Beetle Inspired Aerodynamic Thoughts

[aerohead]

The new Beetle has 33% aft-body based on the 'Template',here are some comparison Cd's based on the roof/body style for a car of 36% aft-body:
*New Beetle with separation at top of backlight,pseudo-Jaray style-Cd 0.39
*Jaray-Zeppelin/'Template' style ( no separation )--------------------------------- Cd 0.215
*Kamm model J-form pseudo-Jaray with separation @ 21% aft-body-Cd 0.33
*Kamm model K-form style ( no separation but steeper than template )--------------------------------------- Cd 0.25
* Walter Lay model 18-degree roof slope/12-degree plan-taper----- Cd 0.1975
As you can see,the Cd for a vehicle with an aft-body of this dimension is all over the map,and is a function of clean vs separated flow which has to do with tangent angles of curvature at any given point.
The Beetle,for it's aft-body length,could have Cd 0.215 by filling in the void below the 'template,' just like Carl Breer attempted with the 1934 Airflow,and AUDI does exactly with the deployed-spoiler on the TT.

[ChazInMT]

I have read through the sticky's. Have you read this?

Aero Testing Porsche 993 & New Beetle, AutoSpeed



It is what I'm basing my thoughts on, please tell me where they are wrong in this article so we can discuss it. Otherwise what I'm putting up is solid thinking. What I'm saying explains both why flow is attached on the New Beetle and why the "Ideal Aero Template" is still 100% valid as the shape we all want to be in. Flow looks attached on the New Beetle (don't know a darn thing about the Old Beetle). If flow is attached on this New Beetle shape, you have to admit that the current "Hive Thinking" in this forum needs to be examined.

[aerohead]

Originally Posted by ChazInMT

I have read through the sticky's. Have you read this?

Aero Testing Porsche 993 & New Beetle, AutoSpeed



It is what I'm basing my thoughts on, please tell me where they are wrong in this article so we can discuss it. Otherwise what I'm putting up is solid thinking. What I'm saying explains both why flow is attached on the New Beetle and why the "Ideal Aero Template" is still 100% valid as the shape we all want to be in. Flow looks attached on the New Beetle (don't know a darn thing about the Old Beetle). If flow is attached on this New Beetle shape, you have to admit that the current "Hive Thinking" in this forum needs to be examined.

Chaz,I have no explanation for the orientation of the tufts.I wish they'd set off a smoke-bomb attached to the car's rear.
Historically,there has never been a car built which can maintain attached flow at exit angles beyond 22-degrees.
My suspicion is that there are vortices,well formed,on each side of the roof/greenhouse,which may explain the tufts.
As to the lift,that will be a function of the low pressure associated with the separated flow acting upon the top surface of the rear slope and higher pressure,slower air under the car.
If you'll check out the 'FLOW-IMAGES' thread you see some examples of separation and wakes.
Stores closing gotta go,will catch up,Phil.

[Patrick]

Contrary to their drawing, the flow appears to be separated at these points:

[ChazInMT]

If you're referring to the 2 tufts on the back, they're stuck on themselves. They look the same in both pics. Did you read the article? Where are they wrong in the article and why?

[Patrick]

Did you look at the picture? Apparently not.

[KamperBob]

FWIW this looks steeper than 22 degrees.



I can't help but wonder about contributions from both sides feeding the rear end. Could that support a higher angle?

I also think about how much less steep is the ridge of a hip roof compared to the sides. Point being that flow vectors with some transverse component might be considered effectively shallower with their azimuth taken into account. When working in any field (electric, magnetic, etc) cosine projections into a principle plane usually offer great advantage but maybe there are cases where a fresh or even naive look reveals something previously missed. (shrug)

Cheers
KB

[aerohead]

I got a blueprint of the New Beetle off the web,enlarged it,and put under the 'Template'.
The Beetle is of 32% 'Template' aft-body.
Other cars which share the New Beetle's aft-body percentage are:
* Prius-II ----------------------- Cd 0.26
* Insight-I---------------------- Cd 0.25
* M-B Boxfish------------------- Cd 0.19
----------------------------------------------------------------------------
Certainly,there are more criteria to define the aerodynamic performance of a car's body than it's aft-body architecture.
Curiously,the Boxfish,which demonstrates the lowest drag of the 4-cars considered,just so happens to fit the 'Template' curve perfectly.

[aerohead]

Originally Posted by KamperBob

FWIW this looks steeper than 22 degrees.



I can't help but wonder about contributions from both sides feeding the rear end. Could that support a higher angle?

I also think about how much less steep is the ridge of a hip roof compared to the sides. Point being that flow vectors with some transverse component might be considered effectively shallower with their azimuth taken into account. When working in any field (electric, magnetic, etc) cosine projections into a principle plane usually offer great advantage but maybe there are cases where a fresh or even naive look reveals something previously missed. (shrug)

Cheers
KB

This photo,I believe is of one of Ludwig Prandtl's Jaray combination form models being tested at AVA Gottingen.
It is a pseudo-Jaray
'fastback'.Here's the rub.Yes the tufts are attached well down the back but it's not a deal-breaker for us.
Here is Dr.Hucho's explanation for what is going on.It's from page 18 of chapter-1 of his second edition,and he's speaking about these forms and specifically about the Beetle and Airflow:
(the Fastback )"produced two distinct longitudinal vortices.Due to the downwash induced by these trailing vortices,the flow along the longitudinal mid-section of the car remains attached over a long path;however,a high vortex-induced drag is produced so that the total drag is higher than for true Jaray shapes."
Hucho then comments on the fact that Cd 0.6-0.8 for boxy shapes were typical of the time and the Cd 0.4-0.5 of the pseudo-Jaray still an improvement.

[aerohead]

Originally Posted by ChazInMT

If you're referring to the 2 tufts on the back, they're stuck on themselves. They look the same in both pics. Did you read the article? Where are they wrong in the article and why?

According to Hucho,the tuft study fails to reveal the longitudinal attached vortices spinning off both sides of the back of the Beetle which produce the downwash which allows the tufts to remain attached at the longitudinal mid-section of the car.The attached vortices are induced drag which doesn't show up visually but does explain the Cd 0.39 vs Cd 0.21.
It's an example of what we can expect when we exceed Mair's 22-degree limit,which historically remains a universal constant.