How much airflow required to fill the wake?

[Cycle]

Originally Posted by ChazInMT

OK, yer right. Looks like you and Coanda have this all sorted out, what do I know. Dopey me. Good Luck. (Aerohead just said pretty much the same thing as I did in different words fwiw.)

Except rather than saying "You can't do that", Aerohead discussed how it could be done. And it is done... on motorcycles, airplanes, racing autos and even artillery munitions, as the examples provided in this very thread attest to.

I'm not looking at propelling the bike solely via air flow, I'm looking to at least partially fill the wake to lessen wake drag. On a small bike of limited power, every bit of energy wasted has a proportionally larger effect upon fuel economy than it would have on, say, a 200+ HP motorcycle that has no problem going 180 MPH naked.

Already, in just the little I've so far done with the bike (friction reduction via hybrid ceramic bearings in the rear gears and wheels, 15% taller rear gearing, tungsten disulfide in the engine and gear oil, and pre-heating the block prior to running the engine), I've gone from a historical average over 9574.6 miles of 65.851 MPG to the last tankful being 94.478 MPG (43.47% increase). And even that could be improved... I couldn't resist opening it up to WOT for 20 miles on one trip (because 85 MPH on a small bike is so much fun... watching the faces of people on the freeway as you buzz past them on a scooter that looks like it wouldn't do more than 50 MPH is priceless), and there were two trips climbing those steep Oakland hills.

Now imagine what a proper aerodynamic body will do for it, one that not only pushes as little air aside as possible, but also fills the wake to lessen wake drag. On top of that, imagine what a lighter frame, saving about 70 pounds of weight will do.

[Xist]

How do you plan on converting the 2/3s waste heat into moving air?

[Cycle]

Well, for the exhaust, it'll be via a Coanda Effect nozzle. That will mix the engine compartment air with the engine exhaust while providing more air than the engine exhaust alone can provide, while also helping to pump cooling air through the engine compartment when the bike isn't rolling, so things don't overheat.

The heat from the engine compartment, along with the engine exhaust heat, will expand the air, helping to fill the wake.

The radiators will be in the cross-over ducts, used to mitigate side-wind effects. They'll exhaust near the widest part of the body (lowest pressure area) in vertical slits that put the air out parallel to the aft part of the body. Thus, a warm (less dense, thus lower friction) laminar boundary layer to reduce skin friction, that hopefully stays attached all the way to the tail.

The tail will be a narrow Kamm to reduce bike length and side-wind effects, with the engine exhaust-driven Coanda nozzle in it to pump the engine compartment air and engine exhaust into the wake.

One thing I wonder about... you see on some cars those small corner ducts at the front to grab the air and direct it where the car designers want it to go... would putting something like that (with a gap that's only as wide as the boundary layer) at the back of the bike, and directing that air around the corner of the Kamm tail before smoothly exhausting it into the wake have a beneficial effect?

I'm thinking something along the lines of a shroud that grabs the boundary layer air and puts it through a shape that's the same shape as the Coanda nozzle, but a bit larger... thus you get a "Coanda nozzle within a Coanda nozzle" effect to pump that boundary layer around the corner of the Kamm tail and into a small wake.

[freebeard]

I'm with you on this. I think not so much to fill the wake as to reduce its size. Here is the concept applied to a Beetle:



It's really a Kamm-back, but instead of a flat truncation it has a half-round inside a Coanda nozzle to encourage the closure of the wake without the extra length.

[Cycle]

I wish I had the artistic ability to produce even a picture as simple as that. Unfortunately, I can see what I want it to look like in my head, but it never translates to paper or the screen.

[freebeard]

That is a case of what I can do, not what I'd like to do. I'm semi-literate when it comes to 3D rendering. I did make the primitives for the Kamm-/boat-tails and the Moon disks are flattened spheres. But they're just tacked on instead of integrated. I did get to pick the color of the car.

Essentially, I'm in a very similar position vis-a-vis what's going on in my head not coming out the way I want it to.

Since the subject is wake-filling, here's another example:

[Cycle]

Well, I just ordered "Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering, 4th Edition" by Wolf-Heinrich Hucho from Amazon.com. Got it new in hard-cover for only $57.71. So I expect I'll learn a lot and make changes to the planned design based upon what I learn.

I installed Z88 Aurora... and my eyes immediately glazed over. Have no idea how to start on that, but I'll muddle through.

[freebeard]

So ≠ uncomplex?

I looked at the Wikipedia page and the host website. It looks like you start with the pre-processing, and the import options for geometry include .stl and .dfx. I'm not sure what the 'FE structures' are. So you start by finding tutorials and preparing the geometry of your bike frame and body.

Good luck

[Cycle]

Oooh, I found OpenSCAD. I can learn programming languages pretty quickly, and for some reason, my brain likes the idea of using programming to define the objects. And OpenSCAD exports in the formats that Z88 Aurora uses, so after I get the frame mocked up in OpenSCAD, I can test its ability to handle load in Z88 Aurora (or export the model in DXF format from OpenSCAD, then have someone help me to test it in an FEA program, if I still find Z88 Aurora inscrutable).

That gets around my brain's block on putting what it sees into a graphical model, and allows me to alter the model without messing everything up, since it's all defined in code.

{EDIT: Even better, I only have to model half the frame, then mirror it to the opposite side using one line of code. Loving this program.}

[freebeard]

How does it work? Constructing a model with programming statements. I can see (per RBF) everything is amplitudes and angles; but how do you construct a data structure of some arbitrary constellation of component parts? Start with some high-level description?