# Teardrop Ratios - above deck

Discussion in 'Hydrodynamics and Aerodynamics' started by kach22i, Jul 8, 2013.

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### kach22iArchitect

As I understand it the optimum shape for coefficient of drag is more or less a teardrop shape.

In free air it's a L/H ratio of 3:1

In conjunction with a ground plane it is 5:1 because of the ground pressure wave.

And if a protrusion on a moving body 10:1, again because of the interaction with adjacent surfaces.

I get these ideas from this lengthy thread in another forum, see link below with attached image.

http://ecomodder.com/forum/showthre...amlining-template-part-c-9287.html#post116392

However, just about all road cars use a scaled down version of the 5:1 ratio, and based on wind-tunnel images seem to achieve good attachment (see link below).

Car Wind Tunnel Data
http://bigmike.marlincrawler.com/forum/index.php?topic=33.0

Overlaying the aerodynamic template for proximity to ground plane on sleek looking car, the template fits, but only when scaling it down (I'm told a big no no - just line it with with high point of roof)

http://s184.photobucket.com/user/kach22i/library/Aerodynamics?sort=3&page=1

Bringing this discussion/observation back to boats, super boats, a racing class..........

http://s184.photobucket.com/user/kach22i/library/Aerodynamics?sort=3&page=1

Again, same thing with cars, a super aerodynamic one below.
http://s184.photobucket.com/user/kach22i/library/Automobile 2 - Odds and Ends?sort=3&page=1

My question:
Why do the "rules" of aerodynamics as discovered and published in the 1930's resist application in the automotive and boat designs shown?

Are all the racing boat and car designers incompetent and following nonsensical styling exercises when they have rules from the 1930 which they can follow (Hucho/Kamm/Jaray and so forth)?

.........and or can the aerodynamic template be scaled down because it is the ratios which are important?

Last edited: Jul 9, 2013
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### PetrosSenior Member

economy does not sell cars, styling and performce are what consumers want. consider the best selling cars are actually SUVs, mini-vans and trucks. I like small cars, but there have not been any sold in the US since the early '80s (the Honda CRX HF only weighed about 1700 lbs, early cars were as light as 900bls). The new Fiat 600 is a throw back in size and style but it has a massive 2 liter engine, and weighes in at over 2000 lbs.

We also have better ways to analyze shapes than in the 30's, so there are other means to get low drag.

Bottom line is, if you do not sell your customers what they want to buy, you will not stay in business. The best selling cars are not super high fuel economy, so that is not a major consideration for the car manufacturers either.

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### HakimKlunkerAndreas der Juengere

As the pictures show, the principles are applied-and were.
Only: Not everyone likes the looks.
What is designed, must be made, must be wanted, must be sold.
And if the product at last is not perfect - who cares if the respective owner is happy with it?

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### kach22iArchitect

I appreciate the focus on cars and styling but I used those examples because of the ground plane pressure similar to a water surface pressure plane involved.

The old Honda CRX and newer Toyota Prius and Honda Insight do follow the aero-template (and were/are pretty good sellers).

However the racing boats where one would assume aerodynamics are of prime concern, do not.

My conclusion is the template can be scaled down to good affect, but I not found published documents to support this conclusion.

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### Mike GrahamJunior Member

You should keep in mind several things

Sometimes efficiency in one area hurts performance in others. In some cases, optimal aerodynamics may create a vessel that is heavier, more expensive, less reliable, harder to see out of, uglier, or otherwise less optimal. With boats, aerodynamic drag is usually a very low-priority concern and negative impact in many other areas is serious.

An incredible amount of money and effort is put into aerodynamic analysis of production cars and race cars, computing the aerodynamic properties to a level of fidelity not even dreamed of in the 30s. Even with race cars, where appearance is of minimum importance, designers don't arrive at the shape you think they will--I suggest that it's much more likely that their solution is better given their constraints than some hand-waving about abstract principles can give.

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### gonzoSenior Member

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### kach22iArchitect

Yea, that final shape is pretty close to the aero-templae but again, not quite. Which begs the question, how close is close enough?

http://s184.photobucket.com/user/kach22i/library/Automobile 2 - Odds and Ends?sort=3&page=1

Mike Graham, I'd love to dump the 1930's research if I suspected it was faulty or that the laws of physics have changed since then. However I'm not qualified to go that route.

I do have a set of eyes and powers of observation given training as an artist, illustrator or "just draw what you see" sort of HS and college experiences.

And that argument might best bashed around in this old thread (sore feelings on both sides still abound):

Theory verses practice, why such a big difference?
http://s184.photobucket.com/user/kach22i/library/Aerodynamics?sort=3&page=1

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### Mike GrahamJunior Member

I never suggested that any particular research was faulty. I certainly did not claim that fundamental engineering principles were faulty or the laws of physics have changed. I'm really sorry if my post did not make that clear.

What I've said is that

1. There are other concerns when designing a vessel than drag, and
2. There are superior analysis methods in use.

You do not seem to have taken either point to heart, and I think you ought to.

On the first point, realize that aerodynamics is not the only consideration when designing a vessel. Weight, cost, and other performance characteristics are also important, even without considering aesthetics. Beyond these non-aerodynamic concerns, there are even aerodynamic concerns that can compete with drag reduction. For example, spoilers on racecars increase drag, but improve the overall performance by generating downward forces. The whole bodies can do the same; for example, shovel-shaped noses do this.

On the second point, you seem to completely mischaracterize the reality of modeling by suggesting we have to throw out a principle for others to be in use. All engineering models have assumptions -- the optimal teardrop shape comes from making pretty severe ones about the quality of the flow, the totality of the shape, and the interaction with any boundaries of the fluid domain which we know are not completely right. Through modern engineering methods, both analytical and experimental, we have additional analysis we can do. These are performed constantly. You are oversimplifying the issues here.

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### Mike GrahamJunior Member

This is not some abstract question. Remember always, engineering is done with numbers. We can and do actually quantify the drag of various designs.

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### kach22iArchitect

I did not mean to come off as rash or dismissive and appreciate your valuable input.

I'm thinking based on many examples that the optimal or idealized 3D teardrop shape is best, but attached flow can be achieved at half that length.

The resulting suck-back or disturbed flow in the wake from the increased arc closure must surely be measurable and therefore graphable as a relationship.

I would assume that the changing or undulating ground plane of the water as the boat hops/bobs in and out of the water while planing is one major difference between high speed boat and automotive models.

This is great stuff to fall upon, thanks.

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### DCockeySenior Member

Remember that most flows are 3D. Only looking at a 2D section is frequently misleading. A body which is an extrusion of the template shape, say 1/2 the length of the template in width with flat sides, is likely to be higher drag than a truncated shape of the same length with a less taper on the top and a vertical base.

A lot of good, fundamental work on low speed "bluff body" aerodynamics was done in the 1970's and 1980's. A good reference is Aerodynamics of Road Vehicles, edited by Wolf-Heinrich Hucho, Butterworths, 1986.

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### kach22iArchitect

I've looked up "Bluff Bodies" before and studied vortex generation (on my own), and couldn't agree with you more.

I don't have any of the books which are often recommended, but have read excerpts and have quite the collection of PDF papers on my computer. It's just a hobby for me, but I enjoy learning especially things which may be considered subjective arts.

Some images on the topic I've posted before.

ScienceDirect.com - International Journal of Heat and Mass Transfer - Heat transport from a bluff body near a moving wall at Re=100

http://www.sciencedirect.com/science/article/pii/S0045793011002726

http://www.sciencedirect.com/science/article/pii/S0889974611000399

The all important corner vortex formation - see below.

Influence of the Slant Angle of 3D Bluff Bodies on Longitudinal Vortex Formation
http://fluidsengineering.asmedigitalcollection.asme.org/article.aspx?articleid=1433689

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### Leo LazauskasSenior Member

In another thread, David also warned that spinning wheels also have significant
effects on the flow that must be considered.

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### FAST FREDSenior Member

"The best selling cars are not super high fuel economy, so that is not a major consideration for the car manufacturers either."

A major consideration is the fines and penalties , per car, if the mileage mandates of the ruling burorats is not met.

EZ to pay a few thousand in fines for a turbo Ferrari , but not for peoples cars.

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### kach22iArchitect

If anyone can lead me to a website or PDF article regarding the ground plane or water plane "pressure wave" which mandates a longer body than "free air" I'd appreciate it.

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