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Old 01-27-2013, 07:55 PM   #1 (permalink)
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Undercarriage airflow for high set vehicles, 4x4,s SUv's, Pickup's etc.

I stumbled across some threads questioning the benefits of excluding underflow air on these types of vehicles and have been trying to get a better picture of what is going on there.

As far as I can see the air dam is a "dirty fix" for a "dirty problem", that being primarily the "Aero torture chamber" of mechanical parts as Aerohead describes it.
So if we clean this up a bit, with some underbody panelling,belly pans, then I imagine this would be superior to an air dam.

It seems more and more car makers are shifting to this solution rather than ground hugging air dams, even on std road vehicles there is a gently downward inclined undertray starting from the front bumper to front axle assembly or further.

I also read that there is a fringe area between 9-14" ground clearance where it is questionable whether air should be excluded or allowed to flow under the vehicle. In my case I have 12" clearance under the bottom of the axles, the only things lower are the pumpkins (diff housing's) and the more I look at it the less sense keeping the air out makes, even with all the mechanicals. The only way would be to increase frontal area with a low air dam and full side skirts and that doesn't really make sense.

Information around this seems to be pretty scant, firstly most aero information is tainted with "top speed performance" concepts, and then most of it is in relation to regular road vehicles. I tried looking at the trucking industry and again, they have different goals and constaraints, although they are taller wheel vehicles, because of their load bearing nature the axle/suspension structures are so bulky that they have less under axle clearance than I do.

So I imagine this problem needs to be considered in the form of duct modeling.
I have 12" ground clearance at 54" wide, considering the mirror effect of the ground, this would represent a duct 24" high and 54" wide.
For a regular car, the same width but only 6" clearance would be 12" high x 54" wide.

Now I really don't know that much about airflow in ducts, but I do know that Volume (V) : Surface Area (SA) ratio is an important factor and the higher vehicle has nearly twice the volume for only a small increase in SA, 8.3:1 vs 4.9:1.
In addition to that we know boundary layer gets thicker as we go down the length of the vehicle, As an arbitary figure, assume it gets to 1" at the rear outflow, the numbers then become 22"x52" and 10"x52", that brings us to 7.9:1 vs 4.2:1 ratios, both got worse, but the lower vehicle dropped 14% while the higher vehicle dropped less, only 6%.

Now ignoring wheels and their turbulance for this exercise, airflow through the taller duct would be twice as good at the very least than that in the lower duct.

So if we think of the 3d model, body of rotation etc. with fixed external dimensions and we put a small hole in the middle and go through the stages of making the hole bigger, there must be a point where drag begins to fall as the hole gets bigger. until it becomes less than baseline eg:

No hole = Baseline
Small hole = Baseline +1 drag
Hole +1 = Baseline +2 drag
Hole +2 = Baseline +3 drag
Hole +3 = Baseline +2 drag
Hole +4 = Baseline +1 drag
Hole +5 = Baseline drag
Hole +6 = Baseline -1 drag
Hole +7 = Baseline -2 drag

So what are these numbers and at what point is it better to let air go under freely than try to restrict it?

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Old 01-27-2013, 08:22 PM   #2 (permalink)
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I just approached the same issue in a different may in my F150 aerocap thread.

My approach is, does shielding the differential cause an unsafe rise in temperatures, and how can I manage the airflow around the diff to increase streamlining and give a clean airflow out the back?

Since we're basically asking the same question, but you're approaching it from a more technical view, maybe the conversation should be here?
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Old 01-27-2013, 08:31 PM   #3 (permalink)
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The diff will be fine. I've never even noticed tractor diffs getting hot, and they are doing real work.
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Old 01-27-2013, 08:37 PM   #4 (permalink)
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Quote:
Originally Posted by plasticuser View Post
I just approached the same issue in a different may in my F150 aerocap thread.

My approach is, does shielding the differential cause an unsafe rise in temperatures, and how can I manage the airflow around the diff to increase streamlining and give a clean airflow out the back?

Since we're basically asking the same question, but you're approaching it from a more technical view, maybe the conversation should be here?
I think all the fundamental mechanical aspects take first priority, so diff & transmission cooling are higher up the list than aerodynamics, but with these resolved then the question is how to best approach the undercarriage airflow.

I could not/would not do a full cover belly pan, I have live (solid) axles, so articulation is always going to be a problem, I have done front from bullbar (pushbar) to front axle, then will do the sides (done partial there already) between axles, then from rear axle back, so I will be left with the two axle gaps and the centre tunnel gap, this should be plenty of exposure for transmission and diff cooling.
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Old 01-27-2013, 08:57 PM   #5 (permalink)
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A full cover belly pan works quite well for my F150. I can stay under the driveshaft most of the way, then notch around it so it has freedom to move. This would also keep airflow entirely away from the spare tire and tow hitch.

My ideas include making a heatsink that bolts on to the outside, and other various passive cooling ideas.... if it is even necessary. I'll collect temperature data first and see what happens.
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Old 01-27-2013, 08:58 PM   #6 (permalink)
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Belly pans seem to get mixed results, some report expected improvements, others none or even negative results, so there is definately more devils in the detail.
Apart from getting the approach angles for the front undertray and the departure angles right on the diffuser one must also consider what is going on with the boundry layer.

Even though the underside may be a mess and is causing turbulance it may well be that the boundary layer is set by the lowest point, say the axles and these may be attached at their lower limits as the vehicle travels over and skims at these points.
If you go and hang a belly pan the full length of the vehicle, even if it is at lowest axle height, this will result in the boundary layer transferring to this surface and then adding to effective frontal area, which may be more than enough to offset the gains of reduced turbulance.
The difference is a flow through a rough walled larger pipe compared to that of a smooth walled smaller pipe, there is a point where the larger pipe, even if rough flows more air.

So my thinking is a balance between getting a smooth underside, but with a minimum reduction of existing ground clearance, the limits may be somewhere approaching the lowest point, (axles?), but not all the way and almost certainly not exceeding them.

I need to do some reading on ducts, anyone here have some ideas on how ducts operate and what primary constraints exist?
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Old 02-08-2013, 04:10 PM   #7 (permalink)
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Just lifted this from another thread.
Most of the stuff I read hear on the undercarriage airflow is like the old fire training line, "get down low and go, go, go."
But then every now and then a comment like the one below pops up, but I haven't seen any numbers that quantify at what point it is better to go up, rather than down to reduce drag.

Quote:
Originally Posted by tasdrouille View Post
Ground effect is the key part. The Aptera isn't exactly your average ground vehicule. The 0.15 reference is for a basic bluff body with wheels at regular car height. The higher from the ground you go, the lower the Cd will be for a given shape.
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Old 02-09-2013, 05:16 PM   #8 (permalink)
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to pan or not to pan

Quote:
Originally Posted by Tesla View Post
Belly pans seem to get mixed results, some report expected improvements, others none or even negative results, so there is definately more devils in the detail.
Apart from getting the approach angles for the front undertray and the departure angles right on the diffuser one must also consider what is going on with the boundry layer.

Even though the underside may be a mess and is causing turbulance it may well be that the boundary layer is set by the lowest point, say the axles and these may be attached at their lower limits as the vehicle travels over and skims at these points.
If you go and hang a belly pan the full length of the vehicle, even if it is at lowest axle height, this will result in the boundary layer transferring to this surface and then adding to effective frontal area, which may be more than enough to offset the gains of reduced turbulance.
The difference is a flow through a rough walled larger pipe compared to that of a smooth walled smaller pipe, there is a point where the larger pipe, even if rough flows more air.

So my thinking is a balance between getting a smooth underside, but with a minimum reduction of existing ground clearance, the limits may be somewhere approaching the lowest point, (axles?), but not all the way and almost certainly not exceeding them.

I need to do some reading on ducts, anyone here have some ideas on how ducts operate and what primary constraints exist?
*Drag (pressure drag) is a function of pressure regain at the vehicle rear.
*Kinetic energy of eddies and turbulence cannot be recovered in the form of pressure regain.It is lost forever to heat.
*A properly executed belly will provide for minimum turbulence in the under-body flow,allowing (especially with a diffuser) velocity to be traded for pressure as the airstream slows along the diffuser,imparting a higher base pressure rather than vacuum.

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