A paper on aft-end underbody diffusers

[Focus-Ak]

Originally Posted by aerohead

Without wheels,the model seems way too simplistic to dovetail any results into real world expectations.

I agree in large part.

Originally Posted by aerohead

The info on the Focus car would have been more useful.

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.

[California98Civic]

Originally Posted by Focus-Ak

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

[freebeard]

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:

...To achieve good balance, all the aerodynamic devices (front wings, diffusers and rear wings) need to be carefully designed, and optimised as a package to enable the car to be ‘tuned’ for different circuits based on their specific aerodynamic requirements, often a compromise between high straight line speeds (low drag) and high cornering speeds (high downforce).

...The depression at the diffuser inlet is a significant source of downforce and has the effect of also reducing the underbody pressures forward of the diffuser inlet, increasing the underbody flow rate, as can be seen from the pressure results in Figure 1.4.5.

...The drag was however, increased by the diffuser at low ride heights over the region in which the downforce production increases rapidly, but a drag reduction over the flat floor was observed for small diffuser angles (1°-5°) highlighting the potential for application as a drag reduction mechanism.

...Centreline pressure data was examined and found that a change in downforce in the flat floor configuration was observed as the ride height was altered such that at very high ride heights (representative of freestream) the model lift coefficient was close to zero but that as ride height was reduced this became increasing negative, identifying the downforce production due to ground proximity.

...Results will be presented in the form of non-dimensional coefficients as a function of diffuser angle (α), non dimensional ride height, (h1/H), where H is the overall height of the body, non dimensional diffuser length (N/h1) and diffuser area ratio given by the equation:

...As the ride height is increased the areas of low pressure around the diffuser inlet and endplate region increase due to increased diffuser pumping and stronger vortex structures, which is confirmed by the increased downforce and drag (Figure 3.3.1 and Figure 3.3.2) and more negative depression observed in the centreline pressures (Figure 3.4.2).

...As Chapter 3 showed that the plane diffuser is largely attached, the improvement must be due to an improvement in one of the downforce mechanisms rather than any changes in the separation characteristics, because a drag reduction is observed whereas increased vortex strength and associated downforce improvement would be expected to increase the drag

...The two-channel diffuser (Figure 4.3.5a) has an increase in drag coefficient seen at high angles (22° to 30°) and high ride heights (low non-dimensional lengths) which may be attributed to an increased vortex drag component consistent with the increase in downforce in this region.

...It was identified that, for the two- and three-channel diffusers, the improvement is limited to distinct regions; the two-channel (Figure 4.3.7.a) improvement is seen at 16°-19° at high ride heights, while the three-channel (Figure 4.3.7.b) improvement occurs between 22° and 28° and between 10° and 16° at low ride heights.

...At a fixed ride height, it can be seen that for the multi-channel configurations the pressure distributions are very similar to the corresponding plane pressure distributions around the front edge and flat underbody where little or no difference is observed.

...The vortex core position for the 25° diffusers is presented in Figure 4.5.9 and shows that at h1/H=0.0903 (28mm) the position for the three- and four-channel diffusers is closer to the end- plate than the plane and two-channel diffusers, consistent with the increased diffuser pumping and increased vortex strength.

...As the diffuser angle was increased the vortex core remained in a similar position each time but increased signs of separation were observed at the diffuser surface, in the same configurations where reduction in downforce and increase in drag were seen in the force measurements.

...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.

[skyking]

you'd do a high angle for downforce?

[freebeard]

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.

[Vman455]

Originally Posted by California98Civic

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

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."

[California98Civic]

Originally Posted by Focus-Ak

.... 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.

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

[JulianEdgar]

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.)

[aerohead]

Originally Posted by skyking

you'd do a high angle for downforce?

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.

[California98Civic]

Originally Posted by aerohead

... The 2019 Nissan-Renault Alliance ALPINE A 110 gets away from using a spoiler with this trick,plus the 'template' roofline ...

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



Also has some box cavity elements, no?