Teardrop aerodynamics... in reverse!

[winkosmosis]

Originally Posted by MazdaMatt

You can't confuse hydrodynamics and aerodynamics. They work pretty differently. We're not trying to pull your leg here. Simply google "lowest cd shape" and you'll find a million references to an airfoil (aka teardrop). Dig deeper and you'll see why the bulb is better than a flat front or a pointy front. Better yet, search around this site. This dead horse has been beaten into a unrecognisable bloody spatter.

Well the lowest cD shape would be an infinitely long cylinder with 0 diameter, so any real world discussion of optimal shapes would involve length and volume constraints.

The way I see it is that the shallower the angle of the rear taper of the object, the lower the cD. But the front end also has to cut into the fluid and then allow smooth transition to parallel flow, and then stay attached to the rear taper.
If you have a constraint on length, sometimes the best thing to do is to get the rear taper as gradual as possible, which means less length available for the front end, so you end up with a hemisphere.

But if the hemisphere itself was the optimal shape for a front end, planes would be designed that way-- but they aren't. Their noses are shaped more like a half oval, or parabola. They are pointier than the theoretical teardrop.

In car design, you don't get to use as shallow a rear taper as physically possible, because you have to deal with design constraints. So you get it as shallow as you can, and then you design the front of the car based on the constraints created by mechanicals, windshield, driver, etc. There isn't much disadvantage to making the front pointy, because you'd only be adding a little more length. Compromising with a blunt front doesn't allow you to make the rear any shallower.

Since the pointy front would normally take the shape of a horizontal crease, it's not really affected by crosswinds.

[trikkonceptz]

Throwing my back woods logic into the mix... I would have a hard time arguing with mother nature as far as ideal shape at sub sonic speeds. Thereby making the water droplet moving through air the most aerodynamic shape under atmospheric pressure.

[MetroMPG]

Originally Posted by winkosmosis

The purpose of the bulb is to change the shape of the bow wave so it doesn't interfere with hydrodynamics at the rear of the ship. It's optimized for a particular speed

Bulbous Bow

You're right, that was a bad example.

A better example would have been modern submarines designed primarily for fast, quiet underwater operation. They feature a rounded, not "pointy" bow.

But Matt is right: we need to be careful trying to comparing ordinary road vehicles to aircraft, watercraft or even speed record-setting land vehicles. The design rules change depending on the speed & size of the vehicle (in the same fluid), and between fluids of different densities.

[hypermiler01]

Your basic mistake is assuming that the front of the water drop is aerodynamic. It isn't.

Surface tension of water pulls it into a perfect sphere. This has nothing to do with aerodynamics.

And the front of the water drop isn't pointy because those water molecules are being pressed back by air pressure in front, and pulled back by the surface tension.

If the front of the water drop were shaped like a high velocity bullet, it would fall faster.

Because the pointy nose does not cause as much of a pressure increase in the air in front of it.

So you ARE arguing with nature.

[hypermiler01]

Originally Posted by MetroMPG

You're right, that was a bad example.

...we need to be careful trying to comparing ordinary road vehicles to ... even speed record-setting land vehicles. The design rules change depending on the speed & size of the vehicle (in the same fluid), and between fluids of different densities.

How so? Land speed cars are designed specifically for minimum air resistance at subsonic speeds, which is exactly what we want here.

The only reason that every car couldn't be shaped like a land-speed racer is because parking and speed bumps would be hell.

[Bicycle Bob]

IHMO, the last LSR vehicle with an optimized cd was the Railton Special. A lot of engine guys just design for low frontal area and mostly teardrop shapes, and they are usually into downforce, too.

[hypermiler01]

And nobody in the past 70 years has tried to optimize their cD for reduced air resistance? Even though the setting of records and winning of prizes depends on it?

[Bicycle Bob]

John Cobb drew a nice shape and packed his machinery into it, and held the record far longer than anyone else. Mickey Thompson didn't even use a tapered tail, and missed the record by a technicality even after adding superchargers. The Summers Bros. used a more slender, tapered shape, with more HP and less frontal area to finally get Cobb's record. These days, traction, acceleration, and stability/downforce dominate LSR designs.
If you want to see how much engine men think about airflow, just try to find a streamlined roll bar. A bit of cardboard and tape can halve the drag on those, and proper fairings are far better than that.

[hypermiler01]

Found a graphic illustrating the air pressure on the surface of a sphere traveling through air.

The reason that cars, fish, and planes have pointy noses is because they lower the air pressure in front, which causes less resistance to forward motion.

[Frank Lee]