It was a curious test object that the researchers led by Frank Janser from the FH Aachen maneuvered into the wind tunnel of the Department of Aerospace Engineering. Normally, aerodynamic characteristics of aerospace, automotive and motorcycle objects are recorded here. This time, however, it was a 1.20 meter high Nordmann fir, which was to reveal its air resistance here. According to the researchers also such a small tree was suitable, because the results can be extrapolated to large copies. For the test special precautions were necessary, the researchers report: so that the pine needles at the air speeds of more than 80 kilometers per hour do not fly around in the wind tunnel, they have previously fixed this with several doses spray paint.
Better anchoring seems necessary
"We measured which back pressure builds up at different flow velocities, " says Janser. From this, the so-called resistance coefficient (or cw value) can be determined, a dimensionless aerodynamic coefficient, explains the expert in fluid mechanics and industrial aerodynamics. The measurement in the wind tunnel showed a drag coefficient of about 0.8 - for comparison: in a modern car, this value is 0.3 to 0.35, while a truck also has a value of about 0.8. Ultimately, the researchers come to the conclusion: So far, one has assumed a much lower drag coefficient and therefore often has the anchoring too weak.
The calculations based on the values from the wind tunnel tests in the case of a ten meter high Christmas tree: He should be anchored with at least ten to twelve tons of weight, the researchers say. At the initiation of the investigation was also the organizer "Markets and Action City City eV" involved, which organizes the Christmas market around the cathedral and the city hall of Aachen. The results of the experiment have now been incorporated into preparations for the 2017 Christmas Market, which begins on November 24th. Apparently, Aachen now decorate particularly storm-proof Christmas trees.
- FH Aachen