Wind tunnel babble...

[windstuffnow]

  While waiting for the real thing I've been designing a wind tunnel for future testing of different models.  I've come up with most of the components ( some still need to be purchased ) but one thing I'm having a hard time finding is the inlet diffuser air straightner.

  The air straightener is basically a honeycomb layout of about an inch or two thick and its only purpose is to straighten the air as it enters the contraction tube.  So far I've found nothing that would work.  Anyone have any ideas of where or what could be used for this purpose?

  I've put together a couple of cardboard tunnels using house fans to watch the inlet air and how it flows through the tunnel, not pretty but great initial knowledge.  Using some insense at the inlet tells the whole story of what is going on around the inlet as well as traveling through the test chamber.  I'll bet I spent an hour of watching the smoke trails in various positions of the tunnel.  It's quite surprising how clean you can make it.

  In any case for the larger one I'll definately need some straighteners of some type...

  So far the larger design has been down graded because of the space required to build it as it should be.  The original design ended up being around 32 ft long to keep the diffuser angles below 6 degrees.  I've reduced that to 14.5 ft and will have to change the inlet and outlet curves to make up the difference.  It will be using 2 30 inch jet fans that are rated at 8550 cfm each at 600 rpm but I'll be running them up to 1100 rpm giving me about 35,000 cfm through the test area ( about 30 mph free flow ).  With a 2x2 turbine inside it will most likely be go to 20+ mph.  

  I'm hoping to have this done shortly after the real wind tunnel tests to see how the results vary using the same machine.  I should be able to come up with a reasonable "fudge" factor between the two.

  My ideas on the "honeycomb" so far are few, as follows...

Box fan grates stacked making up the thickness

PVC tube cut to length and glued in place (expensive)

Sheet metal stips slotted to form the honeycomb

Brain lock - can't vision what something like that would be used for... any suggestions would be greatly appreciated...

.

[ghurd]

How about 2' x 4' fluorescent light diffusers?

Plastic, maybe 3/4" thick, maybe intended for suspended ceilings.

Same idea as the box fan grate, just bigger.

Big box stores may have them.

Commercial lighting places would for sure.

G-

[Arno]

What about rolls of plastic gutter guard [6" wide  25' long] for keeping leaves out of your gutters? Very cheap, 99cents a roll.

arno

[electrondady1]

what about cardboard tubeing?  cut to lengh and glued together.

[wdyasq]

Newspring endrolls and/or thin ply in 'eggcrate' pattern.

Ron

[monte350c]

Hi Ed,

Just did a quickie search on ebay for "poster tubes". It turned up a bunch - for example

 "This auction is for one box of 77 poster/picture tubes with end caps(tube dimensions are 24 1/2" long and 2" wide). "

The current bid is at $15

Ted.

[veewee77]

Go get a stack of PVC pipe and use your chop saw to cut them all about two inches long and then glue them together with pipe glue. If you start at the bottom and lay across, you should be able to glue and stack all the way up. . . You'll get about 110 pieces from a 20 foot joint of pipe.  You'll need something for support when the wind is blowing so put a thing about 2 feet away wind-side of it and use fine wire or twine to go out to the honeycomb and tie it off. Use one about every square foot or so. . .

Did I lose ya?

Make a honeycomb pattern with them.

Doug

[MountainMan]

Get everybody to donate their used toilet paper rolls for a couple of weeks...

jp

[inode buddha]

Ryerson stocks 'glass grating which is sold for non-skid catwalks and floors in factories.

[windstuffnow]

  Thanks all for your comments and suggestions.  Unfortunately, I still haven't located anything that would work as yet.

  I did find the glass grates but the cross beams are 1/4" wide which would cause more turbulance.  Looked all over for the fluorescent panels and haven't been lucky enough to get a hit.  Finding thin wall PVC is a bit of a stretch as well.   The poster tubes could be interesting as well as donated toilet paper roll tubes but it would probably take a month just to glue together enough to cover an area of 34" x 68" that I need.  I'd rather spend the time doing something a little less tedious and purchase something pre-made.  

  The fluorescent honeycomb idea seems to be what I'm after but all I can find on the net is angled units ( 20,40, 60 degree diffusers ).  Just not hitting the right keywords on my google's yet.  They don't have to be real strong because its sandwiched between wire screen which also helps to straighten out the air.

  The search goes on....

.

[inode buddha]

For what it's worth Ed, the grates we used to use at work had 1" sq. holes and were 2" thick. Kinda like a highway storm drain grate. I do know that they make them (IIRC the Mfr's name was Ryerson-Tull; they do steel processing and fab).

[bob golding]

hi ed,  cant add to the grating ideas,but  did you know you can buy smoke sticks from builders merchants. they are used for checking leaks in flues and air ducts. produce a lot more smoke than insence,just dont smell so nice.... looking forward to your results.

bob golding

[bparks]

it would take some work, but you could make this as thick as you'd like.  The corrugated plasic roofing panels at lowes or home depot that are kinda shaped like trapezoids, instead of wavy.  Could cut those in strips & glue or them together into a honeycomb.  

[sahlein]

Hi,

Go to McMaster-Carr and search aluminum. There is a hexagonal honeycomb

material of thin aluminum that will do what you want.  The walls of the "cells"

are very thin and wouldn't hurt the flow like thick walls would.

Joe

[maker of toys]

for thin-wall cut-and-stack:  EMT conduit.  or type m (low pressure) copper.  or even soda straws; cut off the bottom and top of the box, stack, repeat.

as for the fluorescent diffusers-  every big-box home center I've ever been in has a rack of such things, and about 1/2 of them have the open-grid type.  I think I'd just wander around the lighting dep't for a while and see what I saw.  or try a lighting store.

otherwise: cheap furnace filters?  the nylon excelsior type might not do, but a cheap high-flow pleated fiber/paper unit might work.  Plus I think they come in 34" dimensions. . . the seams between them might be an issue, though.

maybe strips of corregated roof tin/fiberglas panels or the new-style lexan stuff?  (cut strips across the corregations.)

Poster board/construction paper cut with slots every inch; slide two sets of such pieces togeather at 90' to each other to make a grid. slots keep them from moving relative to each other.  lots of cutting, no gluing.  maybe get the local elementry kid/s involved as scissor operators?

(laughing)  cardboard 6-pack beer bottle holders with the bottoms cut out?  easier to stack, at least until the rate of beer consumption catches up to the stacker. . .  <G>

-Dan

[MountainMan]

I think the poster tubes are the best answer.

They don't all need to be glued together.  Instead, have one person gather up a bundle of ten to twenty tubes in their arms, while another person wraps some string around the bundle and ties it off or wraps some masking tape around the bundle.  Just need probably ten such bundles.  Probably take less than half an hour to complete that part of the project.

Probably a lot cheaper than the flourescent light diffusers and since they are longer, they would work better, me thinks.

jp

[tecker]

 I just told everyone I know to hook me up with their unabused tp rollers .If there's a windfall I'll  part with some  . Hope this doesn't lead to blue room graffiti

[windstuffnow]

  Hi bob,

    I'll have to look into that, I was thinking of buying a small 400 watt fog machine and adapting a 1" inlet tube to it.  That will have to be later though.  

Great idea!

Thanks!

.

[windstuffnow]

  Lots of good ideas!  I especially like the beer boxes... I'm afraid I'd never complete the project though... I can get a heck of a buzz on drinking one beer.

  I checked into the aluminum honeycomb.. it's a bit pricey for my shallow pocket book.  I'm trying to build this thing with a shoe string budget ( That is, less than a shoe string would cost ).

  The cost of fluorescent panels at our local outlet is 10 bux each for a 2x4x 1/4" thick.  I'd need a minimum of 3 deep by 3 across... still a bit pricey.  I may end up making them out of some old lumber in the back barn, cut strips then cut slots to fit them together.  Kind of like the old heat transfere grates in the old houses used to use for moving heat from the lower level to the upper level only thinner.  

  .

[wdyasq]

Ed,

You may be able to use the cheap 'door-skins'. We get them for $6US.  Could be cut into strips, gang slot cut and assembled.

Ron

[ghurd]

How about 8' fluorescent bulb protectors?

Thin clear plastic tubes.

Surplus place up the road has tons of used ones for a buck or 2.

G-

[monte350c]

Hi Ed,

Home Depot over here has fluorescent light diffuser panels 2 x 4 feet that go in suspended ceilings. I can get the price the next time I'm there. They're about 1/2" squares.

The poster tubes would be fairly easy - the pack of 77 listed in that ebay auction would be about 18" square so they would fill the opening fairly quickly.

Good luck with this!

Ted.

[windstuffnow]

  Hi Ted,

    I checked the price around here for the fluorescent "eggcrate" covers and they want 10bux each for them and their only 1/4" thick.  I'd need a minimum of 3 deep and 3+ to fill the opening, fairly expensive.

    I'm still looking for inexpensive alternatives... I'm not sure cardboard would hold up very well.   There are some extreem weather and humidity changes where it will be built...  It will be in a dry barn but it's not a conditioned barn so things tend to vary with the weather.

    Thanks for all the help though.

    I did run down to the local Steel,aluminum and plastics co here and they did have some fiberglass grate, unfortunately it had 1/4" wide cross bars which would most likely create more turbulance than it reduced.

    I'm thinking I'm just going to build it up and see how turbulant it is and play with some ideas to help straighten the air flow.  I'm thinking it doesn't have to be perfect like the tunnels to make intricate measurements on lift and drag in the store bought tunnels.  Basically I'm after consistant wind speed readings so I can get a more accurate output reading from the turbines I test.  Most wind turbines never see "perfect air" anyway.

    I did find some PVC tube in my back barn, a little larger than I would like but I believe there is enough to make a grid the size I need and add some 1/4" wire screen on both sides should do a reasonable job of keeping the air pretty straight... we'll see !

.

[BigDogues]

Okay Ed, Here is how I would do it.

Have you ever seen the cardboard separators that are in a wine case? You could create the same thing using some inexpensive very thin luaon plywood or paneling (I have seen it as thin as 1/8") that can be bought at Home Depot or Lowes or some other home center. If you purchase two 4' X 8' sheets you can rip each of them into 16 strips approximately 3" wide and 96" long. Now stand all the strips up on edge and clamp them together. Next gang saw the strips every 6" half way through using a saw blade that is as wide as the material is thick. Now you can take all of the strips and a little glue and assemble yourself a grate that is 8' x 8' with 6" square cells that is 3" thick. (Is 3" thick enough or too much to straighten the flow? I have no idea, you may know better than me.) My example is not exact in calculation (you would actually have a strip left over if you left the edges of the grate open) and I'm not sure of the size of grate that you need. If you buy three sheets you could make your cells 4" square on an 8' x 8' grate. Four sheets would yield 3" cells etc. I would also ease all of the edges along the strips with some sandpaper BEFORE I gang sawed them. Be sure to use plywood and not Masonite as it is not strong enough for this job.

Good luck with your tunnel, I always wanted to build one myself so I guess I am living vicariously though you.

Hope this helps,

Phil

 

[bparks]

WindStuff - you have probably already solved this, i haven't been on here much lately.  I came across the honeycomb stuff today though & thought I should add a link here,

https://secure9.nexternal.com/shared/StoreFront/default.asp?CS=plastiweb&BusType=BtoC&Count1
=900371730&Count2=817512154

3/8" x 23.75" x 47.750" White Eggcrate Styrene Lighting Panel - $12.42 plus shipping

not sure if 3/8" is really thick enough.