Dynamic effects of transient flows with cavitation in pipe systems

摘要

In pressurised transient flows with cavitation or water column separation, the presence of free gas affects the elastic wave speed, as well as the propagation of transient pressure waves. Several transient tests have been carried out to calibrate simplified numerical models developed using the Method of Characteristics with localised gas cavities and variable wave speed to describe the presence of gas. The developed analysis aims at the parameterisation of special dynamic effects induced during a fast transient. This analysis enables the distinction of different types of system responses and to better understand associated phenomena. Collected transient data were compared with numerical results taking into account pressure variations in time. These analyses include the characterisation of the most relevant issues, such as maximum and minimum observed pressures, oscillation time period and damping effect. In order to analyse the effects of air entrance from the exterior it was considered a case with a leak. Numerical results have shown that the localised cavity model (or vapour-liquid model) simulates reasonably well the shape of observed transient pressures. As this type of model is easier to implement and to use in practice, its accuracy was improved by incorporating additional dissipative-dispersive terms in the basic fluid equations, in order to describe these special dynamic effects.