PIV Measurements of Turbulent Water Flows Over Fixed Low-Angle Compound Dunes Under Reversing Currents
Résumé
This study investigates the influence of surface roughness, current intensity, and direction on tidal flows over asymmetric low-angle dunes using high-resolution particle image velocimetry in a laboratory flume. Experimental measurements reveal that, consistent with previous studies, low-angle dunes generate substantial turbulence through flow expansion and shear, despite inducing only intermittent flow separation. Enhanced surface roughness and current intensity significantly increase turbulence, leading to permanent flow separation, akin to that observed over angle-of-repose dunes. When the flow direction opposes the dune's morphological orientation, weaker turbulent stresses are observed; however, flow expansion at the crest generates sufficient turbulence that persists downstream, impacting the flow field over the next dune. Hydraulic roughness parameters estimated from single and double log-law velocity profiles offer a quantitative assessment of the variability of form roughness along the dunes during both ebb and flood tides. Ejection-sweep cycles dominate turbulent flow in both aligned and opposing flow conditions, albeit with distinct spatial distributions. This study highlights crucial aspects of the interaction between dune morphology and reversing tidal currents, demonstrating that asymmetric low-angle dunes can induce significant form roughness across the different tidal phases. These findings have important implications for flow resistance and sediment transport in tidal environments.
Plain Language Summary Tidal sand dunes are underwater bedforms shaped by tidal currents, influencing water flow and sediment transport in estuaries and coastal areas. Understanding how these dunes interact with reversing tidal currents is essential for predicting changes in water dynamics and sediment movement. This study used advanced imaging techniques in a laboratory flume to investigate how low-angle dunes, similar to those found in nature, affect water flow under varying conditions, including changes in current direction, intensity, and the mean grain size of the seabed sediment. Our experiments revealed that low-angle dunes can generate turbulence and influence flow resistance by creating regions of flow expansion and separation, depending on the mean grain size of seabed sediment, tidal current intensity, and flow direction relative to dune orientation. Furthermore, the interaction of multiple dunes in a sequence showed how turbulence generated by one bedform affects flow over the next, highlighting the importance of studying entire dune fields rather than isolated features. These findings have significant implications for managing sediment dynamics, assessing the stability of offshore structures, and predicting changes in response to tides, storms, and sea-level rise.
