The surface erosion of soil samples caused by an impinging jet can be analyzed with the Jet Erosion Test (JET), a standard experimental test to characterize the erosion resistance of soils. Here 1Benseghier, April 24, 2019 we specifically address the flow characteristics of a laminar impinging jet over the irregular surface of granular beds to discuss the pertinence and relevance of commonly used empirical estimations based on the self-similar model of a free jet. The JET is here investigated at the micro-scale with a coupled fluid-particle flow numerical model featuring the Lattice Boltzmann Method (LBM) for the fluid phase combined with the Discrete Element Method (DEM) for the mechanical behavior of the solid particles.We confront the hydrodynamics of a laminar plane free jet with the results from a parametric study of the jet impingement, both on solid smooth and fixed granular surfaces, which take into account variations of the particle size, of the distance from the jet origin, and of the jet Reynolds number. The flow characteristics at the bed surface are here quantified, including the maximal values in tangential velocity and wall shear stress, which can be regarded as the major cause for particle detachments under hydrodynamic solicitation.We show that the maximal velocity at the impinged surface can be described by the free jet self-similar model, provided that a simple empirical coefficient is introduced. We further propose an expression for the maximal shear stress in laminar conditions including a Blasius-like friction coefficient that is inversely proportional to the square root of the jet Reynolds number.To conclude, we finally analyze the JET erosion of different cohesionless granular samples, confirming that the threshold condition at the onset of granular motion is consistent with the Shields diagram and also in close agreement with previous experimental results.

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