Influence of pipe length on cavitation surge frequency in a cascade

Cavitation surge is a system instability that occurs in pumping machineries in which the unsteady characteristics depend on system components such as the pipe length. On the other hand, under typical boundary conditions of numerical simulations of internal flow, it is supposed that a uniform flow extends infinitely upstream and downstream from the boundaries. Therefore, a numerical simulation of the influence of the pipe length of fluid machinery cannot be realized just by changing the position of a boundary of the computational region. In the present study, a boundary condition that takes into account the pipe length was used to numerically simulate unsteady cavitation in a three-blade cyclic cascade. The resulting cavitation surge frequency became lower when the upstream pipe length becomes longer, which is consistent with previous findings. Also, the resulting cavitation surge frequency showed good agreement with empirical frequencies of the actual cavitation surge of pumps, although the empirical frequency is a rough prediction because it covers various system components. Additionally, the theoretical frequencies were predicted by estimating quasi-steady and local cavitation compliances from the present results. The theoretical frequency takes into account the upstream pipe length. As a result, local cavitation compliance was relatively effective for prediction of the theoretical cavitation surge frequency.