Race car diffuser design

Most diffusers I've dealt with on race cars, have vertical sides, and strakes, but that was dictated by the rules. On street cars, I have seen the sides diverge as well, like the Enzo, and F430.

-Dave
Everything should be designed as simple as possible, but not simpler.

Thanks,

I have a diffuser that I'd like to copy for the Reynard, but looking closer at it, the divergence angle starts at ~7 degrees, but it increases, and ends up ~20 degrees. This occurs over about 16 inches. From a side view, it is not a straight 7 degree angle, but curves upwards, with the angle smoothly increasing.

Is this typical, or desirable? As the air slows down, can a larger divergence angle be used?

Gary

The reason why to contain the angle divergence of the sides, is that it influences the pitch an roll behaviour of the car; since there are no skirts ( as happened during 70's and 80's), the flow must be control by the geometry of the entire diffuser, then by the angles of longitudinal and transversal surfaces.
Usually these values are fixed by the rules of that specific formula you are running in. For Le Mans protos the angle of sides if fixed to 7 degrees, for example..

About the exit angle of the diffuser; the overall behaviour of a diffuser is strongly influenced by the entire car body and, in particular for the exit, by the position, type and angle of attack of the main rear wing.

Bye
Cpinz

Thanks Cpinz,

I haven't had a chance to get back to the diffuser yet. By the sounds of it, there isn't one clear cut answer for what is right in every situation, not that that is surprising.

As for regulations, I don't have any (relating to diffusers anyway). The Reynard runs in Formula Libre, so free reign there, and my Europa will run some vintage stuff, so can't have a huge, obtrustive looking diffuser, but that's about it for restrictions.

At the moment, I'm thinking the throat will lead into two 'tunnels' on either side of the transaxle, hopefully with minimum of suspension stuff in the way, with the top and both sides diverging at between 7-14 degrees (likely lower in the range for sides, higher for top).

Am I correct in thinking that as the diffuser area gets larger (ie closer to the rear) that the divergence angle can increase slightly?

With the Reynard, there is a two element wing behind the diffuser, what sort of relationship should there be between the two? things like height of the diffuser edge relative to the wing leading edge, distance back to the wing, etc? Should the lower element be in the air coming out of the diffuser, or above it?

Lots of questions yet, but I feel like I'm not completely lost anymore. Thanks for any and all info.

Gary

ps. If anyone is interested, the Reynard can be seen at

RacingGary, how are you getting on with those designs ?
I'm in the process of designing a Diffuser myself, for my Lotus Elise S1. So I'm looking for any info that could help me with my research.
I'd like to use a mixture of Aluminium and Carbon Fibre, and thought about integrating a very small oil cooler too...

Purepwr

why alluminium ? If you are thinking to a sandwich structure use nomex; it costs less than alluminium , weights less, has mechanical properties that fully satisfy a diffuser design. An alternative could be high density foams (pvc ) or ,better, Rohacell type material.

If some part is too close to the ground place there some kevlar to protect the carbon.Add inserts in the sandwich to fix it to the car by screws.

Cpinz

The Car comes from the Factory with a small aluminium (sheet) Diffuser. This serves as a cover for the muffler (mid engined Car), and provides about 15Kg of negative lift.

I think it be a bit longer, and "release" the airflow under the car, slightly further along the length of the Car.

I wanted to use thin alu sheet in a U shape (thus creating fins at the edges), with 3 smaller fins (Carbon Fibre), equally spaced in between the two edge fins.


Do these fins need to be big ?

Should the edge fins be squared off at 90° or would it be better to have them slightly rounding off ?

The whole point is to slow air down right ?

I had also thought about using a fat alu fin in the middle, and make it work as a heatsink for the Engine sump (but maybe I'm being optimistic)

Question for you guys about diffuser design:

Should you seal it off at the carburetor so no air escapes? Also, do you want the volume large enough so that there is essentially zero velocity, thus converting most of the dynamic pressure to static pressure?

I understand that for a duct, say for a radiator, you need an exit and you need to maintain some velocity there, perhaps equal to the local external velocity. However, I might be a little shaky on my theory.

I guess you can't say "dynamic" without a silly link being attached to it. Maybe "pressure due to the motion of the air" would work?

Sorry EngJW, I don't understand your question.

Why do you have a carburetor involved in your diffuser design?

A little more clarification may help some of the more experienced guys give you some hints.

Gary

Carburetor?- I was thinking the duct was a ram air intake for the engine and had a carburetor at the end of it, but maybe I should have said fuel injection or something else. The other type of duct I was thinking of could be for a radiator or intercooler where ram air pressure is not needed. My thinking could be confused because I am way out of my field.

The thread was actually about rear diffusers. Mounted at the rear underside of a Car, to clean up air flow and reduce drag/reduce lift/add downforce.

Yep, and getting back to it, I'm still curious if the angle of divergence can increase as you 'go back' (get closer to the outlet) in the diffuser?

As mentioned above the one I have goes from ~7 degrees to ~20 degrees over 16 inches... this seems like an excessive change to me, and could (would?) lead to detached flow. But, is it a valid theory, just taken to extremes (too far) on this example?

Thanks,
Gary

The effect of a diffuser couldn't be considered as stand alone; it's effect is dependent by the way the rear wing works, the effect of rotating tires, presence skirts or side pods.
20 degrees in so short length, probably, must be forced to remain attached using sharp edges in the channels to promote strong vortices that energizes the flow ( even if this is used with lower angles too ). In my opinion,if you have fixed lenght of diffuser, a main role is made by the rear wing with is extraction capacity....rules permitting.

Cpinz

cpinz makes an excellent point. A diffuser (or underwing, or undertray, or underbody) is simply part of a ground effects system. The diffuser's function is to create a low pressure region under the body while producing as little drag as possible. The resulting downforce provides beneficial traction force for the tires. A properly designed underbody can produce downforce more efficiently than using front/rear wing elements. In fact, the front/rear wing elements typically seen on race cars are used mostly to balance the downforce front to rear. Most of the downforce being produced by the underbody.

The other aero components that can assist the diffuser are a "splitter" in front and "skirts" or "vortex generators" on the sides. They will discourage air from migrating under the car. The location and height of the rear wing (if allowed) can also improve the performance of the diffuser. Routing the exhaust outlets (and heat exchanger outlets if possible) into the tunnels can add substantial energy to the airflow in the diffuser. And of course, putting louvers over the wheel wells (on a fendered car) will help to extract air from inside the fenders.

The other important aspect to getting the diffuser to function effectively is designing a suspension system that will maintain proper ride height and ground clearance during cornering (roll), braking (dive) and acceleration (squat). I'm sure there is nothing more disconcerting to a driver than to lose traction while braking into a corner, due to loss of downforce because suspension dive upset the aero balance on the car. Most aero-dependent race chassis use a "three spring" suspension that is stiff in dive, but soft in roll.

Of course, most racers are cheaters by nature. They'll push the rules as far as they can, until they get caught. So any clever ideas that you think you might have, have likely been tried before.

But have fun anyway!

Thanks! I do understand that the diffuser is only one portion of the aero system as a whole. It's the area I'd like to learn about at the moment, once I have a better grasp of the diffuser, and how it works in relation to the rest of the aero, I'll move on to tweaking other bits.

You mention the location/height of the rear wing improving the performance of the diffuser. This is the sort of info I need to learn more on. Where is the optimal position for the rear wing? Should it be placed in the airflow that is coming out of the diffuser, or just above it, or??? How far back from the diffuser is 'best'? I realize there are compromises involved, but a general rule is what I'd like to know.

Once I have some free time (probably not til October!) I'll start building and see what I can come up with.

Thanks everyone,
Gary

I suppose anybody can use google so there is maybe little point in me posting few links, but here goes anyway. (Click-on pictures)