diffusers

does a diffuser create a low pressure area below itself to provide downforce or does it diffuse the air coming from under the car so that the air that is already under the car is allowed to exit faster, thus creating a lower pressure under the car itself.

so basically, does it cause the air to pull down on the diffuser itself or does it help create a lower pressure beneath the car, or both.

i ask cause i think im going to try to make one out of sheet aluminum but if there is going to be a lot of force on the diffuser it will just break off cause i dont think i want to fasten it all that securely (welding and cutting body panels)

does a diffuser create a low pressure area below itself to provide downforce or does it diffuse the air coming from under the car so that the air that is already under the car is allowed to exit faster, thus creating a lower pressure under the car itself.

so basically, does it cause the air to pull down on the diffuser itself or does it help create a lower pressure beneath the car, or both.

i ask cause i think im going to try to make one out of sheet aluminum but if there is going to be a lot of force on the diffuser it will just break off cause i dont think i want to fasten it all that securely (welding and cutting body panels)

The rear diffuser doesn’t create a low pressure area under the car it actually creates a higher pressure. On a non-ground-effect vehicle it servers to reduce form drag and lowers the overall vehicle coefficient of drag. On a ground-effect vehicle it not only does the above, but it also improves the efficiency of the ground-effect system.

The diffuser has an increasing (from front to rear) cross-sectional area and as the air flow travels through the diffuser its velocity is slowed. The slower the air travels across a surface the more pressure it exerts on the surface and vice versa. So the slower air flow in the diffuser cross-section has a higher pressure than it would if the velocity had not been reduced.

i guess if i woul have just considered bernoulis principle for a minute i could have figured that out.

so basically it increases the vacuum beneath the car on a ground effect vehicle?

now as far as a ground effect vehicle is concerned, if i were to get effective sides skirts and a lower front lip maybe even a splitter for my mr-s, would a diffuser be of any use to me.

i guess if i woul have just considered bernoulis principle for a minute i could have figured that out.

so basically it increases the vacuum beneath the car on a ground effect vehicle?

now as far as a ground effect vehicle is concerned, if i were to get effective sides skirts and a lower front lip maybe even a splitter for my mr-s, would a diffuser be of any use to me.

Check this out:

http://www.mulsannescorner.com/diffuser.htm

After more research it would seem I was not entirely correct in my previous post. Although the diffuser does slow the flow velocity for mixing with outside air, there is a sharp drop in pressure at beginning of the diffuser channels. So it seems that a diffuser is capable of creating downforce. Also on a vehicle with a large rear wing the effectiveness of the diffuser can be further increased.

A diffuser if designed properly can generate downforce and reduce drag, so I would think you could find a use for it. It is just up to you on how much time and effort you want to put into optimizing it for you application.

oh, thanks thats a really good link.

if anyone has any links or info about diffusers used on production cars for race use i would really apreciate it.

Get a copy of "Racecar aerodynamics" by Joseph Katz or some similar books, there the aerodynamics of racecars are described.

With a diffuser you don't want to use a low front lip, if you look on a racecar using ground effects it rather have a risen nose to allow air to travel under the car.
Skirts can be a good thing, but also a bad thing, this was noticed by Porsches engineers when they designed the 956. When they added skirts the car lost the downforce created by the diffusor.

Another problem is that you also want to add downforce not only to the rear wheels but to all wheels.

I would think the german DTM cars are as modified as a "stock car" can be when it comes to aerodynamics.

I've been studying aeronautical engineering since I was 12. Thusly, aerodynamics is my main concern. Cars have become a more recent side ambition, in part because a good friend of mine plans to build his own cars the same as I plan on building airplanes in the future.

I laugh at some of the stupid things the automotive industry does in regards to aerodynamics, so here's a general breakdown: horsepower is not a concern in going fast, because all types of racing have proven that you can get more hp than you'll ever need. The real problems in going faster are: traction and aerodynamic drag. At high speeds cars want to lift off the ground, and here's why:

There are two types of lift: Bernoulli's lift, and compression lift. Bernoulli's principal: as speed increses, pressure decreases. Most airplane wings have a curved upper surface and a flat under surface. The air has further to travel over the curve than the flat, and thus has to speed up to meet the air traveling under the wing. This creates low pressure. Think about a car now. It has a nice, slick skin over the top and a generally curved shape up, over the cab, then back down, but the underside is gennerally ignored, leaving frame, pipes, wires, and so forth in the wind. Thus, the air under the car is slowing than on top, lifting the car off the ground, causing the problem with traction.

So, diffusers are intended to help the air under the car rejoin the air behind the car more smoothly, relieving this problem. Thing is, they create a bit more drag in themselves. If you were to simply make a smooth skin under the car, you would do better. If you were to limit the air coming under the car in the first place, by skirts and lower bumper (but not completely cut it off), then create a tunnel where there is more volume near the rear of the underside than in the front, the air will have to accelerate to fill the space, thus creating lower pressure.

Then, instead of using a wing, use a skirt to horizontally divide the air from above the car and behind it (like some Saleen body kits) and use the drag from behind the car to SUCK the air out from underneath the car, you've increased the above effect. This is the sort of thing that most automakers are only BEGINNING to grasp.

Well, that's my rant on auto-aerodynamics. Hope it help's ya.

thanks. i am very farmiliar with aircraft aerodynamics as i have an A&P and an associates in specialized tech for aircraft construction/repair and i am currently studying to me and aeropsace/aeronautical engineer (undecided so far). this was jsut basically for me to try to make some aero parts for my mr-2 spyder out of aluminum. i think side skirts will be super easy and maybe a front lip. it would be great if i could make the entire underside flat with aluminum or actually make some tunnels and strakes if i do it with fiberglass.

Fenris i agree with your post and its true, it seem many people tend to ignore underbody aerodynamics which are every bit as important as top side.

But i would most certainly not atribute this to incompetent engineering. I beleive bad underbody aerodynamics to be the prouduct of an higly cost competitive industry where expenses must be reduced at all costs. Its quite complicated (read expensive R&D) to create smooth underbody aerodynamics considering all the parts residing there (suspension, wheels, trasmission tunel, brake cooling guides etc...).

If you were to simply make a smooth skin under the car, you would do better. If you were to limit the air coming under the car in the first place, by skirts and lower bumper (but not completely cut it off), then create a tunnel where there is more volume near the rear of the underside than in the front, the air will have to accelerate to fill the space, thus creating lower pressure.


i belive this is a good example of what you are saying:
http://img51.exs.cx/img51/9181/Underbody.th.jpg (http://img51.exs.cx/my.php?loc=img51&image=Underbody.jpg)

that is the underbody of the new Ferrari F430 suposedly one of the most aerodynamic cars on the road. (click on the thumbnail to enlarge).


SaabJohan is quite correct, the downforce must be properly distributed over all 4 wheels otherwise you'll end up with a severily unbalanced car.

Also I fully disaprove of "eyeball aerodynamics" meaning adding parts to a car because it would seem they help airflow. IMO its dynamics are too complicated to simply guess them and proper calculations are necessary folowed by proper wind tunell testing. One such example is the common belief that lowering the tailgate on pickup trucks (a common trick done at the drag stip) will lower the CD. It certaily looks that way; however, wind tunnell testing showed that lowering the tailgate will create severe turbulence at the back of most pickup trucks and actually increase its CD.

i belive this is a good example of what you are saying...


Just wondering what the gradient represents (pressure, velocity, etc?), and I am assuming red implies a lower magnitude and blue is a higher magnitude.


…then create a tunnel where there is more volume near the rear of the underside than in the front, the air will have to accelerate to fill the space, thus creating lower pressure..


If the flow is considered incompressible and mass continuity is maintained, then doesn’t the flow decelerate as it reaches the section of increased volume?



Most airplane wings have a curved upper surface and a flat under surface. The air has further to travel over the curve than the flat, and thus has to speed up to meet the air traveling under the wing. This creates low pressure.


There is some interesting research that disproves the air traveling faster along the top of the airfoil just because it is longer. Basically, a low pressure zone is created on the top of the airfoil as the air tries to follow the contour of the airfoil. The air following the airfoil is attributed to a phenomenon known as the Coanda Effect. The low pressure causes the air to slow over the top of the airfoil with respect to the bottom. So it is the pressure that causes the velocity change, not the other way around.

Just wondering what the gradient represents (pressure, velocity, etc?), and I am assuming red implies a lower magnitude and blue is a higher magnitude.

well unfortunatelly the place i found this photo does not give any details. However looking at it, I would venture to say its pressure. This would fit best the pattern for the nose airfoil and the expanding tunnel following it: high pressure (red) in the nose and as the air disipates in the tunnell pressure drops into green.

If that is correct it seems interesting how they focused the low presure areas between the wheels. it would be also interesting to note the high pressure area right at the back egde of the read diffuser, perhaps its where the undecarriage airflow starts to meet the top side one.

Its definitely a good find, and a very interesting picture. Thanks for posting.

When it comes to production cars there are cost limits, but in most cases downforce isn't a wanted effect. Imagine a Nissan Micra with a downforce of 1000 kp just so grandma can drive to the store and buy some milk while almost being glued to the road.

Using ground effects is more efficient than wings but still the drag can be twice as high resulting in a terrible fuel consumption. But a flat underbody can also be used to lower the drag of the car.