How much airflow required to fill the wake?
 

If one knows the dimensions of their aerodynamic body, is there a way to calculate the size of wake it'd leave, then calculate how much airflow would be required to fill the wake at any given speed, given the air temperature and thus density?

I'm thinking in terms of pushing heated air into the wake to fill it, to reduce drag.

Would OpenFoam be able to calculate this? I plan on mocking up my bike frame and body in the Z88 Aurora FEM program to ensure the frame is strong enough and the body fits well. Then I'll figure out some way of exporting the mockup from Z88 into OpenFoam to test aerodynamics.

I have absolutely no idea how to do any of that, I have no experience with FEM or fluid dynamics programs, and I have no artistic ability whatsoever... so this is likely to be the hardest thing I've ever done in my life. Expect plenty of profanity.

Not sure what to make of your question, somewhere your train of thought has gotten off on a side rail and it sounds like you're asking how to pump enough air into the vacuum behind a vehicle to make it "go away".

The fundamental error I see in this is that you are forgetting the air is just sitting there stationary to begin with, a calm day, middle of nowhere, you are ramming your vehicle through it. The air must move out of the way to go around you, how it is returned to being calm again is where we attempt to improve things. This is where the template can be looked at for ideas, since it is the shape that air will follow with the least amount of energy required. Anything else will create more and more turbulence thus requiring more energy....it take energy to make the air swirl about, the more it swirls, the more energy it needed to do that, the energy is seen as higher Cd values and lower MPG's.

You can't really "Add energy" to air to get it to fill a wake and expect efficiency gains from that because it would take a butt load of power to move that much air and the power has to come from the vehicle thus increasing the energy required and dropping the MPG's.

Hope that makes sense.

The wake is a low pressure area, yes? So filling that low pressure area with air to raise the pressure should minimize the wake and thus wake drag, yes? Now, if we got that air from the front of the vehicle, and used the vehicle exhaust to "pump it" (via a Coanda nozzle) into the wake of the vehicle, would we not be using otherwise wasted energy (the exhaust) to not only fill the wake with hot air (reducing wake drag), but to grab some of the air at the front (reducing frontal pressure drag)?

In my case, the air will be scooped at the stall point at the front of the bike, run through the engine compartment to cool components there, then exhausted into the wake by being "pumped" through the Coanda nozzle. I got the idea from John Britten putting the radiator air and engine exhaust into the wake of his superbike, and racers tilting their muffler exhaust tips to fill the wake, giving them a bit more speed.

The Coanda nozzle is there to get the air moving faster, and to pump air through the engine compartment (which will be closed and separate from the rider space for sound isolation) even when the bike is stopped. The air flow coming off a Coanda nozzle is pretty smooth, as can be seen in the Dyson bladeless fan.

It would be hard to quantify. I'd design your cooling system your way, but not trunctuate your aerodynamic efforts too much in anticipation of a huge gain.
In other words, hedge your bets. Make your wake small to begin with, and then see how it fills with your blown air. You cannot deny TANSTAAFL :)
You can do tuft testing which I think is your only recourse to see how it works.

The idea is quite clever, but not new. In fact, it has certain application:

https://en.wikipedia.org/wiki/Base_bleed

For your purpose I agree with skyking. First reduce drag by creating as small wake as possible using fairing and other "conventional" techniques, than fill the rest with exhaust gases and see what happens. I personally would not use anything else than exhaust gases, since everything else to scoop and duct more air into wake will most probably create additional drag.

Good point! It is along the lines of TANSTAAFL, that air moving through the body is indeed subject to drag. He won't just be using exhaust, he has cooling air needs as well.

I have had a few Cessna 310's, and the early models had an exhaust augmenter system that used the exhaust in a venturi effect to draw cooling air through the nacelle.
Every other twin engine had cowl flaps to control cooling air, and the early 310's did not.
Different power settings would provide the necessary change in cooling for the aircooled engines.
Read here about augmenter systems.
Exit Area Too Small ?????? - Page 7 - VAF Forums

4th plane down is a B model with overwing exhaust and muffler, augmenter.
https://en.wikipedia.org/wiki/Cessna_310

Later models used underwing exhaust and still had an augmenter, until the last few models or the factory turbo planes which had drag inducing cowl flaps.

I can't put it in any other words....

If you add energy to the air by "Pumping it" the energy has to come from somewhere.

Your engine exhaust is a squirt gun against the Hoover Dam, the volume of air needed is HUGE to accomplish what you're asking. HUGE volume of air will need a substantial energy source.

For the amount of time, energy and money as you would put into such a system, you could just design it into an aerodynamic shape from the get go and have an efficient vehicle.

Bottom line....It takes energy to move air, the object of the aerodynamic game is to move it as little as possible as we plow through it.

Think about how much power it takes for a fan to blow air across a room, it's moving maybe a few hundred cubic feet per minute at 15 MPH. What you want to do is asking for thousands of cubic feet at 60 MPH. But you know what, even if you could get unlimited free energy, this concept of "Filling the Vacuum" is doomed from the start because that's not the way it works.

Follow with me.

You plow through the air.....It is only trying to get out of your way and get back to where it was before you came crashing through it. The car is moving, the air is stationary.

The more you move the air as you plow through it, the more energy it takes, just like a fan requires more power to move a larger volume of air at a higher speed.

So you're saying you're not only going to move a huge volume of air to plow through it, but you're also going move another huge volume of air to compensate. And you'll be more efficient.

Here's another thing you can do, explain how this would work on a boat?

On a boat going through the water, you can't just pump a bunch of water into the wake to make it go away and make it more efficient, you would require as much energy or more to fill the wake as it takes to get the boat moving at speed to begin with. So how can 2 engines running at full power be more efficient?

I see you're probably talking about motor cycles here, so you're doomed from the start in aerodynamics due to the fact that you're shoving a big lumpy sack of potatoes through the air at high speed.

The aerodynamic complexity of a motorcycle would melt several supercomputers running in parallel trying to make mathematical sense of it if you could come up with the formulas to input in the first place.

airflow
 

If you can approximate the road load horsepower necessary to propel the bike at your design velocity,then you might think in terms of a high-static-pressure fan powered by an electric motor of a horsepower equal to up to 60%-70% of your total road horsepower, required to produce enough thrust into the wake to compensate for the base pressure drag of the wake,depending upon the bike/rider aerodynamic combination.
An HVAC contractor can calculate the air volume from the HP and duct dimensions/jet size,assuming smooth walls.

Yes, it comes from the engine exhaust... at highway speed, it'll be putting out about 567 liters of air per minute.

The Coanda Effect nozzle multiplies the air flow, especially considering that we're pushing high pressure air from the front of the bike, heating it up in the engine compartment, then exhausting it into a lower-pressure area.

Dyson claims a 15x increase in airflow over what their hidden fan itself provides. At even 10x, I'd get 5671 liters of air per minute (not taking into account the fact that the air is hotter due to absorbing engine compartment heat, and the exhaust heat isn't factored in, either). That's about 200 CFM.

On a small aerodynamic bike, that'd go a long way toward filling the wake.

Yeah, that'll be done, but while the entire bike is being built from the ground up, why not include those small areas of energy savings that make it that much more efficient? It's a project bike, the aim is to get as high a fuel efficiency as is possible.

Except that is the way it works. That's why the examples provided by others in this very thread work to reduce wake drag.

Yes, because we're (at least partially) negating that low-pressure area at the back of the bike that's literally "dragging" on the body of the bike like an anchor, in addition to moving some of that high-pressure at the front of the bike that's literally "dragging" on the body... pressure and wake drag. The more you get rid of them, the more efficient you'll be.

Imagine, if you will, a Ski-Doo... except the jet is powered from otherwise wasted energy.

It wouldn't be "2 engines running at full power"... you do realize that approximately 2/3rds of the energy in a gallon of gasoline are expelled as heat, rather than being used for propulsion?

You're making the assumption that the bike won't be designed to be aerodynamically efficient, just as you're making the assumption that the energy to pump that air into the wake isn't otherwise-wasted energy.

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.

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

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.

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:

http://ecomodder.com/forum/member-fr...ed-stinger.png

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.

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.

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:

http://ecomodder.com/forum/member-fr...56-aerobug.png

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.

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 :)

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.}

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?

A smaller version of:

Jet Beetle

With , just enough thrust to overcome some % of the drag.

:D

For instance, the main member of my frame:
"color" is the standard RGB notation, but rather than going from 0 to 255, it goes from 0.0 to 1.0.

"rotate", of course, rotates the described object.

"translate" moves it.

"difference" allows you to cut out parts of the described object, in this instance, it creates a tube of 50.8 mm diameter with 3 mm walls.

The next thing I've got to figure out is how to create triangles... it uses the "polyhedron" command, and that's a bit complicated. I need that to create the frame gussets.

I took a look at some Youtube tutorials. I see you can do Boolean operations on primitives. I don't see any ability to touch individual edges or vertexes. For instance, how would you bevel an edge?

But if it works for you that's great. I looked at a number of programs but the only one that fits my understanding is Wings 3D.

I know that the topic of this particular thread shifted a bit, but i want to post this for future reference to show others what has been done:

http://ar.v.mfstatic.cz/GetThumbNail...ight=120&q=100

In early 50's there was a racing car called Tatra 607/2 monopost built to F1 specifications of that time. Its air-cooled V8 engine vas first cooled by two fans, but later they applied ejector technology, what is using stream of exhaust gases to such the air through cooling. That saved approximately 20 hp of this 200 hp engine. You can find reference here:


TATRA 607 F1,Formule 1, construct, okruhy, závody, grand prix | Constructors F1,

https://en.wikipedia.org/wiki/Julius_Mackerle

Pet motoru z ceskoslovenska (czech only, sorry).

Now why I am still sceptical:

- they used exhaust gases to replace cooling fans only. so applying to the part of aistream that already was "draggy", not adding another ducting to move more air to fill the wake (and Ledwinka knew enough about wake, as he worked with Jaray)

- although i don't doubt the exhaust gases were able to transport large volumes of air, I suppose they didn't produce much of "sucking pressure". Air cooled engines have cooling fins much less dense than common water-cooled radiator is. Therefore i worry that it would severely reduce performace in common water-cooled radiator cooling

- the idea you proposed is not new in its essence . It has been done - the T607 is living example - and it is well documented. But it was - despite its remarkable results (T 607 were unbeatable cars of its time) - abandoned. IIRC it was because it works properly only in narrow set of conditions. Have too low rpm and your engine will overheat, have revs too high and you will overcool.

- From the above stated reasons I must conlude, that it is probably idea not worth to pursue - otherwise bike disegners would already applied it.

I dont want to be a spoilfun. You asked us our opinion and each of us tried to
answer as honestly as he could. It is definitely doable, but it is too much effort with too uncertain result. But, feel free to go ahead.

BTW - Anybody knows how to solve Navier-Stokes equations? If they solved it in the fifties...

Thanks for that.

I saw a short article in Popular Science/Mechanix years ago about a Porsche race car that had a similar installation. Sorry, no linky. :(

That didn't mention cooling problems, it said it was abandoned because it was LOUD.

Edit: My thinking was to retain the cooling fan and reuse the normally dumped hot air. I don't know if it would increase or decrease the backpressure in the exhaust. A Porsche-style fan would allow tuning because they are available with five to eleven fan blades.

A rear or mid-rear engine or radiator should have the advantage here.

Go back and read my links about the Cessna 310 and others. They effectively used the venturi effect and the exhaust gasses to vary the cooling flow, eliminating cowl flaps.
The big downfall of the early models was:
A) it was noisier with the exhaust over the wing, drumming on the aft fuselage. This was immediately known.
B) corrosion of the rear wing spar. That took a while to rear its ugly head, but the gasses leaked out of the muffler and got into the skin laps on the rear spar and caused a lot of damage.

You essentially use difference to subtract the edges:
I Heart Robotics: OpenSCAD Tip: Round 1 of 3 - Basic Rounding
I Heart Robotics: OpenSCAD Tip: Round 2 of 3 - Advanced Rounding

There are also libraries that let you create these sorts of complicated shapes much more easily, but the one I tried for triangles used deprecated commands, so it was outdated.

Not one to give up easily, I sat down last night at work and went through the polyhedron section of the user manual. In doing so, I learned how to create pretty much any shape I want. It's complicated, but very precise. You have to divide up any shape you want to create into a series of faces (the surfaces of the shape), then divide those faces into triangles that are delineated by the corner points. So for instance, the front face of a cube would be denoted by two triangles to make a square. Once you create the shape in the size you want, you can translate and rotate it into position.

That Wings 3D looks good... if only I had even an iota of artistic ability. My clockwork brain thinks in code, not artistically.

seifrob -- TFA makes it sound like a compound system, rather than a replacement of the axial fans.

Not saying you're wrong, though.

Cycle -- I've done 'arty' things and 3D modelling doesn't remind me of that. In Wings you are constructing a string of operations on primitives, you just choose from right-click menus instead of writing the code out directly. It's still sensitive to the order of operations. You find yourself combining, rotating and separating objects.

Artistic comes in at the point of texturing and lighting for renders.

My first polyhedron!

In OpenSCAD, if you go to the 'View' menu and select 'Thrown Together', then click the 'Preview' button on the toolbar, any badly oriented polygons in the faces will be highlighted. It helps to draw on paper the shape you want, then label each corner, starting with zero. That makes it easy to order the face polygons such that the vertex ordering is correct.

Using this technique, I can create the odd shape I need for the frame gussets... imagine looking at a triangle with one corner lopped off flat. That creates a small space between the gusset and the tube joint. That way you're not welding all the way into the corner where the joint is, which is difficult to do and doesn't do much to transmit the stress in the frame anyway.

First models, in later revisions was system tuned to eliminate fans and save approx. 20 hp.(the source is thr last reference link i provided.)