Solidworks CFD simulations

[aerohead]

Seems like your learning curve is accelerating!
It boggles the mind, at how much numbers-crunching is going on inside the software, to be able to illustrate the effects of these subtle changes to the car's architecture.
Impressive!

[Vekke]

Well its pretty easy to use so far. Still need to learn few things as next is rotating tires and wheelwell and deflectors...

Got some nice progress on the Cd 0,158, my diffusor was too short so I put the start pretty much where it is on the EQXX.



EQXX with diffusor extension gives Cd 0,177

[tomi_k]

The Cd 0.158 shape start to look more or less like Onorato's shape on page #4 on below publication....

https://repository.lboro.ac.uk/artic...rag/13516313/1

[Vekke]

Thanks for sharing that study. Cool data and havent seen that before.

Yes shape is similar however in my short study of few CFD runs you need that similar dip also underside the get lowest drag like see on the top. You also need to scale the dip to length to get best values.

friend who does these cfd for living calculated that last model and they got Cd 0,116 on professional software. that model Lt/Ltmax is 0,315. if you put that to the onorato chart seen on that pdf study its pretty good result. again I am still not confident in the result and friend will also check can he find why difference is so big between two programs

[aerohead]

Originally Posted by tomi_k

The Cd 0.158 shape start to look more or less like Onorato's shape on page #4 on below publication....

https://repository.lboro.ac.uk/artic...rag/13516313/1

I revisited the publication. It turns out that what Onorato presented is a von Mises profile, invented by Richard Von Mises, in 1915.
It's a 'fast' aft-section contour which could experience burble-point, then stall, compared to, say, a Clark-Y profile at the same angle of attack.
The 'reflex-camber' at the rear literally acts as a 'spoiler', re-accelerating the flow to mitigate the adverse pressure gradient of the higher than allowed pressure rise which threatens separation and stall.
GM used it with their 1993 Impact-EV1 LSR to reduce lift and drag.

[Vekke]

Audi A2 basic shape.

Rear spoiler is good for delta Cd -0,005
Diffusor is good for delta Cd -0,015
Total impact in Cd -0,02

[Phase]

Originally Posted by Vekke

Audi A2 basic shape.

Rear spoiler is good for delta Cd -0,005
Diffusor is good for delta Cd -0,015
Total impact in Cd -0,02

looks almost like what im trying to do with my ioniq

[aerohead]

Originally Posted by Vekke

Audi A2 basic shape.

Rear spoiler is good for delta Cd -0,005
Diffusor is good for delta Cd -0,015
Total impact in Cd -0,02

According to SAE Paper 2013-01-2414, from Graz University of Technology, a drag reduction from a modification in side elevation only ( top and bottom ) will produce only 65% of the aerodynamic benefit potential, compared to top/bottom/and sides, a 34.7% drag penalty.
Which is in the neighborhood of Wolf Heinrich Hucho's comment that, only 'non-ventilated cavities' ( boat-tails and box-cavities ) produced drag reductions.
Suggesting that the Mercedes-Benz IAA, fully-enclosed tail extension is more efficacious than the Mercedes-Benz EQXX's.

[Vekke]

Final looks.

I also tested the rake angle impact and it was big.
+1,0 degrees gave delta Cd +0,0147
+0,5 in most calculations
0,0 one run delta Cd -0,003

So at least in the Audi A2 0,0 seems to be the best. For future it would have been best to start with that rake angle optimizing and after that the roof spoiler and diffusor angles.

That curvy dip diffusor does not work in the A2 body at all. Tried also to add curvy extension and that did not help either. Straigth seem to be best for this shape.

[Phase]

Originally Posted by Vekke

Final looks.

I also tested the rake angle impact and it was big.
+1,0 degrees gave delta Cd +0,0147
+0,5 in most calculations
0,0 one run delta Cd -0,003

So at least in the Audi A2 0,0 seems to be the best. For future it would have been best to start with that rake angle optimizing and after that the roof spoiler and diffusor angles.

That curvy dip diffusor does not work in the A2 body at all. Tried also to add curvy extension and that did not help either. Straigth seem to be best for this shape.

have you done any tests with rough underbody versus flat underbody and how much of a difference that makes to cd?