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Cavitation
Cav.i.ta.tion \ kav’ i ta’ shun \ n [1. the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure. 2. such a pocket formed.] (Webster). Like any little boy with a penchant for watching things explode, Professor Joe Katz has had a lifelong fascination with the problem of cavitation.
In a water environment, cavitation occurs when the pressure in a certain region falls below the vapor pressure of the surrounding water, causing the sudden creation of a cavity of gas that then rapidly and explosively collapses. Situations like this occur around propeller blades, around hydro-turbine blades in hydroelectric power generation, and in pumps. The result is a very noisy, very destructive event. The Glen Canyon Dam had to be shut down after only 15 years of operation because cavitation events from the turbine blades ate through a three-foot wall of concrete. The noisy belch of a cavitation event around a ship propeller can be heard underwater from 70 to 100 miles away; an inconvenient “here I am” announcement for naval vessels that prefer to roam silently.
Probably the most dramatic example of cavitation occurs in the oxidizer fuel pump in the space shuttle’s main engine, which operates at about 80,000 HP (for comparison, a tractor-trailer is about 500 HP) with a 14-inch impeller. During the space shuttle’s design phase in the 1970s, instabilities caused by cavitation in this pump caused repeated engine blowup. Even now, cavitation-related instabilities in liquid fuel rocket pumps are a major problem, and cavitation control is a primary design factor in these engines.
Because situations that lead to cavitation involve turbulent flow and a very rapid phase change—on the order of microseconds—the phenomenon has proved very difficult to study in the laboratory, and, as a result, difficult to model. Profs. Katz and Knio and their team go “small-scale” to tackle this problem. They simulate real cavitation situations by creating carefully controlled flow conditions in JHU’s experimental fluid dynamics laboratory. They study the cavitation phenomenon, as well as the flow that caused the cavitation, in detail at very high magnification using microscopy, PIV techniques (see Turbomachinery, above), high-speed photography (up to 3,000 frames per second), and noise and pressure measurements. Data obtained from these explorations of the relationship between flow structure and cavitation may someday allow engineers to develop a way to control the phenomenon.