2010 Audi A7/ SAE Paper/ 'Want Low Drag?'

[freebeard]

6) a 'notchback' GOODYEAR blimp would be challenging.
7) a 'squareback' GOODYEAR blimp would be challenging.
8) a 'raked-back' GOODYEAR blimp would be a challenge.
9) GOODYEAR blimps are not a challenge, as their engineering quanta have been known and cataloged since the 1920s.

Today we have a new example, the Celera 500


http://www.aero-news.net/images/cont...500L-0820d.jpg

Why do you think the body has max cross section at 50% instead of 30%? Maybe the pusher prop?

[aerohead]

Its a 'laminar' form.
At altitude, under 'flight' conditions, high above Earth's boundary layer, an aircraft can operate under 'calm', sub-critical Reynolds number conditions.
If an aircraft is very smooth, in very rarified, low-density, low-viscosity air, it's possible to experience a laminar boundary layer and low surface friction drag, up to the location of 'first minimum pressure', which is code for maximum cross-section, as beyond that point, the LBL is now in an adverse pressure gradient of diverging streamlines, flow velocity reduction, and rising pressure, impossible for a LBL to survive. And the LBL immediately transitions to turbulent boundary layer ( TBL ) and concomitant high surface friction drag.
So the term ' laminar' only regards the forebody of the structure. Wings included.
So the trick is to 'delay' the location of minimum pressure by moving the thickness rearwards as much as is practical on these vehicles with tremendous surface area, compared to automobiles.
Shell Mileage Marathon, SAE Economy, World Solar Challenge, and IHPVA bicycle speed record vehicles can take advantage of this effect, as they essentially have sub-critical Reynolds number frontal areas at the speeds they compete at.

[AeroMcAeroFace]

So despite following the template, the car still had separation? And it was caused by a very thin light bar. So, doesn't that mean that applying a centreline template could be useless unless very small protuberances, such as lights, boot lid gaps or window gaps are taken into account?

[aerohead]

Originally Posted by AeroMcAeroFace

So despite following the template, the car still had separation? And it was caused by a very thin light bar. Attachment 29748 So, doesn't that mean that applying a centreline template could be useless unless very small protuberances, such as lights, boot lid gaps or window gaps are taken into account?

Yes, the drop-off created the sudden alteration in cross-sectional area, and positive adverse pressure spike trigger which Hucho tells us always to avoid in an aft-body, and the elevation of the glass, below the streamlined pathway, precluded any reacceleration of the flow to cross back over the spike threshold. Without a constriction to the flow downstream, there's no opportunity for reattachment.
The flip-up spoiler is introduced to provide the flow constriction.
If the roofline had simply continued on, without deformation, like the Honda, Gen-II Insight, there never would have been any separation.
Protuberances would be okay, as they wouldn't constitute a constriction of the body cross-section.
Gaps are okay, as they're not a 'ventilation', as a reverse-step would constitute.

[freebeard]

The Aerodynamics of Celera 500L

This states that the prolated spheroid has a 15% advantage over a 'bi-hemispherical cylinder' which I assume is a hot dog shape. This is by volume, not frontal area.

My question is why it looks faster backwards. I surmise the area ahead of the propellor acts as an invert Coanda nozzle. Laminar flow all the way to the tail. Maybe a little Meredith Effect?

[aerohead]

Originally Posted by freebeard

This states that the prolated spheroid has a 15% advantage over a 'bi-hemispherical cylinder' which I assume is a hot dog shape. This is by volume, not frontal area.

My question is why it looks faster backwards. I surmise the area ahead of the propellor acts as an invert Coanda nozzle. Laminar flow all the way to the tail. Maybe a little Meredith Effect?

I'd call it a prolate ellipsoid.
Hot dog is exactly correct. Hoerner has a Cd for that.
Volumetric drag coefficients were also very common for airships and submarine outer hulls.
Just beyond the hatch on the side with the porthole would be about the limit for LBL on the fuselage. It would pop over to TBL beyond there.
All the wings and stabilizers would behave in kind. LBL up to max thickness, then, pow!
Some of the reflexive tail region will be filled in with the sloughing boundary layer.

[Cd]

Fascinating video.
My first thoughts are of the "backwards" shapes of H.P.V.s.
At 6:31, it is stated that the ventral fin reduces drag from the fuselage upsweep.
Interesting !

[IRONICK]

https://www.youtube.com/watch?v=pdKp1Lb6cDA

https://www.youtube.com/watch?v=F5mTBkJXb-s

https://www.youtube.com/watch?v=RQJ_F9ADV2Y

https://www.youtube.com/watch?v=1LLXZHaBQbg

https://www.youtube.com/watch?v=tesD4F-HOxs

https://www.youtube.com/watch?v=YdvM9NeB5-k

LaFerrari's Rear Diffuser In Motion

https://drivetribe.com/a4ec1f0a-542d...7-ca019268d0c1

[JulianEdgar]

Originally Posted by IRONICK

[etc]

Interesting. But keep in mind a few things:

1) In the videos I watched, the airflow was all faked / simulated - no smoke testing in a tunnel for example. Also, some of the descriptions of how things are meant to work seemed a bit strange.

2) The Cl and drag figures seemed a bit odd - are the Cl figures meant to be negative?

3) Rumour only - someone who should know told me that Ferraris never meet their advertised aero specs when tested in other company wind tunnels.

4) Ferrari, as far as I know, never publish any peer-reviewed tech papers on their cars (and if I have that wrong, I'd love to see some).

Worth watching - but I'd suggest with a skeptical and analytical eye, perhaps.

[Vman455]

Originally Posted by JulianEdgar

3) Rumour only - someone who should know told me that Ferraris never meet their advertised aero specs when tested in other company wind tunnels.

I laughed out loud at this. Reminds me of the time I was laying on the floor at an auto show, taking pictures of the underside of an SUV, when one of the booth attendants asked who I was working for.