Turbulent transport in the outer region of rough-wall open-channel flows: the contribution of large coherent shear stress structures (LC3S)

Abstract

Acoustic Doppler velocity profiler (ADVP) measurements of instantaneous threedimensional velocity profiles over the entire turbulent boundary layer height, δ, of rough-bed open-channel flows at moderate Reynolds numbers show the presence of large scale coherent shear stress structures (called LC3S herein) in the zones of uniformly retarded streamwise momentum. LC3S events over streamwise distances of several boundary layer thicknesses dominate the mean shear dynamics. Polymodal histograms of short streamwise velocity samples confirm the subdivision of uniform streamwise momentum into three zones also observed by Adrian et al. (J. Fluid Mech., vol. 422, 2000, p. 1). The mean streamwise dimension of the zones varies between 1δ and 2.5δ. In the intermediate region (0.2 < z/δ < 0.75), the contribution of conditionally sampled u w events to the mean vertical turbulent kinetic energy (TKE) flux as a function of threshold level H is found to be generated by LC3S events above a critical threshold level H max for which the ascendant net momentum flux between LC3S of ejection and sweep types is maximal. The vertical profile of H max is nearly constant over the intermediate region, with a value of 5 independent of the flow conditions. Very good agreement is found for all flow conditions including the free-stream shear flows studied in Adrian et al. (2000). If normalized by the squared bed friction velocity, the ascendant net momentum flux containing 90 % of the mean TKE flux is equal to 20 % of the shear stress due to bed friction. In the intermediate region this value is nearly constant for all flow conditions investigated herein. It can be deduced that free-surface turbulence in open-channel flows originates from processes driven by LC3S, associated with the zonal organization of streamwise momentum. The good agreement with mean quadrant distribution results in the literature implies that LC3S identified in this study are common features in the outer region of shear flows.

Shear velocity estimates in rough‐bed open‐channel flow

“…It is therefore well suited for field measurements. The wall similarity method can be further simplified by only measuring a time series of u 0 , if the empirical relations between the turbulence intensities exist Raupach, 1981, Hurther et al, 2007. We will investigate the validity of these relationships in our analysis.…”

“…Measured profiles were cut at z/h = 0Á9, since a thin boundary layer in the near surface water layer is generated by the instrument housing (Figure 1b; Hurther et al, 2007). The bottom level was identified from the velocity information (u = 0), and confirmed by the corresponding change of backscatter intensity.…”

Coherent structures in flow over hydraulic engineering surfaces

“…Several observations report structures resembling LSMs that grow up from the wall and reach the surface (Roy et al 2004, Hurther et al 2007, Nikora et al 2007. Surface lengths of 3-5h are reported, but observed widths of 1h are somewhat smaller than the 1-1.5h width of turbulent bulges.…”

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).…”