The Liquid Drop Impact as a Source of Sound and Vibration

The liquid drop excitation of structures is studied with respect to the structural acoustic effects. The pertinent literature is surveyed and it is found that the impact phase of the drop contact is thoroughly treated whereas the flow phase has not been paid much attention. A model employing two simplified geometrical shapes of the drop is developed and the approximate force pulses and force spectra are determined. It is found that the liquid drop to rigid solid contact results in a force spectrum which is flat up to a frequency given by the ratio of the impact velocity to the drop diameter. For non-rigid receiving systems it is found that the influence of the receiver can be assessed from a comparison of the flow impedance of the drop with the point impedance of the receiver. A comparison of the ordinary mass impedance of the drop, seen as a particle, with the point impedance of the structure is found erroneous, leaving a misleading force spectrum. Based on the theoretical analysis, some aspects of the structural acoustic effects of liquid drop impacts are studied experimentally. The study which is limited to low drop speeds, i.e. in the range of 2–10 m/s, shows that the force pulse has a high repeatability. Further, the results reveal no significant influence of the surface tension or adhesive forces upon contact with a receiving structure. The experiments with a high mobility receiver demonstrate a catching effect which lowers the peak force and prolongs the duration of the pulse. Finally, the dependence of the pulse on the thickness of a water layer on the receiver is examined. It is found that the kinetic energy of the drop is transferred to the water layer whereby the force exerted on the receiver may be enlarged as compared with a dry impact.