Lotus E20 VD

[bhall]

Let's not fight over taxonomy, but a diffuser in the form fitted to the rear underside of current F1 cars creates downforce has a low pressure in it (even lower than normal ambient pressure) so it creates a force pulling ground and car together.
(crucial for that is sealing against flow from the side, and as LMP front diffusers have that I think it is fair to call them just that)

I'm sorry, but I simply cannot agree with that. If a diffuser has lower than ambient pressure, how could it possibly vent? Wouldn't it draw in ambient air or create god-awful drag as the forward motion of the car stripped it of the air that it's desperately trying to grip (figuratively speaking, of course)?



A "traditional" diffuser accelerates flow under the floor ahead of it, and that creates low pressure under the floor, which is downforce. The peak of low pressure is at the mouth of the diffuser. From there, pressure only rises.

This has, however, helped me identify a gap in my explanation. So, that's great.



The VD's diffuser (green) is extracting air (red) that's already been vented from the duct (blue) due to the duct's high pressure and the tendency of pressure to follow the path of least resistance. The diffuser does this by creating a peak of low pressure at its mouth, which is immediately behind Exit 1 that's venting high pressure air from within the duct. The pressure gradient here is then high>low>high, and that keeps the duct from becoming completely pressurized.

Does this make more sense now?

[superdread]

I'm sorry, but I simply cannot agree with that. If a diffuser has lower than ambient pressure, how could it possibly vent? Wouldn't it draw in ambient air or create god-awful drag as the forward motion of the car stripped it of the air that it's desperately trying to grip (figuratively speaking, of course)?

The air behaves kinetic (every particle has it's own impulse) so as a diffuser has a lower pressure than the environment the particles are slowed down but not enough to stop them.

It doesn't have to have pressure as low as that, but it is lower than the flow over the diffuser and certainly lower than the one under the floor. (Similar to the floor this has to create DF, and it is more than the same length of floor.)

And not to get me wrong, there is a drawing effect lowering underfloor pressure ahead of the diffuser, but it itself does create DF.

[bhall]

I understand what you're saying; I just don't necessarily agree with it in the context of the conversation. However, this portion of the conversation nonetheless helped me identify an area I was missing, and for that, I'm very grateful.

[gato azul]

with a bit good will, one could say, that the Lotus apparatus around the exit of Vent 1 has some similarities to this solution:



but it utilizes only 1/2 of it.
Interested parties, can look/google for Diffuser-Augmented Wind Turbine (DAWT)

@bhallg2k

Don't worry, I don't want to "steal your thunder", it's all good.
Our different views revolve around, how to view the volume under the engine cover.
You draw/view it as a plenum with on entry (two if you consider the two "ears" separate) and two exits Vent 1 & Vent 2 (VD).
I may see it a bit different, but no need to argue about it.
Some of your explanations and terms used (while I think I know what you have in mind), may confuse some people.
Maybe we need to clear a bit up around flow, velocity & pressure, and how they are related, and what is causing what.

this would be an example

pressure always follows the path of least resistance

I know, what you want to say, but it's maybe not the best way to phrase it, the same goes for the balloon example.
I know what you want to explain with it, but it's not the best analogy - IMHO

If you want to put some numbers to it, flow through an orifice if maybe a starting point, at least for an "approximation"
there will be perhaps some different opinions about the discharge coefficient of the VD slots, and which kind of pressure you discharge into (ambient vs. lower then ambient argument).
This will be important to define the maximum flow rate through the whole duct/system

Have fun

[superdread]

Ok, well, let me just say that your explanation of diffuser pressure takes a big --- all over Daniel Bernoulli, but that's just fine by me. (And I mean that in a very lighthearted way.)

This portion of the conversation nonetheless helped me identify an area I was missing, and for that, I'm very grateful.

I don't see how. Lower than ambient pressure, means lower velocity than ambient (the car's speed, so not 0) but still all the air can drain out as the diffuser has a higher cross section than the floor.

Still, this is not necessary for my understanding, as it is quite clear the diffuser has a lower static pressure than the underside of the floor, and the flow over the diffuser has at least the same pressure as the one over the floor, so when the floor creates downforce the diffuser must as well.

Other explanation would be, that the flow is certainly swept upwards by the diffuser and by conservation of impulse something has to be forced down (and that can by exclusion only be the car.

And number three, the diffuser profile is shaped like a wing, even if flow above and under the wing are independent from one another it creates downforce (a 'normal' wing still creates downforce if the upper side is blown, or the lower).

All of these make sense to me, so take your pick.

(they are not strictly explanation, but rather plausibilisations (that's not a word, I know))

In the naming dispute, a diffuser in it's basic principle does not include creating any downforce but the lower one on a F1-car does. The expansion "nozzle" arrangement of the duct at the beam wing can by all means called a diffuser (but this one creates DF as well it expands more upwards than downwards so conversation of impulse... yadda yadda)

[bhall]

Don't worry, I don't want to "steal your thunder", it's all good.
Our different views revolve around, how to view the volume under the engine cover.
[...]

I'm sorry if my tone came off as argumentative. That's not my feeling at all. I think I'm just a bit frustrated with my inability to convey what it is I've been visualizing in my head. Despite that trouble, I'm very confident that what I see is an accurate representation of what's been implemented on the E20 thus far. The differing views are good, because it's helping me - well, all of us - nail down the particulars.

[superdread]

Don't worry, I don't want to "steal your thunder", it's all good.
Our different views revolve around, how to view the volume under the engine cover.
[...]

I'm sorry if my tone came off as argumentative. That's not my feeling at all. I think I'm just a bit frustrated with my inability to convey what it is I've been visualizing in my head. Despite that trouble, I'm very confident that what I see is an accurate representation of what's been implemented on the E20 thus far. The differing views are good, because it's helping me - well, all of us - nail down the particulars.

To make clear how an effective unproductive (we're not trying to develop a product here, this is just for gain of knowledge, i.e. fun) conversation works:
Everyone takes a stance and finds arguments to support ones own and 'disprove' the ones from the others. Opinions migrate, it can get heated or die down. But in the end everyone has thought about his and other arguments and learned something, not necessarily something about what was discussed.

You have refined your theory about the flow accelerated and concentrated sidepod vents. I try to find a kinetic explanation for diffusers (or something like that, I'm not quite sure). At least it wasn't boring.

[bhall]

[...]

I don't see how. Lower than ambient pressure, means lower velocity than ambient (the car's speed, so not 0) but still all the air can drain out as the diffuser has a higher cross section than the floor.

Still, this is not necessary for my understanding, as it is quite clear the diffuser has a lower static pressure than the underside of the floor, and the flow over the diffuser has at least the same pressure as the one over the floor, so when the floor creates downforce the diffuser must as well.

Other explanation would be, that the flow is certainly swept upwards by the diffuser and by conservation of impulse something has to be forced down (and that can by exclusion only be the car.

And number three, the diffuser profile is shaped like a wing, even if flow above and under the wing are independent from one another it creates downforce (a 'normal' wing still creates downforce if the upper side is blown, or the lower).

All of these make sense to me, so take your pick.

(they are not strictly explanation, but rather plausibilisations (that's not a word, I know))

It was unfair for me to say that your definition takes a big --- all over Daniel Bernoulli, which is why I edited it. That visual just cracked me up.

I just have difficulty following your explanations. That seems to be a theme of this thread, though. So, I suppose it's only natural.

At any rate, I need to give this a rest. It's time for my OCD to go to sleep. Or something.

[hardingfv32]

Back to the original question: Why the requirement for a diffuser at E1? Why wouldn't the proposed theory under discussion work just as well with a simple larger opening at E1?

I would claim that the use of a diffuser to draw flow from a 'cannon' style cooling system exhaust is not as efficient as a larger opening. This is why we do not see it used by other teams with 'cannon' style cooling system exhaust.

I would there for claim that the use of a diffuser at E1 has a purpose that has not yet been articulated. If we assume that the E1 diffuser is an integral part of the Lotus system, then it should have a definable function that is more complex than simple flow extraction.

Brian

[gato azul]

it's all good mate - no offense taken, and I don't think you come across as argumentative.
I guess we will have to wait and see, if in the future some photos emerge, which give a better idea/view about the routing of the ducting, connecting the inlets (ears) to the vents.( v1 & v2).

as for the quote:
I would say flow tends to follow the path of least resistance, I know it's perhaps a minute detail, and I'm sure that it is all clear for you in your mind, but being a bit more specific perhaps helps others to follow your thoughts/ideas - IMHO

[superdread]

Back to the original question: Why the requirement for a diffuser at E1? Why wouldn't the proposed theory under discussion work just as well with a simple larger opening at E1?

I would claim that the use of a diffuser to draw flow from a 'cannon' style cooling system exhaust is not as efficient as a larger opening. This is why we do not see it used by other teams with 'cannon' style cooling system exhaust.

I would there for claim that the use of a diffuser at E1 has a purpose that has not yet been articulated. If we assume that the E1 diffuser is an integral part of the Lotus system, then it should have a definable function that is more complex than simple flow extraction.

Brian

It expands the flow mostly upwards, so at least it intensifies flow under the main wing creating more downforce and reducing drag (upside, underside flow confluence is smoother).

[hardingfv32]

I am trying to understand the big picture. Why bother with the E1 diffuser if in all likely hood it does not provide an efficient method of removing flow from the 'cannon' type cooling exhaust. One would think that it bring special attributes to the Lotus system under discussion. What are they?

Brian

[bhall]

I am trying to understand the big picture. Why bother with the E1 diffuser if in all likely hood it does not provide an efficient method of removing flow from the 'cannon' type cooling exhaust. One would think that it bring special attributes to the Lotus system under discussion. What are they?

Brian

The whole point of Exit 1 is so that the duct doesn't completely pressurize. By default, that means Exit 1 will never be overwhelmed.

That said, the system's diffuser is necessary to this end, because Exit 1 would be overwhelmed without it if the vent is appropriately sized. Remember, this system must accumulate pressure before it begins to work. That means Exit 1 must be large enough to never be overwhelmed, but small enough to allow pressure to escape from Exit 2. If Exit 1 is too large, the VD can't accumulate enough useful pressure. If Exit 1 is too small, the VD can't release enough pressure to prevent the duct from becoming completely pressurized and choking. This is the pain-in-ass tuning part, getting the sizes of the inlets and exits just right.

1. I'll explain this once more. Exit 1 must be small enough to enable the duct to accumulate the necessary pressure for the system to operate. In other words, it can't just bleed pressure on a 1:1 basis as it's drawn in. On the other hand, it must also be large enough to vent enough pressure before the system reaches its absolute maximum capacity and chokes. To fulfill both functions passively, i.e. without any direct driver intervention, a diffuser is used, and I know you know how a diffuser works.

Exit 1 is different than the so-called "cannon" vents found on Red Bull and other cars, including the one previously seen on the E20. Those vents release air that's been inducted from the side pods for cooling. Exit 1 allows the VD to maintain positive pressure, relative to ambient, without the duct becoming completely pressurized and unable to accept more air flow. The ability to accept flow at all times is crucial, otherwise the inlets choke and become veritable air dams.

Three times, chief. Agree or disagree, but do not ask again.

(Heaven forbid, you might have some input of your own.)

[hardingfv32]

Exit 1 allows the VD to maintain positive pressure, relative to ambient, without the duct becoming completely pressurized and unable to accept more air flow.

(Heaven forbid, you might have some input of your own.)

So why is a diffuser required for this purpose instead of say a larger duct opening?

My contribution is to help fill in the 'many' blanks in your theories. If the theory is valid we should be able to solve for some of the blanks that I am asking about.

Frankly I see no reason for the use of a diffuser at E1 in the theory of the system you have proposed. Lotus only uses the diffuser with DDRS type system. Thus I have doubts that the system operates as you propose. I feel an understanding of why the diffuser at E1 is used will give us insight into the correct theory of operation of the Lotus system.

Brian

[hardingfv32]

What exactly is the FloViz indicating in the center of the bottom of the main element (wedge shape)? Is this indicating a lack of flow or increased flow inside the wedge region?

Brian