Exhaust Blown Floor - Forward Exhaust Exit

[machin]

Thought you guys might appreciate a CFD plot from my simple F1 car to discuss the underfloor pressures....




Green is ambient pressure, blue is low pressure, red high pressure.
This is a 2009 model with a very simple double diffuser, so not totally as per today's rules.... but might help the discussion....

[Richard]

SLC - You've been very clear and articulate in this - thank you!

Apologies for the schoolboy error about the diffuser ramp. Of course the underfloor airflow returns to atmospheric pressure as it exits the car.

[Raptor22]

The blown diffusers of last year actually didn't really blow that much air into the diffuser itself. Some designs did blow exhaust through the starter motor hole, but the main benefit was to blow exhaust along the outboard edge of the ramp - the primary effect of this was to change the inboard tyre squirt.

There was some part in energising the flow at the diffuser roof though, no? I thought the purpose of the exhaust gases was to improve the flow quality in this region?

Also what is inboard tyre squirt?

Yes, probably. I can't quite be bothered to go back and find pictures of all of the 2010 exhaust designs, but you are right, by increasing the velocity of the air on the top surface of the diffuser ramp the flow out of the main diffuser outlet will be improved. Although the whole purpose of the diffuser ramp is to raise the flow's static pressure, by reducing the pressure at the diffuser TE you will locally encourage flow to be pulled out. This can help a diffuser ramp which is on the border of separating, for example. The diffuser TE lip serves a similar function - acting as a Gurney flap it creates a discrete drop in static pressure just behind it. Furthermore, by using the exhaust to blow into it, the effect can be strengthened again.

The tyre squirt is the sludge of air that gets squeezed out from under the tyre contact patch. As the tyre rolls forward the air that is displaced out of the way is of extremely low energy. The contact patch produces a "squirt" of this air on either side. The inboard squirt is the squirt that gets pushed in towards the diffuser, and this sludge of air presents itself as a blockage at the diffuser TE plane, and this is messy in several ways. One of the reasons for the RBR car being so good even without a double decker at the beginning of 2009 was it had great control over this squirt - using several vortex generating devices on top surface of the floor just ahead of the rear tyre as well as a saw-tooth style edge on the diffuser flank near the rear drum.

You want to keep the inboard squirt as small as possible and as far away from the diffuser as possible.

And cheers for your support Raptor22, I know we've had our differences in the past

Never a problem. Life is about agreement, disagreement. As long as we try to keep some dignity to anything then life is pleasant.

Respect.

[BreezyRacer]

I'm interested to hear more about your testing of this phenomenon though. What was the setup? How did you reduce the "leading edge airflow," and how did you measure the downforce?

I love that .. "phenomenon".

Anyway we did I believe 23 diffuser undertray/diffuser tests purely on an open wheel car without wings but with a full floor and we controlled leading air patterns with splitter shapes and inlet opening sizes (not F1 rules that mandate a stepped floor). We even went so far as to test induced airflow into the leading edge area. Results clearly showed that less entry airflow was the way, and that leading edge shaping was very critical for the vortexing you are speaking of. Incidentally they also showed that diffuser box shape is far more important than diffuser volume and tire isolation is critical.

This was all done in CFD with an aero engineer from a major F1 team so the testing itself was quite correct. Funny but when we were testing he wasn't sure about the more entry airflow or less entry airflow either. That's why we tested it .. numerous times. We did a lot of work on leading edge shaping.

More important than my testing is the simple observation of current F1 design. Currently almost everyone is ducting the highest velocity airflow into the undertray via tea tray inlets along the side. That's your purest highest velocity airflow. Then there are the bargeboards .. they don't just divert high pressure to the sides .. they also create a vacuum of sorts on the backside of the barge board that stabilizes and minimizes airflow entering under the sidepods. Would a bargeboard make any sense if you wanted MORE airflow under the car. More airflow = lift.

[SLC]

I'm interested to hear more about your testing of this phenomenon though. What was the setup? How did you reduce the "leading edge airflow," and how did you measure the downforce?

I love that .. "phenomenon".

Anyway we did I believe 23 diffuser undertray/diffuser tests purely on an open wheel car without wings but with a full floor and we controlled leading air patterns with splitter shapes and inlet opening sizes (not F1 rules that mandate a stepped floor). We even went so far as to test induced airflow into the leading edge area. Results clearly showed that less entry airflow was the way, and that leading edge shaping was very critical for the vortexing you are speaking of. Incidentally they also showed that diffuser box shape is far more important than diffuser volume and tire isolation is critical.

Did you specifically set a boundary condition that dictated the mass flow entering the underbody? Or were you assuming that by placing devices ahead of the floor inlet you would change the mass flow in turn?

More important than my testing is the simple observation of current F1 design. Currently almost everyone is ducting the highest velocity airflow into the undertray via tea tray inlets along the side. That's your purest highest velocity airflow. Then there are the bargeboards .. they don't just divert high pressure to the sides .. they also create a vacuum of sorts on the backside of the barge board that stabilizes and minimizes airflow entering under the sidepods. Would a bargeboard make any sense if you wanted MORE airflow under the car. More airflow = lift.

Bargeboards don't "divert" high pressure. They create a high pressure. More correctly, they create a large pressure differential across themselves (high pressure on front surface, low pressure on rear surface). The low pressure on the rear surface is created by both lateral acceleration across the rear surface, as well as through a rapid total pressure loss over their top and bottom edges. They are just large vortex generators.

The addition of bargeboards actually increases the mass flow under the car due to the added vorticity.

More airflow = higher pressure differentials = more kinetic energy = more low static pressure. This leads to more downforce

[ringo]

I think that some people are disregarding/misjudging the actual size/position of exhaust relative to floor, and where it's influence on air stream begins.

Most of the air that reaches below floor in the same way and volume just as on cars without front exhaust, BUT instead of escaping sideways, with Renault's way, it is maintained below floor much longer. Therefore, more of it reaches the diffuser than what on normal car would escape sideways.

That's the right drawing man.

[BreezyRacer]

Did you specifically set a boundary condition that dictated the mass flow entering the underbody? Or were you assuming that by placing devices ahead of the floor inlet you would change the mass flow in turn?

More important than my testing is the simple observation of current F1 design. Currently almost everyone is ducting the highest velocity airflow into the undertray via tea tray inlets along the side. That's your purest highest velocity airflow. Then there are the bargeboards .. they don't just divert high pressure to the sides .. they also create a vacuum of sorts on the backside of the barge board that stabilizes and minimizes airflow entering under the sidepods. Would a bargeboard make any sense if you wanted MORE airflow under the car. More airflow = lift.

Bargeboards don't "divert" high pressure. They create a high pressure. More correctly, they create a large pressure differential across themselves (high pressure on front surface, low pressure on rear surface). The low pressure on the rear surface is created by both lateral acceleration across the rear surface, as well as through a rapid total pressure loss over their top and bottom edges. They are just large vortex generators.

The addition of bargeboards actually increases the mass flow under the car due to the added vorticity.

More airflow = higher pressure differentials = more kinetic energy = more low static pressure. This leads to more downforce

First, while I am not a CFD expert the person I was working with is. We assumed nothing and I realize that using CFD for undertray design can be tricky with emulating ground speed, boundary flows, etc. Ride heights and pitches were maintained throughout testing to arrive at sound results.

As for the bargeboards they divert high pressure from the front of the sidepods to the front of the bargeboard .. they do not create high pressure .. and they do create the low pressure on the backside to vortex, like any high/low incidence would create. Without the bargeboard you would just get a lot of turbulence running under the floor unless you can isolate upper body and lower body flows with an extended splitter.

Anyway, we're just not going to agree on this .. that's what testing is for ..

[SLC]

Did you specifically set a boundary condition that dictated the mass flow entering the underbody? Or were you assuming that by placing devices ahead of the floor inlet you would change the mass flow in turn?

More important than my testing is the simple observation of current F1 design. Currently almost everyone is ducting the highest velocity airflow into the undertray via tea tray inlets along the side. That's your purest highest velocity airflow. Then there are the bargeboards .. they don't just divert high pressure to the sides .. they also create a vacuum of sorts on the backside of the barge board that stabilizes and minimizes airflow entering under the sidepods. Would a bargeboard make any sense if you wanted MORE airflow under the car. More airflow = lift.

Bargeboards don't "divert" high pressure. They create a high pressure. More correctly, they create a large pressure differential across themselves (high pressure on front surface, low pressure on rear surface). The low pressure on the rear surface is created by both lateral acceleration across the rear surface, as well as through a rapid total pressure loss over their top and bottom edges. They are just large vortex generators.

The addition of bargeboards actually increases the mass flow under the car due to the added vorticity.

More airflow = higher pressure differentials = more kinetic energy = more low static pressure. This leads to more downforce

First, while I am not a CFD expert the person I was working with is. We assumed nothing and I realize that using CFD for undertray design can be tricky with emulating ground speed, boundary flows, etc. Ride heights and pitches were maintained throughout testing to arrive at sound results.

As for the bargeboards they divert high pressure from the front of the sidepods to the front of the bargeboard .. they do not create high pressure .. and they do create the low pressure on the backside to vortex, like any high/low incidence would create. Without the bargeboard you would just get a lot of turbulence running under the floor unless you can isolate upper body and lower body flows with an extended splitter.

Anyway, we're just not going to agree on this .. that's what testing is for ..

Ah ok, yes, as you put it there I agree with you - they are diverting high pressure away from the sidepod onto themselves.

But I still maintain that increasing the mass flow under the floor will lead to increased floor performance. I've been developing T-tray and bboard devices for many years both in CFD and in the tunnel and I trust my results (and so does my boss!).

[BreezyRacer]

Well SLC I had thought we came away from our project with a clear idea of what works. Certainly our results were good .. but maybe I'll need to keep an open mind on this point.

For our particular project we were working with low ride ride heights (we grounded our testing at 25 mm with no pitch) and the problem was that under heavy braking we could have the undertray within about 10-12mm, which gets into boundary layer problems and diffuser choke off around there. Therefore we always tested with some size of inlet port. Maybe the low ride heights we were testing at had something to do with it. It's quite different than a 50 mm height as is found in F1.

Our best work came with moderate inlet ports and a splitter sculpted to create some vortexing. Most people don't know how sensitive undertrays are to inlet behavior and we ended up spending a lot of time on that.

[Richard]

But I still maintain that increasing the mass flow under the floor will lead to increased floor performance. I've been developing T-tray and bboard devices for many years both in CFD and in the tunnel and I trust my results (and so does my boss!).

It is all relative. get too much air under there and you've created an air brake, not enough and the diffuser stalls.

I guess the more mass you can squeeze through there, then the more it has to accelerate to get the mass through the gap?

Once past that leading edge you want to minimise leakage into the low pressure zone from the side? Would R31 exhaust avoid feeding directly into that path?

However, a stream of hot air down the edge will speed up the underfloor airflow, just like the EBD last year seemed to blow the edges of the diffuser, not directly into the air flow ?

[forty-two]

For what my opinion is worth, I didn't initially think that the concepts which SLC has now explained were the truth of the matter.

I have now changed my mind. if I remember rightly, SLC made a number of comments on here about 12 months ago regarding the F-Duct, and they all turned out to be right, so that in itself inclines me to listen to what he has to say!

It's things like this which make me really love F1, and if it weren't for you lot I wouldn't have anyone to explain what the heck is going on! Thank you all!

Edit: Typo

[Raptor22]

higher mass flow under the floor increases downforce if released without shock via a Diffusor.
Higher mass flow without the diffusor leads to increased drag.

The diffusor is the magic.

[ringo]

How about zero mass flow with a vacuum under the car?

Most of the assumptions some are making here are with the atmospheric total pressure.

If you introduce something else into the picture with higher energy, such as an exhaust system, it has a higher total pressure than the free stream. The proportion of static to velocity pressure wont have to fall within the limits of the free stream total pressure.
Therefore the assumptions wont be clear cut as turning an exhaust under the car will increase down-force becuase the velocity is higher.

Conversely If the car was perfectly sealed along the floor and a vacuum was under it, it wouldn't have to be moving at all to stick to the ground. No velocity needed, just the pressure of the atmosphere over the top surface area of the car, like a plunger.

Not to say one assumption is right or wrong. But special situations change the the rules of thumb. This thing has to be tested to be sure.

[Giblet]

Oh wise ones, I asked about something a couple of pages ago, but nobody answered yet.

What if any heating of the rear tires will happen with this setup, and do you think Renault will have an advantage in restarts with keeping their tires warm without having to wear them as much with all the hard acceleration and 'burnouts'.

[BreezyRacer]

Oh wise ones, I asked about something a couple of pages ago, but nobody answered yet.

What if any heating of the rear tires will happen with this setup, and do you think Renault will have an advantage in restarts with keeping their tires warm without having to wear them as much with all the hard acceleration and 'burnouts'.

Obviously everyone does not feel the same about where this exhaust flow will wind up but I don't believe it will have any affect on heating the rear tires. I'm imaging that the exhaust flow will be outside the floor well before that.

OTOH, I could see standing starts being a little harder than normal on heat, as in the heat from the exhausts collecting in the radiator inlets while standing still.

I really think Renault have one-upped the field with this design but we shall see .. they could be the "Red Bull" of 2011.