Turbulence anisotropy around bridge piers in seepage affected sand bed channel

Abstract

In this work, experimental investigations have been pursued to analyse the Reynolds stress anisotropy in the flow around the bridge pier for no seepage and downward seepage. Experiments were conducted in the non-uniform sand bed channel with circular piers of 75 mm diameter. The streamwise velocity and Reynolds Shear Stress was observed to be maximum near the edge at upstream and near the bed at downstream of pier. The strength of reversal flow diminished with downward seepage. The turbulent kinetic energy at upstream of pier found to be decreased with seepage. Decreased Strouhal number with seepage indicates the diminishing strength of wake vortices. The results present the estimation of the deviation measure from the isotropic turbulence in terms of Reynolds stress tensor for whole flow depth (within and above the scour hole zone) at the upstream and downstream sections of the pier. The streamwise profile of anisotropy tensor within the scour hole zone of the upstream section demonstrates a lesser anisotropic stream in the presence of seepage flow, while transverse and vertical components of anisotropy tensor provide the higher anisotropic stream. The results are quite the opposite in the case of the downstream section of the pier. The present study also analysed the anisotropic invariant maps in terms of Lumley triangle, Eigenvalues, and the invariant functions for the whole flow depth. The anisotropic invariant maps inclining to be two-component isotropy within the scour hole zone for both the section of the pier. With the increase in flow depth that is at the edge of scour hole, the data sets of anisotropic invariant maps show a trend of one-component isotropy, while it has an affinity to develop three-component isotropy near the free surface. Invariant function measurement presents better two-component isotropy within the scour hole zone and quasi-three component isotropy in the outer zone of scour hole for the upstream section of pier. The experimental results provide a qualitative understanding of the evolution of the Reynolds stress anisotropy tensor in the pier-affected alluvial channel.