2nd Gen Non-Engine/Transmission Explain this one science/math guys

[Pit Bull]

I've had both. the 4. 10's in an automatic and the 3. 55's in my 6 speed. I suggest you drive it for awhile and then decide what works best for your driving. I liked both because of the trucks they were in.

 

[Hohn]

Originally posted by nwilliams98

Hohn

I have to disagree with you on the truck height effecting drag point. I've done tons of simulations on light trucks using CFD and have found that changes in ride height have a significant impact. The stagnation points on the axles are exacerbated with ride height increases as well as there being a larger frontal section.

Nate

Please explain how this is so. I can see how a lifted truck would have more drag, because the axles are lower relative to the body. But simply putting on marginally larger tires (with no other changes) should have almost no effect. The increase in "frontal area" that would result from larger tires is VERY small-- since only the bottom half of the tires (or whatever is visible from the front) would factor in. The relationship of the axles to the body has not changed.



Aside from this, the only thing I can see as affecting the stagnation points would be the proximity effect of the ground. But this proximity is really only in effect when you are VERY close to the ground (near the boundary layer).



Exactly what do you mean by the stagnation points being "exacerbated" (made worse?)? Do you mean it is moving? Won't the stagnation point only shift if there is a change in the flowstream direction relative to the axles? I never thought of a stagnation point as something that gets better or worse, it just moves.



Exactly what kinds of modeling did you do in these experiements? To be clear, I referring to only the installation of larger tires and the effect on steady-state aerodynamic drag.



EDIT: I also said nothing about "drag point", nor am I familiar with the term. I am talking specifically about drag in these forms: pressure drag, vortex drag, and viscous drag. As I understand aero, only a change in one or more coefficients of drag OR a change in frontal area would cause in increase in drag. Since frontal area is essentially unchanged, which drag coefficient is increasing??

 

[Hohn]

Originally posted by JStull

Nate,

I agree with what you say. You sound like a mechanical engineer also. Anyway, the tests I have seen done in wind an water tunnels suggest the same findings.

-Jason

Water tunnel finding would be useless. Air is both compressible and elastic-- two things that water is NOT. Therefore, the fluid dynamics of each are VERY different indeed.



Hohn

 

[Oil Burner]

Hohn,

I agree that it would be impossible to get a perfect dynamic similarity in a water test as opposed to air. But fluid flow findings, I feel, are very valuable and can yield an accurate description of the general operating characteristics of a system. I am only considering a truck traveling at 65 mph or so, not a plane buzzing by at mach 1. In the case of the abovementioned aeronautical engineering problem, I would indeed stay away from the water test. Your point is well taken.

-Jason

 

[Hohn]

Thanks, J. I am not trying to be anal here, just trying to get an ACCURATE understanding of what someone is trying to say. I like to either learn something new, or have more evidence for something I already think is the case.



I did take an aero class, but it was only one class (at the AF academy) and I am a Political Science guy by major, so I don't have a deeper understanding of aero. But I find myself very interested in techie things, and get a lot of enjoyment when the engineers on here and the other tech types share their knowledge with all of us.



TDR rules.



HOHN

 

[Oil Burner]

Hohn,

I love these discussions! Its just hard to find good entertainment like this anymore. TDR is the best!!!! If life isn't a challenge, then you aren't applying yourself enough.

-Jason



p. s. The two rules of mechanical engineering are: 1. You can't push a rope. 2. Stuff rolls down hill.

 

[nwilliams98]

Hohn,



I should probably try to write more articulately. I just posted to point out that ride height has an effect on pressure drag (very little effect on viscous). The exact amount of change it will have is probably not more than a few percent.



By going to 315's he raised is truck a couple of inches. This in effect made the frontal area larger. The equation you mention is accurate to some extent Fd=Cd*A*V^2. What I have found in numerous 3d simulations is that this equation is of little value when you are changing geometric configurations. Changes in shape or ride height can drastically effect drag and the results are often counterintuitive. I personally think the aero guys need to abandon teaching this now that we have CFD (computational fluid dynamics).



The simulations show that large stagnation regions develop on the front of the axles. This, in concert with, the very large ground effect at 70 mph causes alot of drag. (by the way, the ground effect is larger than textbooks suggest). This drag gets worse when the truck is higher because more air is directed toward this poorly streamlined region.



I'm not a CFD expert but I am doing part of my Mech E thesis in CFD at the Naval Postgraduate School. The more I work in this field the more I think that all of the variables in 3D flows are unknowable. I can't tell you how many times Iv'e launched a simulation thinking that my new design is going to reduce drag and it does the opposite.



Anyway, Im glad to see there are people interested in this subject and to hear about another academy guy. I'm USNA 98.



One more thing, the Army's National Automotive Center is funding my thesis and so far I've been able to reduce the drag on the theoretical light trucks by about 35%. How much do you guys think someone would pay for this to be installed on their truck?

 

[nwilliams98]

Jason,



Are you the guy that did water tunnel tests on the model Ram up in New England?



Nate

 

[Hohn]

Originally posted by nwilliams98

Hohn,



I should probably try to write more articulately. I just posted to point out that ride height has an effect on pressure drag (very little effect on viscous). The exact amount of change it will have is probably not more than a few percent.



By going to 315's he raised is truck a couple of inches. This in effect made the frontal area larger. The equation you mention is accurate to some extent Fd=Cd*A*V^2. What I have found in numerous 3d simulations is that this equation is of little value when you are changing geometric configurations. Changes in shape or ride height can drastically effect drag and the results are often counterintuitive. I personally think the aero guys need to abandon teaching this now that we have CFD (computational fluid dynamics).



The simulations show that large stagnation regions develop on the front of the axles. This, in concert with, the very large ground effect at 70 mph causes alot of drag. (by the way, the ground effect is larger than textbooks suggest). This drag gets worse when the truck is higher because more air is directed toward this poorly streamlined region.



I'm not a CFD expert but I am doing part of my Mech E thesis in CFD at the Naval Postgraduate School. The more I work in this field the more I think that all of the variables in 3D flows are unknowable. I can't tell you how many times Iv'e launched a simulation thinking that my new design is going to reduce drag and it does the opposite.



Anyway, Im glad to see there are people interested in this subject and to hear about another academy guy. I'm USNA 98.



One more thing, the Army's National Automotive Center is funding my thesis and so far I've been able to reduce the drag on the theoretical light trucks by about 35%. How much do you guys think someone would pay for this to be installed on their truck?

Undercarriage parts of the truck play a HUGE role in the lack of aero efficiency. Congrats on the progress- I couldn't have come easily. I agree that all of the variable in 3d flows are almost unknowable- there just SO many factors, and so much interaction, it becomes some amazingly complex math.



I had an aero prof that showed me some of his PhD work on modeling the air flow behind an AIM 120- AMRAAM-- the math was so advanced a Cray was chewing on it for days... .



I think I will stick with something a little simple like AirTabs!!



Hohn

(Usafa '00)

 

[nwilliams98]

Hohn,

Did you know my cousin Pete Raber? He was 00 too. He played soccer for the first couple of years and then decided to try and do some normal cadet stuff instead.



Nate

 

[Hohn]

I think I do. I remember a Raber that had a rather prominent forehead. In fact, I think his was more like a five-head, or maybe even a size 6! J/K. I had a few classes with him. I am not sure he went by Pete, but I am pretty sure there was only one Raber in our class!! Raber was a good guy who was known for pulling pranks.



Hohn

 

[Oil Burner]

Nate,

That wasn't me that did the fluid modeling on the ram, although I wish I could have been there. I did some fluid tests at Penn State on various vehicle components but that is as far as I got to full vehicle modeling. Keep up the good work on your thesis! Those are awesome results so far!

-Jason

 

[RustyJC]

Originally posted by JStull

p. s. The two rules of mechanical engineering are: 1. You can't push a rope. 2. Stuff rolls down hill.

Sounds like the two rules of civil engineering:

1. 1/4" per foot

2. Manure flows downhill.



Rusty

 

[Oil Burner]

Rusty,

Good one. that also reminds me of the difference between engineers and architects:



Engineers build the weapons, archetects build the targets.



-Jason