Time-average cavitation characteristics in U-shaped notch hydraulic spool valve interpreted via effective flow area evolution

Cavitation-induced erosion is a critical challenge in U-shaped notch (USN) hydraulic spool valves. However, the coupled influence of notch depth and valve opening on cavitation morphology remains unclear. In this study, time-averaged cavitation characteristics in USN valve ports is investigated systematically over multiple notch depths and valve openings. A geometric effective flow area analysis is combined with experimentally validated numerical simulations. Specifically, the effective flow area is quantified as a function of valve opening and used to interpret the migration of the controlling throttling section along the notch. Three cavitation flow regimes are examined: shallow-notch, deep-notch and valve-orifice migration flow. Results indicate that cavitation is governed by the interaction between shear layers and low-pressure vortices. At small openings, shallow notches promote outlet-induced low-pressure vortices associated with a wall-attached jet. In contrast, deep notches favour bend-induced separation near the upstream bottom region, leading to stable attached cavitation. With increasing valve opening, the effective throttling section migrates. This migration reorganizes the internal pressure gradient. It suppresses vortex-dominated regions, enhances boundary-layer separation and wall shear, and shifts cavitation inception from the notch bottom toward the upper throttling edge. These findings provide a unified physical interpretation of cavitation regime transitions in USN spool valves and offer guidance for notch design aimed at cavitation control.