Hydrodynamics of confined impinging jets and the assessment of soil erodibility

Analysis and prediction of soil and water management problems require accurate assessment of the erodibility of cohesive sediment. The Jet Erosion Test (JET) has been developed to quantify the erodibility of soil and other cohesive sediments based on theory derived for circular impinging jets. The JET is now an ASTM standard used commonly around the world, yet the physical characteristics of the impingement jet in the apparatus have not been evaluated in detail. A recent study by the authors demonstrated that a confined environment produces variations in jet properties, in comparison with impinging jet theory. Here, we report on detailed measurements of shear stress in the JET apparatus acting on a flat plate representing a soil surface to be tested. The flow field was quantified at high temporal and spatial resolution with 2D particle image velocimetry. Results show that (1) applied surface shear stress is significantly larger than that predicted by impinging jet theory and assumed by JET methodology, (2) the maximum scaled wall shear stress increases with increasing nozzle velocity, and (3) the wall jet characteristics are asymmetric within the apparatus. These results provide the basis for understanding the erosion process in the JET. This study should improve confidence in employing the JET for determining the erodibility of cohesive sediments in field applications.