Damage threshold velocities for liquid impact

Rain erosion is a major cause of strength and transmission loss in IR ‘windows’ in high velocity flight. The Cavendish Laboratory's Multiple Impact Jet Apparatus (MIJA) is able to simulate high velocity rain impact accurately and reproducibly under laboratory conditions using a series of discrete water jets. Quantitative erosion data in the form of Damage Threshold Velocity (DTV) curves can be obtained, giving the lower limit of damage to the material under study. Damage threshold curves are presented for five different materials carried out using three different nozzle orifice diameters to create 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 have a flat front, and (ii) the drop (‘round-fronted jet’) model. Both methods are found to give predictions well within the bounds of experimental error. 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 orifices are presented validating the use of models for the various nozzle diameters.