An Experimental Study of Water Surface Features in Response to Rain

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Water surface features induced by the impact of raindrops on a deep water pool are studied experimentally in an artificial rain facility. Artificial rain is produced by a rain generator that consists of a rectangular tank with an array of 738 hypodermic needles attached to its bottom and that is mounted at various heights above a deep water pool. In this thesis, three rain intensities and four raindrop impact velocities are used while the diameters of raindrops remain approximately the same. For comparison with some of the results of the rain experiments, a set of single drop impacts on a quiescent water surface were also performed.

In the single drop impact experiments, cinematic shadowgraph methods were used to measure the drop diameter (D) and velocity (V ) just before impact, to observe qualitatively the water surface response and to measure the height of the vertical water jet (stalk) that is typically part of the water surface response. It is found that the stalk height varies with impact Froude number (Fr = V^2/(gD), where g is the acceleration of gravity) in three different ways depending on the Froude number range.

In the rain experiments, the drop diameters and velocities are measured with a cinematic shadowgraph technique while the temporal evolution of the surface profile along the center plane of the target water pool is measured with a cinematic Laser Induced Fluorescence (LIF) technique. The history of rain-induced stalk height and the profiles of the rain-induced surface waves are extracted at each instant in time. It is found that the stalk height varies considerably in the rain field and the average stalk heights are less than in cases with single drop impacting a quiescent surface at the same Froude number (Fr). The stalk height distribution correlates with the rain intensities rather than the impact velocity. Occasional bubble en- trainment was observed at the lowest raindrop impact velocity (Fr = 500) while bubble entrainment only occurred for Froude numbers greater than 1800 in single drop experiments. Furthermore, surface waves outside of the rain field propagate faster than that inside the rain field.

Radar backscattering powers from raindrops, surface waves in front of the rain field and the water surface features inside a rain field are measured. The measurement results show that strong radar return signals are observed from the water surface inside the rain field while the radar return signals from both raindrops and the surface waves in front of the rain field are weak. The experimental results also show that the radar return intensity increases as the rain intensity increases from 85 to 300 mm/hr. In addition, it is found that the attenuation of the radar backscattering from the rain field is likely correlated with a high-water-content layer of secondary droplets generated in the rain field.