Freesteel Blog » Micro-wind tunnel tests on the wind meter

Micro-wind tunnel tests on the wind meter

Monday, August 10th, 2015 at 5:24 pm Written by:

I failed to take a photo of the experimental set-up which the guy on the Newcastle stand at Manchester Makefest let me shove my airspeed probe into. The airspeed probe is described here and some interrupt timing misery to do with it (which I don’t know was properly solved) is reported here.

This is the track of the windspeed when it’s in the device, and then when we put a card across the air intake that roughly halved the flow.
windtunneltrace
The time interval is 5 seconds for every green vertical line.

The readings where it is flat and high (off the left of the diagram) have an average of 7.11m/s, standard deviation 0.11 over 60 seconds. The middle low section has average 3.26, sd=0.05 over 35 seconds. The final high value when I took the card off the intake was 6.99m/s, sd=0.077 over 45 seconds.

The tailing off of the wind speed measurements is probably an important factor.
windtail
Squashing the graph down the Y-axis and plotting the green lines at one second intervals, we can compare the two incidences where I put a card over the intake and halved the wind flow.

It takes about one second to settle, and the two curves don’t match up.

I can’t tell if this is due to the inertia in the propeller sensor, or inertia in the wind tunnel device when I cover it up making it sound like a blocked vacuum cleaner.

At least half is due to the latter, or the curves would be a better match because they’re smooth enough and the propeller inertia doesn’t change.

We’d need another way to more quickly vary the intake into the wind sensor. For example, we could rotate it slightly so that it picks up less wind. The engineer suggested introducing some controlled friction into the system to dampen the spinning so it responded faster. Alternatively there’s a pitot tube.

windtailcurve
Of more immediate concern is the unexplained wandering around of the wind speed sensor by as much as 6% across multiple seconds of time. This can’t be turbulence in the device as it wouldn’t have such lasting effects.

Also, there are short bursts of zig-zagging every half second indicated by the red marks.

I’ve seen these zigzag effects before with the temperature measurements, probably caused by the interference with other devices on the same microcontroller, going into and out of phase with their interrupt cycles.

I don’t know of a mechanism for voltage changes to affect the rotating fan blades (like they did the thermistors), although something could be introducing small delays into the detection of the interrupt signals.

This would take a lot more building of separate interrupt-driven dedicated microcontroller circuits to test the theory. And then that doesn’t answer the 6% wandering.

I got to get back to other parts of the project, and look at this later when I have use for these measurements as well as a test rig.

Maybe there’s other technology, such as a thermal anemometer or a beautiful sonic anemometer that measures windspeed and direction instantaneously for a mere $2700. I haven’t got that kind of money to squander at the moment, but it does show what’s available.

OMG, what’s this publication I’ve just uncovered:

Sonic anemometer and atmospheric flows over complex terrain: measurements of complex flows

Three measurement campaigns and the use of sonic anemometry under specific conditions are described in this work. EBEX2000 was an international energy balance field experiment in San Joaquin Valley USA, were different sonic anemometer types, and heat and momentum flux measurements, were analyzed and compared. The second case was a complex coastal flow at Madeira Island, Portugal. The complexity of the flow compromised the performance of an existing wind farm. The use of post-processing techniques, such as Fourier and wavelet spectral analysis allowed the detection, and unveiled, the existence of coherent structures and other specific features of that wind turbine site. The flow over the mountainous terrain of Madeira Island is also presented for the latter case, where sonic anemometer measurements were executed at wind energy resource assessment phase.

I found some excerpts of the book where they are discussing the instruments. Taking some liberties with the text, there is:

As discussed in chapter 3 the sonic anemometer measurements have to be corrected due to transducer shadow effect and overestimation of measurement due to flow acceleration through the transducer array…

The cup anemometer systematically overestimates the mean wind velocity compared against the sonic… Wyngaard (1981) showed that cup anemometers respond faster to wind speed increases (u > 0) than wind speed decreases causing the anemometer to overspeed.

My anemometer is a propeller, which requires it to be pointed in exactly the right direction. This is not going to help when there is yawing of the glider of up to 80 degrees.

The interesting thing about atmospheric flows over complex terrain is that good glider pilots have the experience to guess the locations of up-currents by visual inspection and from what they know of the wind direction.

In the future the swarms of cooperating autonomous cargo gliders which connect the world together using zero energy (unlike the plans for drones) will be able to not only rely on thermal updrafts, but could also use dynamic lift by reference to weather stations dotted at critical places around the landscape to inform the flow model accurately enough to fly downwind with exactly the right height to clear the next tree line.

I’ve got to climb out of this rabbit hole now before it sucks me in for the rest of the week.

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