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Comparison of artificial and natural rainfall

: Naßhan, K.

Boone, M.M. ; Deutsche Gesellschaft für Akustik -DEGA-, Berlin; Acoustical Society of the Netherlands:
NAG/DAGA 2009, International Conference on Acoustics. Proceedings. Vol.2 : Rotterdam, 23 - 26 March 2009; Including the 35th German Annual Conference on Acoustics (DAGA)
Berlin: DEGA, 2009
ISBN: 978-3-9808659-6-8
Deutsche Jahrestagung für Akustik (DAGA) <35, 2009, Rotterdam>
International Conference on Acoustics <2009, Rotterdam>
Conference Paper
Fraunhofer IBP ()

There are two important differences between natural and artificial rainfall. While drops in natural rain have reached their terminal velocity, they are still in the phase of acceleration in the rain noise laboratory. The maximum number of drops per unit area and unit time in natural rain is at drop diameters around 2 mm, whereas artificial rain according to IS0 140 consists mainly of 5 mm thick drops. It is shown that an accurate consideration of aerodynamic resistance is essential for properly modeling the motion of free falling drops. In particular, the dependence of the drag coefficient on the Reynolds number, e. g. on diameter and actual velocity, has to be included. The impact energy of a drop on a plate or membrane is calculated according to B. A. T. Petersson from the velocity and the shape of the drop. First estimates show a difference of 15 dB to the results obtained with the assumption of a constant drag coefficient. This might explain the difference between measured and calculated rain noise levels encountered in previous investigations.