Hood splitter for my vw

[aerohead]

Originally Posted by winkosmosis

Look at the article. They say that increasing hood angle reduces drag if it prevents separation. Bubble = separation.

The issue for the hood angle would be at the leading edge,not the cowl area.If you'll go the the FLOW-IMAGES thread you'll find many wind tunnel images which illustrate acceptable flow over the windshield in spite of cowl ventilation induction.
The article might mention that the entire car has a bubble out in front of it.This can be clearly seen in any wind tunnel photo.

[aerohead]

Originally Posted by winkosmosis

The "bubble" by definition isn't as good as sheetmetal/glass. It's high pressure and that pressure is acting on the vehicle, applying rearward and downward force.

If you're saying the high pressure bubble isn't a source of drag, you might as well say the low pressure bubble at the back of a vehicle isn't a source of drag. So does that mean boattails are useless??

The 'bubble' travels along with the vehicle.It imparts no pressure.

[winkosmosis]

Originally Posted by aerohead

The issue for the hood angle would be at the leading edge,not the cowl area.If you'll go the the FLOW-IMAGES thread you'll find many wind tunnel images which illustrate acceptable flow over the windshield in spite of cowl ventilation induction.
The article might mention that the entire car has a bubble out in front of it.This can be clearly seen in any wind tunnel photo.

Read the whole thing. There is a whole section about nose design and separation. They're talking about the hood and windshield angles in the part I was referring to and even showed a diagram of the "bubble".

A flow image doesn't tell you what the vehicle's drag is. Look at the example of the fine looking smoke trails... looks great but doesn't tell you that the Cd of a Ford Escape is 0.4.... And how about the VW Beetle Frank posted? The lines look great, but the Beetle has a Cd of 0.49!

[winkosmosis]

Originally Posted by aerohead

The 'bubble' travels along with the vehicle.It imparts no pressure.

You'll have to explain that because I don't see how it's possible.

Air is flowing around the stagnant bubble, applying force to it, and so the bubble itself is high pressure. How can it impart no pressure? You're saying you get something for nothing.

The whole car is surrounded by a boundary layer so does that mean there's no drag? The body of the vehicle is traveling with it. Does that mean the body imparts no force to the uniframe?

Are you familiar with the pitot tube? It's the device that aircraft use to measure airspeed. Airflow increases the pressure inside the tube, the higher the speed the higher the pressure. The air inside the pitot tube is a "bubble" moving along with the plane, and it imparts pressure, otherwise the device wouldn't work.

[3-Wheeler]

Originally Posted by winkosmosis

...Are you familiar with the pitot tube? It's the device that aircraft use to measure airspeed. Airflow increases the pressure inside the tube, the higher the speed the higher the pressure. The air inside the pitot tube is a "bubble" moving along with the plane, and it imparts pressure, otherwise the device wouldn't work....

I'm not trying to be picky here, because I understand the point your trying to make, and you are making a good point.

However, the pitot tube, or at least the pitot tubes I have seen at work, work on the differential pressure between the static pressure (potential energy) and the dynamic pressure (kinetic energy).

Static pressure would indicate the generalized pressure rise inside the duct that feeds the flow meter pitot tube, and the air velocity component measures the air pressure *reduction* due to the high velocity air. Yes, I did say pressure *reduction* or vacuum if you wish.

I should point out as a matter of air flow discussion, that the higher the air velocity inside a closed conduit, the higher the *vacuum* generated. This is exactly how vacuum aspirators work. They take compressed air and convert it into a vacuum due to a tiny restriction flow path inside the device, that highly accelerates the air stream, which in turn creates a high vacuum. I have to demonstrate this phenomena to new staff in the lab from time to time as some of them have a hard time believing the effect.

And so it is with pitot tubes that measure the air stream velocity. I believe that the really accurate tubes measure the differential pressure, and not just the stagnation pressure as alluded to above.

Hope this helps, Jim.

[3-Wheeler]

Originally Posted by aerohead

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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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Hi Aerohead,

Your point above is well taken, I really did understand what you were initially trying to say, but I did have to at least make a comment about the mass of the air molecules.

Another example that shows just how significant that mass can be is taking a look at the ordinary air nozzle, you know, the one used to dust off metal parts after machining them?

The compressed air nozzle is very effective at directing a blast of air at the object which needs the chips blown off for whatever reason.

If the air mass were *not* an issue and was strictly dictated by pressure alone, the blast of air would not jet out from the nozzle, but propagate out in a hemispherical pattern in all directions from the nozzle. In this case, pointing a nozzle at an object to be cleaned would have only a slight effect on it, and in fact blow just as much air toward the user as away from them as well.

But the fact that this does not happen means that mass is a larger effect than directed in the previous post.

Jim.

[donee]

Hi All,

Hood/Windshield angle has to have some kinda optimum region. But its more complicated than a 2D model can determine. This discussion is with respect to primarily one plan or two intersecting plane front end designs.

Here is why. A steep angle results in the formation of vorticies at the upper corners. A shallow angle results in momentum effects across the whole top of the car. Or at least that is what I would look to verify in a CFD model if I was starting such an analysis.

The steep angle results in flow attachment in the center of the windshield/roof transition becuase the air slows as it reaches the top of the windshield. The slowed air at the edges starts to turn outboard pickus up speed and then has momentum to shoot beyond and outboard around the upper A-pillars. Then tumbles into the vortice.

The shallow angle results in fast air hitting the windshield/roof transition, and it cannot turn in time to stay attached. The result is a slight bubble at the central portion of the transition, which increases the effective cross section area. As the airspeed is great there, the flow never goes turbulant, just shoots up a little before turning. At the edges, the air never gets slow enough to start turning much, and then when it hits the A-pillar, it just wraps around. I saw this in the 3rd Gen Prius salt pattern on its upper 1/3 of the A-pillar. Paterns, I never saw on the 2nd Gen.

Now, I am writing like I know this for sure, but I do not. This is just a proposed hypothesis.

I think the angle is critical. And its somewhere between the 2nd Gen Prius, and the 3rd Gen Prius angles. Because in my 2nd Gen Prius, the turbulators on the upper 1/3 of the A pillars improved air flow. And in the 3rd Gen, they disturbed air-flow. But, there are other differences between the two - the inset of the windshield on the 3rd Gen. Its a 3D problem. And there is no getting around that.

Most cars in the steep angle regime. So, improvement really is not going to be made at the central zone of the transition, because the air stays close to the body there. The improvement to be made is in the upper 1/3 of the A-pillar wrap-around air, with the turbulators. Which prevent the vortice from forming, and keep the effective cross-section minimized.

An exception to this might be the older Saabs with the curved windshields. Which does not have an intesecting plane design. The curve probably prevents the vortice from forming, and being steeper, the air over the top is closely flowing to the roof. With this design, there might be improvement to be made at the lower 1/3 A-pillar, such as large fillet.

And an improvement in a new design using the planar features, would be to round over along the windshield/roof line, the sideglass/roof line, and A-pillar/sideglass lines. This requires more complicated metal forming and glass forming, all joining at that corner. The side door might actually have to be clipped on its forward upper corner, to accomadate the 3D curves. Which is probably why its not been done yet.

[winkosmosis]

Originally Posted by 3-Wheeler

I'm not trying to be picky here, because I understand the point your trying to make, and you are making a good point.

However, the pitot tube, or at least the pitot tubes I have seen at work, work on the differential pressure between the static pressure (potential energy) and the dynamic pressure (kinetic energy).

Static pressure would indicate the generalized pressure rise inside the duct that feeds the flow meter pitot tube, and the air velocity component measures the air pressure *reduction* due to the high velocity air. Yes, I did say pressure *reduction* or vacuum if you wish.

I should point out as a matter of air flow discussion, that the higher the air velocity inside a closed conduit, the higher the *vacuum* generated. This is exactly how vacuum aspirators work. They take compressed air and convert it into a vacuum due to a tiny restriction flow path inside the device, that highly accelerates the air stream, which in turn creates a high vacuum. I have to demonstrate this phenomena to new staff in the lab from time to time as some of them have a hard time believing the effect.

And so it is with pitot tubes that measure the air stream velocity. I believe that the really accurate tubes measure the differential pressure, and not just the stagnation pressure as alluded to above.

Hope this helps, Jim.

According to the diagrams here, the pressure inside the tube is higher than static pressure Pitot Tubes

Look at the equations. They depend on stagnation pressure being higher than static, because you can't take the square root of a negative number
V = \sqrt{\frac{2 (p_t - p_s)}{\rho}}
http://en.wikipedia.org/wiki/Pitot_tube

Since the tube is pointing into the airflow, it has to be higher pressure inside than outside. Stick your head out the car window and you can feel the pressure increase in your mouth. Ram air intakes on cars wouldn't work otherwise.

[aerohead]

Originally Posted by 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.

* The favorable pressure gradient at the front of the vehicle helps maintain attached flow.It travels along with the vehicle.It has nothing to do with Newtonian physics.It in itself imparts no drag to the vehicle.
* drag is predominantly about pressure differentials.
* Air does not exert a rearward force on the vehicle.
* It is the pressure differential between the base pressure of the turbulent wake and the forward stagnation point which creates the drag ( I'm ignoring skin friction,we can't do anything about it ).
* The 'vacuum' behind the vehicle,acting in concert with the forward stagnation pressure creates the net retarding force.
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* The Prius' front end is cheap to build.It's not low drag.It's not a model for low drag.It's an attempt to not pay the piper for the more expensive compound windshield used on all high performance low drag vehicles.
* Your diagrams are circa 1986,yes?
* You may track down Chuck Torner and David Holls.Chuck was Director,Advanced Engineering,General Motors Corporation.He was involved with the AERO X.David was Director,Advanced Design,General Motors Corp.,also involved with AERO X.Ask them about your nose/cowl/windshield diagrams.
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* Below 250 mph,no compression of the air takes place.
* You are spot-on with respect to the energy required to displace air,that's where the lion's share of your net fuel dollar is going when on the highway.
* Frank Lee is correct in what he says.As the air reaches the point of max. cross-section it will attempt to reach equilibrium as it was before the vehicle came along.
* If the body allows it,the air will ramp-down off the back of the aft-body without separation with little perturbation.As the air descends the tail it is giving up velocity pressure for static pressure and will exit at a pressure close to the undisturbed air mass of the outer regions.The atmosphere really doesn't push it back together,it just marches back to equilibrium.There will be some disturbance of course,if for no other reason than entropy,as a consequence of the surface friction and viscous shear of the real gases.
* The boat-tail is the deceleration ramp which allows the air to slow,gain pressure,without separation.

[aerohead]

Originally Posted by winkosmosis

The blister can be shaped to optimally, the bubble is shaped according to the airflow and the pressure... like a raindrop, which people think is "teardrop" shaped, but in reality is not aerodynamic at all.



We know it's enough force to matter because cars designed to be as aerodynamic as possible have a shallow windshield and a steep hood to the point that they're nearly the same, eliminating any bubble.

20° average slope windshield

If you'll do a search for this thread and location you'll find a 3-series photo montage of the 1st-gen Golf/Rabbit.This is earlier than Varn's Jetta,and you'll see that with the angled hood/windshield intersection,the air just goes around all of it.There is no discernible 'bubble' except in front of the 'ideal' nose.The air goes around the blunt nose as if it were shaped like a Hershey's Kisses.