Damage threshold velocities for liquid impact

Rain erosion is a major cause of strength and transmission loss in IR 'wind ows' in high velocity flight. The Cavendish Laboratory's Multiple Impact Je t Apparatus (MIJA) is able to simulate high velocity rain impact accurately and reproducibly under laboratory conditions using a series of discrete wa ter jets. Quantitative erosion data in the form of Damage Threshold Velocit y (DTV) curves can be obtained, giving the lower limit of damage to the mat erial under study. Damage threshold curves are presented for five different materials carried out using three different nozzle orifice diameters to cr eate the impacting jet. Two approaches are made to predicting the DTV value for each specimen using only data from the standard 0.8 mm nozzle: (i) the 'cylindrical jet' approach, in which the impacting jet is considered to ha ve a flat front, and (ii) the drop ('round-fronted jet') model. Both method s are found to give predictions well within the bounds of experimental erro r. The former predicts the damage threshold better when the smaller (0.6 mm ) nozzle is used and the drop model the threshold when the larger (1.2 mm) nozzle is used. High-speed photographs of jets from differently-sized orifi ces are presented validating the use of models for the various nozzle diame ters. (C) 2000 Kluwer Academic Publishers.