Erosion Mechanisms below a vertical plain Jet on a Non-Cohesive Soil Bed

This experimental research is focused on fail mechanisms in non-cohesive soil beneath the jet with a Jet Erosion Test(JET). A vertical plain jet impinges fluid to a sand bed, which causes zero shear stress beneath the jet averaged over time. Many erosion theories would say that no pick-up happen at that point. E_c=u/(√(H⁄d) √(ΔgD50 ))Large range of erosion parameter was done in this research with video recordings to the activities beneath the jet. Tests varies with those parameters outflow velocities, stand off distances(SOD) and grain sizes. Comparisons are made to Rajaratnam, who has executed similar JET with varies erosion parameter too. The cavity depths and hill heights correspond well as function of the erosion parameter. Cavity widths have deviations for values above 4.0, which might cause by different grain properties or by the limitation of flume width. Secondary flow limits the width growth over time.Weak deflected jet showed only surface erosion by micro turbulence at the soil bed till erosion parameter 1.0. One smooth bed was formed between two hill tops. Static bed did not exceed the concerning sand during and after start. Increasing further the erosion parameter (1.0-5.0), soil deformation was also seen with ejection beneath the jet. Bearing capacity by Prandtl's theory was insufficient and failed the soil bed. Consequence, the inner cavity exceeds the internal friction angle of sand with dynamic erosion. Erosion parameter larger than 5.0 had a wider cavity. Grains in soil translate and rotate in the start phase by insufficient bearing capacity. Vortices sweep around the jet with chaotic particle transport in the inner cavity. Theories of Van Rijn, Meyer Peter Müller and Van Rhee underpredict for JET. The pick-ups rates are in all cases lower, which are based on the shear stress on bed. The impinging jet has different flow field and failure mechanism, which can be better related to the jet momentum.