Aeromodability of a VW Bug?

[Gerrelt]

Originally Posted by aerohead

I believe that the smoke is the more accurate indicator of the flow.
The air should detach right at the window trim and then crash against the rear of the car so as to allow the engine to harvest it for cooling.
Once the tangent angle on the roofline exceeds 22-degrees(as measured against the horizon)the flow cannot remain attached.
Attached-vortices can hold the tufts against the body but it;s a 'false-signal' when reading only the tufts.
Smoke is the preferred visualization tool.

Hi Aerohead,

Sorry, I have to disagree with you. The air does not "crash against the rear of the car".
The air normally detaches at the top of the rear decklid, creating a low-pressure area behind it. Making it hard for the engine cooling fan to get air.
Volkswagen added more and more vents in the rear decklid trying to fight this.

I don't think wool tufts give a false signal. It precisely tells what's going on. Without the spoiler the airflow is still attached to the body, with the spoiler the tufts tell that there is turbulence behind the spoiler.

Greetings,
Gerrelt.

[Gerrelt]

Everybody who's into car aerodynamics should read this chapter of this book:
The Automotive Chassis: System design, chapter AN OVERVIEW OF VEHICLE AERODYNAMICS
Skip over the formulas!

[Sven7]

Originally Posted by CFECO

Rc ?

You can see a small opening on the driver's side of the car.

[aerohead]

Originally Posted by Gerrelt

Hi Aerohead,

Sorry, I have to disagree with you. The air does not "crash against the rear of the car".
The air normally detaches at the top of the rear decklid, creating a low-pressure area behind it. Making it hard for the engine cooling fan to get air.
Volkswagen added more and more vents in the rear decklid trying to fight this.

I don't think wool tufts give a false signal. It precisely tells what's going on. Without the spoiler the airflow is still attached to the body, with the spoiler the tufts tell that there is turbulence behind the spoiler.

Greetings,
Gerrelt.

Is not the cooling air inlet above the top of the rear deck?
Do you think Dr.Porsche guessed that this would be an appropriate location to site the cooling/combustion air inlet.
From the smoke flow study photographs you can plainly see that the flow has completely separated from both the Golf/Rabbit and Scirocco.
In the graphic for the simplified slant back models you can also see that the only reason the air is attached along the centerline of the incline,is that the attached longitudinal vortices are holding the flow there..
Low drag cannot be achieved with attached longitudinal vortices.And they are present because the pseudo-Jaray 'fastback' is too steep to support attached flow and the violent pressure differential between the top and side flow is inducing these vortices.
This is why we are told to avoid this type of roof like the plague.
Mair's research of boat tails determined that aft-body curvature beyond a tangent angle of 22-degrees cannot support attached flow.And that is with a body of revolution which has as much body side camber as that of the boat tail 'roof.'
If you want to see a modern-day example of a proper Kamm roofline,look to today's Bugatti Veyron.Any of the three models.

[metromizer]

Cool stuff...

[metromizer]

Cool stuff... is the original poster, GTkid, still participating in this thread?

As a side note, I've been involvement with the silver stretched and narrowed chop-top beetle shown a few pages back, built and driven by friend Dick Beith (great guy, lots of Bonneville history there). All built in his little shop beside the house, his first attempt at such major body mods on an 'egg'. I also helped him out a little at the Bonneville WOS event a couple of months ago in September. It has a full aluminum belly pan. One mod not very obvious from pictures, is his attempt to 'fill the low pressure void' behind the car with higher pressure ducted air (rectangular openings on both sides of the engine lid). I'm not convinced it was worth the fabrication effort.

Great article on the 60mpg bug, too. I'd never seen or heard of that project, and I've been in the VW hobby for 30yrs+. Thanks for posting that!

I've been telling some of the 36hp VW challenge guys that they need to look at ecomodder for ideas, as the same areomods principles apply to what they are doing.

[Gerrelt]

Originally Posted by aerohead

Is not the cooling air inlet above the top of the rear deck?
Do you think Dr.Porsche guessed that this would be an appropriate location to site the cooling/combustion air inlet.

I think at the time they developed the beetle, the 1930-tees, aerodynamics was an upcoming science. And they didn't know what we now know today.
Add to this that the beetle was developed for a TOP speed of 100 km/h, aerodynamics at high speeds as we drive today wasn't interesting.
Yes, the VW beetle was one of the first cars that attempted a aerodynamic body. But if Dr. Porsche would develop the VW beetle today, with the knowledge we have now, I think he would have reconsidered the placement of the vents.

Originally Posted by aerohead

From the smoke flow study photographs you can plainly see that the flow has completely separated from both the Golf/Rabbit and Scirocco.

Those cars have a "sharp" edge at the end of the roof, so seperation is easier.
Look at figure 21.23 on page 143 of the book I posted earlier ( This one ).
That edge is a trick to get the airflow to seperate at the end of the roof.

The VW beetle roof is round, making it easier for the airflow to stay attached.

Originally Posted by aerohead

Low drag cannot be achieved with attached longitudinal vortices.And they are present because the pseudo-Jaray 'fastback' is too steep to support attached flow and the violent pressure differential between the top and side flow is inducing these vortices.
This is why we are told to avoid this type of roof like the plague.

Ah, I think we're talking about two different things. Those vortices are one aspect of aerodynamics, what I am talking about is what Genta calls "streamlines in the symmetry plane".
Both contribute in aerodynamic drag.

I still think the wool tuft tells a valid story and the flow is attached. If the flow wasn't attached there, the wool tufts would be going all places, and not nicely in straight lines to the bottom.
The rear of the VW beetle is not steep enough to get the flow detached, creating lift and drag. It's the same effect what caused the Audi TT to be retrofitted with a spoiler on the rear.
The effect can be seen in this picture:

Up to 30 degrees the flow stays attached, steeper then that, the flow is detached.

Maybe we should agree to disagree.

[gtkid2002]

Metromizer - Yeah, I'm still here. I'm just out of the country right now visiting friends I don't see too often. And earlier I was helping my mother move from WA to CA, so I've been lurking really. It's interesting to read the debates on everything, and the fact that there's still so much life in such an old car style. I'm guessing all of the talk of vents and such would be different if the body style was different.

But I still think 80mpg is possible with mostly stock equipment. I'm not sure if ripping out two cylinders still renders it "stock" (might be a little cheat there), but I still think the initial goal is achievable. If it's possible to build the body around a roll/crash cage, awesome and all the better. If it's possible to use most of the original glass, also good. I'm still trying to sketch up and think of little things to add to this. One of them was using an electric peltier to generate power from waste heat from the exhaust for either battery trickle charging, or to even power small things like cabin lights. Hackaday had an article about a candle heat recycler using one. I've kicked around the idea of playing with one before, but that's the first time I've ever seen one used on a high power (4W I think) LED. Which wouldn't be bad if I used several of them.

I think another goal of this that I'd like to hit should be to incorporate as many small ways to recycle waste energy as possible. Unless it takes an extravagant amount of time to get a watt or two saved, then I think the route should be taken.

[CFECO]

Peltier power, from what I have seen is a looser in this application. Maybe look at some of the Drag race fiberglass VW Bug bodies, lowered and stretched, and light. Perhaps kick out the windshield to a steeper angle, and with fiberglass it would be "easy" to modify, stretch out the rear to get close to the Template.

[Frank Lee]

But I still think 80mpg is possible with mostly stock equipment.

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