A discussion on deformation of solids by the impact of liquids, and its relation to rain damage in aircraft and missiles, to blade erosion in steam turbines, and to cavitation erosion - Disintegration of raindrops by shockwaves ahead of conical bodies

Abstract

When an aircraft flies at high speed through rain the impact of raindrops on the forward facing surfaces of the aircraft may cause severe erosion damage depending on the size and number of the drops, the speed of the aircraft and the time of flight in the rain. However, before the raindrops reach the aircraft surface they have to pass through a region where they are subjected to relative air velocities caused by the airflow round the aircraft surface. This is particularly applicable to supersonic flight when, in the region between the shockwaves and the aircraft surface, the raindrops may be exposed to air velocities large enough to disintegrate them. The raindrop disintegration is not an instantaneous event; it takes short but finite time and appears to be an erosion process whereby droplets are torn off the surface of the main drop until it is completely reduced to a fine mist. The degree of disintegration of a drop by the time it reaches the aircraft surface will depend on the magnitude of, and the exposure time to, the air velocity. For supersonic flight this time depends on the distance travelled by the drop between the shockwave and the aircraft surface. The experiments described had the object of determining the time required for high speed airstreams completely to disintegrate water drops. An empirical relation is postulated between D , the drop diameter, V , the airstream velocity and t , the time for complete disintegration. The paper considers a conical body at supersonic velocity in a raindrop environment, the body being of a shape typical of that envisaged for supersonic aircraft design. From the derived empirical relation for the time of disintegration of water drops the size of drops to be completely disintegrated when approaching the surface of cones of different vertex angles has been calculated for a range of flight Mach numbers. An experiment giving partial justification for computed results is described.