Non-Equilibrium Erosion and Suspenion By a Forced Impinging Jet

Abstract:

Traditional approaches to sediment transport are typically based on assumptions of fully developed, equilibrium conditions. Many flows, however, are dominated by the presence of intermittent, coherent large-scale structures, which may not satisfy such assumptions. The current study examines the erosion rates and suspended load produced by a strongly forced air jet impinging on a mobile bed of fine glass beads. A parametric study is conducted using a range of mean flow and forcing conditions (forcing amplitude and frequency) to elucidate the role of the dominant structure on the transport process. Single-phase PIV measurements were used obtain the initial characteristics of the flow (prior to significant erosion). The results show that the use of the time-averaged turbulent stress on the bed cannot be used to effectively predict the location or magnitude of erosion. Instead, it is shown that the erosion rate can be related to the location and magnitude of the periodic stress produced by the vortex interacting with the bed. After an initial transient, the erosion for all cases was observed to proceed at a relatively constant rate. The net removal rate was found to correlate closely to the unsteady stress produced by the periodic structures, and predicted the erosion rate for all of the conditions studied when scaled by the integral of the excess wall stress raised to the power 1.2. The turbulent suspended load and two-way coupling was measured by performing two-phase PIV measurements over a mobile bed. Conditions were studied during the early transients with nominally “flat” conditions, and at a later stage with significant bed forms present. The suspended load for the “flat” conditions was nearly an order of magnitude less than was observed for the “eroded” case, and also showed significant increase in flux along the radial direction.

*This work is supported by the AFSOR under grant FA95500810406