Size, shape and dynamics of large-scale turbulent flow structures in a gravel-bed river

Abstract

In this paper, we present a detailed investigation of the size, scale and dynamics of macro-turbulent flow structures in gravel-bed rivers. We used an array of seven electromagnetic current meters with high resolution in both space and time to measure the streamwise velocity fluctuations in a gravel-bed river. The array was deployed successively in various configurations in order to quantify the vertical, lateral and longitudinal extent of the flow structures and to estimate their advecting velocities. To depict the spatial and temporal properties of the flow structures, we used space–time velocity matrices, space–time correlation analysis and coherent-structure detection schemes. The results show that the large-scale turbulent flow structures in a gravel-bed river occupy the entire depth of the flow and that they are elongated and narrow. The length of the structures is 3 to 5 times the flow depth while the width is between 0.5 and 1 times flow depth. In spite of the high roughness of the bed, these values are similar to those reported in the literature for laboratory experiments on large-scale turbulent flow structures. The dynamics of the large-scale turbulent flow structures investigated using flow visualization highlight the interactions between the outer flow region and the near-bed region. Our evidence suggests that large-scale flow incursions trigger ejections in the near-bed region that can develop into megabursts that can reach the water surface.

Turbulence structure of open channel flows over permeable and impermeable beds: A comparative study

“…This condition is best satisfied for the five layer low Re experiment, for which the measurement length covers up to four times the flow depth, which is roughly considered as the length of the largest eddies generated in turbulent open channel flows. [31][32][33] We acknowledge that much larger scales have been recently observed in turbulent boundary layers and pipe flows ͑up to 20 times the boundary layer thickness, see Ref. 34͒ but their measurement and detection are well beyond the capabilities of our measurement system.…”

Animal perception in gravel-bed rivers: scales of sensing and environmental controls on sensory information

“…For example, turbulence has 3-dimensionality, is intermittent in time and space over a range of scales and has rotationality (Nikora 2010). These observations have led to the study of turbulence in a more deterministic way and the identification of characteristic coherent flow structures (CFS) in gravel-bed rivers, including eddies of various scales and types that are distributed partly as a function of relative submergence and flow Reynolds number (see Ashworth et al 1996;Roy et al 2004). It is CFS that entrain, transport and mix chemical signals in rivers, resulting in chemical plumes becoming intermittent and concentrated into spatially and temporally discrete volumes that have been called 'parcels' 'streets', 'filaments' or 'vortices' of odour, separated by odourless water (Atema et al 1991;Zimmer and Butman 2000;Webster and Weissburg 2009).…”

Time-Average Velocity and Turbulence Measurement Using Wireless Bend Sensors in an Open Channel with a Rough Bed

“…The turbulent structure of the flow has been well measured and described for these hydraulic conditions using digital particle image velocimetry, large-scale particle image velocimetry, and ADV measurements (Fox et al 2005;Fox and Patrick 2008;Rodriguez and Garcia 2008;Belcher and Fox 2009;. The structure of turbulence for these conditions consists of connected vortex packets that shed from gravel particles and eject away from the bed to form alternating high momentum/low momentum cells termed macroturbulence in the outer region of the flow (Duncan 1970;Adrian et al 2000;Roy et al 2004;Hurther et al 2007;. Turbulence for these conditions tends to make two general imprints on the instantaneous streamwise velocity signal including (1) Scale decomposition and spectral analysis were used to isolate the imprint of shed vortices and macrotubulence upon the instantaneous velocity time series of VBS and ADV data.…”

“…Inexpensive, wireless sensors also show promise for measuring the mean flow and turbulence in pools and transition zones in streams in order that aquatic biologists can link hydraulic diversity with fish habitat conditions and functioning (Hauer and Lamberti 2006). Further, spatially distributed sensors will be useful for verifying the hypothesized double layer of turbulence in streams, which includes connected vortex packets that eject from the bed, and macroturbulence in the outer region (Duncan 1970;Adrian et al 2000;Roy et al 2004;Hurther et al 2007; as well as measure large three-dimensional eddies induced by channel bathymetry and large obstructions (Kwan 1988;Fox et al 2005;Nezu 2005). Finally, as computational fluid dynamics modeling becomes more sophisticated and applied, sensor network measurements of the flow field could be used to calibrate model parameters (Maier et al 2010).…”