Relevance of Free Jet Model for Soil Erosion by Impinging Jets

Abstract

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.

Generalized Shields criterion for weakly cohesive granular materials

“…The remainder of this paper is organized as follows: Sec. II revisits previous results by the authors [48] in the light of recent numerical results [49] for an accurate estimate of fluid shear stress for an impinging jet. Next, Sec.…”

“…In the light of these recent findings, the previous experimental results by Badr et al [53] on the erosion onset of a granular bed by a plane-impinging jet have been revisited, leading to more 034308-4 realistic values of the critical Shields number [49]. The following section follows a similar approach in the present 3D situation and compares it with the recently published results carried out with cohesionless materials using the same experimental device [48].…”

“…A recent numerical study by some of the authors using the lattice Boltzman method (LBM) has highlighted the relevance of the free jet models to quantify the maximal shear-stress τ m at the soil surface for 2D scenarios of laminar jets impinging on either solid or granular surfaces [49]. There the derived expression for the maximal shear stress includes a Blasius-like friction term, i.e., a friction coefficient inversely proportional to the square root of the flow Reynolds number as observed for a laminar flow over a flat plate [54].…”

“…There the derived expression for the maximal shear stress includes a Blasius-like friction term, i.e., a friction coefficient inversely proportional to the square root of the flow Reynolds number as observed for a laminar flow over a flat plate [54]. In particular, τ m is obtained symmetrically at both sides and at a certain distance from the jet axis, while its magnitude can be simply approximated by [49]…”

Characteristic of Erosion Properties and Relationship With Geotechnical Properties for Red Riverbank Soil in Hanoi, Vietnam

“…The measurement technique for determining the soil erosion properties can be carried out in laboratories using a channel flume set up lateral (Bo et al 2011;Yu et al 2015;Patsinghasanee et al 2018) or on the bottom of a water flow channel (Briaud 2008;Badr et al 2002) with the abundant size of length and width as the shape of the flume. A few research used simple soil columns (Benamar et al 2012), submerged impinging jets (Regazzoni and Marot 2011;Montoya 2018;Benseghier 2020) and calculated the soil erosion properties from the previous experiment equations.…”