Rear diffuser?

[Phase]

Originally Posted by aerohead

Went back in and found no pages!
Look at bottom of 1st paragraph after Figure 8.
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okay! in the PDF, it's on page 8 / 21 after Figure-8

i saw that. they mentioned other sources

their testing showed a drag increase on page 13 of the pdf. any idea why their 7 degree diffusers added drag in every single set up?

[Phase]

https://www.sciencedirect.com/scienc...75389211006924


yet this study shows drag dropping from .28 to .24 with a 6 degree angle. wowzers

https://aia.springeropen.com/track/p...20-00054-7.pdf

this study shows a drop of about .01 though, which is still an improvement

https://link.springer.com/chapter/10...030-86446-0_56


this one says 15 was their best angle. dont see test results for anything below 10 sadly
https://www.semanticscholar.org/pape...6c12216c5bb314

5 degrees for this one dropping .08 cd. crazy crazy
https://dergipark.org.tr/tr/download...le-file/533658

drag also reduced...

[freebeard]

file:///C:/Users/skate/Pictures/ADOBE%20PREMIERE%20BIN/Influence...

That's a local address, not an online address.

[Phase]

Originally Posted by freebeard

That's a local address, not an online address.

Whoops those were the actual pdf files it made me download. Fixed the links to the sites that host the pdf

[aerohead]

Originally Posted by Phase

i saw that. they mentioned other sources

their testing showed a drag increase on page 13 of the pdf. any idea why their 7 degree diffusers added drag in every single set up?

* Carr's 'short' diffuser on the Audi 100-III was @ 4-degrees.
* Carr's 'long', lowest-drag diffuser on the Audi 100-III was 2.8-degrees.
* FIAT's lowest drag diffuser was also @ 2.8-degrees.
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Ahmed and Buccheim's prismatic roof angles were all dependent upon their length, as a function of total greenhouse length.
The same may be true of diffusers.
It's going to do with vortex-drag and induced upwash.
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* All 'high-performance' (sic) sports cars design-in extra drag for the sake of high-speed stability.
* All 'low-performance' ( sic) 'economy' cars reject designed-in drag, as a tool for high efficiency.
The fairly-recently-reported, 2013, Cd 0.11, Cambridge University Eco Racer ( CUER ) has a diffuser which angles upwards as much as 9-degrees, however, this curved surface constitutes 79.7% of the car's total length!
University students privileged enough to have access to multi-core processors and industry-grade CFD could devote some time towards parsing out the subtle differences between 'simple' and 'complex' diffuser designs. And do it within the context of low-drag.
And they might want to study a 'real' 'passenger' car, not some simplified data cloud which depicts no real world mass-produced production vehicle we'd ever directly experience.
The Chinese are doing it!

[Phase]

Originally Posted by aerohead

* Carr's 'short' diffuser on the Audi 100-III was @ 4-degrees.
* Carr's 'long', lowest-drag diffuser on the Audi 100-III was 2.8-degrees.
* FIAT's lowest drag diffuser was also @ 2.8-degrees.
--------------------------------------------------------------------------------------
Ahmed and Buccheim's prismatic roof angles were all dependent upon their length, as a function of total greenhouse length.
The same may be true of diffusers.
It's going to do with vortex-drag and induced upwash.
--------------------------------------------------------------------------------------
* All 'high-performance' (sic) sports cars design-in extra drag for the sake of high-speed stability.
* All 'low-performance' ( sic) 'economy' cars reject designed-in drag, as a tool for high efficiency.
The fairly-recently-reported, 2013, Cd 0.11, Cambridge University Eco Racer ( CUER ) has a diffuser which angles upwards as much as 9-degrees, however, this curved surface constitutes 79.7% of the car's total length!
University students privileged enough to have access to multi-core processors and industry-grade CFD could devote some time towards parsing out the subtle differences between 'simple' and 'complex' diffuser designs. And do it within the context of low-drag.
And they might want to study a 'real' 'passenger' car, not some simplified data cloud which depicts no real world mass-produced production vehicle we'd ever directly experience.
The Chinese are doing it!

So realistically it seems like I need to just fill in the “ parachute” gap between the muffler and rear bumper with a smooth flat surface, and a very slight angle upwards. I may have to make the diffuser a tad shorter and less noticeable in the final design than that cardboard mock-up. Car wash people are already getting hostile with my rear wheel skirts when I come thru the wash lol

[aerohead]

Originally Posted by Phase

So realistically it seems like I need to just fill in the “ parachute” gap between the muffler and rear bumper with a smooth flat surface, and a very slight angle upwards. I may have to make the diffuser a tad shorter and less noticeable in the final design than that cardboard mock-up. Car wash people are already getting hostile with my rear wheel skirts when I come thru the wash lol

According to the IONIQ's blueprint, the bottom of the rear quarter panels are already angled right at 2.8-degrees from the factory.
All you have to do is create a bottom 'surface' over the angled void. Easy!

[freebeard]

Not to sound like a broken record, but a Basalt fiberplate could attach directly to the muffler.

duckduckgo.com/?q=basalt+fiber+plate+high+temperaturei&ia=web

Sukhoi use them in jet engines.

[Phase]

Originally Posted by aerohead

According to the IONIQ's blueprint, the bottom of the rear quarter panels are already angled right at 2.8-degrees from the factory.
All you have to do is create a bottom 'surface' over the angled void. Easy!

That’s pretty much what I did with the cardboard. It was just taped on and flush with the existing angle. It was mostly flat but slightly sloped up. Could barely notice. Only weird thing is this gap that’s lifted. I wonder why Hyundai did this? Styling? Easier for air passing over the muffler to escape and not get trapped?

With the cardboard diffuser attached to the lower sides, it lift about an inch air gap where that horizontal slit is. I figured in my final design I’ll leave that small slit there to allow any possible trapped air underneath to escape and maybe it’ll act as a light vent for hot trapped air around the muffler to leave. Don’t want really hot air pressure building up back there

[aerohead]

Originally Posted by Phase

That’s pretty much what I did with the cardboard. It was just taped on and flush with the existing angle. It was mostly flat but slightly sloped up. Could barely notice. Only weird thing is this gap that’s lifted. I wonder why Hyundai did this? Styling? Easier for air passing over the muffler to escape and not get trapped?

With the cardboard diffuser attached to the lower sides, it lift about an inch air gap where that horizontal slit is. I figured in my final design I’ll leave that small slit there to allow any possible trapped air underneath to escape and maybe it’ll act as a light vent for hot trapped air around the muffler to leave. Don’t want really hot air pressure building up back there

Technically, this 'porosity' would take away from the diffuser's performance.
It needs to be as air-tight as can be made.
Maybe, add material to the bottom of the rear fascia, down until it meets the 'flat' angled diffuser.
I know you're not made of money. Some sort of heat-proof material near the muffler. freebeard's volcanic basaltic spun glass fiber would be wicked cool.
Scrap aluminum. Sheetmetal cut from the discarded door of an abandoned, deceased refrigerator. I've used the sheet metal from worn out gas water heaters.