Scour mechanism and efficiency contrast between under-expanded and expanded air jets in underwater

Selecting an efficient operational mode for air-jet seabed scouring requires understanding how jet expansion state governs both mechanism and performance. This study experimentally compares under-expanded and fully expanded air jets impinging on a non-cohesive sand bed in quiescent water. High-speed imaging reveals two distinct mechanisms: the expanded jet drives continuous viscous shear erosion (VSE), reaching dynamic equilibrium rapidly (0.48-3.9 s) via stable wall-bounded shear flow, whereas the under-expanded jet triggers violent bearing-capacity failure (BCF), requiring significantly longer times (5.9-7.0 s) due to intermittent explosive ejections. Under identical flow input, the expanded jet demonstrated unequivocally superior performance, achieving an 11-24% increase in maximum particle entrainment height, an expansion of cumulative entrainment area by up to 1.9 times, and a remarkable enhancement of the horizontal diffusion rate by a factor of 3.9 to 7.9. Morphologically, its scour profile closely matches the classical shear-driven model ( R 2 = 0.98), while the under-expanded jet yields a concave, non-classical profile ( R 2 = 0.63) with limited downstream transport. The jet expansion state, controlled by standoff distance relative to the Mach disk, thus governs the transition between efficient shear dominated and inefficient explosion dominated regimes, providing a quantitative basis for selecting the fully expanded mode in seabed trenching and similar marine engineering applications.