Hood splitter for my vw

[aerohead]

Originally Posted by winkosmosis

Fighter jets have pretty upright canopies for the sake of visibility, and long "hoods" to house radar... so we should look at what they do.

The F22 has a gradual transition from hood to windshield

Aircraft aerodynamics presumes zero separation,low to nil form drag,and a majority of surface friction drag ruled by wetted area.
Ballistic aircraft aerodynamic drag is ruled by shockwaves.
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Automobile drag is ruled by flow separation and the differential between the forward stagnation pressure and the base pressure of the separated wake.
Moving the separation point back is the only way to achieve the lower drag.
Lowest drag is achieved when there is zero wake.Fully boat-tailed.
While the 'front' of a car is not un-important,it is the 'rear' of the car which is most important.

[aerohead]

Originally Posted by euromodder

http://ecomodder.com/forum/showthrea...ics-16219.html

The value published for the drag reduction inherent from altering the hood/cowl intersection angle is a typographical error which was not caught by a fact-checker at the publisher.
The data is incorrect.I have two other studies which virtually duplicate this research and the value is off by a decimal point,4-point something,not 40-point something.

[aerohead]

Originally Posted by winkosmosis

You're assuming a bubble is just as aerodynamic as metal. Following that reasoning, why put a boattail on a car when a bubble forms back there in the shape of a boattail anyway?

(1) the bubble IS as aerodynamic as metal.That's the point.
(2) a bubble does not form behind a vehicle in the form of a boat-tail unless you're talking about the last 15% of the 'Template' which is superflous anyway do to ground clearance.
Hucho made a grave contextual error when referring to Lay's wind tunnel research and the drag reducing contribution of the tail.
Kamm's comments MUST be taken in context as to the value of a tail
Ditto Fachsenfeld
Ditto Korff
Ditto Alberto Morelli ( he removed only 16% from the back of the CNR banana car,not 50% as inferred in some articles concerning the K-form).
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If you add out to 70% boat tail,based on the 'Template' you can expect no appreciable drag penalty.The air WILL pass over a phantom tail area from 70% rearwards.

[3-Wheeler]

Originally Posted by aerohead

Bear in mind that in streamlining,we're dealing with pressure,not inertia and momentum.
Pressure converted to velocity by dislocation will seek equilibrium,flowing towards regions of lower pressure until de-dislocated,at which time it will return to pressure either by streamlining or viscous attrition.

Hi Aerohead,

I'm not sure that I completely agree with what you said about "pressure" being the only factor in air movement.

Air molecules *do* have mass and because of that also have inertia.

Now the air molecule's mass is very low, but it's still there.

Regarding moving an object through the air, the biggest factor is pressure itself and the fact that the molecules "communicate" with each other at only the speed of sound, and not faster like electrons do in electrical circuits.

Air moving close to the speed of sound becomes a major issue because as you know, the "communication link" between molecules is almost nil due to the high speed. The air molecules can not communicate their localized pressure to surrounding molecules, because they are all moving at the speed of sound, and this is as fast as the pressure pulse in the air can move.

Obviously your Streamlining Template is designed to give the air time to move back into it's originally undisturbed space before our vehicle passes through, via the localized air pressure at the time. And this is partly due to particle mass, local pressure, and the speed of sound, all contributing to the general movement of air back to it's original state.

Jim.

[winkosmosis]

Originally Posted by aerohead

NASA may have begun with the 1963 'oval' nose,as developed by GM researchers and then went from there.
Even with the NASA mods,the airstream will begin to deform long before it reaches the truck as reported by Prandtl in the 1920s/30s.
This cab might have had attached flow before the mods.

There's a weird tendency on this forum to think attached flow is all that matters for drag... I think it's just a bandwagon mentality--- a few members who consider themselves experts constantly talk about attached flow, and that becomes the common wisdom. But what also matters is the energy it takes to move air aside. Just because you have a bubble making airflow look nice in a wind tunnel doesn't mean it's not taking more energy than if the "bubble" was made of metal.

[winkosmosis]

Originally Posted by 3-Wheeler

Hi Aerohead,

I'm not sure that I completely agree with what you said about "pressure" being the only factor in air movement.

Air molecules *do* have mass and because of that also have inertia.

Now the air molecule's mass is very low, but it's still there.

Regarding moving an object through the air, the biggest factor is pressure itself and the fact that the molecules "communicate" with each other at only the speed of sound, and not faster like electrons do in electrical circuits.

Air moving close to the speed of sound becomes a major issue because as you know, the "communication link" between molecules is almost nil due to the high speed. The air molecules can not communicate their localized pressure to surrounding molecules, because they are all moving at the speed of sound, and this is as fast as the pressure pulse in the air can move.

Obviously your Streamlining Template is designed to give the air time to move back into it's originally undisturbed space before our vehicle passes through, via the localized air pressure at the time. And this is partly due to particle mass, local pressure, and the speed of sound, all contributing to the general movement of air back to it's original state.

Jim.

It's not just the mass either though. Moving air takes energy, because air is surrounded by air. Pushing air out of the way of a vehicle means shoving air molecules into other air molecules. It takes energy just like moving air with a fan takes energy.

A more gradual transition is good in the same way a shallower raked windshield is good-- air is moved more slowly relative to the speed of the vehicle.

Look at your vehicle's electric cooling fan. See how the blades are curved? Why don't they just use the flow departure angle for the whole blade and make it flat?
Because it takes less energy to ramp up their air speed gradually over the blade surface, from a shallow angle to a steep angle.

We need to put to rest this nonsense about attached flow being the end all be all except at supersonic speed. A normal fan doesn't spin at supersonic speed.

[Frank Lee]

Originally Posted by winkosmosis

There's a weird tendency on this forum to think attached flow is all that matters for drag... I think it's just a bandwagon mentality--- a few members who consider themselves experts constantly talk about attached flow, and that becomes the common wisdom. But what also matters is the energy it takes to move air aside. Just because you have a bubble making airflow look nice in a wind tunnel doesn't mean it's not taking more energy than if the "bubble" was made of metal.

Citations... or... anything?

[winkosmosis]

Originally Posted by Frank Lee

Citations... or... anything?

Common sense? Take a cardboard folder, hold it at a 15 degree angle and move it through the air and note the resistance you feel. Then angle it to 45 degrees and fold it so that the frontal area is the same. Is the resistance more or less or the same??

Then look at the real world examples. Car windshields- you've never noticed that economy cars have very slanted windshields? Prius? Heck, look at your own picture of the Beetle in the wind/water tunnel. The fact that air piles up and forms a "bubble" proves that flow is not as easy as if the windshield were sloped back. Look at the fan and ask yourself why the blades are curved instead of flat.

I know you are into rear end aerodynamics. Did you ever ask yourself why airflow is better able to stay attached the shallower the rear surface angles are? It's because it takes less energy for the pressurized air to expand toward the surface and flow along it.

Just visualize-- use the spatial common sense that mother nature built into your brain which allows you predict where a ball will land depending on how hard and at what angle you throw it.

But if common sense and visualization isn't enough for you here is an article that talks about hood and windshield angles:

In This Chapter the Modifications That Were Carried Out To

Windshield inclination angles of more than 60o are not practical because of light diffusion. In addition, large highly inclined windshields lead to increased solar heating of the passenger compartment. Two effects are responsible for the favorable, though moderate, influence of a highly inclined windshield on drag. First, the excessive speed in the area of the A-pillar is reduced so that the momentum loss occurring at the point is smaller. Second, the deflection of flow at the transition from the windshield to the roof is smaller. The low pressure peak occurring there is therefore smaller and the positive pressure gradient in the remaining flow is less steep. Hence the momentum loss in the boundary layer is lower, allowing greater pressure recovery in the area of rear end. Therefore even if a strongly inclined windshield does not contribute to a local drag reduction, it helps to improve the flow over the rear part of the car and thus to reduce the overall drag.

Note that even though the language suggests that a strongly inclined windshield doesn't "contribute to local drag reduction", their strong incline is less than 30 degrees from horizontal, much less than the OP's. I believe that's even a shallower angle than the cab forward Honda Civic's.

They say that a steeper hood angle doesn't help drag once it's good enough that flow stays attached, so it seems to me like the bubble that forms when the transition is too severe is the problem.

BTW, that article also has a section on arched roofs, where they explain that even though an arched roof increases frontal area, it can reduce total drag.

[Frank Lee]

Common sense? Take a cardboard folder, hold it at a 15 degree angle and move it through the air and note the resistance you feel. Then angle it to 45 degrees and fold it so that the frontal area is the same. Is the resistance more or less or the same??

Common sense tells me this does not apply here.

Car windshields- you've never noticed that economy cars have very slanted windshields? Prius? Heck, look at your own picture of the Beetle in the wind/water tunnel.

I've noticed that exotics have very slanted windshields, and their Cds are atrocious.

The fact that air piles up and forms a "bubble" proves that flow is not as easy as if the windshield were sloped back.

What is "easy" flow, and does it cause less, more, or the same drag?

I know you are into rear end aerodynamics. Did you ever ask yourself why airflow is better able to stay attached the shallower the rear surface angles are? It's because it takes less energy for the pressurized air to expand toward the surface and flow along it.

I'm not into rear ends any more than the next guy. Wait. That didn't come out right...

I know why flow stays attached at shallower departing angles- it's because on the aftbody, each molecule in the air mass that had been displaced by the forebody can only return to (roughly) it's original position at a certain rate in an orderly fashion. If that rate is exceeded turbulence ensues.

Just visualize-- use the spatial common sense that mother nature built into your brain which allows you predict where a ball will land depending on how hard and at what angle you throw it.

OK, I'm there now, in the field of dreams... this isn't helping.

And now, finally, on to the good stuff (thanks for linking that article! )

In #36 I said:

Originally Posted by frank #36

I can't tell enough from the Camaro pic to comment on it, but that region is known to not have high attached flow... Or, let's see data from the pros; either one will do.

...and the illustration and text you provided support that nicely.

Originally Posted by article

From all these data it can be concluded that the direct influence of windshield inclination on the drag is only moderate

Thank you. That's what I said. That's what Hot Rod Magazine found out too.

Originally Posted by article

Second, the deflection of flow at the transition from the windshield to the roof is smaller. The low pressure peak occurring there is therefore smaller and the positive pressure gradient in the remaining flow is less steep. Hence the momentum loss in the boundary layer is lower, allowing greater pressure recovery in the area of rear end. Therefore even if a strongly inclined windshield does not contribute to a local drag reduction, it helps to improve the flow over the rear part of the car and thus to reduce the overall drag.

Thanks again! See what they are talking about here? It ain't the bubble; it's the transition at the TOP- windshield/roof junction. When I said this:

Originally Posted by frank #38

So I'd wager hood blisters don't really affect the flow upstream and downstream... so then what?

... I meant if you replace the air bubble with a physical bubble, the flow ahead of it- over the hood- should be the same, and the flow after it- upper part of windshield, then windshield/roof transition, then aft- should be the same too. I see no reason for the faux bubble vs. a physical bubble in that location to make a material difference in drag.

Now you are off and running on something else besides the bubble: the windshield/roof transition.

[Frank Lee]

Originally Posted by winkosmosis

With the windshield and hood, putting in the splitter shortens the distance by making it more of a curve instead of a sharp angle

And don't call this thing a splitter, cuz a splitter divides flow and this ain't doing that.