Diffuser Confusion

[hardingfv32]

vonk

I'm not exactly sure what point I have to prove to you. I'll do this thru studies so as not to bring my qualifications into question.

"An important feature of the flow is that the pressure at the base of the bluff body remains
relatively constant as the model height is varied 6. Hence, as the model height is reduced, pressure underneath the model nearest to the ground plane must be “pumped down” 59, which leads to an increase in downforce."

From: Ground Effect Aerodynamics of Race Cars

6 - Senior, A. E., and Zhang, X., 2001, “The Force and Pressure of a Diffuser-
Equipped Bluff Body in Ground Effect,” ASME J. Fluids Eng., 1231, pp.
105–111.

59 - Sovran, G., 1994, “The Kinematic and Fluid-Mechanic Boundary Conditions
in Underbody Flow Simulation,” in Proceedings of the CNR-Pininfarina Workshop
on Wind Tunnel Simulation of Ground Effect, Turin, Italy, May, published
by National Research Council.

A little more effort than a scan may be required to set you straight on this subject.

Brian

[hardingfv32]

1) "but fundamentally if you take a fixed mass of air and increase the volume available, as the airflows, the pressure will drop thus sucking the car to the pavement."

The entrance (throat) to the diffuser will be a low pressure zone? This low pressure zone can cause the regions near it to experience a low pressure gradient?

2) "The bigger the increase in volume the lower the pressure. The higher the airflow then the less negative effect of atmospheric pressure "leaking" back into the rear of the diffuser, thus the added benefit for blown diffusers."

Exactly how is this done: blown the inside or top of the defuser? I have no studies explaining this. I can speak to the subject of blown flaps and how to increase the angle of the diffuser roof and still maintain flow attachment, but none of this is available per current F1 rules.

Brian

[vonk]

Brian, for simplicity, I have put my (blue) responses with your text.

1) "but fundamentally if you take a fixed mass of air and increase the volume available, as the airflows, the pressure will drop thus sucking the car to the pavement."
This is true in a closed system, where the air is confined to itself. It is not true in the open system of a race car, where the air comes in contact with adjacent flows at different speeds and pressures.

The entrance (throat) to the diffuser will be a low pressure zone? This low pressure zone can cause the regions near it to experience a low pressure gradient?
What would create that low pressure zone in an open system?

2) "The bigger the increase in volume the lower the pressure. The higher the airflow then the less negative effect of atmospheric pressure "leaking" back into the rear of the diffuser, thus the added benefit for blown diffusers."
Not true for an open system.

Brian

vonk

[hardingfv32]

What is an open system? The sides are not closed? There can be leakage? I assume the front is open.

Brian

[hardingfv32]

Try this paper. It is easy to get through and there are actual wind tunnel tests verifying the function of the under tray.

Aerodynamic Undertray Design for Formula SAE

http://ir.library.oregonstate.edu/xmlui ... sequence=1

I don't know under what logic you can question the results. Have you ever seen anything posted substantiating or summarizing your ideas on this subject.

Brian

[Gatecrasher]

Just because you state a system is open or closed does not change the fundemental engineering principles. Air pressure can and does change in open systems, just look at a barometric weather map. Although the diffuser is an open system the vast majority of flow into and out of it is well controlled.

The current diffusers are designed on the same principle that "ground effect" cars ran in the 70's. Small frontal cross section allowing air into a large underfloor tray. Since this is banned the engineers do it to the areas of the car that they can now legally get away with.

[vonk]

What is an open system? The sides are not closed? There can be leakage? I assume the front is open.

Brian

In the context of this discussion, a closed system isolates the flow from the environment within defined boundaries, subject only to inlet and outlet conditions. An open flow system is subject to influence by its environment, including contact with other flow. There may be degrees of openness depending on the presence of physical boundaries.

vonk

[vonk]

Try this paper. It is easy to get through and there are actual wind tunnel tests verifying the function of the under tray.

Aerodynamic Undertray Design for Formula SAE

http://ir.library.oregonstate.edu/xmlui ... sequence=1

I don't know under what logic you can question the results. Have you ever seen anything posted substantiating or summarizing your ideas on this subject.

Brian

Haven't had time yet to study the paper, but here's a quote from it:
Like a venturi, the efficiency of an undertray is only as good as the efficiency of the diffuser section. Due to its high visibility relative to the rest of the undertray, there are some common misconceptions in the race car industry to how a diffuser works. First is that the diffuser is what actually creates all of the down force of the undertray and second is that the diffuser expands the air under the vehicle causing lowered pressure. Both of these concepts are false since the role of the diffuser is to slow the air under the vehicle back down to free stream to reduce the drag and increase the overall undertray efficiency, and as it is an open system with gaps around the edges it is unable to expand the air to cause a density change. With these things in mind, it is the diffuser angle and entrance location that drives the undertray performance.


About my ideas, I hope there’s somebody out there who would agree with me. But no, I haven’t seen anything.

[Gatecrasher]

Just because someone wrote their Masters Thesis on the subject does not mean that I would take every word in the paper as true. If it was that way we would no longer need engineers as everything must have already been written and verified. I also did not know Oregon State did this type of work, good for them.

To the Schumi quote before about him driving upside down, that advert was crummy. Also the car was “held” upside down not due to aero down force but more due to centrifugal force. Let him drive that road car upside down in a straight line then see what happens. Top Gear episode in N.I. when Clarkson drove in a tunnel will show you this effect better.

[xpensive]

...
Cut in half, we have the diffuser under a race car.



The flow at the outlet is at atmospheric pressure, therefore pin must be lower.



But, the FIA regulations dictate a flat floor, parallel to the ground with no protrusions, allowing no venturi effect to be created under the car. So, even under ideal conditions the static pressure under the car cannot be lower than atmospheric.

I've been trying to understand your point here, but I'm afraid that I fail miserably, are you saying that the diffuser does not increase the air-speed under the car and therefore does not contribute to lower static pressure and downforce?

If so, your thinking is truly radical and makes me for a second wonder what the DDD was all about and if the current F1 designers have a vague idea of what they are doing?

In reality, increasing air-speed under the car in order do create a lower static pressure than on top (atmospheric) is what it's all about, this is why the FIA itroduced stepped underbodies, the plank and a limit on diffuser xit-area.
You don't need the classic curved venturi-shape to achieve that.

The rest of your post just seems like "Techno-babble" to me, while your statement that Bernoulli's equation only relates to laminar flow is simply wrong, the fluid should preferably be incompressible, but not necessariy so, density constant and friction from viscous forces negligible.

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

[vonk]

...
Cut in half, we have the diffuser under a race car.



The flow at the outlet is at atmospheric pressure, therefore pin must be lower.



But, the FIA regulations dictate a flat floor, parallel to the ground with no protrusions, allowing no venturi effect to be created under the car. So, even under ideal conditions the static pressure under the car cannot be lower than atmospheric.

I've been trying to understand your point here, but I'm afraid that I fail miserably, are you saying that the diffuser does not increase the air-speed under the car and therefore does not contribute to lower static pressure and downforce?

If so, your thinking is truly radical and makes me for a second wonder what the DDD was all about and if the current F1 designers have a vague idea of what they are doing?

In reality, increasing air-speed under the car in order do create a lower static pressure than on top (atmospheric) is what it's all about, this is why the FIA itroduced stepped underbodies, the plank and a limit on diffuser xit-area.
You don't need the classic curved venturi-shape to achieve that.

The rest of your post just seems like "Techno-babble" to me, while your statement that Bernoulli's equation only relates to laminar flow is simply wrong, the fluid should preferably be incompressible, but not necessariy so, density constant and friction from viscous forces negligible.

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

This is from Weber’s flight last year. Would Ed Newey do this if he wanted un-separated smooth flow into a diffuser? He must be chuckling when he reads what people think he's doing.

I’m just thinking out loud, doubting some of what intellectual fellow travelers keep saying. And, per your link, “Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline.” A bunch of those streamlines would be laminar flow.

[xpensive]

...
This is from Weber’s flight last year. Would Ed Newey do this if he wanted un-separated smooth flow into a diffuser? He must be chuckling when he reads what people think he's doing.
...

Why would anyone be pre-occupied with "un-separated smooth flow into the diffuser"? The only thing you want the diffuser to do is to speed up the air-flow under the car vs over the car as much as possible in order to create downforce;

Force = A * Rho/2 * (v1^2 - v2^2), and it doesn't take much; if v1 is 85 m/s, v2 70 m/s and A is 2 m^2, then F = 2800 N,
while the pressue differential is still only 0.014 atmosphere, but it acts with the force equal of 280 kg!

If Newey has found that a concave diffuser does that more efficient than a convex, I guess he thinks he's right then?

It's always interesting to come across someone who thinks he/she knows something nobody else does, makes me wonder how you came to the conclusion that the F1 car's floor does not contribute to the car's total downforce, I'm intrigued?

[hardingfv32]

Vonk

Maybe it would be easier to dispel what YOU think is going on in this area of the car.

So, I assume that you AGREE that there is down-force being generated by this system most of us are calling a diffuser. What is your explanation for how the down-force is being generated.

Your first post mentioned flow above the diffuser. Is this your main theory?

Brian

[vonk]

It's always interesting to come across someone who thinks he/she knows something nobody else does, makes me wonder how you came to the conclusion that the F1 car's floor does not contribute to the car's total downforce, I'm intrigued?

Relax, expensive.

[Just_a_fan]

This is from Weber’s flight last year. Would Ed Newey do this if he wanted un-separated smooth flow into a diffuser? He must be chuckling when he reads what people think he's doing.

I’m just thinking out loud, doubting some of what intellectual fellow travelers keep saying. And, per your link, “Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline.” A bunch of those streamlines would be laminar flow.

Well, for a start, the entry to the diffuser isn't a 90deg angle in the way you imply. The rules allow for a small radius to be formed at the entry to the diffuser but Ferrari came up with the idea of forming the radius the other way so that instead of a nice curved approach there is a stepped-type approach.

Because the rules limit the height and length of the diffuser they also limit the angle of the diffuser. By using a concave style ramp approach you can form the main part of the diffuser at a slightly lower angle - this should give better separation characteristics as I understand it. In effect the air thinks the diffuser is longer and shallower than it is. The vertical fences are there to prevent lateral migration of any separation that might occur. Whilst they might give slightly higher skin friction they more than make up for it by increased diffuser efficiency.