Study on the Influence of Characteristic Scale Ratio of Irregular Wall on the Impact Characteristics of Drilling Water Jet

Abstract The impact of drilling water jets is closely related to the structure of the impact surface. The actual inner wall of the borehole has geological structures such as randomly distributed concave, convex, and irregular peeling surfaces. However, existing theories still have limitations in describing jet behavior under non-uniform wall conditions, particularly in explaining the mechanism of jet energy redistribution due to the coupling effects of concave and convex structures. This makes it difficult to accurately predict the impact effect in actual drilling scenarios. To address this issue, this article systematically investigates the influence of water jet impact characteristics under varying characteristic scale ratios using numerical simulation methods. The results show that concave/convex structures significantly influence jet impact characteristics through scale-dependent mechanisms. For concave structures, when α < 0.15, cavity convergence dominates and enhances bottom shear effect. When α > 0.15, flow separation-induced energy dissipation becomes predominant. Smaller depressions (α < 0.1) expand the jet action range, while larger ones confine the impact area. For convex structures, increasing β creates a “focusing effect” that shifts shear action from uniform to concentrated distribution. At α = 0.35, the vertex shear rate reaches 7.9×105s−1, while the sidewall remains below 1×105s−1. Engineering applications should employ an intelligent operation sequence of “convex first, concave second, with graded processing” using differentiated parameters. This strategy achieves synergistic optimization between local intensity and overall efficiency, providing theoretical guidance for improving drilling performance.