Detection of Coherent Structures in Non-smooth Open Channel Flow (Comment on Quadrant Analysis)

Coherent structures in wall turbulence boundary layer transport momentum and provide a means of producing turbulent kinetic energy. In fully developed flow and above viscous sub-layer, different coherent structures such as ejection and sweep and hairpin vortex can be formed which can be detected by various detection methods such as quadrant analysis and analysis of instantaneous velocity values. In the present study, we aim to explore the accuracy and efficiency of these methods in non-smooth bed free surface flows. To this end, laboratory measurements of flow field above a gravel bed were carried out using particle image velocimetry. Despite the fact that the results of both techniques above roughness elements are in agreement, it was not possible to see any coherency between the results of these techniques just between roughness elements. Such incoherency can be resulted by non-zero mean vertical velocity and three-dimensional nature of flow in non-smooth bed condition which makes application of quadrant analysis in this type of flow challenging. Therefore, using analysis of instantaneous velocity values or modified quadrant analysis can be suggested for correct estimation of sweep and ejection.