A paper on aft-end underbody diffusers
 

I have found a few threads here about projects for making underbody diffusers. Here is a dissertation I found that may be of interest to some ecomodders:

"An experimental study of automotive underbody diffusers", by Lydia Jowsey, 2013. It was a bluff-body wind tunnel study that included variables of diffuser angle, number of channels and ride height. The material is quite interesting. Link:
https://dspace.lboro.ac.uk/dspace-js...013-Jowsey.pdf

At the end are appendices with data tables from full scale tests done in 2009 of a Ford Focus and a Ford Modeo hatchback. Unfortunately, the Focus model was not mentioned (sedan?, hatchback?), and these tables are not discussed anywhere in the document. Cd is provided and is of interest, but much that is in the tables is cryptic. Maybe someone else can make heads or tails out of them, particularly what led to the minimum Cd figures.

"Submitted in partial fulfilment of the requirements for the award of
Doctor of Philosophy of Loughborough University 2013"

Philosophy??

Yeah, I skimmed through the paper. Most of the technical descriptions were above my head.

Can anybody translate this into simple terms?

Quoting from the conclusions:

I skimmed it and the salient point was provided by wdb above. Drag was not the target, maximum downforce was. No need to read into that further as downforce is a real drag :P

But if you read it more carefully, drag is discussed quite a bit and it is covered in several of the charts. Also, it is covered in all the rows of the tables in appendixes A & B.

Doctorate degree. Girl got her Ph.D. and is now Dr. Lydia Jowsey or Lydia Jowsey Ph.D. depending on the circumstance.

Will be reading the article later Focus-AK, thanks for the link.

Yeah, I guess I meant more why is somebody who is getting a PhD in Philosophy doing a paper on this...

But that's not really important. The important question that we should answer is what is the best angle for US.

paper
 

Without wheels,the model seems way too simplistic to dovetail any results into real world expectations.
The presence of wheels can alter the drag by 2X and their addition to the model would significantly affect performance of the diffuser(s).
The info on the Focus car would have been more useful.

It is just an oddity of a long tradition in the academy. It does not mean she studied Kant and Hegel. It means she completed an advanced degree in research. Once upon a long time ago, it was almost all "philosophy" ... think Aristotle and Plato... all the modern fields of inquiry evolved from that tradition, splintering off. Many still have the Ph.D. ... Physics ...

Here is a sjmilar degree from UKansas: Doctor of Philosophy in Aerospace Engineering: Degree Requirements - KU Catalog

James

It's not intuitive. PhD means philosophy doctor, and it goes back a few hundred years to early colleges, where science was considered an arm of philosophy (use of logic, etc.). One science journal a few hundred years old is Philosophical Transactions of the Royal Society (now published in two segments: Home | Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences and Home | Philosophical Transactions of the Royal Society B: Biological Sciences). See also: https://en.wikipedia.org/wiki/Royal_Society.

Dr. Jowsey's Phd is in automotive engineering (an applied science).

And of course, since things mushrooms, there are now PhDs in music, sports, etc.

I agree in large part.

And the data seems to be there in the tables, if it can be teased out. In the Cd column for some of the tests the Cd is shown reduced from 3.42 to 3.0, 3.1 or 3.2.

Haven't figured out how to contact the Dr. to ask questions and for photos of what was done to the Focus. Pitty there wasn't more on that in the dissertation.

A "Lyds Jowsey" is on twitter at @lydsjowesey ... and a Lydia Jowsey, Automotive Engineer, married a Richard Alexander in Sept 2014, a year after this dissertation was completed.

I looked for Lydia Jowsey Alexander briefly on faculty lists and found nothing. But i only barely started and gotta go...

Try tweets

Whenever it looks overwhelming, I look to Mac OSX's Summarize service. In this case abd for some reason at the lowest setting—1%—it captures the entire table of contents. But deleting that leaves this:

...when looking at Sentences. Paragraphs preserves that dense sentence structure. Sentences returns manageable morsels. It's also interesting to move the slider down and watch which parts go a way at a lower setting.

I took a tape measure to my car. From the hard point at the rear torsion housing to the rear valance is 27% of the overall length. But the diffuser angle gets difficult above 6.5%. The engine sump and fanbelt area is only a foot wide so it could be easily boattailed.

If I ever get rear-ended I know what to do.

you'd do a high angle for downforce?

Back around 1980, I had a Beetle with the rear fenders shortened to the legal limit (24" IIRC) and the engine lid shortened to the 'floor' of the engine plenum. That was in film camera days and there was maybe one (badly exposed) picture taken of it.

A 'dual cannon' 4 into 2 header would put the exhaust at the taillight's level. A 6.5° plate would expose 1/2 the valve covers and all the lower engine tin would be stripped away, replaced with a louvered panel. There would be a 12" wide boattail fan-belt cover. It might include an exhaust tip from the two mufflers Y-d together into a Coanda nozzle.

I don't want to cut up the car I'm driving, but the next time someone smacks the rear—Boom! The last time someone tapped me at a stop sign, it actually improved the fit of my back bumper, so I shook his hand and we drove off.

Thank the Germans; they were the first to apply "Doctor of Philosophy" as a catch-all term for any post-graduate research degree in the arts and sciences outside of theology, medicine, and law. This quickly spread to the US, where it persists. The term doesn't actually have anything to do with classical philosophy; in the medieval university system, philosophy was not a part of any course of study, or of the quadrivium and trivium that were prerequisite, and the Germans used "philosophy" (more-generically meaning "knowledge") to replace the previously-common catch-all "arts."

I just received a PM with contact information for Dr. Jowsey. Anybody still wanna reach this author?

I think the paper is a good find, and I have added it to my 'second edition' file for my book. Basically, I think any published data like this is worth assimilating.

But as (I think) Aerohead said: beware models, especially those without wheels.

And then, as Aerohead didn't say, beware entities with wheels not tested in wind tunnels with rotating wheels.

And then as I say, beware any models versus on-road testing.

Hmmm.

I glanced through the 'real car' stuff in the paper but it will take a fair bit to dig out the essential points.

However, based on the research I did for my book, and on the measurements I have made on the road, I think that all 'rules of thumb' like those in the paper need to be treated with great skepticism.

Basically, so much depends on what is happening ahead of the diffuser. Rear wheel wakes? Even front wheel wakes? Surface discontinuities leading to thicker boundary layers?

Even, as I was reading today, it also depends on the wake strength, because a stronger wake (ie lower pressure) creates better flow attachment on a diffuser.

So much better to simply measure real pressures under your real car on a real road.

(And I don't have any questions for the author at this stage.)

high angle
 

Yes. You're looking for separation under the diffuser. The low base pressure of the wake can telegraph this low pressure under the rear overhang,creating a moment (torque),which attempts to rotate the car about the rear axle,creating some of the downforce.
Induced,counter-rotating,longitudinal-vortices on each side of the diffuser creates a low-pressure,upwash from the belly,which acting at a vector,adds additional downforce. The resultant force from both sources helps keep the rear planted at speed.
The 2019 Nissan-Renault Alliance ALPINE A 110 gets away from using a spoiler with this trick,plus the 'template' roofline,as per Hucho, 2nd-Edition,page 281. A 155-mph car.

Quite a beautiful automobile. And that there is quite close to the "template" line, if not right on.

https://ecomodder.com/forum/attachme...1&d=1590607384

Also has some box cavity elements, no?
https://ecomodder.com/forum/attachme...1&d=1590607409

cavity
 

Without an overhead, plan-view, it's hard to tell their intention. They may just be providing a hard edge to force separation should the plan contour be a source of bistable flow, where that line could dance around,causing aeroelastic booming and panel flutter.Also, back-soiling of the taillights might have been a consideration. I think Hyundai had a battle royale fighting to get a upper separation line like the A110.

1. The Alpine 110 uses a full length undertray and rear diffuser. That's why it doesn't need a spoiler. Nothing to do with your trick. Page 185 of my book has a good underside view.

2. Hucho, 2nd-Edition,page 281 does not show your template - not unless your template can comprise three quite different shapes. (In which case, it could hardly be a template.)

Yet again, more misinformation from Aerohead.