I found an interesting literature review concerning underbody diffusers and there is a list of references, some of which are through the SAE.

Here's the article, and if anyone can get me electronic copies of the articles or knows of a way, I'd appreciate it.

http://www.tech.volvo.se/ivs/courses/kurs_...ero_rapp_sp.pdf


Thanks

Tony

you can order it yourself:

sae papers

I'm a member so I get a discount, but it's not worth it for a few bucks.

Wow, I'm used to IEEE where you don't have to pay for papers if you are a member.

Anyway, thanks.

-Tony

UCI has access to the papers at the school libraries. I rifled through and got a bunch one day when I was bored on campus, but that rarely happens now (free time on campus). I'll see if I can access them from home since I'm on a uci only network or if I can swing by the library soon.

So what would an underbody diffuser look like? I'm working on a design to modify the front of my car to reduce lift at high speed. My current design is to copy the tray and side diffusers on the prototype Lamborgini Miura that was in AutoWeek last month. I've been wondering how hard it would be to do something to smooth out the underbody at the same time. Any ideas?
Here's a pic of the Miura. It's hard to see the flat front tray under the grill.

Here's a pic of my car that I hope to mimic that tray and side diffusers to.

I'm thinking it would look something like this...

Basically a pair of venturis to accelerate the air and thereby lower the pressure resulting in downforce. But for my purposes its to suck air through a radiator. The downforce is an added benefit.

-Tony

Tony; is your radiator located in the front trunk? How large will you need for the venturi to move enough air? I guess a rear piece like you show under the engine would be questionable for an aircooled 6. I'm thinking maybe one that stops at the rear of the engine would still let the heat get sucked down.

Porsche ground effects car... 962

QUOTE (drew365 @ Feb 7 2006, 07:41 AM)

Tony; is your radiator located in the front trunk? How large will you need for the venturi to move enough air? I guess a rear piece like you show under the engine would be questionable for an aircooled 6. I'm thinking maybe one that stops at the rear of the engine would still let the heat get sucked down.

I've been thinking about the G.E. concept for awhile.

Some ground effects tubes were inboard (on f1 cars) and they used a duct set at a right angle and pointed straight at the ground, which caused suction at that spot. A few holes pointed straight at the cylinders might do well to keep them cool and remove that much air, at speed. There may not be an application for street use quite like I've stated, since the air is only sucked away when at speed. If anyone wants to try it....

Don't diffusers work more on Boyle's law, than the Bernoulli principle. More volume = lower pressure.

well... yes and no. Bernoulli's says that airspeed increases/ decreases with the size of the container. But a diffuser isn't necessarily about changing airspeed. It's more about controlling direction of air to minimize turbulence. At least this is my understanding.

But with the example that tony mentioned, there is airflow through the CF channels in the rear, fast airflow and directional. Fast air tends to create a low pressure near it and draw more air to it. Perphaps air from the engine bay, through holes in the CF, as long as those holes don't interfere with the airflow through the channels.

Sorry if I jacked this thread. I can start a new one if anyone wants.

QUOTE

Tony; is your radiator located in the front trunk? How large will you need for the venturi to move enough air? I guess a rear piece like you show under the engine would be questionable for an aircooled 6. I'm thinking maybe one that stops at the rear of the engine would still let the heat get sucked down.

My radiator will be located about 5-6 inches from the back of the rear firewall. I'm planning on having the radiator exhaust exit through a slot which goes across the firewall/floorpan seam. It will be about 5 inches wide and about 24 inches across. Air will be forced into the engine bay via a subtle air diverter or scoop and also be sucked through at speed using the venturi. At least thats the idea...

I'm really not sure how large the venturi needs to be. Since I don't have access to a wind tunnel or computational fluid dynamics software I have to hack it. I've thought about making some scale models and a fluid tank for testing. First though I want to research the topic and get some intuition about what to do.

In my mind, its what is after the rear of the engine that causes the most problems. The 914-6 GTs had no rear valence and I believe it was at least in part to reduce drag caused by the 'parachute effect' of trapped air in front of the rear bumper.

QUOTE

Don't diffusers work more on Boyle's law, than the Bernoulli principle. More volume = lower pressure.

I believe Boyle's law is a 'static' gas law, whereas Bernoulli's principle has to do with fluid flow, that as pressure decreases, velocity increases.

QUOTE

well... yes and no. Bernoulli's says that airspeed increases/ decreases with the size of the container. But a diffuser isn't necessarily about changing airspeed. It's more about controlling direction of air to minimize turbulence. At least this is my understanding.

That's partially correct. In the end it is all about reducing pressure under the car which is effectively downforce. The shape of the channel is made such that the air pressure and velocity are kept low and high respectively. This works best (or maybe at all) with laminar flow versus turbulent flow, simply because turbulent air moves slower. From what I've read so far though, maintining laminar flow on any part of a vehicle is next to impossible, so its all an approximation anyway. In any case, the air immediately exiting the venturi is going to to be massively turbulent as that is the low pressure condition.

So really it isn't about direction, but about air velocity/pressure.

-Tony