How much airflow required to fill the wake?

[Cycle]

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.

[ChazInMT]

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.

[Cycle]

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.

[skyking]

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.

[seifrob]

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.

[skyking]

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.

[ChazInMT]

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.

[aerohead]

Originally Posted by Cycle

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.

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.

[Cycle]

Originally Posted by ChazInMT

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.

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

Originally Posted by ChazInMT

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.

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.

Originally Posted by ChazInMT

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.

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.

Originally Posted by ChazInMT

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.

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

Originally Posted by ChazInMT

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.

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.

Originally Posted by ChazInMT

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

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

Originally Posted by ChazInMT

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?

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?

Originally Posted by ChazInMT

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.

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.

[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.)