The Coanda Cowl...its uses and functional benefits for Beetles

[Windisch]

Occasional mention has been made of various considerations regarding airflow drag physics affecting VW Beetle cars (original, not the ‘new beetle’, although they probably apply to some appreciable degree). Among them are references to horizontal strakes added to the hood and abbreviated rear-edge-of-roof spoilers (to help break up areas of drag, as with the Kamm effect). All well and good, but nowhere have I read of anyone factoring into this subject the VW’s engine air-cooling physics, that comprise quite an important aspect of the car’s functional design layout.

Specifically, the latest (last) iterations of the Bug featured louvered vents on both the areas immediately below the Bug’s rear window and in the upper engine compartment lid, the idea seeming to be that air is sucked into the engine compartment to an appreciable extent by virtue of the Coanda effect (whereas air that is flowing over the roof of the vehicle and following the curvilinear shape of that structure is drawn into the louvers and into the engine compartment, where it exits, helping to lower the ambient temperature in that space. Of course, ‘California look’ modifiers have in some cases altered the engine compartment lid to make it stand proud from the body, so as to help cool the car’s engine (but personally, I’ve always thought that look is extremely tacky).

Alternations to the existing airflow dynamics of the Beetle’s upper surface specifically for improving Beetle aerodynamics by my reckoning would seemingly work against the Beetle’s deliberately configured engine cooling process. And, in the case wherein a fiberglass “Coanda Cowl” has been added to a car’s louvers (intended to help catch that Coanda airflow even more effectively), aerodynamic airflow tinkering would seem to reduce significantly (if not negate entirely) the cowl’s cooling enhancement capability.

I would love to see some bright young university student choose this subject for intensive laboratory scrutiny so that we could see some actual analytical quantifications of these considerations, but of course that’s unlikely, given that the bug we all love is now an out-of-production antique (except for that awful resurrected version of the early 2000s, ugh; I will state upfront I am no fan of the ‘New Beetle’).

Any thoughts on this from others here? Particularly with regard to use of a Coanda Cowl device (which I have recently added to my 73 Standard Beetle, and which is once again available here in the United States from a specific source at US$100/each).

As most of you know, the fiberglass cowl in reference was introduced in the 70s as an add-on accessory, intended to help cool Beetles much as side air-scoops were intended to help cool Type 2 vehicles (buses). They seem to have been pretty popular, back then, judging from various articles and images appearing in such mags as the Peterson Publishing VW specialty magazines, but much more recently those fiberglass add-ons seem to have largely done a disappearing act.

At present, the so-called ‘Coanda Cowl’ type accessory is only available from one supplier in Las Vegas, NV (as far as I know; correct me if I’m mistaken), and ordering one (or more can require quite a bit of patience on the part of the purchaser…my two arrived about 2&1/2 months after I ordered them). They are nicely finished in primer-gray, ready for painting your choice of colors and I was quite pleased with mine (I ordered a second one as a spare, should I ever need another). Cost per each is about US$100, which I think is fair enough, all considered.

The Coanda Effect was named for its inventor, design theoretician, engineer and overall genius Henri Coanda, who was born in Romania, and it remains, in addition to the famed ‘Venturi Effect’, one of the most important aeronautical principles of the modern age. Simply put, the Coanda hypothesis states that airflow tends to follow the curvilinear surface it flows over and some believe that the VW Beetle was designed to in part take advantage of that principle. Later on, in subsequent years, specific modifications of the VW Beetle’s engine lid deliberately took further advantage of that effect to help cool the VW engine. The Coanda Cowl thus augments and increases the existing entrainment of airflow into the Bug’s aft louvered vents, hence its great (practical) value.

Although most Bug modifiers added a Coanda type fiberglass cowl merely to enhance the already ‘cool’ appearance of the Bug, back in the day, only later was its functional, very practical use fully appreciated. Today, I would consider it a most useful and desirable addition that provides functional value with a minimum of visual clutter on a bug.

I’d be quite interested to hear from anyone who has some interest in this and/or inputs on the subject (naturally). Since as we all know, “To AIR is diffusion, but to CLEAVE (air flow) is just fine!” (Sorry, couldn’t help myself…!). Anyway, let’s see if anyone wants to belly up to the bar on this subject!

I’ve attached three images here. One that shows an airflow plot over a typical Beetle, the other two a Coanda Cowl on my 73 Standard Beetle.

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1973 Standard Beetle, 1974 VW/Porsche 914-4, 1.8 liter
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Der Volkswagen ist ein Ausdruck der deutschen Kultur der Allrad Art ...

[The only irrefutable evidence we have that intelligent life truly exists elsewhere in the Universe is the fact that so far it has NOT contacted humanity...]

[78BusGA]

Intriguing! Not to rain on your interest however, it has some, concerns that would need addressing. BUT I do like and appreciate the technical lesson and the history given behind the function of the piece. It is Always a good thing to have an educated argument behind a practical concept!

Well, being that this is the first time of hearing of this cowl(been following the VW community for 37 years), I am intrigued by it's inclusion in the aftermarket community as something that might have been overseen by the mainstream, or else possibly deemed unlikable by owners, and probably unwelcome by various engineers.

In the idea that it might help engine cooling airflow, that might be the case with a non-louvered deck lid. The increased air intake pressures supplied by the cowl would increase with speed and wind, though in the circumstance of the louvered deck lid, I'd care to differ.

Given that there is only the cowl over the lower window vent, that also enhances the idea of a vacuum dead zone behind it's bulbous curvilinear form. The shape might in theory enhance the under-swept curl of air behind it, and therefore increase the turbulence behind it, and in the further lower section of the deck lid and bumper.

I would also speculate that while on a non louvered decklid, the increased at-speed pressure from scooped air could possibly be a benefit. However on a louvered deck lid, I could see that being equally negated as the extra air pressure would cause a back flow, out of the vents on the deck lid. Also, if one did not have their engine compartment completely sealed, there would be increased places for leaking air pressure to not find it's way into the fan shroud path.

The shroud and fan design would probably also not benefit as well with an increased force of air being pushed into it. The design is not conducive to the type of linear flow where more air could bypass the spinning fan design to increase flow through the system. It is not a tubular screw blade design that can simply be bypassed by a higher pressure flow. The flat blade ring design of the fan cannot adjust to more air flow and would probably have to deal with extra turbulence of the higher pressure air trying to press it's way past a limited flow design.

And then there is the aspect of, while it could be spoiler-like, it also is not a very finished solution, blocks the rain channels, and impedes the lower 1-2 inches of the rearward view from the window, also being a security and safety hazard. And then with all the detritus it would catch if it was not a garaged vehicle, and the way it is attached, I'd hate to see the rust it would create in a hard to repair area like that after unknown years of placement, if not maintained well.

It is an interesting idea to a solution that could not ideally benefit this kind of system. Like many will tell you, the Engineers have toiled to make the airflow the best it can be for the design it was, well, sort of stuck with. This cowl would benefit more if it also came with a more conducive design to the cooling system that would easier adjust to a high speed, higher pressure variability at speed.

But like I have heard too, more air doesn't not necessarily mean better cooling efficiency.

And that first image is cute! I love the "Engine" represented as a Radiator. Oh diagrams made by non air cooled enthusiasts do make some interesting conversation.

And, off topic, is that a Front bumper on the Rear of your '71?
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Few will understand the passion and majesty, among the nuts and grease.

Tom H.

My '74 Super 1303 thread - https://www.thesamba.com/vw/forum/viewtopic.php?t=764902&start=60

My '78 Kombi Bus thread - https://www.thesamba.com/vw/forum/viewtopic.php?t=769385&highlight=

'72 Superbeetle (retired), '78 Bus Kombi 2.0FI, '74 1303 Superbeetle

[Lost69Convertible]

I’ll start with some basic concepts to consider. Stand in front of the Beetle. What you see is the projected front area of the car. Stand behind the Beetle and you see the projected rear area of the car.

As the Beetle moves through the air, there is positive pressure exerted on the front area and negative pressure exerted on the rear area. Positive and negative pressures are measured relative to static atmospheric pressure.

The difference between the positive pressure (multiplied by area) on the front of the car and negative pressure (multiplied by area) on the rear of the car is drag. All cars experience drag. It can be minimized. But it can’t be eliminated. All cars have negative pressure on the rear area.

Now considered cross section area of the car from front to back. As the cross section area increases, from the headlight area to the windshield area for example, there will be a positive pressure gradient, and the airflow will be laminar.

Towards the rear of the car, the cross section area will begin to decrease. And the pressure gradient will turn negative. This is the difficult part, this is where all the research and design and wind tunnel testing is directed. In general, decreasing cross section and negative pressure gradient will trigger a transition from laminar (smooth) to turbulent (circular vortices) flow. The Kamm Ratio recommends chopping off the rear of the car at 50% of the max cross section area because not much more can be done to minimize turbulent drag.

Looking at the Beetle from the side, I think the lower rear window and the engine lid are less than 50% of max cross section area. But that’s a guess on my part. If I’m correct, the turbulence in this area is large and the pressure is low. I question what an air scoop can accomplish mounted on the rear turbulent area of a Beetle.
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1956 Beetle Ragtop: My Father's car
1969 Beetle Convertible: My first car. I loved it, I lost it, and I never got over it.
1979 Super Beetle Convertible

[rayjay]

Some tuft testing should be the very first thing an experimenter does so he will have an idea of what is going on.

[Lost69Convertible]

[rayjay]

Nowadays with a smartphone or a video camera anyone can tuft test. Or just have a chase car.

[rayjay]

The first iteration of the 917 was also a disaster like the GT40. Drivers were killed in both iirc. On the 917 it was the bug guts patterns that tipped them off that the airflow was missing the spoiler.

[Lost69Convertible]

Today there are powerful Computational Fluid Dynamics (CFD) programs like ANSYS that are much easier to use than 20 years ago. And young engineers have a tendency to rely 100% on CFD. They resist the work of experimental flow visualization. (tufts, smoke)

And that’s how you can get into trouble.
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1956 Beetle Ragtop: My Father's car
1969 Beetle Convertible: My first car. I loved it, I lost it, and I never got over it.
1979 Super Beetle Convertible

[Windisch]

Well, folks, I am gratified to see so many take a kindred interest in the two related areas of Buggy aerodynamics (cooling effects and drag effects) and thanks to everyone for inputting your respective comments. Clearly, aerodynamics as they apply to a land-based vehicle can be quite a complex subject, benefiting most from a careful consideration and analysis by qualified individuals who specialise in such matters (obviously) and further inputs by others. I harken back to my basic flight aerodynamics course, conducted by the Navy in training Naval Aviators (we USAFers used to refer to those brown-shoe Airedales as 'Nasal Radiators', LoL, but mostly only in the O'Club from time to time, since we all had a sneaking respect for anyone who can land a screaming, 25,000 pound hunk of metal on a heaving and pitching 'postage-stamp' sized field at sea!); but I am this reminded that my patchy past recollection of this subject has its limitations...especially since I'm an ex-aeromedical type, far more comfortable with hemodynamic parameters than with physical drag coeffecients (although I did informally study the drag effects of female breasts on competitive swimmers...on a bet!). This is why I'd love to see, as I remarked earlier, some actual lab studies done on this subject (complete with inclusion of various details to consider such as vents, louvers, engine-room/fan flow measurements, et al. Admittedly, I am far out of my comfort zone when trying to factor in all those many important interrelated particulars! [Bug guts splatter patterns? Fascinating!]

Toward this end and in furtherance of all this, I found several interesting website URLs that include a previous SAMBA thread on drag/airflow considerations (below), as well as a YOUTUBE video that features a dynamic study of Beetle airflow patterns (while it is VERY interesting, ground-speed versus airflow pattern effects still requires much study). One thing is clear, slapping a 'Coanda Cowl' on the Bug Body just under the rear window does is not productive of any definitive conclusion about beneficial cooling effects, unless ALL the related considerations are also examine and plotted equally and in some exacting detail. Hence my wish for some bright young physics student to produce such a study! Lacking this, we can only generalise and hypothesise logically and hope to deduce some truths, short of a full aerodynamic study done in a lab.

By the way, even if a Coanda Cowl was beneficial to moving more air into the Bug engine-room, it's pretty logical that the VALUE of that device would vary considerably with speed the vehicle is traveling... En avant!


https://www.gerrelt.nl/section-aerodynamics

https://www.thesamba.com/vw/forum/viewtopic.php?p=9256661

https://en.wikipedia.org/wiki/Automotive_aerodynamics

YOUTUBE video showing VW Beetle drag vectors: https://www.youtube.com/watch?v=LtXOUrTc_dA
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1973 Standard Beetle, 1974 VW/Porsche 914-4, 1.8 liter
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Der Volkswagen ist ein Ausdruck der deutschen Kultur der Allrad Art ...

[The only irrefutable evidence we have that intelligent life truly exists elsewhere in the Universe is the fact that so far it has NOT contacted humanity...]

[Lost69Convertible]

Windisch - I’d like to suggest a few rows of 4 inch yarn taped to your rear window. And another one or two rows on the outside of your cowl. I’m very interested in the result. Have a friend follow you and take pictures.

Let’s see if you have laminar flow entering the cowl. I’ll gladly eat my words

P.S. Anyone that can land on a carrier has amazing ability and nerve.
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1956 Beetle Ragtop: My Father's car
1969 Beetle Convertible: My first car. I loved it, I lost it, and I never got over it.
1979 Super Beetle Convertible

[Windisch]

No need to ever 'eat words', Lost69, when hamburgers are so much more satisfying, LoL!

However, that's a very interesting idea of yours. I may just do a 'tuft-test' on my Bug. Meanwhile, I need to think of a convenient snappy comeback when the Gestapo pulls me over to ask me what exactly I'm up to. LEOS may be our valued public safety buddies, but they never seem to have much of a sense of humor while on duty...

I'll keep everyone here apprised if and when I put this 'flight test' op into action! It's an intriguing suggestion, to be sure! And I'm just enough of a 'closet mad-scientist' to do it!

[PS: I am reminded of a time when I was stationed at a SAC base in NoDak, back in the mid-60s. A good buddy of mine and I had had a few beers and since it was Halloween, we stuck a candle in a carved-out Jack-O-Lantern and placed it on top of my VW's convertible top, proceeding to chuff slowly down the main drag in a light snow. Sure enough, after less than a minute, the local constabulary pulled me over and asked us "What in %$#^&%! are you guys up to? Don't you know it's snowing?" Quick as a Minuteman II missile launch alert, I said "Gee Officer, we were just trying to warm up the pumpkin for some pie baking."

Needless to say, that went over like an aborted launch...]
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1973 Standard Beetle, 1974 VW/Porsche 914-4, 1.8 liter
----------------------
Der Volkswagen ist ein Ausdruck der deutschen Kultur der Allrad Art ...

[The only irrefutable evidence we have that intelligent life truly exists elsewhere in the Universe is the fact that so far it has NOT contacted humanity...]

[Rome]

[Windisch]

Interesting to hear of that previous thread you reference, Rome. Do you have a URL to retrieve it? I'd be interested in taking a look at it.

How sweet it would be if we had a full-on, serious investigation of flow patterns over the rear 50% of the Bug both with and without the devices mentioned to refer to, since we all agree that it is a quite complex proposition (that, in my opinion, is long overdue for some substantial answers).

By the way, to the best of my knowledge, that fiberglass cowl made by Markvfiberglass.com (of Las Vegas, NV) has not heretofore ever been referred to as a 'Coanda Cowl' by anyone. I believe I first made that connection, so that appellation is my own association.

When they added that second series of 4 vertical louver rows to the VW engine lid, that may have (as pointed out) an impact on any real functionality that that cowl may or may not have. In order to control that variable, it would be interesting to experiment with various configs, including perhaps a second cowl placed over the lower (vertical louvers), also facing forward...(?), among other things.

One quite interesting question would seem to me to be is "exactly where does the rear roof airflow separate (boundary layer) from laminar into turbulent motion?" This subject was quite a puzzler, I recall, back in the early 50s, when the inlet ducts for the Lockheed F-104 Starfighter were being investigated for best efficiency in variable speed states (since unlike later supersonic planes like the SR-71 and Convair F106, they had fixed, non-variable geometry). Of course they had multi-million dollar aerodynamic test facilities on hand to investigate such things (even back then).
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1973 Standard Beetle, 1974 VW/Porsche 914-4, 1.8 liter
----------------------
Der Volkswagen ist ein Ausdruck der deutschen Kultur der Allrad Art ...

[The only irrefutable evidence we have that intelligent life truly exists elsewhere in the Universe is the fact that so far it has NOT contacted humanity...]

[Windisch]

By the way and speaking of the Herrod Helper devices, the more I search the internet, the more on this whole nebulous matter (of Beetle aerodynamics) arises. Here's another YOUTUBE (brief) video presentation on tuft-testing a Beetle (vortex generators/indicators) that is useful.

https://www.youtube.com/watch?v=CGmqdXKgBiQ
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1973 Standard Beetle, 1974 VW/Porsche 914-4, 1.8 liter
----------------------
Der Volkswagen ist ein Ausdruck der deutschen Kultur der Allrad Art ...

[The only irrefutable evidence we have that intelligent life truly exists elsewhere in the Universe is the fact that so far it has NOT contacted humanity...]

[Rome]

Here you go, Windisch- https://www.thesamba.com/vw/forum/viewtopic.php?t=...mp;start=0

[modok]

I agree with Rome. it's a scoop.
Check out the old empi side scoops, they work better.

I believe most of the air separates about halfway down the back window.

Tho the herrod helper does help anyway, how?
maybe.... keeps the air attached longer by blocking re-circulation currents coming up.

[Windisch]

Rome, thanks for that URL on the fender well side-scoops. That's some real genius, IMO. Now if someone would just start making the parts for that set-up for retail sale, so as to save all the rest of us who were born with 5 thumbs on each hand (and THAT'S really 'thumbthing' to rue!) from an inept attempt to replicate that beautiful work!
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1973 Standard Beetle, 1974 VW/Porsche 914-4, 1.8 liter
----------------------
Der Volkswagen ist ein Ausdruck der deutschen Kultur der Allrad Art ...

[The only irrefutable evidence we have that intelligent life truly exists elsewhere in the Universe is the fact that so far it has NOT contacted humanity...]

[Windisch]

[vwracerdave]

The Herrod Helper is a long lost secret that needs to make a comeback. It broke up the air traveling over the engine deck lid and the negative pressure at the bottom. It stabilized the Bug at highway speeds. It helped cool the engine a few degrees and also increased 2-3 better MPG.
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2017 Street Comp Champion - Thunder Valley Raceway Park - Noble, OK
2010 Sportsman ET Champion - Mid-America Dragway - Arkansas City, KS
1997 Sportsman ET Champion - Thunder Valley Raceway Park - Noble ,OK
Featured in Dec. 2001 HOT VW's Magazine page 63

Watch my racing video's http://www.youtube.com/user/okvwracer/videos

[modok]

The only favorable aerodynamic quality about the bug body is it's appearance.

The bad news is it's hard to make any simple improvement.

The good news is it's hard to make it worse.

Good to hear your enthusiasm, maybe you will find a trick nobody thought of yet.