Effect of flow confinement on the hydrodynamics of circular impinging jets: implications for erosion assessment

Abstract

The fluid dynamics of a water jet impinging on a flat surface in confined conditions were studied using particle image velocimetery. The experiments were meant to replicate conditions expected in a jet erosion test (JET) designed to assess cohesive sediment erosion parameters in field applications. High-resolution two-dimensional velocity vectors were measured in a plane passing the jet centerline including free jet, impingement, and wall jet regions within a fixed-wall box. The general flow behavior in the free jet and wall jet regions is in good agreement with the behavior of impinging jets in an unconfined environment. Results show that the entrainment coefficient, however, is lower than values in unconfined conditions, lowering lateral spreading rates. The rate of momentum transfer also increases along the axial direction since the confinement causes secondary flow and recirculation in the box. Wall shear stress is calculated based on extrapolation of Reynolds shear stress and turbulent kinetic energy, where the latter procedure provides more consistent results with expected scour hole shape under an impinging jet. This wall shear stress distribution shows higher values near jet impingement in comparison to previously reported distributions, especially as formulated for the JET under unconfined conditions. The maximum value of wall shear stress is found to be about 2.4 times greater than the commonly accepted value in the literature, and also occurs at a position closer to the impingement point. The shear stress at the impingement point is also close to its maximum value, which is consistent with the expected scour hole shape beneath an impinging jet. These findings have important implications for the use of jet impingement theory to assess sediment erosion, especially in the application of the JET.