The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation

[1] Results of a detached eddy simulation (DES) are used to better understand the effects of the mean flow three-dimensionality and secondary currents on turbulence and boundary shear stresses and the mechanisms through which the momentum and Reynolds stresses are redistributed in a strongly curved 193 degrees bend with fixed deformed bed corresponding to the later stages of the erosion and sedimentation process. The ratio between the radius of curvature of the curved reach and the channel width is close to 1.3. The large channel curvature and the point bar induce flow separation near the inner bank and the formation of several strong separated shear layers (SSLs), where production by mean shear dominates. DES shows that in addition to the main cell of cross-stream circulation developing in the deeper part of the bend, several streamwise-oriented vortices (SOV) form at the inner bank. DES satisfactorily captures the distribution of the streamwise velocity and streamwise vorticity in relevant cross sections compared to experiment. Comparison with a Reynolds-averaged Navier-Stokes (RANS) simulation shows that DES predicts more accurately the velocity redistribution and cross-stream motions in the channel. This is because RANS significantly underpredicts the circulation and turbulence amplification inside the cores of the SOV vortices. DES is then used to clarify the influence of the SOV vortices and SSLs on the boundary shear stress. DES reveals the presence of several regions of large amplification of the pressure RMS fluctuations near the inner and outer banks, which can locally increase the bed erosion and affect the bank stability in the case of a bend with erodible banks. The mean flow bed shear stress distribution predicted by DES is significantly different than that predicted by RANS, while DES predictions of the mean flow are more accurate. This means that use of eddy-resolving techniques like DES in mobile bed simulations of flow in curved alluvial channels should result in more accurate predictions of bathymetry.