Turbulent Kinetic Energy Spectra in Single-Sized Sediment Flows with Different Sediment Concentrations Analyzed by Interface-Resolved Large Eddy Simulations

Abstract

Abstract Detailed measurements of motions, including turbulence structures, in high-concentration sediment flows are difficult, and their dynamics remain poorly understood. Interface-resolved large eddy simulation (IRLES) is a numerical simulation method that uses computational grids smaller than individual particles to resolve the flow using large eddy simulation and simultaneously solves the particle–fluid coupled motions. In this study, the turbulence structure of sediment flows was analyzed using IRLES. The IRLES simulations using gravel-shaped particles were conducted for uniform flows of sediment–water mixtures with uniform grain size under three different flume slope conditions: 6.4°, 10.5°, and 14.4°. The simulated particle volume concentrations of 0.086 (6.4°), 0.173 (10.5°), and 0.298 (14.4°) showed good agreement with predictions of the equilibrium sediment concentration equation, with a maximum error E c of 13%. Turbulence energy spectra of sediment flows were obtained at different heights to analyze turbulent structures. The simulations indicated that, in the steep-slope case, the peak of the turbulence energy for high-concentration sediment flows occurred closer to the free surface than in low-concentration flows. To clarify the characteristics of the turbulent energy spectra of sediment flows in comparison with those of water-only flows, an additional simulation was conducted for the 6.4° slope condition in which the bed particles were fixed. In the water-only simulation, the energy spectra, when nondimensionalized by the local turbulent kinetic energy, collapsed onto nearly the same curve at all heights. In contrast, in sediment flows, the decay of the energy spectrum at high wavenumbers became less pronounced with increasing particle concentration near the bottom of the flow. In high-concentration flows, this trend of reduced spectral decay at high wavenumbers persisted even near the free surface, indicating coexistence of vortices across different scales and complex vortex dynamics near the surface.