Central Pressure Differential Reduction Tube...

[gasman]

Warning... lateral thinking ahead...

I've been looking at a lot of info about aerodynamics (specifically mods) lately. Well, (I may just be tired) but it seems like there are two main factors in drag. One (F1) is the obstruction itself (hard to define...) and the other factor (F2) is the after effects (and residual drag) from all obstructions and displaced volume, in the form of a massive pressure differential between the front of the vehicle (relatively high pressure) and the rear (relatively low pressure).

To explain that a little better (and to differentiate between the two), F1 causes distortion by imperfections, and F2 magnifies that by maintaining the pressure differential.

Now, boat tails et alia address both soft of, but it doesn't directly change F2.

I hope I expressed myself clearly enough... but anyway...

My proposal: A central pressure differential reduction tube. The name is pretty self-explanatory... but it is a tube (yes... a physical tube) that is somewhat central (as practical) to the vehicle then looked at directly on (or at the rear) in the X and Y axes (or otherwise as close to a direct A-B {PeakHigh-to-PeakLow} pressure route as practical-- front to back).

The idea is that instead of more smoothly making air flow around the very large obstruction (cab) - the total area of the vehicle when looked at straight on - (which F1 addresses directly), it directly addresses the pressure differential (between high in front and low in rear) through the cab. This would have an artificial effect similar to reducing the area of the cab (reducing the 'void' around which air must flow (F2) vs. simply smoothing the flow around a set void (F1)). This should both increase efficiency, and significantly reduce the residual magnification of the aerodynamic flow factors (making them less important). I hypothesize that, at relative speeds significantly above 0mph, that the effective reduction in cab area will be greater than the ID (inside diameter) of the tube.

Is this too crazy/impractical? Or am I missing something big that makes the whole idea stupid? lol (I've done it before) I will gladly concede that having a tube running front-to-back through the cab might not be the most convenient mod ever... but that is just the very general concept. With modifications for a specific application (to minimize hindrance to passengers, functionality, and so forth), I think it could have potential to hugely affect fuel efficiency. Key word is "think." The actual effect, obviously, is questionable... However. I maintain that it is plausible, and would be a good candidate for someone to try.

Any thoughts? Anyone willing to experiment?

[Frank Lee]

This notion has been entertained already.

[gasman]

hmmm... were there any conclusions?

[Frank Lee]

You could search EM for "duct" and find out: http://ecomodder.com/forum/showthrea...ces-15156.html

[Red_Liner740]

Lemme see if i got this correct. You want a direct connection tube between the point of highest pressure (front bumper) and the point of lowest pressure (aft of rear bumper). For this to work, the tube would have to be straight and travel through the center line of the car??

[gasman]

^^ to be optimized, yes. but it would work to some degree if you had a garden hose piping about under the carpet... though it wouldn't be noticeable/significant. I was thinking a tube (cylinder) around 4-6" in diameter, snaked through, but kept as straight as practical, though it could be a "flat" tube or anything else that reduced the pressure differential enough to have a significant effect (though I haven't done any calculations on what would constitute a significant effect). It should kill two birds with one stone- both reducing the air force pushing the car back, and lowering the pull effect of the low pressure area behind the car, whereas boat tails (for example) basically only affect the back (or dams, the front).

I'm going to read up on the link you posted frank, thanks.

to clarify: I said center line because I assumed the peak (high/low) barometric pressure values would be approx in that area. The main idea is to connect the peak high to peak low, wherever they may be (well... for the most part).

My main concern in the whole idea (atm) is that by effectively reducing the size of the cab), the importance of flow around the outside of the vehicle (outside drag) will be reduced (assuming the idea works as I imagine it, anyway). So, if aero mods on the outside of the vehicle reduced drag by 15%, and a duct is then added that increases efficiency, it then would reduce the gains of the outside mods, (dunno how much... but that's the crux of it).

Anyone have access to a miniature wind tunnel? I used to use them all the time for modifying aerodynamics of racing vehicles I designed back in high school... but they were always on loan from other schools, so I doubt they would have one there now.

[user removed]

Smokey did that a longtime ago in NASCAR, until they found out and made it illegal.

regards
Mech

[gasman]

well... I've read up on all the posts I could find (again, thanks for the link frank. it lead to a lot of info). My conclusion... I still see no reason why this will not work. Everything is still telling me that it will all but certainly work. Take small wind tunnels for example. How do they get laminar flow? They take blocks of honeycomb material and put it behind a fan. To argue that the honeycomb behind a fan would be no more efficient than a same size block with no holes just seems illogical to me. That is essentially exactly what I propose doing (though it is an extreme version). As long as any air flows through whatever duct is made, efficiency is gained (as drag/friction is diminished). Since air flows the path of least resistance, I can not see how it couldn't flow through. The questions then being ones of practicality.

As for chopping...

I see that as an example of the false dichotomy logical fallacy, as I cannot find a reason to support that the assumption of mutual exclusivity (between chopping and ducting) is true (among other reasons).

Whether or not the idea would pan out in practice is another story. But I very much want to know the answer...

[Frank Lee]

Originally Posted by gasman

As for chopping...

I see that as an example of the false dichotomy logical fallacy, as I cannot find a reason to support that the assumption of mutual exclusivity (between chopping and ducting) is true (among other reasons).

It's quite simple; you are assuming the duct has no drag. Well, it does. So you are now ADDING, yes ADDING surface area and thus more skin friction to the vehicle while a top chop REDUCES surface area, skin friction, and more than likely, form drag.

[gasman]

Originally Posted by Frank Lee

It's quite simple; you are assuming the duct has no drag. Well, it does. So you are now ADDING, yes ADDING surface area and thus more skin friction to the vehicle [...]

'Duct drag' is a good point. It also seems to be something that came up on some of those other ducting conversations, and rightfully so. However, I think that is where many of the topics broke down. Personally, I think it's a big misunderstanding. Allow me to explain a little more about how I envision the ducting idea...

First off, I am 100% confident that there will be drag within the duct, that the drag will be significant, and that the drag will increase exponentially with speed. What I'm hypothesizing is that, up to a point, the drag within the tube/duct system will be less than the total drag of going around the whole car. Nature calculates this nicely and the proof would be whether or not air would actually take the path of the tube. If it does, then it proves that it is the path of least resistance, and has increased efficiency... if not, then it just doesn't work. I'm not claiming that there will be zero airflow around the outside of the vehicle (that it would all go through the tube). Nature will do the calculations and go through the path of least resistance, which would lead to air moving both through (hopefully) and around the vehicle.

The energy saved can be calculated easily, as far as I can tell. Take the vehicle efficiency (easier said than done... lol, but can be approximated), measure the airflow through the tube (I just happened to do professional commercial HVAC systems testing while at college), then use a lil math to calculate the energy it would use (in fuel) to generate that multiplied by the inverse of whatever the efficiency factor is. Boom, there is the exact savings (at that speed). Then, just take readings at several speeds (and maybe some more data, like temp, elevation, time of day, etc.) and do a regression analysis. Then boom, a nice little equation that gives savings at any speed (or given variable), to be used to plot long-term savings, or whatever you like.

Originally Posted by Frank Lee

[...] while a top chop REDUCES surface area, skin friction, and more than likely, form drag.

I would like to clarify something... I have not (and don't intend to) attack the idea that chopping reduces drag and increases fuel efficiency. In fact, I'm confident that that concept works just as you (and others) say. I take back whatever I said that made you think that