Structure functions of turbulence through a degraded channel bed

Abstract

This investigation deals with the turbulence characteristics for flow through degraded channel beds consisting of uniform and bimodal sediment particles. Three-dimensional velocity was measured throughout the depth at three different streamwise locations over a degraded bed. The velocity data were then processed to observe the spatial distributions of the longitudinal and vertical velocity components in vector form, turbulence indicator and length scales at different locations. The main intent of the present study was to observe the effect of bed degradation on the intermittency and anisotropy in higher order scaling exponents using the structure functions methodology. The extended self-similarity (ESS) technique was employed to estimate the scaling exponents from higher order structure functions of streamwise velocity component. The results show that below the bed-level (level of the bed surface measured before the degradation started), the scaling exponents deviate more from the theoretical values than that above the bed-level indicating higher intermittency of scaling exponents in the vicinity of the bed. Furthermore, data were analyzed to observe the scaling exponents from higher order mixed structure functions. To this end, the SO(3) symmetry decomposition was utilized to differentiate the isotropic and anisotropic parts of the scaling exponents. The anisotropic scaling exponents obtained from higher order mixed structure functions possess higher values than that of isotropic part. The experimental findings also specify that as one moves close to the degraded bed, the scaling exponents obtained from the higher order mixed structure functions deviate more significantly. However, according to the present experimental results, no significant differences were observed in the values of the scaling exponents and intermittency when they were compared between uniform and bimodal sedimentary beds. It is expected that the results of the present study will essentially lead to new contributions to the existing knowledge in the field of turbulence through degraded and scoured channel beds.