Implosion hydrodynamics

Cavitation has three negative side effects in valves—noise and vibration, material removal, and reduced flow. The bubble-collapse process is a violent asymmetrical implosion that forms a high-speed microjet and induces pressure waves in the fluid. This hydrodynamic noise and the mechanical vibration that it can produce are far stronger than other noise-generation sources in liquid flows. If implosions occur adjacent to a solid component, minute pieces of material can be removed, which, over time, will leave a rough, cinderlike surface. 

The hydrodynamic forces of cavitation - i. e. the shock wave energies produced on bubble implosion. 

John von Neumann, the Hungarian mathematician who had come to the United States in 1930 and joined the Institute for Advanced Study, had been examining for the NDRC the complex hydrodynamics of shock waves formed by shaped charges, technology which was being applied to the American tank-killing infantry weapon known as the bazooka. Like Rabi, von Neumann had agreed to serve as an occasional Oppenheimer consultant He visited Los Alamos at the end of the summer and looked into implosion theory, another warren of hydrodynamic complexity. Ned-... 

Von Neumann soon drafted Ulam to help work out the hydrodynamics of implosion. The problem was to calculate the interactions of the several shock waves as they evolved through time, which meant trying to reduce the continuous motion of a number of moving, interacting surfaces to some workable mathematical model The hydrodynamical problem was simply stated, Ulam comments, but very difficult to calculate— not only in detail but even in order of magnitude. ... 

Carefully spaced prearranged wires contacted by the metal sphere as it imploded supplied information not only about the timing of the implosion but also about material velocities at various depths within the core. That provided direct, quantitative data which the Theoretical Division could use to check how well its hydrodynamic theory fit the facts. The Electric Method group began by measuring the high-explosive acceleration of flat metal plates. Early in 1945 it adapted its techniques to partial spheres and eventually to spheres surrounded by HE lens systems with only one lens removed to access the necessary wires. 

Low-pressure zones in a fluid system under turbulent flow conditions are at the origin of cavitational wear. When the hydrodynamic pressure at some location temporarily drops below the vapor pressure of the fluid a fraction of the hquid is transformed into gas bubbles. The recurrent formation and implosion of these bubbles leads to material damage by local fatigue. If simultaneously corrosion takes place we speak of cavitation corrosion. 

The results of these experimental measurements of the liner trajectory were compared with a numerical model for the implosion obtained from a 1-D, Lagrangian, hydrodynamic code containing a detailed treatment of the equation-of-state of the liner material. Figure 17 shows such a comparison for a 1.4 MJ shot. The point labeled as "abrupt current change" marks the discontinuity in the slope of the current trace caused by the sudden ending of the IL contribution to the voltage when the liner hits the axis. The... 

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