I'm bumping my cleanmpg.com post here as it seems to be a better place...
Sorry: Photos don't swow up in here because my URL is mangled by the swear word detector. Click above likn to see them
Ideas and comments on how to tackle the underbody project and with what material are welcome.
Kitcar: Kudos for the videos, but when you can, do include the test results @#!$. You make me hoping I can reach 30MPG @75MPH
I am nw wondering about trying a front lip (air dam) for the CR-V. Maybe a flexible plastic, foam or vinyl might work (sitll wanna get some off-road capability). That would be good at least for the wheels.
I might also try to apply vortex generators to create air bubbles in the complext parts.
My original post w/o photos:
Anybody around with some aerodynamics understanding to help me out?
I took some photos of my 2002 (Gen'2) Honda CR-V. I think there's room for improvments by covering it... Maybe get some noticeable MPG out of that.
For info, this SUV is meant for long highway drives, followed by light off-roading (not necessarilly well maintained forest dirtroads or paved with potholes, or the likes). I did not meant to buy it for city driving, nor for heavy off roading though I can't exclude to ever have to do with it a little approach drive through rocks or cross rivers...
BTW, if I do tuft testing, what speed is needed? I'd have to attach a camera somehow at different spots.
On the right, we can see the gas tank sticking out protected by a tubing, and in front 2 parallel frame beams.
The muffler and drive shaft (also protected by a V-shaped bent metal rod) probably get very hot so can't be covered much.
The rear wheels are not covered, partly because of clearance I suspect. Note that the clearance is excelent: There's lots of room in the middle so this SUV could pass pretty big rocks with proper driving, to pass them by rolling on top of the high points.
There we can see the bottom of the bumper and the cladding under the engine. I assume that part protects from dust but also evens the flow of air going into the engine for proper cooling.
- The cladding is not uniform, and could be smoothed out.
- There's no lip to create a depression or push air to the side.
- There's no lip in front of the tires to reduce wheel drag and car stability (though it's pretty stable passing trucks).
A close-up of the gas tank and rear. Note: If I create a sloping underbody cover here, I feel like adding room for a portable gas jug in front.
Adding a cover has the added benefit of protecting the brake lines from debris.
The rear passenger side... Not much to do except maybe cover from the rear suspension to the back. I'd have to see what parts are moving with the suspension. The shiny part seems to be a heat shield for the spare wheel well.
Front wheel well passenger side.
The wheel is totally exposed to wind as well as the hole where the wishbone moves. The space is open with the engine bay and wheel well. A big fat lip might help there, or a vortex to create a bypass bubble around the hole without creating too much turbulence.
Rear driver side (and passenger side) wheel shooting from the back of the truck.
Definitively some flattening would help the flow there. The trick is doing that with the large moving suspension frame. I don't have any ideas except maybe using some kind of flexible rugged fabric. Again I don't know what are the cooling characteristics that need to be preserved.
Some details of the body aerodynamics
The wipers are not well protected it seems.
The rearview mirror is kinda flat, and the pilar holding it kinda big, must create drag.
I added some pictures of the '07 underbody from the 360-3D model, to get ideas of what aero mods they did. I wonder how aggressive they were. Probably not very.
BTW, I pulled the photo links from my 1st post here, since they're dead.
The end of the movie with the cab spoiler killed me. Tune in next time for results. Argh! From the random posts I read, you were not happy with it.
For your cab I would go with vortex generators, since your goal is to extend the wake to the end of the cab. Vortekz has a nice template for their fin solution to D/L. And then if it works, buy their product is only $20 and looks very nice on the pics.
If you search porshe 914 and aero tabs or something they got some good race results for specific tracks (it's a targa with flat trunk like the Del Sol).
The airdam seems to work for you, but with all that extra surface I'm surprised. Gotta do the skirts too probably to avoid sucking air from the sides, else probably not as stable?
"Drag is the force encountered when pushing through an air flow. There are two types of drag.
induced drag is the result of generating lift. It is the part of the force produced by the wing that is parallel to the air flow.
Profile or parasite drag is experienced by all objects in a flow and is caused by pushing the air out of the way while moving forward. It has 2 components:
Skin friction is a function of the surface area wetted by the airstream. Any increase in surface area will increase skin friction drag.
The other component of profile drag is pressure drag. Pressure drag is a function of the size of the wake behind in an airstream; it can be reduced by streamlining the object to delay separation of the flow.
Streamlining is an increase in the wetted (exposed) area and hence skin friction, so there is a tradeoff."
I am not sure why lift would cause "induced" drag. Maybe depends on what you call the lift force. For me going through a flow would apply a force on the vehicle, the vertical component would be lift and the horizontal drag of the 2 kinds listed, no 3rd component.
Of that I would agree that the horizontal force (drag) could be decomposed into 2 components: skin friction and pressure.
Pressure would be the force pushing the car as a whole, and correlating it to the wake would mean that when an air particle bounces on a car body's particle there is conservation of energy, and how far it goes tells how much energy was exchanged (aka a big wake means a lot of energy to push the air out).
Skin friction would be for me something supplemental. At the microscopic level, think of a pellet bouncing off the water. If the "pressure energy" was small, the air molecule was deflected only to bounce back on another molecule of the car in a ricochet. This would not waste much energy, but some (when skipping rocks, the wakes are small, throwing the rock straight in makes a big noise, splash and bubbles).
Now the ideal shape of the car would be so the angle the air molecules hit bounce off without ricochet, but minimizing the wake. Ideally, they bounce and then travel parallel to the car's body.
Now I know what I'm saying is not very "fluid mechanics" like, but still.
If there's a hole in the car, you want the air particle to jump across it and when reaching the other side, not land but keep going parallel to the body.
Now, for the wake size...
If the body of the car is ill shaped, but the air is deflected correctly into a teardrop shape travel, then the virtual shape going through the air is optimal.
The energy wasted is that of a virtual teardrop of the size of the wake. So a big wake = a big teardrop.
The Cx of a teardrop is 0.05, so even with a bigger surface, the energy loss would be minimal, or at least LESS than a smaller wake with a less optimal shape like the one of a car.
Conclusion: Making a big wake might not matter much, provided it has a nice clean shape that will reduce the Cx. That is why spoilers work: the wake makes a virtual low-angle wing where the air travel from the bottom and from the top is almost identical, so little lift generated and the air flowing under the body goes faster, it is less restricted and therefore frictions less with the car.
Note: I have no formal training in fluid mechanics. I could be all wrong.