Turbulence in open-channel flows over granular low-tortuosity beds

Hyporheic/surface mass and momentum exchanges are mostly governed by bed tortuosity, porosity and grain size distribution. These determine near-bed mean flow and turbulence. Not many studies explicitly address the differences in turbulent variables between high conductivity and low conductivity bed, which justifies the objective of this paper: to investigate the effects of tortuosity and hydraulic conductivity on the integral length scale, on the parameters of the power spectral density functions (longitudinal and transverse) and 2nd order structure functions and on Reynolds stress anisotropy. Experimental tests were performed in low tortuosity beds (mono-sized glass sphere beads) are compared with the existing database of low conductivity beds of Ferreira et al. (2012), keeping constant the range of values of porosity, Shields parameters and roughness Reynolds numbers. A new database of instantaneous velocities was acquired with Particle Image Velocimetry (PIV) and processed to gather time-averaged velocities and space-time (double-averaged) quantities, namely velocities, Reynolds stresses and form-induced stresses. Turbulence variables obtained from low tortuosity bed are compared with those of the high tortuosity of the existing database, for mobile and immobile bed conditions. The main differences are identified and discussed. They include: increased anisotropy in the flow over high tortuosity beds; smaller integral scale in the near-bed region in the in the flow over high tortuosity beds; different parameters of the 2nd order structure functions. The observations may be used to explain the drag reducing behaviour of the flow over low-tortuosity beds.

This research was partially supported by Portuguese and European funds, within the COMPETE 2020 and PORL-FEDER programs, through project PTDC/CTA-OHR/29360/2017 RiverCure