To reveal the evolution of the cavitation field and the wall cavitation erosion response in a confined variable-cross section micro-hole under time-varying pressure generated by volume alternation, this study investigates an Al 1060 variable-cross section micro-hole with a diameter transition from 1 to 2 mm. A mathematical model of bidirectional volume-alternating cavitation (BVAC) and a three-dimensional numerical flow-field model were established. The evolution of the cavitation field and its flow characteristics were analyzed, the effects of alternating frequency and alternating stroke on the cavitation field were clarified; and cavitation erosion experiments were conducted to examine the correspondence between the numerical flow-field characteristics and the wall response. These results indicate that after five alternating cycles, the BVAC flow field reaches a dynamically stable state and exhibits periodic evolution. The abrupt cross-sectional change induces the formation of large-scale, symmetrically distributed vortex structures in the flow field. Both the vapor volume fraction and the flow velocity increase with increasing alternating frequency and alternating stroke. Cavitation erosion experiments at different alternating frequencies, with the alternating stroke fixed at 20 mm and the erosion time at 6 h, showed the best overall improvement at 40 Hz, with the surface roughness Ra of the large-diameter, small-diameter, and transition sections reduced by approximately 52.9%, 60.7%, and 30.5%, respectively. The differences in roughness improvement among different sections are generally consistent with the regional differences in cavitation action revealed by the numerical simulation.
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