THE ROLES OF CAVITATION/COLUMN SEPARATION DUE TO EARTHQUAKE INDUCED HYDRAULIC TRANSIENTS

The roles of cavitation/column separation due to earthquake induced hydraulic transients in pipe flow were investigated experimentally and theoretically. It is known that when the system pressure during a hydraulic transient reaches the saturation pressure of the liquid cavitation/column separation occurs. When the cavities collapse or when the separated liquid columns rejoin, very high pressures are produced.; The experiment employed pipe mounted between a constant pressure tank and a piston type excitation device. The resulting pressure variation at the piston was recorded continually. Comparison of pressure response for various test runs indicated that the peak pressures at/near resonance frequency were not affected significantly by the excitation velocity amplitude in cases of cavitation/column separation. These results were confirmed by comparison with theoretical analysis.; A numerical method to simulate column separation/cavitation was developed. In order to eliminate numerical oscillation, the flux-correction method has been introduced.; The effects of frictional dissipation were investigated theoretically. Theoretical analysis show that the phase angle of the pressure response changed depending on the friction factor used in theoretical calculation. By comparing the theoretically calculated phase angle with experimental data, the equivalent friction factor was found. Theoretical results which were calculated using the equivalent friction factor show good agreement with both the shape of the pressure response and peak pressure amplitude.; The equivalent friction factor was converted to bubble shear stress which is associated with the non-adiabatic compression and expansion of the cavitation bubbles.; The peak pressures which are associated with the collapse of the large vapor cavities were much larger than would be calculated from the simple water hammer equation with the excitation velocity amplitude. Also, unexpected damage of the pressure transducers was observed during the test.; The results obtained from this analysis suggested that the cavitation/column separation due to earthquake induced hydraulic transients is an important damage mechanism. Also it was found that the energy dissipation is important for cavitation inception especially at the resonance frequency. (Abstract shortened with permission of author.)...