Air-entrained vortex in open intake: Time–frequency analysis and the interaction with subsurface vortices

Abstract

In this paper, the simple coupled level-set and volume of fluid and bifurcation models are used for the accurate prediction of the flow in an open pump intake with a vertical pipe. The continuous wavelet transform, which is suitable for the vortex detection, is applied to the pressure signals near both air-entrained and subsurface vortices. Low-frequency with long duration for air-entrained vortex due to the vortex wandering and broadband with short duration for floor vortices (dominant in subsurface vortices) due to the generation of extreme strong vortex are observed. The vortex motion mechanism is revealed by the analysis of the transport equation of the vertical vorticity's enstrophy. Different from the previous results in which the tilting effect controls the vortex motion, the stretching effect is found to be dominating when it is large enough. When going through the bell mouth, the air-entrained vortex's vorticity changes the sign and strengthens the vortex with the same sign. On the plane near the bell mouth, three vortex patterns, including co-rotating pair, merging, and counter-rotating pair, are observed. The onset criterion of the vortex merging at a/ b =  0.29–0.32 is found to be applicable to the present case. The counter-rotating pair is found to be more stable. Air-entrained vortex serves as an amplification of the strong vortices generated from the subsurface.