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M. Farhat, M. Tinguely, and M. Rouvinez
Ecole Polytechnique Fédérale de Lausanne
Laboratory for Hydraulic Machines, Lausanne, Switzerland
The high-speed impact of a solid on water drops, or jets, may lead to severe erosion in a variety of industrial applications. In the aerospace industry, for example, rain erosion is an important issue for supersonic aircrafts. In the hydropower generation field, severe erosion of large Pelton turbines is believed to result from repetitive impacts of the turbine buckets on the driving water jet. Yet despite a tremendous amount of work already dedicated to this problem, the erosion process is still not yet fully understood. In the present study, we achieved a unique experimental set-up (Figure 1) to investigate the physical phenomena involved in high-speed impact of a solid surface—a bullet—on a water jet, with the collaboration of the Swiss Army. In the experiment, a 9 mm bullet is shot towards a falling water jet of 2.5 mm diameter (the bullet’s speed is varied in a range of 500 to 1800 km/h), and a high-speed camera is used to capture the jet’s breakdown shortly after impact.
Figure 1: Experimental set-up involving a high-speed bullet impacting a falling water jet. With support from the Swiss Army
As illustrated in Figure 2 (3rd frame), the impact of a 500 km/h bullet on the jet interface leads to the generation of a strong, supersonic shock wave that travels within the jet much faster than the bullet itself. As the shock wave reflects on the jet boundary, it turns into a low pressure wave leading to the formation of a large amount of vapor bubbles in a process called cavitation. At this time, the bullet nose is barely inside the jet. When the bullet does penetrate the jet, it undergoes a violent collapse of cavitation bubbles. The present experiment clearly demonstrates for the first time that shock induced cavitation plays a major role on material damage observed for high speed liquid/solid impacts. It paves the way for further investigations of complex and fast phenomena using military projectiles.
Figure 2: High-speed visualization of 500 km/h bullet on a water jet (54000 frames per second)
Acknowledgment: Authors would like to thank the Swiss army personnel for their valuable contribution