Exhaust Blown Floor - Forward Exhaust Exit

[Raptor22]

from the renault r31 thread

n smikle wrote:
It has energy in the form of speed and temperature. When you impede it's motion or diffuser it the pressure will rise. Naturally it is going to drop to atmospheric after some time outside the pipe, but for example if ones uses their hand to block the pipe and you will see the pressure can get high.

There was a pdf on a CFD analysis of the blown diffuser floating around the internet. the pressure behind the exhaust was higher. I do not know how much higher though, but it can be significant if the exhaust is impeded.

Sorry but thats not correct at all. What you are feeling is the pressure rise against your hand not the exhaust pressure. When thehot gas leaves the boundary of the pipe it moves to atmospheric pressure instantaneously. Your hand is feeling pressure due to the velocity of the gas and is not a pressure that will be felt by the underside of the car.

The airflowingunder the car will be accelerated by the faster moving exhaust gas. It is this acceleration of the normal airflow due to the added kinetic energy and thermal energy loss to expansion that accelerates the air flow under the car. That drops the pressure under the car increasing downforce under the stepped bottom.
it is this alone that is doing the magic, nothing else.

Using a exhaust exit in or near the diffusor is slightlyless effective because you impart less kinetic energy to the freestream due to loss of knetic energy due to skin friction. Both concepts allow for accelerating the air flow under the car.

Lotus Renaults solution may have a slightly higher expansion ration resulting a few kilo more downforce but that expansion is now taking place over a much bigger surface area which could negate the benefit.

It doesn't move to atmospheric instantly, if it's presented with an obstruction the pressure will increase. Keep in mind the mass flow of the air and Newton's third law. The pressure rise against your hand is equal and oppossite to the pressure of the gas.Only if the obstruction is present, which in the case of an exhaust outlet, it is not. The exhuast drops to atmospheric with some time, but would only be instant with a propper diffuser on the pipe.
The exhuast is pointed sideways indicating it's intentions are not to fully go under the car. The uncoming air would have to be almost of equal energy and mass flow to turn the gasses right under the car.Its pointed sideways to minimise the pipefriction losses and manage back pressure. The mass of incoming air is greater than that exiting the exhaust. The exhaust gas only has kinetic energy to provide the necessary trajectory to split. No doubt some lost exhaust gas will flow along the side pod but thats simply due o the how the rules restridct the layout of the floor. The bottom of the exhaust exit must still be level with the bottom of the central floor.

That's the thing with this concept as well, it doesn't work the same way at all speeds. It probably has a speed where it works perfectly. The team might be able to angle it for different tracks.perhaps but I doubt it

One weakness is that a car can pull up beside robert or Vitaly and affect one side of their car by interacting with the flow field of the exhuast.

Nonsense.

[BreezyRacer]

I agree pretty much with Ringo's assessment except that imagine almost all the exhaust flow is under the car .. the exhaust flow acts as a barrier to KEEP FORWARD AIR FROM GOING UNDER THE CAR .. thus more low pressure area under the car, and low pressure created as far forward in the chassis as is possible.

The exhaust flow will exit nearly perpendicular to the car and interrupt the forward air off the the sides of the under tray.

You want LESS airflow under the car to create as much low pressure as is possible .. at least until you get so little airflow that there is no flow

The exhaust doesn't block the air from entering below sidepods since it is leveled up with the floor and most of the air passes below exhaust pipe.

Also, the idea isn't to reduce the flow under the car but to increase it. The more air you get under without bottoming the better (the greater difference in pressure between the air on top and below the better = greater downforce).

Skirts, both real ones as well as these virtual serve to prevent air from spilling sideways, since spilling reduces it's amount and speed, decreasing downforce.

I don't want to argue, but I do want to point out that more airflow under the car means less low pressure. Low pressure is what you're looking for. That has born out in the testing I have been involved with too. If you wanted more airflow you could direct more airflow UNDER the car with diverters to flow high pressure to the front of the sidepods floor splitters. Note that they don't do that .. just the opposite .. they divert as much high pressure airflow as possible to the sides ..

[SLC]

I agree pretty much with Ringo's assessment except that imagine almost all the exhaust flow is under the car .. the exhaust flow acts as a barrier to KEEP FORWARD AIR FROM GOING UNDER THE CAR .. thus more low pressure area under the car, and low pressure created as far forward in the chassis as is possible.

The exhaust flow will exit nearly perpendicular to the car and interrupt the forward air off the the sides of the under tray.

You want LESS airflow under the car to create as much low pressure as is possible .. at least until you get so little airflow that there is no flow

The exhaust doesn't block the air from entering below sidepods since it is leveled up with the floor and most of the air passes below exhaust pipe.

Also, the idea isn't to reduce the flow under the car but to increase it. The more air you get under without bottoming the better (the greater difference in pressure between the air on top and below the better = greater downforce).

Skirts, both real ones as well as these virtual serve to prevent air from spilling sideways, since spilling reduces it's amount and speed, decreasing downforce.

I don't want to argue, but I do want to point out that more airflow under the car means less low pressure. Low pressure is what you're looking for. That has born out in the testing I have been involved with too. If you wanted more airflow you could direct more airflow UNDER the car with diverters to flow high pressure to the front of the sidepods floor splitters. Note that they don't do that .. just the opposite .. they divert as much high pressure airflow as possible to the sides ..

I'm not one to argue either, but you are wrong here. For a further decreased static pressure level under the floor, you want as much mass flow as possible. Increased mass flow will mean a higher velocity at the throat of the system (that is, the floor LE), which means a further decreased static pressure level.

[ringo]

Nonsense.

Pressure comes in many forms, static and velocity pressure. An obstruction can easily covert velocity to static pressure.

[Agerasia]

Also pressure from the gas being hot.
Hot gas will displace the air around it, contracting as it cools.

[BreezyRacer]

I'm not one to argue either, but you are wrong here. For a further decreased static pressure level under the floor, you want as much mass flow as possible. Increased mass flow will mean a higher velocity at the throat of the system (that is, the floor LE), which means a further decreased static pressure level.

Well I guess let's try to get the bottom of this shall we and if we disagree so be it. What you want is velocity. Velocity is what creates the pressure drop.

Think of the undertray airflow as being pulled from the diffuser, not pushed from the frontal airflow. Will you get your highest velocities from high pressure or low pressure?

I know the answer because I/we studied this very question. It's easy to look at it both ways and make arguments fro and against. Testing revealed that reducing the leading edge airflow increases the downforce as long as you have enough airflow to keep things going along. With a 50mm stepped floor that problem won't happen.

[Formula None]

It seems the jury is still out in this thread regarding the primary effect of the front exhaust (vortex "skirts" vs increasing mass flow).

I'm finding myself in the increased mass flow crowd. Moving more air results in higher velocities and lower pressure under the car (provided the diffuser can extract this extra air effectively). All the air sucked into the engine is essentially being pumped to the front of the floor. I image a similar effect would be created if an intake ramp were positioned at the front of the floor to cram more air in (again, only effective if the diffuser can actually evacuate the extra air).

Comparison of my 2 lines of thinking (ramp feeding floor (top), engine feeding floor (bottom):




But this doesn't explain why the flow is being split above and below the floor. How that split is biased is unclear. It seems like two counter-rotating vortices would be created, above and below. I don't understand what effect this might have. To me it seems like whatever vortices are set up would be washed out fairly quickly by: on the top, all of the air flowing over the sidepods, and, on the bottom, all the air getting sucked in from the sides and front of the floor.

[manchild]

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.

[SLC]

I'm not one to argue either, but you are wrong here. For a further decreased static pressure level under the floor, you want as much mass flow as possible. Increased mass flow will mean a higher velocity at the throat of the system (that is, the floor LE), which means a further decreased static pressure level.

Well I guess let's try to get the bottom of this shall we and if we disagree so be it. What you want is velocity. Velocity is what creates the pressure drop.

Think of the undertray airflow as being pulled from the diffuser, not pushed from the frontal airflow. Will you get your highest velocities from high pressure or low pressure?

I know the answer because I/we studied this very question. It's easy to look at it both ways and make arguments fro and against. Testing revealed that reducing the leading edge airflow increases the downforce as long as you have enough airflow to keep things going along. With a 50mm stepped floor that problem won't happen.

Absolutely, velocity is what you want.

And yes, technically speaking, the mass flow through the system (and mass flow, seeing as this is all incompressible, is basically just velocity) is determined by the TE height of the diffuser. In a closed system, you cannot force any additional mass flow into the system inlet (floor LE) that the system exit is not willing to accept (and in this case, this limit is the amount of expansion offered by the diffuser ramp).

I don't understand why you ask your question "Will you get your highest velocities from high pressure or low pressure?"

I think it's pretty obvious that the higher the velocity at the floor LE the lower the static pressure acting on the floor lower surface will be. The lower the static pressure acting on the lower surface, the more downforce is produced.

On an F1 car the underfloor system is not simply a closed inlet-throat-exit type of control volume, however. It is complicated quite significantly by the vortex system created by the nose vane, T-tray, canard and bargeboard. There is also the fact that the sides of the floor are open, but we'll leave this.

The vortex system creates a very strong suction peak down the floor surface (I'd say 15%ish of the floor downforce comes from the vortex system alone), and the way in which this is created is quite sensitive. It is very much dependent on the longitudinal pressure gradient the air coming off the edges of the vortex creating devices experiences. This pressure gradient is determined by the suction created by the acceleration around the rounded LE of the floor. The exhausts are blowing directly onto this leading edge, and as a result, you end up with a stronger suction region behind the BBoard, and this suction encourages a stronger vortex system to develop. Thus, the downforce created by the forward portion of the lower floor surface is increased.

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?

[luca]

This just happened to Massa: viewtopic.php?p=221762#p221762
They say it was probably due to an oil leak.

What would happen on a car with front exit exhausts? I guess the fire would be really close to the driver.

[Richard]

Just like the pull-rod debate, there are multiple factors at work with the under floor air flow.

The objective of the under floor rake and diffuser is to expand the air flow to generate low pressure, hence downforce. Pumping exhaust into the diffuser near the rear of the car prevents stalling, and the velocity of the injected exhaust speeds up the air flow. The heat also causes the wake to expand, further decreasing air density and hence pressure (and annoying the following car).

Pumping air in at the front of the floor will have a different effect. As someone else has noted above, the air is sucked by the diffuser, not pushed from the front.

The under floor effects (ie rake) will have created some reduction of pressure under the driver's seat, pumping gasses in there will simply fill the void cancelling out any low pressure. I can't see how ramming more air under the floor at the front helps.

However, the killer the argument against pumping air under the floor is that the exhausts are facing outwards. If they were intended to feed the floor, they would face inwards wouldn't they? That's the evidence, Renault chose to push the air out to the side, not under the car.

So, I tend to agree the prime function is to create a virtual skirt along the floor edge.

[SLC]

The objective of the under floor rake and diffuser is to expand the air flow to generate low pressure, hence downforce. Pumping exhaust into the diffuser near the rear of the car prevents stalling, and the velocity of the injected exhaust speeds up the air flow. The heat also causes the wake to expand, further decreasing air density and hence pressure (and annoying the following car).

The diffuser ramp actually increases the pressure. I know it might sight slightly counter-intuitive, but the purpose of the diffuser (and its geometric expansion) is to increase the static pressure of the underbody flow back up to atmospheric pressure. The low static pressure under the floor is (which is what creates the downforce) is created at two points. First is the floor LE, which acts like a throat and accelerates the air which is being pulled in (it acts just like a constriction in a converging-diverging tunnel system). This acceleration brings the static pressure down below that of atmospheric pressure.

The second point of low pressure creation is the diffuser kink line just ahead of the diffuser ramp. This sharp curvature accelerates the air and you get a strong local suction region. The flow then expands into the diffuser itself, and its static pressure is raised as it nears the diffuser TE.

Yes, pumping exhaust into the diffuser itself adds momentum to the flow. As you say, this delays diffuser stall, and more importantly, allows a more aggressive diffuser profile to be used (which will allow more local suction to be created). In an ideal world this would also allow you to raise the diffuser TE height, but this is of course limited by the regulations.

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.

Pumping air in at the front of the floor will have a different effect. As someone else has noted above, the air is sucked by the diffuser, not pushed from the front.

The under floor effects (ie rake) will have created some reduction of pressure under the driver's seat, pumping gasses in there will simply fill the void cancelling out any low pressure. I can't see how ramming more air under the floor at the front helps.

Yes, strictly speaking the diffuser controls the mass flow. However, the floor of an F1 car is not a closed system and so as I explained in my earlier post, can not be considered in such simple terms. The floor vortex system has a significant effect, and this is affected by several other things in addition to the diffuser expansion.

I don't think you really understand what is mean by low (static) pressure. There is no "void" so to speak... by blowing air into the region you do not "cancel out" the low pressure. In fact, by blowing high velocity, energetic flow (high total pressure!) you actually further decrease the low static pressure along the streamlines.

However, the killer the argument against pumping air under the floor is that the exhausts are facing outwards. If they were intended to feed the floor, they would face inwards wouldn't they? That's the evidence, Renault chose to push the air out to the side, not under the car.

So, I tend to agree the prime function is to create a virtual skirt along the floor edge.

The exhaust nozzles may be facing outwards in a geometric sense, but I don't think the flow ends up along the outboard edges of the floor. The flow at the floor LE has a significant, natural y-component due various reasons (the FW flap vortex is one of them, which, due to its counter-clockwise sense generates a lot of outwash ahead of the sidepod), but the flow ends up being pulled back in to flow under the floor. I realise it may not look like it from the pictures of the car, and it is pretty hard to show this without some flow-vis or CFD results.

The exhaust exits are facing outwards because they are aimed at the rounded LE of the floor. The profile of this LE is curved (this is related to the outwash in this region) and as a result, in order to maximize the acceleration of the air about this surface you want the oncoming flow to hit it perpendicular to the section.

Anyway, i think I'm starting to ramble a bit now

[Raptor22]

PLease listen to SLC.

BTW SLC, the purpose of the diffusor isto raise the static pressure at exit WTHOUT SHOCK. Very important the shock part heh.

Nice work on sticking with this. Itshard to dispell science fiction once it gains traction (oh was that a pun, my bad)

[horse]

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?

[SLC]

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