Drag Optimization of Light Trucks Using Computational Fluid Dynamics

[Big Dave]

Wow!

Get your best reading glasses and put on a pot of joe. This is a treasure trove.

Couple of thoughts:
1. I am encouraged that his canopy and air dam wound up looking a lot like mine. See Pg. 161 & 167. My canopy is a flatter than his, but in its next iteration, it will not be so flat.
2. I see he couldn't keep his air dam all that tight to the ground either. Mine wears to about a 1.5 inch clearance.
3. A fuel economy improvement of 20+% with just two changes in impressive.
4. Side skirts would have helped but he'd have lost his nerf bars and needed a step ladder to get into that truck.

[AndrewJ]

Well, there it is in black & white: bigger is better. Page 107.

[MickF]

I hate to say it - but... YMMV.

Having just read most of the Simon McBeath "Competitions Car Aerodynamics", what has been driven home to me most strongly is that there is no rule of thumb applicable for aerodynamics. You cannot apply what had been done with a truck with a dirty underside (frame, axles etc.) and substantial ride height and apply it to a monocoque body just off the ground. Some of the airflow stuff shown by CFD in the McBeath book just boggles the mind...

I'm thinking about setting up for CFD at the moment (Linux, Gerris solver etc. etc.). I'll only try 2D modelling due to the time required for a 3D matrix, but I think it's going to be worth the effort.

[trebuchet03]

Sweet... I'm downloading at the moment...

I've been sitting on some CFD analysis I've run on a semi truck trailer (simplified model of an entire rig)... I'll be posting it soon

[s2man]

A very good read. I was surprised by the negative X velocities. Especially beneath the truck, where I would expect the high pressures in front of the truck to force air through the engine bay. and maintain some positive velocity. Never mind, he didn't model that - the front of the truck is solid. I noticed the air velocity beneath the truck was about 0 when he raised the dam 4" off the ground. I suspect a full dam with air exiting the engine compartment beneath the truck would also equate to minimal air velocity beneath the truck.

I was also surprised, at first, the air dam had a much greater affect than the bed cap. But then I realized the air was already flowing in our beloved tear-drop shape over the dead air spaces created by the cab and tail gate. And the real issue with the 4wd is all that exposed running gear underneath, which the air dam fixed quite handily. And it was quite interesting that while the cap "provides a negligible benefit on its own", adding it to the air dam raised his FE increase from 7% to 21%. So the whole of our aero mods is greater that the sum of the individual mods.

This has encouraged me to try a full air dam this spring. It seems easier to construct than a full belly pan. But I note, the optimized NASA box truck has a full belly pan.

[bondo]

It is on page 148, table 11., where Capt. Williams records his real world road tests with the air dam alone showing an increase in fuel efficiency of 7.27%. With the addition of the aerodynamic bed cap, the fuel efficiency increase to 21.23%.

In over ten months of real world road testing of the aero cap as the only modification on my truck, a better than 20% increase in fuel efficiency has been realized at speeds over 60 miles per hour.

I am going to have to build me an air dam this spring too!

[Big Dave]

After chopping through this for the third time, I am psyched. My bed fairing oveshot his optimum (pure luck) and is too flat. This is easily rectified on the next try. I'll carry more of a barrel shape along the centerline to the tail gate. Not only will this approach his optimized shape but make room for more stuff to be carried along the center four feet of the bed.

Having lived with an air dam much like Capt. Williams' I have some suggestions to offer.

1. My air dam is fastened to the bumper with nine nut sets. I would suggest attaching the dam (and drilling holes) starting with the center. On the center I would put 6-10 washers between the bumper and the air dam. One the flanking two holes I subtract a couple, and continue on to zero washers. doing this will round the air dam off as viewed from above. This will tend to throw the air off more quickly to the side rather than stagnating (max pressure) right in front of the truck. another thing this avoids. My air dam like Capt. Williams' is falt to the wind and the unsupported part of it vibrates along its length like a clarinet reed. Putting a curve in the rubber dam will increase the stiffness and avoid this vibration.

2. You can start with the air dam as low as you like - I started at 3/4 inch - but the road will eventually wear it to about 1.5" clearance. Mine only dragged on braking and backing into my garage, but it wore the rubber down all the same. Once it got to 1.5 inches the rubbing stopped.

3. I used capscrews and fender washers to attach the air dam, but the fender washers do not spread the clamping load evenly and when the air dam began vibrating the fender washer cut into the rubber. I would recommend a stainless steel or aluminum clamping plate in front of the air dam to even out the clamping pressure and to make the air dam hold its desired shape better.

Next steps:
A tail cone attached to the tail gate to carry the curve on down.
Side skirts and fender skirts.