Experimental Study of Time-Averaged Flow and Turbulence Structures over Low-Angle Tidal Dunes under Steady Bidirectional Flows

Abstract. Large-scale, high-resolution flume experiments were conducted under representative steady bidirectional flows in a large recirculating flume to investigate the time-averaged flow and turbulence structures over two-dimensional (2D) fixed tidal dunes. Two dune morphologies were considered for representing asymmetric dunes with low to intermediate-angle slopes. The dune morphologies are an idealised representation of natural tidal dunes. Specifically, the study aims to characterise in detail the influence of dune morphology on the properties of flow separation zones and turbulence structures above these dunes subjected to bidirectional flows, which are a representation of tidal flows. Results show that a smaller permanent flow separation zone is found over the tested intermediate-angle dune (mean lee slope of 12°, steep face of 22 compared to those found over angle-of-repose dunes detected close to the bed where a larger intermittent flow separation is found. The corresponding turbulent wake expands downwards to the trough before dissipating further downstream. Over the tested low-angle dune (mean lee slope of 10°, steep face of 15°), both small permanent and intermittent flow separations are observed. When the flow is opposed to the dune asymmetry and flows over a very gentle side (4°) with a short steep portion (10°), only a very small intermittent flow separation is detected. Over only a straight gentle side (4°), both flow separations are nonexistent. Neither intermediate- or low-angle dunes exhibit a distinct turbulent wake when the flow is opposed to the dune asymmetry, and a turbulence structure similar to that under a flatbed condition can be observed. Large-scale turbulence associated with both types of dunes is observed through the high occurrence of energy containing ejection and sweep events as revealed by quadrant analysis. Overall, our study demonstrates the significant impact of dune morphology, particularly the mean lee slope and steep face on the emerging flow and turbulence structures above intermediate- to low-angle tidal dunes.