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techntrek View Drop Down
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Direct Link To This Post Topic: r-pod Airflow
    Posted: 15 Oct 2009 at 12:28pm
Sandpiper, if you'll be driving near Shrewsbury, PA, wave as you go by.  We're about 10 minutes south.
 
Outbound - doh!  Thanks for the still pics but most image hosting sites are blocked @ work, too.  Since I have a high speed connection via my phone I've been considering buying a cheap laptop to leave at work.  Others here have done the same.
Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Direct Link To This Post Posted: 15 Oct 2009 at 1:22pm
techntrek  I'll wave at you the entire trip I'll be so glad to get the 177.  I'll be going up I 81 near Harrisburg so you will have to get your binoculars out to see us. Emoticons  I plan to thumb my nose at the State legislators as I go by too.

From what I'm seeing there are enough folks in and around PA to have a R-Pod Jamboree sometime in the spring.  Lots of good camp spots in PA.  We have several close to us here in the North Central  part of PA.
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Direct Link To This Post Posted: 15 Oct 2009 at 3:14pm
We've been camping a lot at Codorus State Park lately (SW corner of York County).  Been there the last 3 weekends, next one we're taking a break, then the next 2 weekends we'll be back again.  We wanted to get more camping in before the end of the season but our girls have practices and games every weekend right now.  Then I remembered that Codorus has a campground and its 5 minutes from where they practice.  Solved that problem!
Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Direct Link To This Post Posted: 15 Oct 2009 at 5:13pm
Thanks Outbound.  This confirms my experiences.  I can "feel" when someone is behind me, within this turbulent area - it causes me to slow down, as if I were gently on the brakes.  I'll pretend to be an armchair engineer and say that I think the spoiler is almost the right idea.  I'm inclined to think that an air deflector, which would be high on the Pod and direct air down the rear of the Pod might work better.  It could grab some of the "clean air" and direct it into the low pressure area at the rear of the pod.  It looks like the low volume of air from underneath the vehicles is trying to (inadequately) fill this area.  The faster we go, the more we are "sucked backwards" because of the low pressure.  The air flowing over the top of the Pod should be moving very quickly (like over the top of an airplane wing).  The Pod is "blunt" enough in the rear (unlike an airplane wing) that there is considerable turbulence.  It makes sence (to me) that the lower mounted spoilers would have no effect.  The higher mounted spoiler, it would seem, would push the turbulence back, but create an even bigger low pressure area.  An air deflector, forcing air into the low pressure area, might be the answer.  It will produce some drag, but, I'm thinking not near as much as the big low pressue area behind the Pod.  Is it possible to try something like this on the modeling software? 
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Direct Link To This Post 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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Direct Link To This Post 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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Direct Link To This Post 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!)
Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Direct Link To This Post Posted: 16 Oct 2009 at 3:40pm
Originally posted by Phil H

Would it be possible for you to try a deflector mounted near the rear of your Tribute's roof to see if that helped?

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.

Originally posted by Phil H

Also I was wondering what speed the simulation was mimicking? Does it say?

Its an increasing velocity... the program is a bit esoteric and I'd have to figure out how to convert it to something useful.
 
Originally posted by Phil H

As an aside, I always wondered if the R-pod's teardrop shape wasn't more aerodynamically efficient if it was being towed backwards???

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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Direct Link To This Post Posted: 16 Oct 2009 at 3:41pm
Originally posted by techntrek

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.

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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Direct Link To This Post 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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