Effects of higher and lower angle isolated dunes on very-large-scale motions in open channel flows

Dunes are ubiquitous riverbed forms, yet how they modify the most recently documented energy-containing turbulent structures—very-large-scale motions (VLSMs)—remains unclear. To address this issue, high fidelity experiments were conducted in open-channel flows (OCFs) over isolated dunes with contrasting morphologies. Time-resolved, long-duration, and high-resolution velocity fields with extended streamwise coverage were obtained using an in-house multi-camera particle image velocimetry system, enabling detailed quantification of dune effects over long distances with fine spatial resolutions. Consistent with canonical OCFs, VLSMs are evident in both dune cases upstream of the lee face, where dune effects are minimal, as revealed by the bimodal features of the streamwise velocity spectrum, indicating two dominant energy-containing turbulent structures: large-scale motions and VLSMs. The scales and strength of these motions are comparable to those in canonical OCFs. In contrast, over the lee face and downstream side, pronounced dune morphology–dependent effects emerge. For low lee-slope dunes, VLSMs persist throughout the flow depth, whereas for steep dunes, their spectral signature is eliminated in the near-bed region and remains only near the free surface. Furthermore, the strength of VLSMs, measured by their contributions to streamwise turbulent kinetic energy (TKE) and Reynolds shear stress (RSS), is substantially reduced: from 60% to 30% for TKE and from 50% to 15% for RSS in the near-dune region. With increasing distance downstream, dune effects weaken, and VLSM strength gradually recovers.