Random Aerodynamic Oddity of the Day Thread

[freebeard]

There are a few other examples of boundary layer turbulence being used to help fix a shape, but they don't get into the popular literature as a trap for the amateur.

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There is no excuse for a land vehicle to weigh more than its average payload.

At least the dimples are in an area where they might have some good effect. Most plaster them all over the hood and front fenders.

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My 1961 VW panel van weighed 1750lb and had a 3/4 ton rated axle.
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[Bicycle Bob]

Originally Posted by freebeard

At least the dimples are in an area where they might have some good effect. Most plaster them all over the hood and front fenders.

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My 1961 VW panel van weighed 1750lb and had a 3/4 ton rated axle.
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If they are needed, it is usually on the front, but this shape has plenty of length to avoid abrupt transitions with. The dimples raise the skin friction considerably, while only letting the wake follow that stylish curve to the transom a few degrees farther. A conventional smooth top and sharp cutoff would be much slicker. This guy is just trying to sell his art with a false rationale.

[kach22i]

Originally Posted by Bicycle Bob

Arrgh! Dimples are not a panacea. They are used on golf balls because their shape is so bad that another mistake can improve it. Specifically, the transition from air getting out of the way to air returning is so abrupt that the air is thrown right off, expanding the wake more, unless it has been churned up to stick on the surface. There are a few other examples of boundary layer turbulence being used to help fix a shape, but they don't get into the popular literature as a trap for the amateur.

You could have just said the only place on that car the dimples would have possibly been a benefit is the rear bumper area where there is none.

That is what I found so odd about that car.

Other than that it's general shape is very sexy and aerodynamic looking.

Originally Posted by freebeard

At least the dimples are in an area where they might have some good effect. Most plaster them all over the hood and front fenders.

I agree the roof dimple location is better than hood or front fender location, but not by much.

I disagree in the roof location at all however, the gently sloping roof and rear window location chosen does not need it.

Originally Posted by Bicycle Bob

If they are needed, it is usually on the front, but this shape has plenty of length to avoid abrupt transitions with. The dimples raise the skin friction considerably, while only letting the wake follow that stylish curve to the transom a few degrees farther. A conventional smooth top and sharp cutoff would be much slicker. This guy is just trying to sell his art with a false rationale.

The front is where high pressure build up occurs (adherence/attachment), dimples do not help anything there. Tripping up the air early on to then flow over the aerodynamic shape you pointed out is defeating the purpose of an aerodynamic shape with attached air flow.

Past the mid-point (high-point on aerodynamic template or in this case sphere) detachment of air occurs, the eddies (turbulent flow) caused by the dimples change the trailing air flow also known as air drag.

Both links below are excellent reading, I'm not providing the links to just reveal or credit the source of the images as I usually do. Good reading.

Surface Finishes -
why are they not used on an F1 car
Dimpled Surface Finish



Boundary Layer


I think we all recognize that the dimples on this car are a styling exercise or marketing exercise and not an aerodynamic engineering exercise.

However the what, where and why of it seems to be of debate. Hence the design is successful in beguiling us, and adding mystery.

NOTE: Attachment below is intended to illustrate that the roof does NOT need dimples to have attached flow, but the rear hindquarter may benefit from added dimples.

The reason a golf ball has dimples all over is the ball's orientation in flight cannot be guaranteed as the ball may spin. If one could maintain non-rotating flight, dimples on the aft half would help, but making it teardrop shaped for attached flow would be even better - but then it would not be a ball.

[Bicycle Bob]

Originally Posted by kach22i

The reason a golf ball has dimples all over is the ball's orientation in flight cannot be guaranteed as the ball may spin. If one could maintain non-rotating flight, dimples on the aft half would help, but making it teardrop shaped for attached flow would be even better - but then it would not be a ball.

I don't have the reference handy, but tests showed that a non-rotating golf ball could maintain attached airflow as well as a dimpled one with a single thin ring added to trip the airflow into turbulent condition at about 2/3 of the diameter. At or past half-way, it was too late to help.

[kach22i]

Originally Posted by Bicycle Bob

I don't have the reference handy, but tests showed that a non-rotating golf ball could maintain attached airflow as well as a dimpled one with a single thin ring added to trip the airflow into turbulent condition at about 2/3 of the diameter. At or past half-way, it was too late to help.

Would something like that cure my wicked slice?

Pros and Cons of Low Spin Golf Balls
https://golf-info-guide.com/golf-equ...in-golf-balls/

[freebeard]

I think we all recognize that the dimples on this car are a styling exercise or marketing exercise and not an aerodynamic engineering exercise.
....
NOTE: Attachment below is intended to illustrate that the roof does NOT need dimples to have attached flow, but the rear hindquarter may benefit from added dimples.

Agreed. Your attachment would be an improvement. It would seem to be a strategy that accommodates cross-wind situations.

[Bicycle Bob]

Originally Posted by freebeard

Agreed. Your attachment would be an improvement. It would seem to be a strategy that accommodates cross-wind situations.

Ahh. I just noticed the extra info on the attachment. IMHO, the area of improvement would be much narrower, and of very little consequence on such an abrupt shape in any case.

[freebeard]

GQ: Take A Tour Of The Biggest, Most Expensive Superyacht Ever Designed

They ghost out [one of] the most interesting part[s], the hydrodynamic form of the hull below the water line. It has a belly with an ogive curve abaft.

[RedDevil]

I think they mean the inverted bow ? If you plow into a wave it cuts through rather than lifts the front. This design is more economical in heavy seas.

Ah no. I should have known.
I do have a belly and an ogive shape abaft as well.

Ogive can mean concave as well as convex. I guess they mean concave here, judging by the middle picture. I'd think that would only work for speed boats but not for ships over 200 meter long, unless it travels at 80 mph or more?

[kach22i]

Since we are including odd looking hulls, try this one out.

https://www.pavati.com/wake-boats-fo...y-pavati-al24/



https://www.pavati.com/wake-boats/al24-wake-boat/


Pavati Wake | Sneak Peek
https://www.youtube.com/watch?time_c...&v=flQRpzEVYAU


There are images of the stern in the links as well.

Stupid inquiry of the day...................if aerodynamics is study of how air flows around objects, and hydrodynamics is the study of how water moves around objects, and they both fall under the category of "fluid dynamics", is there a sub-section of fluid dynamics that explores the differences of the two?

I'm assuming "fluid dynamics" is compromised of the commonalities of the two branches, is there another branch that defines the specific differences?

Here is my thinking: water does not compress (to speak of) so the bow of the boat is more important than the stern reagading hull form, plus water attachment because of pressure/density is a non-issue unless supercavitating hulls/propellers are being discussed.

Air does not compress below 250 mph and that area of study beyond is covered under high-speed aerodynamics, and beyond that supersonic aerodynamics.

The study of the differences between bodies moving through air and water is called what?

The old WW2 torpedo dive bombers must have had an aerodynamicist study the flight of the torpedo being dropped though the air, and another engineer covering the travel through the water, but only a person understanding the important differences could truly understand the interaction of the two, is what I am saying.

Yea, I know, the torpedo's dropped from aircraft and motor torpedo boats in WW2 rarely reached their targets. Maybe this was part of the problem, nobody studied the differences?

Penguins Fly Underwater Like Supercavitating Rocket Torpedoes
https://focusingonwildlife.com/news/...ket-torpedoes/


Is world ready for an undersea missile? Supercavitating torpedo offers speed of 230 miles per hour
https://www.militaryaerospace.com/ar...-undersea.html


https://www.slideshare.net/cpricenaik/supercavitation