This paper examines an unusual annular erosion footprint observed during static exposure of metallic surfaces to an ultrasonically excited pulsating water jet (PWJ). The morphology is characterized by a comparatively preserved central region surrounded by a ring-shaped erosion zone, indicating that cumulative erosive loading is preferentially distributed away from the jet centerline. Circular and crescent-shaped footprints are considered as reference morphologies to illustrate different spatial distributions of PWJ-induced erosion. The annular footprint is interpreted as evidence of peripheral loading rather than direct evidence of a hollow-cone liquid structure, because several mechanisms may contribute to its formation. A hollow-cone-like impact structure remains one possible hypothesis requiring direct experimental verification. Static PWJ exposure can thus reveal spatial characteristics of erosive loading that may be obscured under dynamic traverse conditions. Recognizing such non-uniform impact regimes may improve control of PWJ surface treatment and provide useful guidance for optimizing nozzle and acoustic-system design where a defined spatial distribution of erosive loading is required.
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