Comparison of artificial and natural rainfall

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.