Patterns of turbulent flow structures over dune shaped bed features under the influence of downward seepage

Abstract

The research presents an experimental investigation on change in streamwise flow velocity and turbulent characteristics over a fixed dune shaped bed feature, in the presence and absence of downward seepage. Streamwise velocity, Reynolds Shear Stress (RSS), turbulent intensities, and third order correlation profiles are presented along the flow depth at eleven locations over the dune. Amplification in the values of streamwise velocities, RSS, and turbulent intensities has been observed at the initial sections on the stoss side as well as at the sections on the lee side of the dune under the influence of downward seepage. At the initial and lee side sections of the dune, average values of the streamwise velocity, RSS, and streamwise turbulent intensity along the depth of flow increase by ∼0.2 %−31 %, ∼9 %−50 %, and ∼2 %−30 % respectively, for the 10 % seepage condition as compared to the no seepage condition. For the 15 % seepage condition, there has been found an increase in these value values by ∼3 %−67 %, ∼52 %−150 %, and ∼35 %−43 % respectively, at the considered sections over the dune. Further, it has also been observed that under the influence of downward seepage, bed shear stress increases at the initialand lee side sectionsof the dune. Through the third order correlation studies, occurrence of rushing flow due to seepage was confirmed at the lee side and initial sections of the dune. Further analysis at the wake zone revealed a reduction in the values of second and third-order moments demonstrating greater streamwise flow of Reynolds stress and diffusion of vertical Reynolds stress in streamwise direction as a result of increased velocity owing to downward seepage. Results indicate that the introduction of seepage leads to the creation of large angled dunes with wake zone stretched in the flow direction as well as the same wake zone was found squeezed in the vertical direction.