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2003-12-14 Rev. -12-17, -12-21, -12-29, 2004-01-01, -01-15,
2007-03-04, 2008-01-22, 2010-02-15, -12-11, -12-30
[Search on date pattern to find latest changes, more than one may be found.]
When I went exploring the Internet to get some information and suggestions about shapes of the ends of my whirlies, I found I was having to dig bits and pieces up from the depths of odd web sites like wind energy and weather and human powered vehicles (80 mph). In particular, nothing came up on the lift effects of side mount hemispheres as on anemometers. A spinning cup anemometer is considered a drag instrument (unlike a propeller anemometer.)
In the image, the wind is blowing from the top. There are four forces applied to the whirly as shown. Each force has its own moment arm R (center of rotation to center of force of the cup.) Of the two drag forces F1 is bigger than F2 (next paragraph) and is usually the only force discussed. But F3 and F4 both act clockwise. Whether they are the same force or different depends on the cup shape (like the lack of symmetry here) and what is in the way (many anemometers have a good sized hub.) 2003-12-31
I learned early that the anemometer cup has totally different
drag when it is open side upwind as opposed to dome side upwind. This is
the Drag Coefficient (Coefficient of Drag, CD or Cd),
still exists, page with data now gone] which is "defined as
the drag force per square meter frontal area of the object."] A round cup open upwind
"has a very high CD of 1.42, whereas its CD is only 0.38 if you turn it 180°
around a vertical axis." For other shapes: "A flat plate has a Cd of 1.28 while a prism
(pointed tail) is 1.14 and a bullet 0.295 which is calculated with the formula
Cd = D / (.5 * r * V^2 * A) where drag (D) is measured in a wind tunnel with
density (r), velocity (V) and area (A) being measured."
http://wright.nasa.gov/airplane/shaped.html eSearch of 'wind
drag of a cone' yields this site
has point on ratings down the page of many shapes and discussions of why drag
changes with wind velocity. Also links to other sites. Start of a
history of wind power with some drag discussion
http://www.telosnet.com/wind/early.html 2010-12-11 [Lately I found
While this difference in drag should be enough to make revolving occur, it does not explain the better performance of my units with a flange which adds drag, but which should add to the aerodynamic lift with upstream and downstream (edge on). In the drawing, which represents the 4 extreme positions of a cup (upwind, downwind, cup to wind, dome to wind) the two side positions are drag, where the flange adds to drag equally while the dome has extreme difference as mentioned above. The upwind and downwind positions should be developing thrust (clockwise in this drawing), but how much? And how does having the cup open or flat across the opening make a difference?
I have a good sized squirrel cage blower that I kept out of the house furnace when we changed it recently - 3 speeds. I had planned on using it to replace the blower from the swamp cooler that is mounted in the garage, but a wind tunnel sounds useful.
I went out and wired up the blower to medium and fixed up a switch. Turned it on - lots and lots of wind, maybe too much.
The wind tunnel design will be fairly simple - a lead in section with a grid to straighten the air, a test section and a bit of out flow area, perhaps with another grid. The test section has to allow for testing two kinds of force - drag along the air stream and lift across the air stream. If I make the wind tunnel moveable, I could do a blow up or blow down as well as blow sideways. The up or down could make measuring drag easier as it could be straight lifting of a weight instead of around a pulley. It should be square, I guess for repositioning ease. I will continue to explore the Internet and sketch.
I have been thinking more about building a wind tunnel. With my usual impulsiveness, I pulled some wood that might make a box, then, with my somewhat less usual reserve, backed off from doing anything without a design. During the day I sketched on a design with a completely different shaped box that focused on solving the measurement and balance devices, which evolved to a square box just for this and the blower and grill attached separately. Also e-mailed Texas A&M about their wind tunnel and usage! [$350/hr]
"To measure the "lift" and "drag" of the models in the testing
section, we used two Force probes from Vernier Instruments " http://www.fi.edu/flights/first/makebigger/index.html
[Actually Vernier Software & Technology 13979 SW Millikan Way Beaverton, OR 97005-2886 phone 888.837.6437 fax 503.277.2440 email email@example.com ]
The Dual-Range Force Sensor has two ranges: ±10N or ±50N. It can be easily mounted on a ring stand or dynamics cart (mounting bracket included), or used as a replacement for a hand-held spring scale. Use it to study friction, simple harmonic motion, impact in collisions, or centripetal force. This force sensor is used for experiments in our physics, physical science, and middle school science lab manuals.
|Well, that shoots this idea down, since I would need two, plus the interface, plus the software. Even the individual gauges from other sources cost $45 or so. Back to the balance scale idea.|
|These are the drawings, based on hand sketches done away
from the computer, that work out the problems of a wind tunnel, adding the
balances needed after concluding I did not want to spend the money for
The first design consideration was to be able to store the thing after having built it. That means it has to defend itself (no odd bits sticking out) and fit in a 2x2 foot stack as do my various storage boxes. To the right is the test section. The upper half is fronted with 1/4" Plexiglas for viewing and some strength. The back is 1/2" plywood for strength while the bottom section and top are 3/4" board for the same reason. [As actually built, the upper front and top are a bent 1/4" Plexiglas sheet.] Below is the sequence of blower, transition, straightening grill and test section. The blower is one taken from my old furnace and may be too strong.
In the test section, the following factors are built in: Not being able to accurately remount the test shape, it will be mounted on a turnable mount - a shaft with a square top section and a moderately flexible coupling at the bottom to a handle on a pivot. The force will be measured by pulling from upstream and from the back (lower drawing) so mounting will depend on anticipating the forces. The pulleys are grooved ball bearing replacement rollers for glass patio doors, washed clean with solvent and lightly oiled for ease of movement. I have a choice of putting weights on the scale pans or adjusting the pull on a spring and working out the force per inch of the spring. 2003-12-21
|A lot of additional problems. If the shapes are made of sheet metal, the square shaft mount I was thinking of will interfere with air flow, perhaps too much. I did some calculations with force and e-mailed a guy at NASA who confirmed that we are talking some pretty small forces. I wanted to be sure I had the correct result. A 3"x3" flat plate (75x75 in the calcs) in a 10 mph wind at normal air density has only 1/3 ounce of force! 2004-01-01|
|Flat plate||Cd = 1.28||0.093 N||0.3345 oz|
|Anemometer cup: Open end||1.42||0.10317 N||0.37109 oz|
|Domed end||0.38||0.02761 N||0.09931 oz|
|I see why the Wright brothers did such a delicate balance with a drag plate formula.|
I am continuing by building a basic test bed. Instead of the box above, I
am building a flat plate. The plate has two layers, the upper with a circle cut
free on which are mounted a bar and a pulley. The bar, lower center,
pivots near the edge and supports an upright 532" square tube. On this
tube are to be mounted curved shapes, Plexiglas first, perhaps metal or glass
later. The wind will come from the left. Weight will be placed on
the hook below tied to a string to the center to keep the shape at the center.
By picking up the shape and reversing it, drag in various positions for the same
wind can be tested. The looping C shape is a slot cut in the lower piece
of plywood, which permits the upper disk to be rotated right or left so the
weight can indicate lift, two sides permitting either orientation for
unsymmetrical pieces, like my existing glass. Note that the unit can
not measure drag and lift together or in quick sequence by rotating the piece as
the one above can. [2004-01-15] Ran the wind tunnel for the first time
today, having installed the side and roof panels. Did not try to push in and
reduce wind. The mount is clearly too wobbly and the disk is somewhat off
the centerline. Shows lift, but needs more control and rigidity to be
|2004-01-20 When I ran some further experiments, I feel I could demonstrate that there was reasonable lift, but I also demonstrated that rigidly mounting the shape on a square tube on the arm resulted in odd angles and difficulty adjusting the angle to zero. One solution would have been to reconstruct the arm only to parallel arms with the test form mounted on a connector arm, always perpendicular to the wind, on a pivoting mount. But there were several other irritating factors, like the weight of the hook, limits on centering the arm, etc., that I decided to put the effort into building something closer to the design further above where the model is on top of a pivoting post and measuring is done below.|
|I am changing the design to omit the pulleys and have balance arms directly on the vertical post. Two nights ago, I made up the base disk and epoxied to it tubing for the lower pin and a 1/4" socket drive universal joint with square tubing to make a spline like connection. [drawing and picture below] Upper arm ends are round tubing soldered into holes drilled in square tubing arms to hold nuts. All carefully balanced with extra lead solder. Tonight I found a back wall - 2x2x0.5" plywood and cut the shelves and the end pieces, drilling the shelves to align the pivot hole and the riser tube clearance hole. Glue it up in the warmer part of the day Thursday.|
|2004-02-19 - Did my first rough run with the new wind tunnel balance, very rough. The basic results came out correctly and it seems possible to make some reasonable measurements, but so far the calculations (guessing at wind speed) are only bulk answers.|
| I weighed two full boxes of
nuts at work on the postal scale (to 0.1 ounce) and found that standard
coarse 1/2" hex nuts weight 16.7 gm and 1/4" weigh 3.3 gm, which is nice
because 5 1/4" nuts (16.5 gm) just about equal one 1/2" nut.
The ratio of the horizontal arms to the upright, from the pivot is 0.316, the
gram weight is given under Arm. Adj. and the force in Newtons under N.
As shown, the flat has 0.300N force, the Cup upwind 0.383N, Cup downwind 0.238N and the lift of the cup sideways is 0.093N. Thus the difference in forces of drag is 0.145N or about 1.56 the cup sideways, but there would be two sideways in a 4 cup arrangement, so lift contribution should be about 0.310N vs. 0.145N for drag. Similar work can be done with the P-Curve but it is harder to get reliable figures without more care since misalignment with the tail produces considerable changes. 2005-06-24
I need to buy another bike speedometer to calibrate the anemometer, since doing it in a car alone did not work. I should probably enclose the wind tunnel test section as I felt I could feel a difference on my hand from the open side to the back wall. [Did in fact enclose it] [Did buy another speedometer - 2 actually at Wal-mart - but have not yet calibrated it. Quiet air with enough light to make run in park. 2005-06-24]
The composite image below shows various views and details of the wind tunnel
Contact Mike Firth