Blow Augmented Diffuser 1

[Paul6]

On an enclosed body race car I am designing a diffuser for ground effects. Because of the bodywork, there is no source of high speed flowing over the trailing edge of the diffuser. This is an important feature for entraining air out of diffusers and enhancing the flow. I plan on introducing rearward directed high velocity air at some point in the diffuser. This would blow out (or past) the exit of the diffuser to encourage higher mass air flow through the throat of the “venturi”. The source for this introduced air would be high pressure air from the stagnation point on the nose of the car. I recognize there will be some ducting losses, which will be minimized by reducing the velocity of the transit flow.
The question is, where is the best point at which to introduce the high velocity jet? Should it be at the trailing edge of the diffuser, or further forward in the tunnel?

Paul

 

[matt00000]

If you put a tube from the nose of the car to the rear of the car then the nose of the car will be at 1 atmosphere not at a higher stagnation pressure. i think you might want to rethink you theory a little more...

 

[evelrod]

I am not sure what you mean Matt. At high speed even the most technical airfoil will show pressure variations at the leading edge . A racing car is hardly textbook airfoil and definately shows pressure variations over it's surface, e.g., at the nose, in front of and behind the 'greenhouse' and, at the rear end.
Am I missing something here?

Rod

 

[i278]

If I read Matt right, what he's getting at is if you tap static pressure at the stagnation point, with the intent of dumping it into a lower static pressure area, you're not going to get the big pressure differential you're expecting. The static pressure at the stagnation point is high because exactly that: it's stagnant (it's been brought to a standstill). If you now replace part of the nose of the car with a low loss duct, taking air to parts unknown, the air no longer stagnates at that point--it flows right through, and the static pressure never rises.

Now, of course, this is a matter of scale: if you're putting a little intake on the front of the car to blow air on the driver's face, no one will ever notice the effect. Pulling off enough air to significantly change the flow around the rear of the car is a different matter. However, I think this is a red herring in this situation. Just take the freestream-velocity air you're given and work with that.

My first stab at the problem would be to try to prevent separation of the flow as it goes 'around the horn' at the beginning of the diffuser. I'd take your flow and inject it parallel to the diffuser wall just after the end of the transition from the flat underbody.

If I'm not being clear, just do a web search for 'aircraft slotted flap'. Look at the picture upside-down and that'll give you an idea of something to try. The difference would be that instead of injecting air before the transition, you'd do it after.

Just my two cents--never tried any of this, but I never let ignorance get in the way of an opinion

Regards

 

[matt00000]

Hi, sorry I didnt make mayself very clear because i wanted you to think about the problem some more. i278 explained what i meant though. If you want to introduce a high pressure jet the only place you will get a high pressure is from the (engine)exhausts. and obviously when you corner you lift off and reduce the effect when you need it most!

the idea of the diffuser is to reduce the pressure under the car, this with the effect of the front spliter is what gives you your downforce. if you do a google search for aerodynamics of cars. there was a web page with some diagrams of what happens. i would search for you but i will be late for work if i do!

good luck,
Matt

 

[Paul6]

As i278 suggests, in the simplest form: think of it as an unusually shaped wing with a Fowler flap running inverted just above the ground. The back edge of the flap continues into the bodywork. The bottom flow of the flap is ducted thru to points outside the car. Is Matt suggesting there would be no flow thru the flap to wing slot? 1 Atm on both sides?

In the actual proposed design, the air flow for the flap slot is collected in a chamber with an inlet at the highest pressure point at the front. It turns out that full bodied race cars consistently create a high pressure zone (for other reasons) at the front edge with a splitter . My plan was to use a front inlet opening approximately four times the area of the slot injecting flow into the diffuser, thus maximizing inlet pressure by reducing the inlet flow velocity.

This is my fourth diffuser/downforce system design for race cars, so I’ve got a fairly good idea how they work. This is the first in which I’m considering the introduction of external air flow to augment the diffuser flow. Previous flow injected designs (to my knowledge) have injected exhaust flow typically 1/2 way back the diffuser in a direction 30 to 40 degrees to the diffuser ceiling. This is an attempt to induce increased mass airflow. (Not in vain, as Matt has suggested, but instead, at the most crucial phase of a race car’s operation: the beginning of corner exit, when full throttle is reached. No cornering phase will do more to improve lap times.)

Unfortunately, exhaust augmentation is not an option as this car is built around the latest (rules forced) aero scheme with separate diffusers, front and back. This discussion is about boundary layer control of the front diffusers which exit thru the flanks of the car. i278’s idea of injecting flow just past the diffuser “corner” has a lot of charm, especially as there is a steep bell shaped negative pressure peak centered on the corner of the throat to diffuser transition. This would maximize the pressure differential from the front inlet and give better injection velocity. I suppose the question would be: should the injection be close to the corner at the risk of a reduction in the bell’s peak or should it be further up the slope to re-energize the boundary layer further up the slope when it is more “tired”?


Paul6

 

[i278]

Well, it sounds as though you have a good handle on this. If you're having any difficulty getting an intuitive feel for the situation, I think it may be that you're not keeping total pressure and static pressure separate enough in your head. It seems to be a trap that everyone falls into at some time.

Keep in mind that both the under-car flow and the duct flow have the same source of total pressure--the flow in front of the car. So, if frictional losses are similar for the two flows, the combinations of static and dynamic pressures for both the flows will all work out fine, and you'll certainly have flow through the slot. Just don't expect a jet like out of a compressed air nozzle. The slot is more about 'active redirection' of the flow--making sure the flow continues how and where you want.

So, where to have the slot exit? As I said, my first guess would be just after the transition to the diffuser. If, however, the flow stays attached for some distance up the diffuser wall, you only need to inject the flow just before where the separation occurs. The separation point can only be determined (as far as I know) by experiment. It sounds like you have access to a couple cars. If I were you, I'd break out the masking tape and ball of yarn and stick yarn tufts all over that diffuser wall. Then drive behind the car at speed and see what you have. Who knows; maybe there are some large-scale vortices back there that are already forcing flow attachment for you. I doubt it, but you never can tell with cars.

As far as your inlet goes, make it as small as possible, probably no larger than your exit area. After the inlet, however, your duct should be a diffuser itself. Get the cross-section as big as you have room for, but make sure the included angle stays under 8 degrees, or you'll start getting the same separation problems in the duct that you're trying to solve at the rear diffuser. Keep the duct nice and big, as you propose, and end it with a smooth but short contraction just before the slot.

Regards

 

[matt00000]

Hi Paul,

Now you have mentioned what you have mentiod I see that you have thought about what you are doing! quite condiderably!
what I ment was if you have a hole at the front of the car with a pipe to the back then you will not reach stagnation pressure as (as you are probably aware!) stagnation only occurs when the free steam velocity is reduced to zero on the front of the car. if the air can flow unresticted 'through' the car then it wont be stationary and therefore wont increase in pressure.

depending on the size of the inlet the amount of pressure you generate in your chamber and the size of the outlet are will depend on where the best place to put the outlet is. if it is just to energise the boundry layer then you will need to do some flow vis on the diffuser to find where the flow seperates. if you are unable to do this in a wind tunnel then use oil on the diffuser and you should be able to tell. since you already have three to use.

if you have a high mass flow then placing the outlet near the start of the diffuser will help to generate more downforce over the section of the diffuser.

whilst it is true that exiting the corner with a high degree of downforce is beneficial (as it means is you can put your foot down abit more than you would without it) it is probably of greater benefit to have maximum downforce during breaking for the corner and going into the apex of the corner. very few race cars go off the track exiting the corner. the more the downforce during breaking and turn in the high the entry speed you can have. but i am sure you are aware of that and the exhaust blown diffuser is obviously just a compromise as maximum down force would occur when the car is running a straight with max revs (probably the only time you dont want max downforce)

anyway as i said good luck with your reasearch.

Matt