Impinging Jets in a Confined Environment: Application for Soil Erosion
Ghaneeizad, Seyed Mohammad
MetadataShow full item record
Impinging jets cause soil erosion in many natural and engineering processes such as landscape craters, headcuts, and plunge pools. Impinging jets also have been employed for assessment of soil erodibility, including determination of critical shear stress. Despite these applications, the hydrodynamics of impinging jets have been poorly understood in regards to soil erosion. This study concerns hydrodynamics of submerged impinging jets and in particular the associated shear stress applied on the impingement plane. It comprises a series of physical and numerical experiments in two confined conditions of a relatively large box with confinement ratio (receiving environment area to nozzle area) of 13,950 and a relatively small cylinder with confinement ratio of 2,210. Physical experiments for the box and the cylinder were performed for flat and smooth bed conditions and under various impingement heights and nozzle velocities. Mean and turbulent characteristics of the jet were measured using two-dimensional particle image velocimetry. In general, the flow in the free and wall jet regions followed self-similarity profiles for unconfined conditions. Results, however, showed that confinement altered both the mean and turbulent characteristics. Two major phenomena were observed in the confined conditions: (1) low entrainment because of limited source of fluid to be entrained into the jet, and (2) an increased rate of momentum transfer in the free jet because of the momentum added by the secondary flow. The maximum bed shear stress distribution was found to be 2.4 times larger than the commonly accepted distribution reported in the literature. A normalization parameter was developed for turbulence intensities of impinging jets, to provide a means of scaling turbulence intensities as a function of distance from the nozzle for the free jet region and of the impingement height for the impingement and wall jet regions. Using this normalization parameter, it was found that confinement affects the turbulence intensities in the free jet, impingement, and wall jet regions of impinging jets. For the lower confinement ratio, the radial component of the turbulence intensity increased by as much as four times in the wall jet region when compared with unconfined conditions. A computational fluid dynamics model was developed for the cylinder and validated using measurements. The model was used to explore jet hydrodynamics in eroded bed conditions. Various scour aspect ratios were investigated. As the radius of scours decreases the jet deflection angle increases and results in a recirculatory flow pattern and decline of maximum shear stress. The maximum shear stress, however, increases significantly when the scour radius becomes lower than about four times the unaffected jet half-width at the entrance of the scour region. Results were used to develop a relation for maximum bed shear stress in eroded bed conditions. This research provides a basis for understanding soil erosion processes by impinging jets for prediction, modeling, and assessment purposes. The findings help to consider effects of confinement and scour hole formation on jet hydrodynamics and applied shear stress. The results improve soil erosion prediction for environments such as headcuts and knickpoints. It also provides improvements for soil erosion assessment techniques such as Jet Erosion Test and Cohesive Strength Meter.