Exhaust Blown Floor - Forward Exhaust Exit

[segedunum]

Yer, what Renault are doing I think is trying to ensure that the gas flow maintains some sort of integrity along the length of the car so it creates enough of an effect. The effect of something tends to get minimised the greater the distance between two objects.......

What I didn't think of, and thanks to Ringo's diagrams, was just how quickly the gas cools. They need to do as much as they can with that hot gas in a short time and distance span.

I'm still not convinced by this really and I hope for Renault's sake that they don't spend too much time on it.

[ringo]

Ringo, nice work there... good to see my model being put to some actual use. =D> (looks like you'ev put the wheels in the wrong place though -too far forward?)

Can your analysis software do surface pressures? Afterall flow lines look nice, but at the end of the day all we really care about are pressures on the car....

If anyone wants to see the original CFD analysis on the standard car (with a basic 2009 double diffuser) I posted the pics here:-

viewtopic.php?f=6&t=6770&start=105

these are without FEE:
velocity


pressure:


surface plot, pressure:



bottom:





got a little scale on the side so you can get an idea. The car is moving at 70m/s or 156mph.

note: the profiles are taken at the centre, they'll look very different along the width of the car.

[myurr]

What surprises me in that is the huge area of low pressure over the side pods. I would have thought that this would be the opposite of what the teams want?

I'd be really interested in seeing the difference between that model and one with McLaren style side pods.

[machin]

Nice work again.... is that with the exhaust blowing? How do the under side pressures look without the exhaust blowing?

[BreezyRacer]

Very nice stuff guys .. just 1 question .. what about the high pressure area between the tail and the diffuser? I'm thinking that should be a pretty low pressure area unless your model has open ducts from radiator inlets to the exit.

[ringo]

It's with out the front exhausts.

The sidepods or any point of inflection usually produces a low pressure zone, especially for a convex curvature.
It's unavoidable for designs with hump shapes, such as sidepods and low noses. However having the surface behind that point fall away as quickly as possible can reduce the size of the low pressure zone and recover the pressure.
The redbull sloped sidepods have a reduced low pressure zone on top and it doesn't have much flat surface for the zone to spread out.

The model has radiator outlets similar to sauber's but closer to the diffuser.

[machin]

There are no radiators in the model that's true...

And remember I made it 'by eye' so there's a lot of refinement possibilities!

[BreezyRacer]

There are no radiators in the model that's true...

And remember I made it 'by eye' so there's a lot of refinement possibilities!

Not that it's the right thing to do but when I was modeling we made a panel in there with a holing pattern to "approximate" radiator flows. I couldn't ever really get any flow info from my racing radiator company, BTW, though we asked. In any case the diffuser results are highly suspect with high flows in that region .. You'll also find it might change inlet flows as well ..

[ringo]

There is room for a lot of improvements for real!
but it's too much work to refine over and over. It's a pretty decent model for an eye baller.

what i did the last time, not on an f1 car, was use a permeable surface to imitate the radiator.

I can probably put one in at a later time.

edit:



don't take my word for it, but there was a 13.6% increase in force in Y direction over the area highlighted by the blue line with FEE. Not the whole car!! just that area!! and this is a DDD car model.
I'll soon put up the comparison shots.

edit: Funny enough, there is little difference with the skin surface plots both with velocity and pressure.
I would have to change the colour scale in order to see the minute differences and that would lose the whole point of comparison.
I guess that's the limitation of solidworks, it's probably not so good a free mixing? Or maybe another paremtet would show something different?
Or the mesh settings simply need to increase in further detail ?

Here is another shot. I isolated the exhaust flow this time from the rest of the floor parts unlike last time, so you can clearly see where the flow is going. Explains the temp stickers on the endplate of the R31.

[segedunum]

Guys, while the diagrams are nice and all we really need to see what the effect of this hot gas being expelled is. As xpensive said earlier, unless we can see this creates some kind of useful Bernoulli effect then I'm very sceptical as to what this is doing apart from pumping out hot exhaust gas to be cooled quickly negating any effect you're seeking to create. I can't help but feel that Renault haven't throught this through well enough.

As a slight aside, in that diagram above can anyone see the higher pressure area at the back of the car and the slightly higher pressure area where the yellow dot is between the sidepod and the engine cover? Would anyone like to hazard a guess as to why the RB7's rear end is shaped the way it is?

[machin]

What we would hope to see is a clear difference in the surface pressures... Ringo says they're marginal... So maybe that's all there is to it; a marginal improvement, nothing more. An f1 car is all about adding up lots of small benefits...

[hollus]

I am still missing a factor here.
We all seem to agree that at least some of the exhaust flow goes under the car.
There is also talk of the gas rapidly cooling by mixing with colder gas, so with the hot gas contracting and the cold gas expanding, there is no net effect.
But that is for cooling through mixing with colder gas.
As I see it, for gas "trapped" under the car, there has to be some cooling by contact with the track and the car's floor, both much colder than exhaust flow. So, with all other factors being equal, how much cooling comes from contact with cold surfaces?
As I see it, once the hot air has entered under the floor, its mass content is fixed, but cooling will still reduce its volume creating lower pressure under the car (until enough external air can rush in). Significant?
Note that the mixing with external gas also has to be much slower under the floor than above the floor.

[segedunum]

If it's marginal then I really can't see any point to it. Renault will have probably moved heaven and Earth to get their exhausts routed forward in a way that won't affect reliability and won't have lost them any power. Has the work really been worth it? When you put this kind of thing in place you're looking for significant gains, not small ones. Work put in versus benefit. You would only put the exhausts where they have if you were sure you were going to get a sizeable increase in ground effect and downforce in that area.

I just don't see what the gain of this is versus routing the exhausts around the diffuser, creating a bigger effect by virtue of them being closer together and more enclosed and allowing as large a volume of air as possible to enter that area and be accelerated. Basically what Red Bull is concentrating on. I can't see what Renault is doing will affect the diffuser as much.

It's an interesting idea in theory, but the only way I can see it really working and being worth the effort is if you created a sculpted double floor of a significant area and routed the exhausts inward to create a much bigger Bernoulli effect on it. Now that I've thought about it I share xpensive's puzzlement over what the concept is really trying to do.

[ringo]

hmm well 13% improvement is not bad, but that is compared to a non blown diffuser car.
Most cars on the grid now are blowing somewhere on the floor so that 13 could be smaller relative to one of those blown diffusers.

I think the concept of this FEE is that the hot exhaust spends more time over the floor than if it were at the end of the car. It dissipates more energy into the air.

Then there is the low speed characteristics. This cfd is at 160mph, may it might look different at monaco like corner speeds?

As it relates to cooling on the track, I would have to factor in the roughness of the track and thermal characteristics etc. This cfd test never included thermal exchange with other surfaces, only the air exchanging heat with other gases.
The relative speed between ground and exhaust gases is high, so i think by the time there is any reasonable heat exchange with the ground, the car is already over a different patch of asphalt.
Whatever the case, try as i might, the exhaust is simply not staying underneath, the pipe angle and the influx of high momentum air from under the splitter makes it difficult for the exhaust gas to simply occupy the center of the floor. I posted a picture i think.
I will try the other types of blown floor, such as Torro Rosso and redbull and see what's the difference.

[xpensive]

Again something like the following:

This is my humble thinking; if rho goes down it would also decrease the dynamic pressure under the car for the same air-speed, which in turn should increase the static pressure under the same car. Is this a good thing?

What puzzles me as seg is xpressing, is if the xhaust gases increases the temperature of air passing under the car,
density goes down and so does dynamic pressure, which according to Bernoulli should increase static pressure.

Doing that by heating up the flow in the diffuser would make some sense, as has been tried in the past, but doing the same under the floor where you want as low a static pressure as possible, that simply makes no sense at all to me?

Puzzled.