On the measurement and prediction of rainfall noise

Abstract

This paper concerns the measurement and prediction of rainfall noise with a particular focus on measurements made using the method described in the ISO rainfall noise standard. Rainfall noise is generated by rain impacting on a surface which excites the surface producing noise. Here, several different models for the force produced by a water droplet impacting on a flat inclined surface are presented. These models are designed for the nominal intense and heavy raindrops described in the ISO standard and are validated against experimental measurements. The best-performed models have been incorporated into theoretical methods for predicting the noise produced by rainfall on a flat inclined panel. These methods are used to produce noise level predictions for heavy rainfall on a standard reference test specimen which are compared with experimental measurements and show moderate agreement. These experiments were conducted using the test method described in the ISO standard. Several issues which we encountered implementing this test method are described and a number of suggestions are made to improve this method. Finally, the theoretical models are used to investigate the effect of different parameters on rainfall noise for the purpose of illustrating potential sources of error during testing using the ISO standard method.1 Note that table K1 in Annex K of ref. [5] gives four rainfall type classifications (moderate, intense, heavy and cloudburst). These are attributed to IEC 60721-2-2. However, these classification names and the rainfall characteristics (rainfall rate, typical drop diameter and fall velocity) do not correspond with those given in the current version of IEC 60721-2-2 (published in 2012) which lists 'heavy rain' as having a rainfall rate of < 16 mm.h -1 and 'very heavy rain' as having a rainfall rate < 50 mm.h -1 . No characteristics, other than rain intensity are associated with the different rain classifications in IEC 60721-2-2. However, it is stated that rain, in general, has a droplet size distribution typically 1 mm to 2 mm in diameter, with diameters up to 5 mm to 8 mm in thunderstorms; fall velocities are stated as being typically between 2 m.s -1 and 12 m.s -1 .

Recording Rainfall Intensity: Has an Optimum Method Been Found?

“…The sound of raindrops striking a detector of some kind has been exploited to record rainfall arrivals and allow rainfall intensity to be determined. The acoustic energy released by rain striking a metal roof has been a subject of considerable study in the acoustic design of buildings (Schmid et al [126]), and it is known from that work that the sound energy is linearly related to the logarithm of rainfall intensity. Figure 3 presents an example from Dubout [127].…”

Stress distribution and surface shock wave of drop impact

“…Very few experiments have been performed probing the dynamic properties of drop impact that are directly responsible for drop-impact erosion 18 . As an important dynamic factor, the impact force of liquid drops has begun to attract attention in recent years 18 – 26 . However, it becomes clear from these recent studies that the unusual ability of an impacting drop in erosion cannot stem from its impact force alone, as the maximum impact force induced by a millimetric water drop falling near its terminal velocity is very weak, more than an order of magnitude smaller than that generated by the impact of a solid sphere of similar size, density, and velocity 23 .…”

Effect of Reynolds Number on Impact Force and Collision Process of a Low-Velocity Droplet Colliding with a Wall Carrying an Equal-Mass Deposited Droplet

“…It has found wide applications in many fields, such as spray printing (Lohse, 2022), surface coating (Kang and Ng, 2006), spray cooling (Cheng et al, 2016), material cutting (Sharma et al, 2011), and fluid machines with droplets (Day et al, 2008). Droplet impacts may also cause many problems, such as soil erosion (Planchon and Mouche, 2010;Soto et al, 2014;Nouhou Bako et al, 2016), blade erosion (Keegan et al, 2013;Amirzadeh et al, 2017a,b), and noises from raindrops (Schmid et al, 2021). Studies have indicated that the impact force of droplets on walls is a key factor causing erosion (Lopatnikov and Gillespie Jr, 2014;Cheng et al, 2022).…”