Near Threshold Sediment Transport by a Forced Jet Impinging on a Mobile Sediment Bed

Abstract

Although sediment transport has been extensively studied in the past, flows such as rotorcraft brownout with large-scale coherent structures call many of the simplifying assumptions into question. The objective of this study is to develop a model for the prediction of sediment removal, referred to as erosion, based on independent measurements of the single-phase flow and the evolution of bedforms on the surface of a mobile sediment bed. A series of phase-resolved particle image velocimetry (PIV) flow measurements have been conducted to quantify the stress induced by an acoustically forced impinging jet, analagous to tip-vortices within the rotor wake. The threshold conditions for incipient particle motion are quantified through a series of PIV measurements of the single-phase flow at conditions found to produce quantifiable erosion of the surface. A force balance approach is used to develop a model, following the theory presented by Bagnold (1966), to predict the transport of sediment due to the stress above the theshold. A series of surface elevation measurements are analyzed to quantify the removal of sediment, for the evaluation of the predicted model. An additional series of PIV measurements are performed on a prototype bedform, modeled after the observed bedforms, to quantify the changes in the flow field caused by their developement. The proposed model is shown to provide a better prediction of the observed erosion than classical sediment transport models, especially for cases close to the threshold conditions. For higher speed cases however, the model dramatically over predicts the observed erosion. Several physcially-based explanations are provided for this kink in the trend.