100% Bluff Body Drag Reduction Theory

[Christ]

The first thing I notice about those images (I haven't tried to translate it yet) is that the cylinder is approx 25% the size of the cube, and placed the same distance from the cube as the width of the cube.

Scaling this up (scaled models show basic fluid dynamics, but do not catch the nuances), the cylinder would then have to be scaled, the cube (trailing object) gets scaled up, and so does the distance between them.

There is also (intuitively, I admit) an inversely scalar difference between leading object size and distance from the trailing object. The larger the leading object is, the closer it can be.

Based on those images, I think we're looking at virtually projecting a 10-12* angle from the absolute edges of the trailing object forward, then placing a cylinder at the convergence.

EDIT: Adobe and FoxIt both view the entire file as a picture, so I can't select individual text and xlate it. I'd need OCI and a Japanese text file to do it.

[Frank Lee]

And scaling up the pre-splitter renders the project... pretty pointless I'd think. Kinda the aero equivalent of a perpetual motion machine.

[Rokeby]

Frank Lee,

You may be right. There is a high probability that you are.

I'm still in the "What would this even look like?" stage. This is very much an
data gathering exercise for me at this point. My current thinking suggests
the potential benefit is much smaller than 100% drag reduction.

On the road out in the real world, in constantly changing airflow velocities
and directions -- that would be yaw, right? -- forget 100% drag reduction.
I'd be surprised but pleased to see a 5% drag reduction and ~2.5% MPG
improvement.

As to vehicles this might work best on, these would somewhat approximate
the cube of the cited experimental results. That would be full size vans and
maybe older pickup trucks, for sure not my Prius.

Still, we don't really know if this might work, even a little bit.

Geez, I feel like I'm trying to sell snake oil...
better make that 'perpetual motion lotion.'

Then again, I guess we better not pursue that suggestion.

[Christ]

I forgot to mention that the size and distance of the leading object as a whole is also scalar to the speed at which it's expected to perform... the lower the speed, the closer or larger the object has to be.

As a brain exercise, this is great, I'm interested. I just don't see it being applicable to our world at this point.

[Frank Lee]

Don't get me wrong, I think the concept is way cool... for super high speed and/or underwater stuff.

You'd think general aviation would be on that like bees on honey- they have higher speeds and they don't have yaw unless they're on approach.

[Christ]

Originally Posted by Frank Lee

Don't get me wrong, I think the concept is way cool... for super high speed and/or underwater stuff.

You'd think general aviation would be on that like bees on honey- they have higher speeds and they don't have yaw unless they're on approach.

I can't find pictures, but I could swear I've seen aviation test planes with a ball about the size of a 8lb bowling ball mounted up front on the radar nose, a couple feet in front of the plane, but they were low-res images, and the shape was distorted by the image quality, so it could have been something entirely differently shaped from a bowling ball.

Regardless, at 300+ MPH, I see how it could make a difference, but little more than pictures is all I've ever noted of it, regarding compressible mediums (air.)

The idea works so well in water because you don't compress water, you simply displace it, and because of it's lack of compression, it's pressure doesn't change, so the faster you displace it, the further away it moves, and obviously, the longer it takes to refill that space completely.

With air, the faster you go, the less you displace it, and the more you compress it, which provides for a spring effect, creating high and low pressure zones.

Unfortunately, pressure is where hydro and aero sciences part ways, as far as I have learned.

I'm no professor, though. I've got alot to learn yet.

[Rokeby]

I just realized that a lot of cars have a feature that approximates this idea...

those little airflow deflectors in front of the front and rear wheels.

[Christ]

Rokeby -

The idea is similar, but slightly different. The wheel deflectors are similar because they're a leading object which opens a wake for the tire to travel in, but different in that they're only really there to prevent the tire from compressing air under and around it, creating potential traction issues and instability.

Same principle, different application, I suppose.

Great way to look for typical application, though!

[Bicycle Bob]

"Compressability" is a word that usually shows up in aeronautics at around 500 MPH, although some is alwys present.
Has anyone considered the power used to create the drag reduction? I think the leading object is being given a free ride. Motor paced bicycle records are set using a motorized fairing to set up a pressure differential on the bike, and pedaling to hold position.

[Rokeby]

Originally Posted by Christ

...The idea is similar, but slightly different. The wheel deflectors are similar
because they're a leading object which opens a wake for the tire to travel in,
but different in that they're only really there to prevent the tire from
compressing air under and around it, creating potential traction issues and
instability.
Same principle, different application, I suppose.

The OEM wheel deflectors have their major axis parallel to the ground. To be
congruent with the experimental set up in the OP cite, they should be
perpendicular to the ground, or perhaps more to the point the underbody
of the car. And I suppose there should be a flat and level "ground plane"
under the car as well. The OEM deflectors sort of hang down out of nowhere.
Deflected air is free to go up, down, or around them.

The paper cited in my OP was a set of lecture notes. As such, the material in
it is probably snippets of stuff from many places. There is indication that
identical masking effects on the main body can be achieved with either a
plate or a cylinder.

As noted above, to be consistent with the experiment cited in my OP, the
up-stream plate should be perpendicular to a "ground plane." I suppose it
would hang down to ~ an inch from the ground. It would have to be able to
handle being impacted by stuff; speed bumps/humps, debris, etc.

I'm thinking that a cylinder might fair better than a flat plate hanging own in
front of the wheels... something like a short length of swimming pool
"noodle."

Ultimately though, I suspect that identifying the contribution of the wheel
deflectors to overall fuel efficiency would be difficult for backyard
ecomodder experimenters. I thought that I've read, but can't find, figures
from Volvo that might be that detailed.

Originally Posted by Bicycle Bob

...Has anyone considered the power used to create the drag reduction? I
think the leading object is being given a free ride. Motor paced bicycle
records are set using a motorized fairing to set up a pressure differential on
the bike, and pedaling to hold position.

Bob,

I'm not sure what you are saying here. The original experimental set up
involved a simulation of a building and an upstream "deflector." There was
no consideration of energy inputs to create/sustain the wind. In thinking
about the use of an upstream deflector -- either for a wheel or the whole car
-- energy inputs, that is moving the vehicle, are of interest.

If I understand what you're saying, there should be an expectation that there
will be additional energy needed to move the car with the plate/cylinder
attached. But if the plate/cylinder works, more energy would be saved due
reduced drag from the main body,, whether it is a wheel or the whole car.

Am I even close?