Speed Holes for MPG?

[Logic]

While the results are likely very much car specific, here's someone that has actually done A-B-A tests as he was replacing the bumper anyway:
https://www.ek9.org/index.php?thread...26/post-158601

Bumper with holes
From A to B: 208km/h
From B to A: 208km/h

Bumper without holes
From A to B: 205km/h
From B to A: 206km/h (just barely)

Description of experiment
We drove to a parking spot next to the autobahn. There we launched. I kept accelerating untill a certain spot (next parking spot, few km's) and then we noted that top speed we got there. We turned around at the next point where we could. Then we would go flat out again from the beginning on the autobahn, only opposite direction obviously. Untill we came across the first parking spot, we then noted the top speed again.

Description of results
With the bumper with holes we got to the top speed pretty quick. With the new bumper, we were not only slower, maybe it was even more surprising how long it took to get at the top speed, compared to the bumper with holes. For example, on the B -> A run with hole-bumper we got to our top speed relatively quick and just before we got to the parking spot we had the feeling it might even get to 209, but there was a car that got in front of us. The run without hole-bumper was pretty different. It took way longer to get at 205 and only 100 meters before the parking spot where I'd let go of the gas pedal it went to 206. I have yet to download the G-Tech results, but for my feeling the bumper with holes made some difference in higher rpm's in 4th and 5th gear.

I NB that this is a low hanging, parachute shaped rear bumper without any sort of rear diffuser.
This is (still?) the case on many cars.

Looks aside, what I like about this idea is that this is one of the few aero mods that reduces weight by a tad and weight reduction is THE Eco Mod 98% of the time:

In the UDDS (urban) driving cycle: (98% of driving)
Rolling Resistance (%): 19
Aerodynamic Drag (%): 14
Inertia (%): 68

In the HWFET (highway) driving cycle: (2% of driving)
Rolling Resistance (%): 29
Aerodynamic Drag (%): 47
Inertia (%): 24

https://ecomodder.com/forum/showthread.php/inertia-vs-aero-drag-vs-rolling-resistance-urban-41665.html

[freebeard]

but for my feeling the bumper with holes made some difference in higher rpm's in 4th and 5th gear.
...
HWFET (highway) driving cycle: (2% of driving)

Is there no reinforcing inside that bumper? I'd expect a bar or honeycomb.

[Logic]

Originally Posted by freebeard

but for my feeling the bumper with holes made some difference in higher rpm's in 4th and 5th gear.
...
HWFET (highway) driving cycle: (2% of driving)

Is there no reinforcing inside that bumper? I'd expect a bar or honeycomb.

Yep; Highway, but also the car is now slightly lighter.
Name the aero mods that reduce weight...

Actually; it's a case of aero being more noticeable the faster you are going...
There is some talk of better quarter mile times too on forums discussing these.

And yes; I expect the bumper has some sort of steel 'real bumper' under the plastic.
One can only assume the air was flowing around it from a higher to lower pressure area behind the plastic.
As the U shape (say) of a steel bumper has its strength in the 2 vertical bits of the U, one might drill the steel too, for an even lighter aeromod!?

[freebeard]

Diminishing returns?

https://i.kinja-img.com/gawker-media...jzd2eihqoc.jpg

[LostinTransit]

I need to figure out something for that dead zone at the rear of our van.. since removing the spare tire from under the van, there's this large space under the rear.. I had planned on installing a better exhaust and some sort of rear defuser system. Doubt I'd drill holes in the rear bumper cover, but could just trim off the lower edge of the bumper cover.

my prob is that with the addition of the 19'' wheels and 245/40 tires the van is up higher that stock.. I don't want to lower it anymore, but may need to use side skirts etc to reduce the negative air flow under the van.

Ideally, if I could do it again to make the wheels more aerodynamic is to go with a 42 offset instead of a 40 offset.. but that's extremely tight.

[kach22i]

Nice to see this old thread has some legs to it.

1. Older cars like my 1977 Porsche 911, and maybe even modern pickup trucks that lack rear wheel well liners will benefit from rear bumper removal and or speed-holes because they will actually be venting the rear wheel well pressure.

2. Trapping air to create a vortex air cavity (rear gate or rear bumper) requires energy. I suspect this energy need be initialized just once on acceleration. Therefore this energy surge is economized over length of cruising distance where it can remain static or stable. This would explain the acceleration testing "Logic" has posted.

Sidetrack: For an analogy think of the high initial amperage draw of an electric motor verses the continuous amp draw.

Racing, especially drag racing is going to be dominated by the initialization of air cavity pressurization prior to vortex forming. Best not to have any air-buckets in such cases.

I would love to see an air tunnel experiment or CFD graphic and or video comparing a bucket with into the wind verses opposite the wind. That is to say in example "A" bucket bottom towards wind, and example "B" annular opening towards wind.

To be a "simplified" example, removal to ring loop carry handle and top of bucket bullnose lip would remove unnecessary and distracting issues.

[freebeard]

"To be a "simplified" example..." straight bucket sides so it's not 'faster backwards'.

[kach22i]

Originally Posted by freebeard

"To be a "simplified" example..." straight bucket sides so it's not 'faster backwards'.

It's early, still on first cup of coffee, is that a joke?

I did an image search, did not find what I wanted. However ended up seeing some images of engine cylinders, and it dawned on me that the compression in an Internal Compression Engine is indeed the cycle that consumes energy.

A sealed cavity (cylinder, square or bumper shaped) perpendicular to air flow that traps and compresses air will consume more energy than a surface that deflects or contours the flow over and around it's self.

One thing I found that I did not expect were examples of air flows over flat planes with cavities.

The below was perhaps the most unexpected find.

Flow around a NACA0018 airfoil with a cavity and its dynamical response to acoustic forcing
https://www.researchgate.net/figure/...fig1_225437384

Trapping of vortices in a cavity has been explored in recent years as a drag reduction measure for thick airfoils. If, however, trapping fails, then oscillation of the cavity flow may couple with elastic vibration modes of the airfoil. To examine this scenario, the effect of small amplitude vertical motion on the oscillation of the shear layer abov..............

Turn the below image up side down, and imagine the trailing edge is the inside edge of the bumper.

The Impact of Steady Blowing from the Leading Edge of an Open Cavity Flow
https://www.mdpi.com/2226-4310/8/9/255

Figure 1. Typical flow topology in a shallow open cavity.

Despite this modest amount of research, all I found is that these cavities can reduce drag or increase drag, it just depends on the angle of attack. And a car going up and down hills may be victim to such conditions. Plus add in cross winds and we pretty much have an unstable condition under that car, and that cannot be good.

The case for a belly pan has been made, but the air bucket rear bumper still has no CFD graphic? Come on someone has got to stumble across one sooner or later.

[kach22i]

In order to obtain results on this topic it's not just not rear bumper we should be considering, it's the whole undercarriage.

Google image search term: "cfd undercarriage"

2017
Effect of underbody structure on aerodynamic drag and optimization
https://www.extrica.com/article/19210
By Zhiqun Yuan, and Yiping Wang
Corresponding Author: Zhiqun Yuan

1) The aerodynamic drag increased by 23.4 % because of the complex airflow of underbody. The energy lost greatly due to the separation vortex of underbody and the uncovered underbody structure which was impacted directly by the high-speed airflow. The effects of those were greater for the aerodynamic characteristics.

EDIT:

I think the terminology "3 box" could be useful to use in the future.

Vehicular Aerodynamics
Posted by karthik on June 16, 2013 in Blue Motion
https://gr8autotech.wordpress.com/20...-aerodynamics/

[Logic]

We are talking aerodynamics here, so nothing is certain and the 'rules' change with airspeed, the proximity of the ground, subtle changes in shape etc-etc.

What is certain is Force = Mass X Acceleration, so the less Mass you are 'pulling away'; the less Force (fuel) you need to do so for the same amount of Acceleration.
That's why I linked the one real world A-B-A test I could find of an aero mod that may do both! But is omitted by manufacturers due to undesirable 'Looks'..?

I'm certain that a 'flat' bottom with a diffuser at the right (blended in) angle (2.5 deg IRC) is best from an aerodynamic POV, but that adds weight.

If you're an Aero Hobbyist looking for constant speed freeway bragging numbers; then the above knowledge is something you're liable to reject, but that's not something those just trying to save a buck or 2 on their daily, mostly urban, transport can afford to do..!


(This is an interesting read that highlights how much subtle body shape changes affect aero:
https://www.linkedin.com/pulse/sunswift-eve-unreliable-highly-incomplete-retelling-graham-doig

Looks:
The Speed Holes don't necessarily need to be holes:

Someone should have explained some wheel aero to this guy however!

Related is exhaust exit position:
Schematic view of exhaust positioning and variation of lift (right) and drag coefficients [left].

https://www.sciencedirect.com/scienc...40X?via%3Dihub

The wording in this study seems to have relied on a very early version of Google Translator, translating whatever is spoken on Proxima Centauri B so good luck actually reading it!

What I am certain of:
There's a hell of a lot of exhaust gas coming out of an engine, so using it to both 'fill' this vacuum and draw out the the exhaust makes sense.
Similar/opposite for induction..!

Already discussed here, where water buildup (cold areas) was a challenge. (float valve?):
https://ecomodder.com/forum/showthre...tml#post695293