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  #13  
Old 06-24-2012, 10:52 PM
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Re: Aerodynamics, Anyone?

Like every jeep with a custom stinger?
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Old 06-25-2012, 07:56 PM
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Re: Aerodynamics, Anyone?

Briando, the SRT front bumper is as close as you're going to get, unless you have one custom made for mega $$$. I know what you're after though.....something like some of the NASCAR fronts. You'll also have to lower the crap out of it to get any aerodynamic gains. I've got the SRT lowering springs with the SRT bumper, and have to be real careful on some bumps and the cement parking lot stops. On my last trip I just nudged one a little and bent one of the mount brackets. Fortunately I was near my son's house, and had him fix it.
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Old 06-26-2012, 06:09 PM
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Re: Aerodynamics, Anyone?

I'd like to give you some of my comments on the aerodynamics of our Jeeps. In my undergrad studies as a Mechanical Engineer, I participated in Supermileage races put on by Shell. My college, Cal Poly State University, took the North American Championship prize in 2007 with over 1900 miles per gallon. The following year we bested that and the year after that we received our highest score of 2752.3 miles per gallon with a 50cc engine and a carbon fiber monocoque car that weighed only 95lbs without the driver. Not really real-world-applicable, I know, but I did learn a lot about aerodynamics and engine efficiency from being involved in that car design. We did some wind-tunnel testing and CFD analysis on our computer models. In our wind-tunnel testing, our drag coefficient was around .12 if I remember correctly. Granted, I'm not an aerospace engineer, but I think I at least have a relatively good handle on how to make a car slippery in the air.

Now there are three types of drag you're dealing with for automobiles. There's drag due to the frontal area of the car and there is drag due to length of the car (skin friction). But there is also a third that makes a world of difference, which has to do with turbulent flow and back pressure generated by the spinning wheels and the big blocky shapes of cars.

Let's look at wheels first: Your fat 285's on a lifted truck make a world of hurt for the aerodynamics of the vehicle. It throws the airflow into complete confusion and just when it starts to straighten out again, it hits the rear wheel. The faster they spin, the more disturbance they create. Then there are your spokes. They look super cool, but let's face it - it's basically a giant air blender. We saw some of the biggest gains in mileage by faring the wheels, or having them completely covered except where they contact the ground. Okay, so that's not so practical on your Jeep, but that's partially why that SRT front bumper makes such a difference. It's increasing the frontal surface area of your car (bad), but it's also diverting some of the flow of air away from your wheels (good). Lower side skirts will help for the same reason.

Now the 2nd thing you have to consider is how the flow of air comes back together. You separated the air by driving through it but how it comes back together at the rear makes a huge difference. A flat backed SUV is probably the worst for aerodynamics. The best would be if you attached a cone to your trunk with a 30 degree taper or so. It increases skin friction, but the turbulence avoided from flow separation matters more. The flow separation creates a back-pressure right behind your vehicle, pretty much sucking it back and requiring more power to be overcome. If there is a means of coercing the air down behind the back of the trunk and up from the rear bumper, then you'll see big gains there. Our Supermileage car looked like a teardrop from above. Again, not practical, but you'll see the biggest gains by trying to mimic that tear-drop shape and air behavior. The "air tabs" probably don't make too much difference because the air flow is already turbulent by the time it gets to the back of your car. I think a rear spoiler that diverts air downward or some type of scoop under the rear bumper to deflect air upward will be best for air flow. I believe the Ferrari Enzo does something like that.

So will cleaning up the airflow on the front help? Probably not noticeably. Once the air hits your grill, it's already turbulent. Will washing your car of dirt and contaminants help? Marginally. I've heard of fighter pilots that wipe their wings down with a white cloth to make sure they're clean and they have the best possible airflow. You're gonna see the biggest gains from reducing the effect of the wheels on the airflow and by trying to fix the flow separation at the back of the vehicle.
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  #16  
Old 06-27-2012, 08:12 AM
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Re: Aerodynamics, Anyone?

I have to agree here, look at fast roadcars, they all have diffuser fitted under their readr bumper.
Also look at your car's bottom, i that flat? by no means, so if tiy are able to:

-lower the car
-close and flatten the bottom
-fit a bottom diffuser for releasing air coming from underneath your vehicle

you will probably have a lot more effect than now.

good luck!
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Old 06-27-2012, 10:19 AM
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Re: Aerodynamics, Anyone?

Both great posts, Verminator and wimbers. Trying to reach a happy medium and make "some" improvement, without making it look totally like crap, is probably the biggest challange. I know these things are just big heavy boxes, without too much potential for great improvement without major modifications. Personally, I find this kind of stuff fascinating and just love to tinker with these things, especially if it's different or a "little out of the box." For mine, I need to get some SRT side skirts, or figure out a descent way of deflecting the air by the rear wheels. Maybe a smaller version of the deflector some semi's run down both sides???

Verminator, have you made any efforts on yours to try and improve the aerodynamics, power, or fuel economy? The short time I've been on the hypermiler forum, I've picked up a few hints (I'll admit I don't really know squat about true aerodynamics) about smoothing out the airflow. Over on one of the other forums, we've been having similar discussions. IMO, any and all input regarding this is important. Being retired, I have nothing (almost) but time to do the tinkering. I've got the engine pretty much the way I want, without spending a fortune, have lowered the power to weight ratio significantly (from ~18 down to ~13), and by advancing the cam have moved (only partially successful at this point) the torque range to a more usable RPM range. If I could drop a few hundred pound of weight off it, that would help too. A 5000 lb. box is not real condusive to good fuel economy and/or performance. I love the challenge though.

Guys/gals, keep the good info., discussion and ideas coming.
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  #18  
Old 06-27-2012, 05:27 PM
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Re: Aerodynamics, Anyone?

Quote:
Originally Posted by jeepgcoman View Post
Verminator, have you made any efforts on yours to try and improve the aerodynamics, power, or fuel economy? The short time I've been on the hypermiler forum, I've picked up a few hints (I'll admit I don't really know squat about true aerodynamics) about smoothing out the airflow. Over on one of the other forums, we've been having similar discussions. IMO, any and all input regarding this is important. Being retired, I have nothing (almost) but time to do the tinkering. I've got the engine pretty much the way I want, without spending a fortune, have lowered the power to weight ratio significantly (from ~18 down to ~13), and by advancing the cam have moved (only partially successful at this point) the torque range to a more usable RPM range. If I could drop a few hundred pound of weight off it, that would help too. A 5000 lb. box is not real condusive to good fuel economy and/or performance. I love the challenge though.

Guys/gals, keep the good info., discussion and ideas coming.
My big challenge is that I bought this Jeep as an off-road vehicle and my commute to work is less than 2 miles. Heck, I don't even drive to work sometimes. I haven't put a whole lot of thought or energy into making it more aerodynamic. However, I did notice a difference when I took the front air dam off. But I've got mine on 32's and a 2-3in lift so I'm not doing myself much of a favor if I'm trying to make this thing more fuel efficient.

If I did, I'd lower it, get the SRT package all around, and clean up the underbody. I might consider making some type of rear diffuser/deflector also. Flow under the car is interesting. Your ground clearance has a major effect on the aerodynamics. I read a book in undergrad about "ground effects" on wing profiles. They found that when near to the ground, a standard wing shape (bump on top) actually generates downforce and not lift. When high enough off the ground, it begins generating lift. The reverse is also true. An inverted wing near the ground generates lift and generates downforce away from the ground. The transition zone was somewhere around 4-8inches, depending on the shape. Well guess what? Most of your vehicles probably have ground clearance in that transition zone! So I'm actually unsure what effect cleaning up the underbody would have on the jeep in general and I'd want to do some experimenting.

What we used to do is tape small 2 inch pieces of brightly colored yarn all over our vehicles and run at our target speeds. Then we'd video tape the motion of the yarn in a chase car in order to get a visualization of the airflow. You can easily tell where the air is turbulent and where the flow is still streamlined so that adjustments can be made to the shape of the vehicle. Sure beats renting a wind-tunnel.
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Old 06-27-2012, 08:27 PM
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Re: Aerodynamics, Anyone?

Good idea with the yarn. I got a lot of comments/questions when I taped the front.....wonder what people would say with bright yellow yarn taped all over it. Ah, who cares? Maybe I'll try it before long.

I just took a short trip today to check out some of the things I recently changed. A very pleasant surprise! I take this same trip maybe 6-8 times a year, so I know what the WK usually does. This is almost the same trip I outlined in Post No. 1. I used to get around 21 mpg EVIC, and when the taped off front and windy conditions, the EVIC read 22.8 mpg. Today, over the same highway route, the EVIC read 24.0 mpg at the bottom of the off ramp. About 7 miles of city driving and the drive home, without resetting the EVIC yielded 23.2 mpg. On the outbound leg, that's .4 mpg better than the "tapeoff" trip. I filled up before and after the trip using the same pump, so I know this is accurate. The trip this time was 158.1 miles on 6.7 gallons of non-ethanol fuel. That figures out to 23.597 mpg, which is the best I have gotten to date, since installing the Sidewinder cam.

Now, here are the things I did or changed since the "tapeoff" run. See previous mods. in my signature.
--Removed 12 Airtab vortex generators from the front of the roof (I put them there to try and quiet down the sun roof wind noise.), and in front of the roof rack rails. I really think this helped alot as, the Airtabs, in additon to supposedly helping, also create drag.
--Replaced the tape with plexiglass covers for the fog lights and the lower SRT grill. See pix in Post 5, this thread. I used cutouts from the tape job as patterns for the plexiglass.
--Took 2 of the Airtabs I was able to save and slapped them on the front fender, in front of the side mirrors. I fold the mirrors on the highway, unless in traffic or when passing. When folded the front facing surfaces are almost perpendicular to the body, or curve in toward the side window. As of now, these are temporary. See pix below.
--Used Home Depot round weather stripping to fill areas between the hood and headlights, below the headlights, and between the hood and grill. See pix below. May have to do a little more tinkering with these. On this one, stick your hand between the headlight and hood (05-07's only) and see how big the gap is there.
--Waxed the entire vehicle.

I am having lots of fun with this, and will definitely continue tinkering. So far, this increase in fuel economy is the most I've gotten from any single mod., except for the cam. This was also much cheaper....less than $50, if you count the price of the Airtabs in the rear, which have been on there for about 2 years. The other mods. were less than $25, and took maybe 4-6 hours of labor, including making the plexiglass covers.

Keep on tinkering!
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Old 06-28-2012, 12:39 AM
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Re: Aerodynamics, Anyone?

You may want to try moving that airtab from infront of the mirror to the side or top of the mirror, or creating mini air tabs to go around the side view mirror. The reason I say this is because when it's in front, you're creating the turbulent vortex flow before it hits the mirror, which does you no good. What you want is for the turbulent vortex flow to continue off the back side of the mirror instead of creating a back-pressure behind the mirror.

I really want to do some flow-visualization with smoke around my side view mirrors. I've always wondered what the flow pattern REALLY looks like and what REALLY works in reducing the turbulence.

Keep us posted with your results!
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Old 06-28-2012, 07:22 AM
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Re: Aerodynamics, Anyone?

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Old 06-28-2012, 04:18 PM
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Re: Aerodynamics, Anyone?

Those airtabs are supposed to go right at the end of a surface- top rear of roof,rear fender( by taillight) etc to help "smooth" the air coming off the vehicle. I dont have them on my truck, but i know some who do and they say it rides/ handles(not as in corners but crosswinds) much better.
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Old 06-28-2012, 08:57 PM
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Re: Aerodynamics, Anyone?

Verm, by putting the Airtab there I was trying to deflect the air away from the mirror. With the mirror folded it leaves a pretty good size flat surface on the folded side for the air to hit directly. I was hoping that the vortices caused by the Airtab would pretty much go around the mirror and not hit them directly. What I think I might do is take you recommendation and tape some yarn pieces all around the area and either run air from the compressor or a fan over it to see what happens. I think, except or the flat end of the mirror, the rest of it is probably not a big aero. problem and wouldn't create a lot of back pressure on the aft side. It may not be worth the effort, but another option may be to put some sort of deflector in front of the mirror. What do you think? Edit: Also, what do you think can be done with the drag caused by the windshield wipers, other than removing them?

4.whoa, they do help a little with stability, especially in crosswinds and when being passed in the other direction by big trucks. The plexiglass covers I made for the SRT front bumper seem to help a little too. Right now I am just trying all kinds of things to see what works and what doesn't. Since I don't have a very good understanding of this stuff, Verminator, and others are trying to walk me through it.
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Old 06-29-2012, 02:15 AM
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Re: Aerodynamics, Anyone?

Quote:
Originally Posted by jeepgcoman View Post
Verm, by putting the Airtab there I was trying to deflect the air away from the mirror. With the mirror folded it leaves a pretty good size flat surface on the folded side for the air to hit directly. I was hoping that the vortices caused by the Airtab would pretty much go around the mirror and not hit them directly. What I think I might do is take you recommendation and tape some yarn pieces all around the area and either run air from the compressor or a fan over it to see what happens. I think, except or the flat end of the mirror, the rest of it is probably not a big aero. problem and wouldn't create a lot of back pressure on the aft side. It may not be worth the effort, but another option may be to put some sort of deflector in front of the mirror. What do you think? Edit: Also, what do you think can be done with the drag caused by the windshield wipers, other than removing them?

4.whoa, they do help a little with stability, especially in crosswinds and when being passed in the other direction by big trucks. The plexiglass covers I made for the SRT front bumper seem to help a little too. Right now I am just trying all kinds of things to see what works and what doesn't. Since I don't have a very good understanding of this stuff, Verminator, and others are trying to walk me through it.
I think I agree with 4.whoa that the tabs should be placed at the trailing edge of a surface and not at the beginning. There are two types of flow, laminar and turbulent. If at all possible, we want the whole car to be laminar flow. However, that's pretty much impossible. The fact is that probably within inches of the front grill, your airflow will start to transition to turbulent. When it hits something with greater surface area (i.e. your windshield or your side mirrors), it will become laminar again while the pressure is still high. Once the flow comes around the top end, it will start to go turbulent again. So if you place the airtab infront of the mirror, you're making the flow turbulent prior to hitting the mirror. It does you no good. The air still has to move around the mirror regardless, so you should aim to keep it laminar flow whenever possible. Also, I think an air compressor will not work well for this application. A large fan might, but you want to ensure you're pulling the air, not pushing it. If it's being pushed into the car, it is already turbulent airflow because of the fan blades spinning and chopping it up (buffeting). That's why wind tunnels always suck air with their fans and don't blow it forward into the chamber.

Regarding the windshield wipers, I think the effect is negligible. It's so buried in that cavity behind the hood that the flow of air is not effected enough by it. Most of the flow will not follow the shape of your car exactly. It will take the path of least resistance. Also, the laws of physics state that the velocity of any fluid (and we consider air a fluid) is zero at the boundary condition. In other words, if you had a little device that measured airflow, you could put it directly on top of your hood, drive 80mph, and it would read 0mph. Raise it up an inch and you'll probably get something pretty close to 80mph, if not faster because of other reasons. So the air going around your windshield wipers is most likely only going a fraction of the speed of the rest of the air. Combined with the fact that they're buried in a corner where the hood meets the windshield and you probably have a pocket of relatively still air in there that doesn't effect the aerodynamics of your car. That's why those pesky leaves always stay stuck in there. There's barely any airflow even at 80 mph. Hope that makes sense.
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