r-pod Airflow |
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Topic: r-pod Airflow Posted: 18 Oct 2009 at 6:22pm |
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A comment was made about airflow over a pickup truck... I used a 2010 Ford Extended Cab with
a length of about 221 inches. The first image is an 'open box'...Flow Illustrator does a two-dimensional analysis, so the PU box looks like an open box with a tailgate on the end: suprisingly the flow was quite smooth over the R-Pod...the open box not doing much to the flow. There were large amounts of pulsing behind the R-Pod and upward from the pavement behind the rear (suggestion...cover the box!) The second image is a 'closed box'...a covered box...there were some lower frequency pulsing at the top rear of R-pod and some rear pavement upward pulses The third image is with a truck 'topper' covering the box...there was also some pulsing at the top rear of the R-Pod the rear upward pulses were more gentle Putting a 20 inch wind deflector at the back of the pickup cab suprisingly caused both pulsing at the top rear and lower rear of the R-Pod...not very efficient flow Moving the wind deflector to the rear of the topper really smoothed the flow...no upper pulsing, no longer period pulsing, with a nearly constant low pressure zone at the lower rear (probably the most efficient flow of any of the cases!) |
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Posted: 17 Oct 2009 at 10:59am | ||
Wow! That's really amazing!
I've noticed that when driving on wet pavement for the first time, I almost thought the brakes might be on as the spray that was burbling off the back of the trailer was so fine it looked like smoke. I'll have to try this program with the Tiguan when I have a minute. I can see by the illustrations that it is definitely an advantage to be pulling the trailer with an SUV as it 'fills' the gap between the car and the trailer more aerodynamically. I would imagine this would also apply to a 1/2 ton pulling the trailer with a cap on it as opposed to one without. with It would sure be interesting to see what a standard tandem box shaped trailer would look like using this program. Thanks for the quick lesson in aerodynamics! Doug , Linda , Scruffy , Bert and Ernie |
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Posted: 16 Oct 2009 at 9:22pm | ||
Note how many pressure pulses are produced by simply adding the A/C to the roof. There is a build up not only much higher at the upper rear...but also a pressure field out in front of the R-Pod.
I think that the A/C on the RP 170+ units with its longer roof line and using a longer and higher TV helps keep the flow much smoother (see Outbound's flow images at the beginning of this thread). It appears that by moving the A/C to the roof of the RP-151, Forest River really decreased the efficiency of the RP-151s. I wonder how much less mpg I will be getting from the 2010 Pod versus the older units? |
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Posted: 16 Oct 2009 at 9:16pm | ||
Flow...with and without the A/C...with the spoiler at the rear and a 30" spoiler at the rear
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Posted: 16 Oct 2009 at 9:14pm | ||
What a difference an air-conditioner makes!
The previous Flow Illustrator runs I did were for an R-Pod 151 with the air conditioner positioned in the forward cabinet...no real obstructions on the roof line. My 2010 RP-151 has the A/C on the roof. It is 10 inches high by 37 inches long and positioned 68 inches from the front of the unit. It extends about 1/8 of the total height above the roof. It is generally a rectangular box obstructing the air flow. I included an 18 inch deflector mid-way back, and at the rear of my TV. I also ran a rear deflector of the largest size I could find in a catalog (30 inches). Note that the air conditioner completely messes up the air flow over the R-Pod! There are pressure waves both in front and behind the R-Pod. Flow Illustrator is a two-dimensional analyzer and does not take into consideration that the A/C does not extend all the way across the roof. So the actual effect should be less spectacular than what is shown. The other images will be on successive submissions. |
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Posted: 16 Oct 2009 at 3:41pm | ||
For my trials, I have the A/C on top of the r-pod. I took out the antenna as it doesn't cover a significant width of the r-pod and thought that it would skew the results. |
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Posted: 16 Oct 2009 at 3:40pm | ||
I'll give it a try this weekend, but I don't think it will help much; there doesn't seem to be much of an airsink between my Tribute and the r-pod. But, we won't know for sure until we give it a try.
Its an increasing velocity... the program is a bit esoteric and I'd have to figure out how to convert it to something useful.
I'm thinking that its not teardrop-shaped enough and that less slope on the rear end would be better. But, I'm a statistician, not an engineer, and have absolutely no background in this. So, trial and error it is... |
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techntrek
Admin Group - pHp Joined: 29 Jul 2009 Location: MD Online Status: Offline Posts: 9059 |
Posted: 16 Oct 2009 at 3:03pm | ||
By looking at tsunami's images I just wondered - are you guys including the stuff on the top of the 'pod? A/C, antenna, etc.?
Also I thought you were testing deflectors on the rear of the 'pod itself.
Again, sorry, I can't see the other images here at work and I keep forgetting to take a look at home. So I'm only going on what I can see from tsunami's images. (thanks for putting them up here so I can see them!)
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Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Posted: 16 Oct 2009 at 8:07am | ||
Fantastic information, Outbound!
I had previously done a bit of research online with regard to a tow vehicle mounted roof deflector but what I had read online seemed to indicate that the deflector needed to be fairly close to the towed vehicle to be the most efficient. A tractor trailer roof mounted deflector would be only a few feet from its trailer while with the R-pod's sloping roof it would be 6-8 feet away. I was going to try mocking something up but feared what would happen if it let go from the tow vehicle roof rack and smacked into the front of the R-pod. Would it be possible for you to try a deflector mounted near the rear of your Tribute's roof to see if that helped?
Also I was wondering what speed the simulation was mimicking? Does it say?
As an aside, I always wondered if the R-pod's teardrop shape wasn't more aerodynamically efficient if it was being towed backwards???
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Posted: 15 Oct 2009 at 9:01pm | ||
I used Flow Illustrator to check the smoothness of flow over my Jeep Liberty and an R-Pod
151. A few comments first on Flow Illustrator: The working image really needs to be in a 1:2 proportion to get a 'quick calculation'. Also as stated the silhoutte image should be in RGB color (tho' you are just using black for the vehicles and roadbase, and white for the rest) and saved as a 24-bit bitmap (.bmp). A very large number of users are running FI...there are literally hundreds of flow videos of vehicles now on Youtube.com The size of the image is also important...too small and FI seems to choke, too large and it takes so long to compute that the servers may time out! I found that a silhoutte image of 1 1/2 inches tall by 3 inches long, at 300 to 600 dpi worked very well. I downloaded side views of a Jeep Liberty and a R-Pod RP-151 from the internet, and imported them into Photoshop Elements. I knew my Jeep was 14.5 ft long and the R-Pod was 16.2 ft. I resized the R-Pod so that it was at the same scale as the Jeep. I photomerged the two images together...and added enough white space around them to make a 1:2 ratio image. I then changed the colors to 2 colors, white & black. But then carefully blackened out some of the white in the Jeep and on the R-Pod side. I then resized the finished image to be 1.5 by 3 inches and saved it. I also determined the scale of the image using the length of the Jeep to draw onto the top of the Jeep a wind deflector. I first chose a length of 18 inches (roof-top deflectors are typically in heights of 15, 24, and 30 inches) as a compromise height. I drew four more images: a deflector at the front of the roof, one mid-way, and one at the rear...then after running FI...I added another image with a deflector about 30 inches high. 1) The top image is air flow over the Jeep and R-Pod without a spoiler. Blue is ambient air pressure. Green is a 'smoke tracer' and pink is lower than ambient pressure. Note the 'waves' over the rear of the R-Pod...waves require energy to form...and this energy comes from your tow vehicle's engine. Also note the 'double wall' column forming behind the R-Pod. You are towing a 'mini-vortex' down the road behind the R-Pod 2) Using an 18 inch spoiler on the front of the roof decreases the 'waves' but spreads the disturbed air about as high above the R-Pod as does no spoiler. The windshield forces air up over the Jeep and a front spoiler just sits in that air movement but does smooth out the air flow over the R-Pod somewhat. The rear disturbance is shrunken somewhat 3) Putting the spoiler at mid-point on the roof makes a noticeable difference. The air layer moving over the R-Pod is much thinner even tho there are some smaller waves. The rear disturbance doesn't change much 4) A rear spoiler has about the same effect as a mid-mounted one Notice also small pink blobs at the rear top of the R-Pods...these are small low pressure parcels of air which form and burst like bubbles...again using up energy. 5) Suprisingly, using a 30 inch high spoiler on the rear doesn't change the flow over the R-Pod much differently than the shorter spoiler. The rear disturbance also grew and collapsed as did the others...but there were noticeably less 'bubbles' forming and collapsing. I have not yet run FI on an R-Pod 170+ (which has a much longer and more gentle rear roofline) to see the effects of deflectors. I will submit the results if no one else does. If you run your own Flow Illustrator movies you will see the rear disturbance grow and collapse repeatedly. If you 'draft' behind a large 18-wheeler, it is this low pressure column that is literally pulling you forward toward its back. If you are driving a smaller-lighter vehicle you may also have noticed a pulsing vibration in the air between you and the 18-wheeler. This is the rear disturbance forming and collapsing. I recommend you try using FI for your vehicle. You may get much different results with a different roof height and length of your TV. |
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