| Turbulent flow over an erodible bed is influenced by complex interactions between the fluid, the grains of sediment, and the dunes that form in response. These dunes are frequently three-dimensional in shape. While the turbulent flow response to realistic two-dimensional dune shapes is well studied, such studies have not been made over three-dimensional dunes. Therefore, measurements were made of unidirectional water flow over fixed, artificial, sinuous-crested 3d dunes in the tilting-recirculating flume at the UCSB OEL. Certain aspects of the flow were found to be consistent with those over 2d dunes. These included a largely 2d free surface response and the majority of turbulence production from the shear layer downstream of dune crests. Other aspects of the flow showed 3d influence. Friction coefficients averaged 50% higher for the 3d dunes. Secondary circulations were responsible for much of this increase, offsetting lower Reynolds shear stresses. A depth-integrated model did not adequately predict the flow response. While the total boundary shear stress could not be predicted using the spatially averaged mean velocity, it was successfully predicted by analysis with the complete spatially averaged conservation equations for mass and momentum. These results suggest the need for more robust modeling and measurements to account for turbulent flow response to 3d dunes in the field. |
