Electroaerodynamic drag reduction...

[aerohead]

Originally Posted by Cycle

Found this... very interesting concept:
https://en.wikipedia.org/wiki/Meredith_effect

And the Meredith paper:
http://naca.central.cranfield.ac.uk/...rc/rm/1683.pdf

I've got some reading and thinking to do. Can this even be done on a bike? The P-51 Mustang example stated the Meredith Effect worked at 430 MPH duct speed.

Hoerner published the thrust for the cooling system on the Messerschmitt Me-109,I don't have that with me,but again,we're looking at a 500+ mph aircraft where compressibility effects are active,not so in low speed aerodynamics.
Also,imagine the maximum heat flux coming from a V-16 aero engine at max power setting.

[aerohead]

Originally Posted by Cycle

For the insulation... would it be enough to have the duct made of material that doesn't transfer heat well (fiberglass molded into the shape of a duct but made an integral part of the body panels), or would it be necessary to actually insulate it?

My plan was to make the ducts go down under the bottom of the bike. I'm not sure if the cross-over section could be made common between the two ducts (instead of two distinct and overlapping ducts)... it would seem it could be, since the air flow wouldn't really want to change direction very easily, it'd continue on to the opposite side of the cross-over section and out the opposite side of the bike.

The reason I put the vertical exit slits (I suck at anything artistic, so the drawing is understandably very rough) just past the widest part of the bike is because it sort of acts like an atomizer nozzle (venturi effect)... the air is just coming off its fastest speed at the widest section of the bike, then the body starts to narrow. Before the pressure has much of a chance to increase, the air speed is still fast, giving a sort of venturi effect upon those exit slits, and sucking the air out of them. Because the air exiting the ducts is better aligned with the narrowing section of the body, it'll stick to the body hopefully all the way to the tail, providing a thin wake that will then be filled with the warm air from the engine compartment, and the engine exhaust.

Because the ducted air is warmer than the surrounding air, its density is lower, which should help to lower skin friction drag a bit.

Would this work? It seems intuitive, but then, aerodynamics isn't always intuitive.



There'd still be a bit of flow in the ducts, which is why I angled them backwards at a 15 degree angle. They're not completely obscured in straight-line riding. I was operating under the assumption that at 75 MPH, a 15 MPH wind at 90 degrees to the bike is equivalent to a 76.5 MPH wind hitting the bike at 10 degrees, which effectively "opens up" the ducts at that angle to receive the full blast of that 76.5 MPH wind. But in straight-line riding with no cross-wind, a small part of the inlets would still be getting air, and you'd have the venturi effect described above.

It would be critical to acquire a pressure profile around the body envelope.You run the risk of air traveling forward through the duct instead of the other way around.
It's like running to the beach in a woody with the surfboards sticking out the open rear,and front windows rolled down.The base pressure is greater than at the A-pillars and exhaust fumes travel forward to the front seat area.Without wind wings flipped out to scoop air in,you're asphyxiated.

[aerohead]

Originally Posted by Cycle

I was under the assumption that according to the Bernoulli equation, the stagnation point is where the forward motion of the vehicle has caused the air to come to rest (or even reverse direction and create eddies), and therefore stagnation pressure is highest at that point where the air is stopped.

https://en.wikipedia.org/wiki/Stagnation_point
"On a streamlined body fully immersed in a potential flow, there are two stagnation points—one near the leading edge and one near the trailing edge."

The stagnation point at the rear of a vehicle is otherwise known as the wake. It causes wake drag... does it not cause the same effect (in this case, pressure drag) at the front of the vehicle?

Here's a research paper that discusses frontal pressure drag:
http://www.ara.bme.hu/oktatas/letolt...cleaerodyn.pdf

If we removed that high pressure stagnation point and transferred it to the rear of the vehicle, would it not act to reduce forebody pressure drag (and coincidentally wake drag), especially if we're heating and expanding that air before we put it out back of the vehicle?

Here's a good discussion of it:
Does putting an opening on the front of a vehicle nose increase drag? - Aerodynamic engineering - Eng-Tips

As long as the vehicle has a front,it will have a stagnation point(s).It's not the issue.
Pressure drag is the differential between the front and the back.
Streamlining eliminates the wake which renders the highest possible base pressure,eliminating pressure drag altogether,leaving only skin friction drag and drag associated with the sloughing boundary layer.

[freebeard]

So 62.72 CFM * 1.0756 = 67.46 CFM, making lots of assumptions.

That seems reasonable. My 1600cc engine is supposedly produces approx. 1500cfm of cooling air and 1500cfm of exhaust.

I was thinking about the radiator air at max body width. In the wake I'd mix the exhaust and air at the last possible moment, in a stainless steel muffler tip that looks like a high-bypass jet engine.

The Dyson fan may be more or less efficient than you computer cooling fan. It's moving air at 55mph in a still room, in a moving vehicle more velocity might be required. But 1200cfm from 400watts would consume ~1/2 horsepower.

Note the circle is about 1 meter in circumference and the slot is 1.3mm. A 1 foot circle would be at the end of a long boattail.

[Cycle]

Originally Posted by freebeard

That seems reasonable. My 1600cc engine is supposedly produces approx. 1500cfm of cooling air and 1500cfm of exhaust.

I was thinking about the radiator air at max body width. In the wake I'd mix the exhaust and air at the last possible moment, in a stainless steel muffler tip that looks like a high-bypass jet engine.

The Dyson fan may be more or less efficient than you computer cooling fan. It's moving air at 55mph in a still room, in a moving vehicle more velocity might be required. But 1200cfm from 400watts would consume ~1/2 horsepower.

Note the circle is about 1 meter in circumference and the slot is 1.3mm. A 1 foot circle would be at the end of a long boattail.

That'd be for mixing more air into the wake. They make some Coanda Effect fans that aren't round, they're oblong. I was thinking something like that for the tail air outlet. The engine exhaust would stand in as the fan in this example.

For the air through the cross-over ducts, I have no idea, nor even any idea of how to calculate it. I know my current radiator is about 6 inches by 8 inches and sits at the very front of the bike, and has an air scoop of about 8 inches by 10 inches that forces air into it. That's why my cooling fan has only ever come on twice in the ~10,500 miles I've ridden the bike. I can tell when the fan comes on because it forces warm air out through vertical slits right next to my legs, whereas usually the air flows down through the bodywork and under the engine, through the faired underbody, then out the back.

I suppose I could calculate how much fuel my engine uses, the energy content of the fuel, what the estimated engine efficiency is, how much heat it's throwing out the radiator, then back-engineer that to the radiator size and the airflow through the radiator at road speed, but it'd again be based upon a lot of suppositions and assumptions.

[freebeard]

If you are looking for a way forward here, I'd suggest some reading at Autospeed.

AutoSpeed - Technology, Efficiency, Performance

The proprietor, Julian Edgar is a fan of the Dwyer Magnehelic Differential Pressure Gauge and shows how to use it to size vents, ducts and airboxes, and intake and exhaust runners. Why you need a bellmouth in your plenum.

There's a lot there (768 articles) but a lot of it is on suspension, electronics, etc. For your purposes maybe AutoSpeed - Building an Ultra Light-Weight Car, Part 2

[Cycle]

Originally Posted by freebeard

If you are looking for a way forward here, I'd suggest some reading at Autospeed.

AutoSpeed - Technology, Efficiency, Performance

The proprietor, Julian Edgar is a fan of the Dwyer Magnehelic Differential Pressure Gauge and shows how to use it to size vents, ducts and airboxes, and intake and exhaust runners. Why you need a bellmouth in your plenum. :thumbup:

There's a lot there (768 articles) but a lot of it is on suspension, electronics, etc. For your purposes maybe AutoSpeed - Building an Ultra Light-Weight Car, Part 2

I have a lot of experience with Magnehelic gauges... they're on every single air handling unit at work to measure filter differential pressure. I can pick a couple of them up from eBay.

I especially like their "Building Ultra Light-Weight Tubular Frame Vehicles", "Working With Tubular Frames" and "Making Parts Lighter" series:
AutoSpeed - Building Ultra Light-Weight Tubular Frame Vehicles, Part 1
AutoSpeed - Building Ultra Light-Weight Tubular Frame Vehicles, Part 2
AutoSpeed - Working With Tubular Frames
AutoSpeed - Lightening Parts

[Cycle]

As regards the bellmouth opening that'll provide cooling air to the engine compartment (and ultimately air that'll fill the wake of the bike), I just wanted to archive this here:
AutoSpeed - Building and Testing an Airbox
"Bellmouths

The sharpness of the radius of the lip that surrounds a bellmouth is important. If it is too tight, the airflow ‘unsticks’ on the transition around the corner and the intake flow is reduced.

In Axial Flow Fans and Ducts (R. Alan Wallis, 1983, John Wiley and Sons, ISBN 0-471-87086-2) the minimum radius of the bellmouth lip is specified as being best between 0.25 and 0.3 times the diameter of the tube. So for example, a 7.5cm tube should have a bellmouth that has a radius of curvature that is 1.9 – 2.3cm. (Where did these figures come from? 7.5 x 0.25 = 1.9, and 7.5 x 0.3 = 2.3).

To turn it into data that can be more readily used, the diameter of a disc that can be nestled inside the outer face of the bellmouth lip should be 0.5 – 0.6 times the diameter of the tube. Most bellmouths are in fact smaller than this – these represent figures to be strived for."

So the curve of the bellmouth should be 0.5 to 0.6 times the inner diameter of the tube the bellmouth is feeding, to prevent air from detaching as it enters the bellmouth.

[freebeard]

Happy to help.

[aerohead]

Originally Posted by aerohead

Hoerner published the thrust for the cooling system on the Messerschmitt Me-109,I don't have that with me,but again,we're looking at a 500+ mph aircraft where compressibility effects are active,not so in low speed aerodynamics.
Also,imagine the maximum heat flux coming from a V-16 aero engine at max power setting.

I completely blew it on the Messerschmitt data.
*It's only a 380-mph airplane.
*1,200 hp
*The exhaust pipes provide 140-pounds of thrust @ max. power setting.
*There is no cooling system thrust as with the North American P-51.
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*For the P-51 to have thrust from the Allison V-16 cooling system it must be in transonic flow,where compressibility effects are at play,otherwise the thrust would project forward through the heat exchangers as well as rearward,cancelling it out.
*The compression of the high speed flight would be acting like the shutters of the ram-jet engine of the German V-1 buzz-bomb,allowing zero thrust to pass 'backwards' out the intake slot.
*So in an automobile,unless we're traveling at least 250-mph in standard air,we'd have no chance to capitalize on any cooling system thrust.