Hood splitter for my vw

[Christ]

D don't forget that if the start and end points are the same for a pair of curves, the one with the wider radius will also cover more linear distance, which can account for an increase in fe...your using the same quantity of energy to get from a to b, but using that energy over a longer distance on a wider curve.

[winkosmosis]

Originally Posted by Christ

D don't forget that if the start and end points are the same for a pair of curves, the one with the wider radius will also cover more linear distance, which can account for an increase in fe...your using the same quantity of energy to get from a to b, but using that energy over a longer distance on a wider curve.

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

[donee]

Hmm...,

The rain paths are different than I saw when I had tufts on my 2006 Prius. There was still a side ways component to the air velocity in the upper 1/3 of the A pillar area of the windshield.

The hump in the middle might be allowing the air on either side of the bump, on the windshield to squeeze into a lower pressure area formed behind the bump. Thus the inward velocity on the inboard side of the rain patterns. This air being inward, reduces pressure along the A-pillar, and avoids the spillage up high, but promotes it down low.

This seems to be backward to what Toyota did in the Gen III Prius, where they hollowed out the hood in the central area near the windshield. Interestingly, I took the turbulators I had on my Gen III in the upper 1/3 A-pillar area (similar to the Gen II) when I noticed the salt stains were straight back above and below the turbulator, and there were were no stains behind the turbulator. Indicating flow detachment. The windshield of the Gen III seems to be inset further in this area. And that seems to be helping the air go straight, and cascade smoothly when it goes over the edge of the body on either side of the windshield.

This does not mean what your doing is wrong, it just means the two circumstances are, most likely.

The tank averages are useless. You need need to do A/B/A coast down tests. The wind and temperature changes this time of year will completely skew the tank mileage averages....

[aerohead]

Originally Posted by Varn

Since I made my hood splitter I have been rewarded by getting 50 mpg over 500 miles. My previous best was the last tank was 47 mpg. Now my last two tanks, 900 miles have been at 49 mpg average, the average since I bought the car has been 42. My max speed is usually about 65 with usual cruising speed of 61 mph. The has weather been getting warmer above the 50's by mid day. I always use the same pump for fill ups.

I am confident that the mileage is accurate. I have a GPS to measure the miles and I use a good technique to try and get the same fill each time. I'm generally fully loaded on the highway, 5 cats and 3 medium dogs. The trunk is packed to the top with clothes, food, guns and ammo and bible. Pretty much what the average doctor and nurse carry when fleeing from Illinois. The Jetta has a lot of space for what it is. We had to drive into town today to pick up some drainage tile. We got a 100' roll into an otherwise empty car today. It doesn't weigh anything but imagine 100' of 5”o.d. tubing snaked all over the trunk and back seat. I had the rear seat back out.

From what I have heard the newer diesels are more efficient than my 1986 engine is. I can't do much about it. I don't have any special instrumentation to help drive efficiently. There isn't a computer on this vehicle that I can tap into. I don't find out what makes it run more efficiently except in 4-500 mile increments.

I am thinking about aerodynamics and don't believe the scenario of that the front is good enough and the name of the game is entirely at the back. I do understand the importance of closing the rear but am working on better equalization of the pressure at the front. Reducing the high pressure spikes there of importance as well. .

My car is rated at 34 mpg as corrected by the EPA. Except for 3 small pieces of coroplast it is stock although the car has narrow winter snow tires on it. Getting 40 mpg in this car is easy, just drive it at 70 mph. Getting 50mpg was not that difficult. Guess I have to try for 55.... probably it is going to be exponentially harder. I am really getting grumpy with paying 3.74 a gallon for diesel.

Varn,Ford got the Probe-IV to Cd 0.15 and change with only front mods so experimenting up there might be fun.
You know of course that Ford followed with the Cd 0.137 Probe-V which employed aft-body mods.
1957 MG EX 181 Cd 0.12 pumpkinseed.
1987 GM Sunraycer Cd 0.089 pumpkinseed.
2009? NUNA Cd 0.077 with extreme aft-body streamlining.
The 'Template' produces Cd 0.08 in ground-effect.Cd 0.13 with 'standard' wheels.Below Cd 0.13 with wheel fairings.Skinny wheels?

[aerohead]

Originally Posted by winkosmosis

There's a reason some cars (90s Camaro) have deflectors in front of the wipers.

Do you think the deflectors operate when the wipers are 'parked',or when at full extension?

[aerohead]

Originally Posted by winkosmosis

Well of course it's not known to have attached flow-- because on most vehicles it's a sharp transition, so a bubble of air forms there.

This truck didn't have attached flow from the front to the sides until NASA put curved fairings on it

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.

[aerohead]

Originally Posted by winkosmosis

Well the blister looks too narrow, but I think a gradual transition between the hood and the windshield would obviously allow air to flow smoothly and stay attached, rather than forming a high pressure bubble. I've seen people on this forum say that the bubble means aerodynamics in that spot don't matter, but it seems to me that it's the opposite.

Look the Aptera, Prius, and other aerodynamic cars. The hood is almost the same angle as the windshield so there probably is smooth continuous flow. The question how close to the same angle ideal you can get with a fairing that doesn't connect to the windshield.

Making the transition gradual with a curve has to help. It takes energy to force air to change direction, but less energy if you direct it gradually. Just look at intake pipes, sewage pipes, etc etc... Gradual curves, not right angle elbows.

The bubble is the locked-vortex that the other air simply skims over.It's also the fresh air source for the cabin and the fan doesn't have to work very hard to bring it in which helps the alternator load( i.e. mpg ).
With respect to Aptera,the majority of air goes around the car,not over.
The Prius,at Cd 0.25 is not demonstrating remarkably low drag for its length/height ratio so I don't know if the zero-zero windshield slope is any quantum leap.
R.G.S White reported the full convex semi-hemispheric windshield as lowest drag as used in Varna,Sunraycer,MG EX-181,M-B C-111 III,Renault Vesta II,McLaren,Honda P-100,GeeBee R-1,USSR' Alpha/Akula/Charlie-class nuclear subs,USS Macon/Akron/Hindenburgh,etc..
Hucho mentions,as others,that if you've got attached flow around the windshield and on to the backlight header,then concentrate on the aft-body,moving the separation point back as far as you dare ( boat-tailing ).

[aerohead]

Originally Posted by winkosmosis

Yeah and at the rear the smoke stream always follows a nice smooth gradual curve downward and rearward. Put a brick in the wind tunnel and you'll see the same thing. Works great for advertising materials!

You realize of course that the sharp-edged brick would have fully separated flow and Cd 0.85,yes?

[aerohead]

Originally Posted by Varn

Interesting smoke around the beetle. A top down view would be interesting considering the external fenders.

Nice thing about the multiple lines is that you can see compression by noticing the spacing of the lines at the front of the car
Since lowering the windshield might be prohibitive filling in front of there might be a start to lower the pressure over the roof of car. The smoke trail is moving upward at about 20 degrees.

On the 86 jetta the trunk lid is pretty high but still might be a bit low to be ideal. Thankfully it is squared and not rounded like the early model vw. unfortunately the front of the jetta is also very square. The hood angle is 7 degrees.

I'm going to throw a caution-flag out on this image.I'm guessing that this is a 1/24-scale model in a student wind tunnel and they've slowed the air velocity down below that critical for proper Reynolds number to get a nice image but the boundary layer is laminar,not turbulent,and flow is separating prematurely.
Elsewhere there is a photo of a full-scale Beetle in the VW tunnel at critical Reynolds number and the flow is completely different!Flow is attached to the top of the backlight.
It changes the whole discussion!

[aerohead]

Originally Posted by winkosmosis

It always takes less energy to change direction with a larger radius... compare coasting your car through a 90 degree corner vs a wide sweeping curve.

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.