Needle-free drug and vaccine delivery devices have been developed by the Department of Engineering Science at The University of Oxford in collaboration with PowderJect Pharmaceuticals plc. In them, micro-particles of pharmaceuticai agent are accelerated, by a supersonic flow of gas in a nozzle, to velocities sufficient for them to penetrate the outer layers of the skin. If the gas flow is over-expanded relative to the atmosphere to which it discharges, shock waves form inside the nozzle, which cause the flow to separate from the wall. The resulting gas-particle flow field is highly non-uniform, leading to suboptimal drug or vaccine delivery and even damage to the target skin.