Turbulence characteristics of flow in an open channel with temporally varying mobile bedforms

Abstract

Turbulence of flow over mobile bedforms in natural open channels is not yet clearly understood. An attempt is made in this paper to determine the effect of naturally formed mobile bedforms on velocities, turbulent intensities and turbulent stresses. Instantaneous velocities are measured using a two-dimensional particle image velocimetry (PIV) to evaluate the turbulence structure of free surface flow over a fixed (immobile) bed, a weakly mobile bed and a temporally varying mobile bed with different stages of bedform development. This paper documents the vertical distribution of velocity, turbulence intensities, Reynolds shear stress and higher-order moments including skewness and turbulent diffusion factors. Analysis of the velocity distributions shows a substantial decrease of velocity near the bed with increasing bedform mobility due to increased friction. A modified logarithmic law with a reduced von Kármán constant and increased velocity shift is proposed for the case of the mobile bedforms. A significant increase in the Reynolds shear stress is observed in the mobile bedforms experiments accompanied by changes over the entire flow depth compared to an immobile bed. The skewness factor distribution was found to be different in the case of the flow over the mobile bedforms. All higher-order turbulence descriptors are found to be significantly affected by the formation of temporally varying and non-equilibrium mobile bedforms. Quadrant analysis indicates that sweep and outward events are found to be dominant in strongly mobile bedforms and govern the bedform mobility.

Effect of aspect ratio on developing and developed narrow open channel flow with rough bed

“…For transitionally rough flows, + values range between 2.0 and 4.7 and, similarly + values for fully rough flows are between 7.0 and 7.9. The velocity shift values are comparable to the values available in the literature; 9.7 to 11(Balachandar and Patel, 2002) and 10 to 15.5(Hanmaiahgari et al 2016) and 10 to14(Krogstad et al 1992). The velocity shift values in the present experiments are slightly less because the experiments barely cross the hydraulically rough flow threshold.…”

“…The proposed empirical equation is developed by using data from the present experiments, immobile bed experiments in Hanmaiahgari et al (2016), Balachandar et al (2002), and Balachandar and Blakely (2004). The coefficient of correlation ( Velocity profiles presented in Fig.…”

A Two‐Layer Turbulence‐Based Model to Predict Suspended Sediment Concentration in Flows With Aquatic Vegetation

“…Previous studies have shown a characteristic constant velocity profile above a certain distance from the bed in emergent vegetated flows (Nepf & Vivoni, 2000;D. Liu et al, 2008;Stoesser et al, 2010 (Schultz & Flack, 2007;Hanmaiahgari et al, 2017).…”

Low relative-submergence effects in a rough-bed open-channel flow

“…For the vertical skewness S w = w 3 / w 2 3/2 , positive values are found, with a similar behaviour for h/k = 3 and h/k = 2. In open-channel turbulent boundary layer flows over highly submerged rough beds, Al Faruque and Balachandar (2011) and Hanmaiahgari, Roussinova, and Balachandar (2017) show highly antisymmetric vertical profiles of the streamwise and…”