As described in http://people.csail.mit.edu/jaffer/SimRoof/Convection/Measurements, I am trying to construct a wind tunnel in order to measure forced convection from a rough plate.
Yesterday I assembled my cardboard fan housing onto the test chamber. It was difficult to do alone; the process ended up involving several bungee-cords. With only 27mm of flow-straightener (will eventually be 150mm), I was not expecting smooth flow.
Comparing operation between the lowest and highest fan settings, there was only a 20% increase in flow through the chamber. So I will not achieve my original target of 10m/s flow. Perhaps I can achieve 5m/s.
Between the fan and the flow-straigtener I put two interlocking boards at right angles to block the substantial rotational flow from the fan (the boards also eased assembly). The flow was thus split into four quadrants, each with substantial rotational flow. The anemometer showed 4:1 variations of flow in each quadrant of the chamber. I was able to easily sense the flow variations by moving my hand around inside the chamber.
I need to cut the interlocked boards so that they aren't as deep, and add additional radial vanes to try to eliminate the gross pressure variations before the flow enters the flow-straightener. Alternatively, would a second flow-straigtener at the fan work better than radial vanes?
For the next test I removed the interlocked boards for suppressing the rotation.
In the test chamber the flow pattern was simpler. Fairly uniform wind speed in a ring around the center. It drops precipitously in the center and at the left and right edges. The momentum of the rotating air coming out of the fan drives it to the outer rim, depleting the center.
Rotation in the test chamber was at low levels. So even a short depth of egg-crate stops the rotation, but does not equalize the flow across its width. This suggests that there should be egg-crate directly in front of the fan to stop the rotation, an airspace to let the air pressure equalize, followed by egg-crate to straighten the flow (leading into the test chamber).
I performed two other tests. When I removed the egg-crate, the flow was slightly stronger but the rotation was much higher in the test chamber. Putting window-screen between two layers of egg-crate cut the flow by nearly half, but did not make the flow uniform.