Spectral analysis of turbulent flow and suspended sediment transport over fixed dunes

Laboratory measurements of turbulent fluctuations in velocity and suspended sediment concentration were obtained synchronously over fixed two-dimensional dunes in a sediment-starved flow. Contour maps of turbulent flow parameters demonstrate that the flow separation cell and a perturbed shear layer are the main sources of turbulence production and that the distribution of suspended sediment is controlled by spatially dependent macroturbulent flow structures. Spectral analysis reveals that peak spectral energies generally occur at 1-2 Hz for the streamwise velocity component and 2-4 Hz for the cross-stream and vertical velocity components. Spectra show larger and better defined energy peaks near the shear layer. Peak spectral energies for suspended sediment concentration occur near 1 Hz throughout the flow. Squared coherency values for cospectral analysis of velocity and sediment concentration are insignificant. Integral timescales for velocity range from 0.20 s for the streamwise component to 0.06 s for the cross-stream and vertical components. Integral length scales for velocity range from 0.065 to 0.135 m for the streamwise component, which is comparable to flow depth, and from 0.020 to 0.030 m for the cross-stream and vertical components, which is comparable to dune height. For suspended sediment concentration, integral timescales and length scales are similar to the streamwise velocity component.