Comparison of flow characteristics of plane jet impingement on a solid plate and on a sand bed

As compared to the well-researched case of a plane jet impinging on a solid plate, relatively rare attention was paid to the impingent on an erodible sand bed, which induces continuous bed transformations and interactively affects the jet development. The present study measured the flow of an impinging plane jet on a solid plate and on an erodible sand bed, respectively, by using particle image velocimetry technology, and then comparatively investigated the flow structure, main jet development and downstream wall jet development for the two cases. The results revealed that the jet impingement on the sand bed has a longer free jet region than that on the solid plate due to enlarged separation distance induced by localized scouring. The width of the plane jet impinging on the sand bed is larger than that on the plate by as high as five times the nozzle width, due to intensified interactions with the complex vortical structures in the concave scour hole. The impinging angle even decreases to negative values near the sand bed due to upward deflection of flow induced by bedform transformation. For both cases, however, the normalized streamwise mean velocity profiles exhibit universal self-similarity at different zones of jet development: specifically, the main jet and the wall jet satisfy exactly the same exponential function and the power law function, respectively.