and details to keep this simple.

Drag Force = F = 1/2 (Coefficient Drag) (Density) (Projected Frontal Area) (Velocity)^2

The above is useless to you unless you know how to cancel out units. If you actually want to use it for some reason or another ask and I'll post a way to work it all out into Newtons and then convert to pounds or mph or what not.

Drag Coefficient = It is a dimensionless number that shows how much drag there is on any particular shape. A completely streamlined body would have about .04 while a cube would be around 1.05. Most trucks are around 1.0 I'm not sure of the exact numbers for Rx7s, but I do know however That the body of a 1981 Porsche 944 has around .035 and our cars would be less than that.

Density = the density of the fluid flowing over whatever object is present. In our case that is air, so it is air pressure.

Projected Frontal Area - Area of the shadow of the vehicle if it were uniformly lit from the front. Basically If you took a head on picture of the car, and took that outline. This is a two dimensional number.

Velocity = Vector speed of the air (if it is flowing int he same direction of the car or against the car makes a difference) +/- the speed of the car (plus or minus once again depending on the car and wind direction)

All of that will get you the force drag at any given time.

On high pressure days there will be more of a drag force on your car. But you have to take into account that the higher pressure may allow you to reach greater speeds.

That's all just to give you a basic idea of what goes on while you drive.

There are more complicated things going on as well. Once air has begun to hit the body of your car, it forms somewhat of a cushion of turbulent air that makes incoming air go even further around your car. There are a few ways of combatting this. The best of which is a fully streamlined design, but here is another example:

Golfballs are spheres, we all know that. But why do they have dimples?
Think of a perfectly streamlined design. A teardrop. How can a golf ball morph into a teardrop shape while it flies? It doesn't, but the dimples trap turbulent air, and create an area of high pressure in the shape of teardrop starting at the very front of the ball. When the ball rotates more turbulent air is caught and continues the spin on the ball spinning around like a water wheel dumping air behind the ball.
Without those dimples golf balls wouldn't travel straight, they'd spin around and fly off in random directions given a small gust of wind. With the dimples the ball flies much more predictably.

If your car has a huge opening in the front with a radiator in it, turbulent air will get caught in front of the radiator just the same, and hot air will become trapped there, instead of allowing cold air to flow over it. This is the reason many people turn to V-Mounts. Instead of V-Mounts one can create a shroud that makes sure air funnels straight towards the radiator and pushes the lower pressure (hot) air away.

Having an intercooler or an oil cooler mounted in front of your radiator will create a pocket of high pressure air right behind whatever it is in front of your radiator. Making a dead spot in the radiator where warm air just stays and isn't pushed of vacuumed out. This is another reason people V-Mount. The air flowing upwards over the radiator created a suction over the entire face of the radiator since air takes the path of least resistance. If you were to put a vent or a reverse scoop above and slightly behind a v-mounted radiator, you will have effectively created a current that flows from the bottom of your lip over the radiator and out of your hood, to be caught up in the flow over your car and dragged away.

Think about it, and you will see why people create fenderes that cover their rear tires. How spoilers create downforce, and how drafting works.

I have to go eat lunch before my next class.