Diffuser Confusion

[hardingfv32]

Vonk

Empirical assertions.... That is that at heart of this forum. We try to develop understanding of a subject using observations and logic. You clearly have difficulty in this type environment.

Approximations using non-representative conditions are the best you are going to get on most subjects. You have to have the skills to interpret the results with the limitations that the studies create.

As an example: "like funneled inlet into the gap from an upstream stagnation region, instead under a splitter from undisturbed free flow. The measured pressure reductions under the bluff bodies may well have been due to boundary layer choking."

The data clearly states the they are talking about the deferential between the diffuser's inlet to exit pressure. Stagnation upstream is not relevant to the results of this study. I doubt very much you will understand this, unfortunately.

The viewers of this forum would not require much intelligence if we always had "Pertinent data from a car on the track".

Brian

[vonk]

In all honesty, I think the mods should interfere when a member persists to question established and proved science.

expensive,

I like your signature. But I also like mine.

I started this thread because I felt that some of the “established and proved science” expressed on this forum and elsewhere, represents inadvertent Techno-Babble. I know this is sacrilege, but it's also not surprising, given the complexity of the subjects at hand. People without formal training in the applicable fields have no choice but to accept statements by people who “should know” as established science, though their explanations are usually couched in popular language without proof. This leads to popular acceptance of the most plausible explanations as fundamental facts. Like mixed metaphors, they can become confusing. Thus, the title of this thread.

While speaking about fundamental facts, you know some recent posts accuse me, among other things, of a lack of “fundamental grasp of the subject”. In defense, let me say that I’ve been around a while, developing a passion for physics and hands-on work with hardware during my experience in the aerospace industry and in amateur sports car racing, where I developed and drove a number of cars as owner/mechanic. My MS in aeronautical engineering (RPI) has been helpful in this. Most of all, it has taught me to question, question, question. Yet I make mistakes. Those who don’t, please let me know how you do it.

Cheers,

vonk

I agree with this post, but not your others. The difuser increases in volume. This alows the air to expand and return to ambient speed and pressure, this expansion allows acceleration under the car reducing static pressure. If the turning moment off the top of diffusers was the diffusers only means of generating downforce, then why oh why would they place all the strakes inside of it. Why? to seperate areas with turbulence from areas with more laminar flow, so the turbulence doesn't slow the whole flow. Why would they need that if they weren't generating downforce from the underfloor?.

Pierce89,

This would be true if the diffuser fully enclosed the flow as shown in the top picture. Unfortunately it is open to outside pressure as shown in the bottom picture.





Teams appear to be attempting to fix that problem by appropriate outside flow conditioning.

However, even a fully enclosed diffuser could not “create” lower pressure at its inlet. It must “receive” flow at the lower pressure in order to diffuse it to higher pressure. I think the talk about diffuser “suction” is misconceived, as this would require work to be done by the diffuser (pressure difference X volume flow rate). Holding a funnel into the free stream outside your car would not create lower pressure at its inlet. Instead you’d get backflow into the big end. The bottom line seems to be that the low pressure to be received by the diffuser must be created under the car. The general consensus seems to be that the air under the car accelerates. I’m unclear by what mechanism, other than boundary layer choking and/or shaping of the step between the two floor planes, this would be achieved. I don’t think by diffuser suction.

As for the strakes, I believe their purpose might be to control the outflow from the diffuser under various backflow conditions. Ferrari, for instance seams to like deflecting the outflow laterally. How important is the interface between the diffuser outlet and the car’s wake?

[hardingfv32]

Vonk

1) You are so fixated on the bottom of the diffuser being open.
The "Experimental study of multiple-channel automotive underbody diffusers" which you claim you read clearly indicated with actual lab data the a diffuser functions with a gap (ground clearance). What parts of the study are you questioning.

2) Are you claiming there is no air flow under a car with adequate ground clearance to not invoke the dreaded "boundary layer choking effect"? So assuming there is flow under the car then there is pressure at the inlet of the diffuser. So what is the issue?

3) As demonstrated in "Experimental study of multiple-channel automotive underbody diffusers" the strakes do not have to be curved to function. Their primary function is to prevent stalling with high angled diffuser roofs. There could be benefit to a laterally widening diffuser exit to the use of curved strakes.

Brian

Brian

[vonk]

Vonk

Empirical assertions.... That is that at heart of this forum. We try to develop understanding of a subject using observations and logic. You clearly have difficulty in this type environment.

Approximations using non-representative conditions are the best you are going to get on most subjects. You have to have the skills to interpret the results with the limitations that the studies create.

As an example: "like funneled inlet into the gap from an upstream stagnation region, instead under a splitter from undisturbed free flow. The measured pressure reductions under the bluff bodies may well have been due to boundary layer choking."

The data clearly states the they are talking about the deferential between the diffuser's inlet to exit pressure. Stagnation upstream is not relevant to the results of this study. I doubt very much you will understand this, unfortunately.

The viewers of this forum would not require much intelligence if we always had "Pertinent data from a car on the track".

Brian

Brian,

Are you speaking for this forum? I wonder whether all members would agree with your policy statement limiting inquiry to such subjective determinants. If so, I’ll apologize for having wasted your time and find myself a forum that wants to go further. I enjoy a tough debate. No need to become angry and personal.

vonk

[atanatizante]

Could be used this on a F1 car ?

http://en.wikipedia.org/wiki/Coand%C4%83_effect

[hollus]

Vonk, am I correct that you agree with everybody else in how a diffuser would work in a car with perfectly sealing side skirts, when the air either side of the floor is completely excluded from entering under the car?

[vonk]

Vonk

1) You are so fixated on the bottom of the diffuser being open.
The "Experimental study of multiple-channel automotive underbody diffusers" which you claim you read clearly indicated with actual lab data the a diffuser functions with a gap (ground clearance). What parts of the study are you questioning.
I don’t recall having said it doesn’t work. The question is how well.

2) Are you claiming there is no air flow under a car with adequate ground clearance to not invoke the dreaded "boundary layer choking effect"? So assuming there is flow under the car then there is pressure at the inlet of the diffuser. So what is the issue?
If you’re talking about porpoising (sp?), I don’t think that’s a problem. Boundary layer choking near the end of the floor might be put to good use. See below.

3) As demonstrated in "Experimental study of multiple-channel automotive underbody diffusers" the strakes do not have to be curved to function. Their primary function is to prevent stalling with high angled diffuser roofs. There could be benefit to a laterally widening diffuser exit to the use of curved strakes.
Agreed.

Brian

I’m riding with the Undertray, watching what goes on underneath:

1. Laminar air enters under the splitter.

2. Obeying the “Non Slip” principle at the splitter, a velocity profile develops.

3. Since air is viscous, a laminar boundary layer begins to form.

4. The boundary layer becomes turbulent.

5. As the boundary layer grows in depth, it narrows the available flow cross section for streamlines that are not involved in the boundary layer. This is known as Boundary Layer Choking. Without a diffuser, the static pressure at the outlet would, of course, be ambient. So, according to the conservation of mass and Mr. Bernoulli (OK in the laminar region), the mean outflow velocity of the laminar flow is increased as the ratio of effective inlet/outlet flow areas. This evidences itself in a steepening of the outflow velocity profile since at the pavement the no slip condition is maintained.

6. The process is critically dependent on the ratio h/L and vehicle speed. If h is too small, the boundary layer will touch the pavement and laminar flow will disappear. If h is increased, the relative narrowing effect at the outlet will decrease.

7. Attaching a diffuser completes a venturi. The diffuser receives the low pressure air at position 5 (orange arrows) and diffuses it to ambient pressure at position 6. It creates no suction.

[vonk]

Vonk, am I correct that you agree with everybody else in how a diffuser would work in a car with perfectly sealing side skirts, when the air either side of the floor is completely excluded from entering under the car?

You might have what I described to Brian:



I’m riding with the Undertray, watching what goes on underneath:

1. Laminar air enters under the splitter.

2. Obeying the “Non Slip” principle at the splitter, a velocity profile develops.

3. Since air is viscous, a laminar boundary layer begins to form.

4. The boundary layer becomes turbulent.

5. As the boundary layer grows in depth, it narrows the available flow cross section for streamlines that are not involved in the boundary layer. This is known as Boundary Layer Choking. Without a diffuser, the static pressure at the outlet would, of course, be ambient. So, according to the conservation of mass and Mr. Bernoulli (OK in the laminar region), the mean outflow velocity of the laminar flow is increased as the ratio of effective inlet/outlet flow areas. This evidences itself in a steepening of the outflow velocity profile since at the pavement the no slip condition is maintained.

6. The process is critically dependent on the ratio h/L and vehicle speed. If h is too small, the boundary layer will touch the pavement and laminar flow will disappear. If h is increased, the relative narrowing effect at the outlet will decrease.

7. Attaching a diffuser completes a venturi. The diffuser receives the low pressure air at position 5 (orange arrows) and diffuses it to ambient pressure at position 6. It creates no suction.

[hardingfv32]

Vonk

Your last post is a clear indication you cannot communicate an explanation on this topic with your "Techno-Babble". What does this have to do with dIffusers? Are you claiming that this dreaded "boundary layer choking effect" prevents the diffuser from functioning? What is the connection to the diffuser discussion.

Your quotes:

"I think the talk about diffuser “suction” is misconceived"

"The general consensus seems to be that the air under the car accelerates. I’m unclear by what mechanism, other than boundary layer choking and/or shaping of the step between the two floor planes, this would be achieved. I don’t think by diffuser suction."

So what is it, the diffuser doesn't work or doesn't work well. How are we to read your statements?

Do you have any knowledge of flow under a flat plate in ground effect beyond boundary layer choking effect?

Brian

[xpensive]

Members, can't you tell the Virginian is just a troll, probably having great fun with engaging so many people like this?

Moderators, I'm disappointed you're letting this embarassment ride?

[flynfrog]

the term autogyro of diffusers comes to mind

[n_anirudh]

Vonk,
What happened to your shear drag theory??

Well, I doubt if you have even bothered to calculate the tickness of the boundary layer and then apply it to a flat plate/undertray.The entire length of the under tray would have a turbulent boundary layer. The ride height for an F1 car is 20-40mm depending on fuel load etc., while the BLT would be a max of 2-4 mm. I doubt if the flow chokes in these modern F1 cars.

Please start with a good book. i'd suggest J.D.Anderson or even Aerodynamics for Engineers. Maybe you should refresh ur knowledge first before trying to put forth your theories.

[Pierce89]

the term autogyro of diffusers comes to mind

Yes!! That's it exactly. =D>
and to vonk: the diffusers expansion creates the accelration in front of it as air expands to fill increasing volume. Having to traverse an increasingly adverse pressure gradient is the main enemy to larger volumes, while steeper roof angles suffer from separation problems.

[vonk]

Vonk,
What happened to your shear drag theory??
You rightfully killed it. You (and only you) immediately called me on my mistaken boundary layer on the pavement (I have since altered the picture). What was I thinking?

Well, I doubt if you have even bothered to calculate the tickness of the boundary layer and then apply it to a flat plate/undertray.The entire length of the under tray would have a turbulent boundary layer. The ride height for an F1 car is 20-40mm depending on fuel load etc., while the BLT would be a max of 2-4 mm. I doubt if the flow chokes in these modern F1 cars.
By "choking" I don't mean blockage. A 2-4 mm boundary layer in a 20-40 mm gap would cause a cross section reduction of about 10%, yielding a static pressure drop of about 20%. What’s wrong with that?

vonk

[vonk]

Vonk

Your last post is a clear indication you cannot communicate an explanation on this topic with your "Techno-Babble". What does this have to do with dIffusers? Are you claiming that this dreaded "boundary layer choking effect" prevents the diffuser from functioning? What is the connection to the diffuser discussion.

Brian,

This has everything to do with diffusers. The diffuser is necessary to make a venturi out of the whole thing.

Please look at it again. Specifically, where am I wrong? If it’s still techno-babble to you, we’ll just have to agree to disagree.




I’m riding with the Undertray, watching what goes on underneath:

1. Laminar air enters under the splitter.

2. Obeying the “Non Slip” principle at the splitter, a velocity profile develops.

3. Since air is viscous, a laminar boundary layer begins to form.

4. The boundary layer becomes turbulent.

5. As the boundary layer grows in depth, it narrows the available flow cross section for streamlines that are not involved in the boundary layer. This is known as Boundary Layer Choking. Without a diffuser, the static pressure at the outlet would, of course, be ambient. So, according to the conservation of mass and Mr. Bernoulli (OK in the laminar region), the mean outflow velocity of the laminar flow is increased as the ratio of effective inlet/outlet flow areas. This evidences itself in a steepening of the outflow velocity profile since at the pavement the no slip condition is maintained.

6. The process is critically dependent on the ratio h/L and vehicle speed. If h is too small, the boundary layer will touch the pavement and laminar flow will disappear. If h is increased, the relative narrowing effect at the outlet will decrease.

7. Attaching a diffuser completes a venturi. The diffuser receives the low pressure air at position 5 (orange arrows) and diffuses it to ambient pressure at position 6. It creates no suction.



vonk