Hood splitter for my vw

[aerohead]

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.

Jim,my pressure argument was directed towards winkosmosis' analogy of a structure going around a right-angle curve or around a smooth curve in the context of Varn's windshield.
I have wind tunnel photos of Varn's car.There is no separation anywhere in the fore-body.The radii chosen by VW are adequate for completely attached flow.I believe this has been the situation since the decade after the 1st Arab Oil Embargo.
Looking through Hucho's book,Hucho says ( paraphrasing):
* pressure drag is the largest component in the aerodynamic drag.Its minimization is the true objective of motor vehicle aerodynamics.
* on the drag problem of a body ....the front has only minor influence....the main drag originates from the rear.......... it is not important to find a proper front........ but it is very important to design a rear body surface which brings the divided streamlines smoothly together.Optimum shapes are 'streamlined' bodies having a very slender rear part.
* lower drag can only be achieved by extending the length of the vehicle's(sic) body.
* the drag coefficient for... cars may be plotted against vehicle length... (I)f the evaluation is limited to vehicles that are developed for the lowest possible drag coefficient,this expected trend is in fact confirmed.
* the fineness ratio of the ( Cd 0.15 ) ARVW is L/H=5.53...... this corresponds to an effective fineness ratio in free air of 2.27.This approaches the drag minimum recognizable in Fig.4.119.
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* Momentum does not appear in the index of the book.It is mentioned in remarks about body camber.
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* Inertia is only mentioned tangentially in Chap.13,dealing with CFD codes and the Euler equations,Navier-Stokes equations,Equation of continuity,Mach number/Reynolds number,Bernouli Theorem,and Laplace-Potential Equation
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Hucho basically blames all our problems on pressure-related effects.
We get to cheat on the front as the favorable pressure gradients there tend to hold the boundary layer against the body.
-------------------------------------------------------------------------- Our challenge is in the contour of the aft-body where no favorable pressure gradient exists,outer-flow velocity has already peaked,pressure is beginning to increase and ideal inviscid flow theory dictates that portions of the boundary layer must have their velocity retarded at locations where their velocity is already zero,triggering the flow separation which begins the turbulent wake with its low base pressure which make up the elephant in the living room we call pressure drag.
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With respect to the Template,it is based on the streamline body Hucho recommends.It has the very slender rear part.It brings the streamlines back together.It extends the body.It is separation-free.
In free air the Template has Cd 0.04 ( from Hucho's table of Cds )
In ground reflection,the Template produces a body of Cd 0.08( after Ludwig Prandtl)
Adding conventional wheels pushes it to Cd 0.13.( after Paul Jaray/Wolfgang Klemperer.
By 'integrating the wheels into the body'(Hucho) the Cd can fall below Cd 0.09 ( GM's original Sunraycer with wheel fairings )NUNA is at Cd 0.077.
Your Varna should be around Cd 0.11 according to the IHPV folks at Battle Mountain.
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The aft-body,if not very much like the Template,WILL trigger flow separation and initiate the formation of the tubulent wake.Exactly what Hucho says is paramount to avoid.
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With the exception of Jeep CJ and Hummer H-1 type vehicles,I can think of very few late model vehicles which do not provide the minimums for well attached fore-body flow.

[aerohead]

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.

"Fluid seeks to avoid very large velocities while negotiating sharp edges,and forms surfaces of discontinuity instead." Ludwig Prandtl,phD.,Gottingen Wind Tunnel,Germany.

[aerohead]

Originally Posted by winkosmosis

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.

Blade pitch varies as a function of differing angular velocity as regards position along the blade.Blade root and blade tip are traveling at remarkably different velocities.In order to provide a uniform acceleration to the air mass,the blade must be pitched.

[Joenavy85]

Originally Posted by winkosmosis

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.

a general rule of thumb: moving alot of air slowly is more efficient than moving a little bit of air quickly. that's the reason why helicopter rotors are as big as they are

[winkosmosis]

I think the paraphrasing you posted is ignoring the fact that the favorable pressure at the front of the vehicle is exerting a rearward force. That's what drag is all about-- the surrounding air exerting rearward force on the vehicle. That's why turbulence creates drag, that's why the vacuum at the back of a vehicle creates drag.

Otherwise, how do you explain the Prius's front end? Why didn't they use more traditional looking front bodywork? The article I posted shows diagrams of different nose shapes, and they quantify the effects of noses, hood angles, and windshield angles on drag.

BTW, it's not exactly momentum I was talking about... It's the fact that it takes energy to push aside fluid when you're surrounded by the fluid. The vehicle has to compress a large volume of air. Look at the smoke lines from a wind tunnel-- a lot of air has to be displaced, and of course displacing it more slowly by using gradually sloped surfaces costs less energy. Frank Lee said earlier in the thread that the air molecules are trying to get back to where they were. Well they don't remember where they were, they're just being pushed back into the area behind the vehicle by the higher pressure surrounding air.

Just like the gradual slope at the front makes displacing air cost less energy, the gradual slope of a boat tail makes replacing that air cost less energy and so it's easier for the atmosphere to push those molecules back into place.

[winkosmosis]

Originally Posted by aerohead

Blade pitch varies as a function of differing angular velocity as regards position along the blade.Blade root and blade tip are traveling at remarkably different velocities.In order to provide a uniform acceleration to the air mass,the blade must be pitched.

Look at the blade... I'm not talking about pitch variation from inside to outside. It's curved from the leading to the trailing edge of the blade. Can you not see that by looking at it?

[winkosmosis]

Originally Posted by aerohead

"Fluid seeks to avoid very large velocities while negotiating sharp edges,and forms surfaces of discontinuity instead." Ludwig Prandtl,phD.,Gottingen Wind Tunnel,Germany.

Originally Posted by Joenavy85

a general rule of thumb: moving alot of air slowly is more efficient than moving a little bit of air quickly. that's the reason why helicopter rotors are as big as they are

EXACTLY. That's why a shallow windshield and hood, moving air out of the way (upward) more slowly, are more energy efficient. That's why the Prius has a 20° and a hood with almost the same angle, and all those surfaces are smooth and gradually curved.

[winkosmosis]

I googled "hood windshield transition".

2009-2011 Dodge Ram pickup trucks
The cowl screen smoothes the airflow transition from hood to windshield, reducing turbulence and wind noise

Car Aerodynamics 101
Vehicles with steep windshields can benefit from a hood fairing to help smooth the transition of air between the hood and windshield.

Wait a minute! What's this in front of the GM Precept's windshield?

[aerohead]

Originally Posted by winkosmosis

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




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.

inclined windshields are low cost.They do not reside on low drag cars.No low drag car has a simple inclined windshield.
If the objective of the car maker is to channel a lot of air over the roof,then the incline is beneficial.
You know that Honda and Toyota cars which use this also have speed limiters to throttle top speed .
You may also be aware that when Bugatti,Lamborghini,and Imperator use this windshield they build a lot of induced drag into the car to keep them earthbound.
* Riley's Tri-Magnum wasn't fast enough to be in jeopardy
* Toyota's first minivan had limited speed
* Mercedes A-class isn't at risk
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The greatest economy cars all have compound windshields
* M-B C-111 III
* ARVW
* Honda P-100
* Renault Vesta II
* GM Ultralite
* GM Impact/EV-1-2
* GM Solaraycer
* Dymaxion Car
* 1935-1/2 DeSoto Airflow experimental
* Jean Andreau Peugeot 402
* 1927/8 Chrysler Jaray prototype'
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Roof camber can only help if normal roof height is maintained and frontal area is maintained.
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Can you please explain 'pressurized air'?

[winkosmosis]

What do you mean "no low drag car has a simple inclined windshield"? The Prius has a 20 degree windshield.

Or are you talking about flat vs curved? No one has even mentioned flat windshields.

The more sloped windshield doesn't increase drag.... Those supercars have shallow windshields just like the Prius, because they reduce drag. They aren't building their cars with anti-aero principles. They're designing them to be as aerodynamic as possible, with as little lift as possible, or even downforce. And designing for downforce doesn't mean you suddenly don't care about reducing drag.

Indycars and F1 cars have high coefficients of drag because of their downforce surfaces, but the other surfaces are as aerodynamic as possible. An airplane has a low coefficient of drag when the wings are parallel to airflow, but in the real world they generate lift by flying at an angle, which means their drag is pretty high. Yet airplanes generally have fuselages designed to be as aerodynamic as possible.

By pressurized air I mean air that is at higher pressure than the surrounding atmosphere.

Edit: The article specifically states that you can reduce drag by adding arch and frontal area. This is common sense, like I said in the other thread. If you decrease Cd by 10% and frontal area only goes up by 5%, voila you decreased drag.